1 - Repositories

PERFORMANCE AND RECOUERY UNDER
PROLONGED
VIBRATION
TAREK ííl. KHALIL, B.IYl.E., (Yl.S. in I.E
by
A DISSERTATION
IN
INDUSTRIAL
ENGINEERING
Submitted to the Graduate Faculty
of Texas Technological College
in Partial Fulfillment of
the Requirements for
the Degree of
DOCTOR OF PHILGSOPHY
Approved

t (S- /
No.-"^ ^•-
(2. uii.''. t2..y
ACK^jüÜli •.nr:ílE')T5
í am cje.jnly i-irieíitra to ür . (T!. '^ Ayoub fjr hii"^
direntior o^ thio disc^ertatlon and t.ú tt^ Í otrer members
oí^ my ?.dv.isoiy conmittee, Drs, Richarc^ A. Dudek, Jeny
D. Rarrsey, Brisn K. Lamberl, and Charles G. Halccib, r or
their helpful advice. Ifianks alço gü te Hflr. James L.
Cibbs for hir, assistance in ::onstruction of the equipi.ent.
This research luas supported by THEIYIIS Contract No.
DAAD05-59-C-01Ü2, betujeen the U.S. Department oF Defense
and Texas Technological College, R. A. Dudek, Project
lYlanager. The contents do not necessarily reflect the official
opinion or policy of the Department of Defense or the
Department of the Army. Reproduction is authorlzed for any
purpose of the U.S. Government.
11

ACKNOliJLEDGIYlENTS
LIST OF TABLES
TABLE OF CONTENTS
Page
ii
v/i
LIST OF ILLUSTRATIÜNS v/iii
Chapter
I. INTRÜDUCTIGN 1
Statement of the Problem 1
Purpose and Scope 4
Revieu; of Literature and Previous Research, , 5
Sources of Vibration and Typical
Frequencies , 7
Subjective Reactions to Uibration
and Tolerance Limits . . , . , 10
Simulation of Human Subjects 17
lYIechanical Simulation of Human
Response to Uibration 19
Effects of Vibration upon Performance , . 27
Tracking Tasks 28
l/ibration Effects upon Vision, Visual
Acuity and Reaction Time 32
Vibration Effect upon (Ylotor Performance
and Body Configuration , , , 36
Summary 38
II. EXPERIfriENíAL EQUIPIYIENT AND flílE ASUREíílEN T 39
Equipment for íílaintaining the Independent
Variables 39
iii

: \j
Page
Vibration Platform 39
Accelerometers 40
Equipment for lYleasuring the Dependent
Variables 45
Performance Console 45
Hand Rest and Control Assembly Device . . . . 46
Seat 48
Signal Playing Apparatus 48
Change of Polarity Circuit 50
Analog Computer 50
Dynagraph Recorder 53
Supporting Equipment for Controlling
the Variables • 56
Signal Recording Apparatus 56
UJide Range Oscillator 56
ITlagnetic Tape Recorder 59
Earphones 59
lYieasuremen t 59
III.
EXPERIfílENTAL DESIGN 61
The Task 61
The Variables ..... 66
Independent Variables 66
The Dependent Variables ..... 75
The Controlled Variables 77
Statistical Design of the Experiment ...... 79

V
Pjge
First Analysis of Variance "9
Second Analysis of Variance ^ 1
Third Analysis of Variance 63
Subjects , 83
Experimental Routine 84
Safety Precautions . 88
IV. FINDINGS AND INTER PRETAT lON S 91
First Analysis of Variance Results 92
Second Analysis of Variance Results 113
Third Analysis of Variance Results 117
V. CONCLUSIONS AND RECOmnflENDAT lONS 127
Conclusions 127
Environmental Effects ..... 127
Period Effect 126
Effect of Ujork/Rest Schedule
by Period Interaction 129
Effect of UJork/Rest Schedule upon
Increase of Error Score due to Vibration
• 132
Transmissibility of Vibration ...... 134
Subjective Reaction to Vibration 135
Recommendations for Further Research .... 135
Summary • 139
BIBLIOGRAPHY 141
APPENDIX 155

LIST OF TABLES
Table
Page
1. Summary of Factors 80
2. Expected Hílean Square for Experimental Design . , 82
3. Subject Information . , 84
4. ANOVA for Absolute Error Score (Normal and
Vibrational Environment) 93
5. lYleans and Standard Deviation of Environment
Effect on Absolute Error Score (Normal
and Vibrational Environments) 94
6. (Yleans and Standard Deviations of Period
Effect on Absolute Error Score (Normal
and Vibrational Environment) . . . . . . . . 94
7. ííleans and Standard Deviation of UJork/Rest
by Period Interaction Effect on Error
Score (Normal and Vibrational Environment). , 99
8. ANOVA for Difference in Average Error Score
(Vibration and Recovery) . , , 113
9. (Yleans and Standard Deviations of UJork/Rest
Schedule Effect on Difference in Average
Error Score (Vibration and Recovery) 114
10. ííleans and Standard Deviations of Period
Effect on Difference in Average Error
Score (vibration and Recovery) 116
11. ANOVA for Difference in Average Error
Score (Vibration) 119
12. lYleans and Standard Deviations of UJork/Rest
Schedule Effect on Difference in Average
Error Score (Vibration) 119
13. ANOVA for Percentage Increase in Error Score
(Vibration) 120
vi

VI 1
Pige
14. (yleans and Standard Deviations of UJork/Rest
Schedule Effect on Percentage Increase
in Error Score (Vibration) 122
15. Natural Frequencies for Various Parts of the
Body 156
16. Specifications of the Accelerometers Used
in this Study 157
17. Calculation of the G Levei Transmitted 158

LIST GF ILLUSTRATIÜNS
Figure
Page
1. Average acceleration measured under operational
conditions for commercial and
military vehicles 9
2. "Short-time" tolerance curves and "long-time"
recommended limits 16
3. The simplest model of the human body is a
one-mass-spring system luith damping 21
4. Parallel and series models for duplicating
the humar> body impedance 21
5. The modulus of the impedance of one subject
at varied body postures compared luith the
impedance of a purê mass (muj) and of a
one-mass-spring system ujith damping 24
6. Phase angle of the impedance for one subject
at varied body postures compared luith
phase angle of a one-mass-spring system
luith damping 24
7. The vibration platform drive mechanism ..... 41
8. Accelerometer mounted on vibration platform ... 43
9. Accelerometer mounted on subject's hip and
vibration stop button 43
10. Accelerometer mounted on subject's head and
earphones 44
11. Accelerometers, amplifiers, pouer supply and
suiitch 44
12. Cathode ray tube and signal lights 47
13. Signal playing apparatus, change of polarity
Circuit, vibration platform activator
and signal siuitch 47
Vlll

IX
Paqe
14. Hand rest and control assembly device 49
15. Subject's controlling position 4?
16. Schematic of the CRT sujeep circuit 51
17. Schematic of the change of polarity circuit ... 52
18. Schematic of the analog computar integrating
circuit 53
19. Vieuj of the analog computer shouiing integrating
circuit connections . 54
20. Dynagraph recorder 55
21. Signal recording apparatus, oscillator
and voltmeter 57
22. Tape recorder, oscillator and voltmeter 57
23. Voltage regulator 58
24. Vieuj of signal and vibration monitoring
equipment 58
25. Generalized illustration of a continuous
closed-loop man-machine system; such
Systems usually are referred to as
tracking systems 63
26. Vieuj of signal recording apparatus shouiing
cem profile 64
27. Recordings on a dynagraph recorder chart .... 65
28. Selected u/ork/rest schedules 69
29. Performance on tracking tasks in relation
to "long time" recommended limits ...... 72
30. Vibration facility 85
31. Subject^s station 89
32. Experimenter station 89

X
Pago
33. Plot of means of absolute error score versus
environment
34. Plot of means of absolute error score versus
period
35. Plot of means of absolute error score versus
uiork/rest period interaction
36. Plot of means of absolute error score versus
period—UJ/R schedule I
37. Plot of means of absolute error score versus
period—UJ/R schedule II
38. Plot of means of absolute error score versus
period—UJ/R schedule III
39. Plot of means of absolute error score versus
period—UJ/R schedule IV
40. Plot of average error score versus time—
Ul/R schedule I
41. Plot of average error score versus time—
UJ/R schedule II
42. Plot of average error score versus time—
UJ/R schedule III
43. Plot of average error score versus time—
UJ/R schedule IV
44. Plot of means of difference in average error
score versus uiork/rest schedule (vibration
and recovery)
45. Plot of means of difference in average error
score versus period (vibration and
recovery) .
46. Plot of means for difference in average error
score versus uiork/rest schedule (vibration) .
47. Plot of means for percentage increase in error
score versus ujork/rest schedule (vibration) .
48. Recording of a subject's hip acceleration
before and during task performance
95
96
98
100
101
102
103
106
107
108
109
115
118
121
123
125

XI
49. Typical response of monitoring accelerometers . .
50. Example learning curve obtained for subject
number 5 during six experimental trials. . . .
51. Plot of absolute error score versus minute
readings—UJ/R schedule I
52. Plot of absolute error score versus minute
readings—UJ/R schedule II
53. Plot of absolute error score versus minute
readings—Uj/R schedule III .
54. Plot of absolute error score versus minute
readings—UJ/R schedule IV
age
158
159
160
161
162
163

CHAPTER I
INTRODUCTION
Statement of the Problem
UJith the increasing sophistication of our society's
technology, ever greater concern is being accorded the
concept of system design.
The criterion of success in
design is usually the overall performance of the system,
or system output.
Recently conslderable interest has been manifested
in the application of the concept to man-machine systems.
An understanding of both man*s capacity and limitations,
and the factors affecting them, is essential in designing any
successful man-machine system.
This is especially true since
the occurrence of recent developments in Space, industrial
and military situations, in u/hich man, the monitor link in
any man-machine system, must perform efficiently for long
periods of time.
Such a situation has been encountered in
many modern systems and u/ill be met Uíith in future ones;
situations in luhich personnel cannot be added or exchanged
during extended spans of time, and in luhich, during these
interims, highly efficient performance by creuj members is
of the utmost importance.

Adopting a definite ujork/rest schedule throughojt
UJhich operators' phases of ujork and rest ensure a high
probability of efficient work performance, is one solution
to this problem. UJork/rest schedules have been investigated
in situations in ujhich durations of missions luere thirty
hours or more, and the requirement for each ujas around the
clock performance of one or more tasks.
Some environmental
factors, such as vibration, impose limitations upon man's
safe exposure period.
For example, an hour of continuous
vibration such as is experienced during many missions, is
considered a long period of exposure at certain
frequencies
and leveis of acceleration (g). (Íílaintaining high levei
performance by means of the most efficient u/ork/rest schedules
for creui members during missions is of serious concern
to system designers.
Vibration is one of the basic problems confronting
the design engineer.
It is usually imparted from surrounding
equipment to its operator or to the occupant of a vehicle
or space.
UJhen man is operating a vehicle, whether on the
ground, the sea, or in Space, he is exposed to vibration.
The vibratory conditions may last as long as the mission
itself and the vibrational effect upon the operator may be
a major limiting factor in obtaining
the highest levei of
performance from the system.
The effects of vibration upon human performance are
particularly
relevant to military and industrial situations.

and have not yet been ujell investigated.
Basic research,
including a study of the effects of intersessions as wall
as intrasessions of vibration, is believed to be greatly
needed.
either.
The nature of recovery is not well investigatea,
The term "recovery" here may include task as well
as environmental recovery.
Hopefully, an understanding of
the nature of recovery will lead to better system designing.
In discussing the importance of investigating
the
problem of performance and recovery under vibration
Johnston
says (1969):
Performance decrements during and immediately
following exposure to whole-body vibration and
time for complete recovery are not cleariy defined.
Although comfort leveis have been fairly well
established, and recently much work has been done
on performance effects, little work has been done
in relating the duration of vibration exposure to
human performance and in studying the recovery period
following vibration exposure.
Analysis of performance as a function of the duration
of vibration is not well developed in the literature, while
postvibration and previbration periods are seldom even
discussed.
Of those who did discuss some of these points, were
Harris and Shoenberger
(1955), who said:
In addition, studies should be undertaken to
determine the effects of long-term vibration. It
is possible that an adaptation effect takes place
by the individual learning to overcome the mechanical
interfering effects of vibration; however, it is also
possible that the individual becomes more sensitive
to vibration due to fatigue. In fact, both adaptation
and sensitization may occur at times in both experimental
and operational situations.

Harris and Shoenbercer also emphasized the need
for investigating intiasecsional and intersessional
effects of vibration in studies of long-term situations.
To provide greater insight regarding some of
the problems discussed above, it is evident that some
basic research is required.
It is hoped that the present
study will affect a contribution to the subject, and that
its results may be applicable to practlcal situations.
One such application is the designing of a crew work/rest
schedule for the achievement of best performance during
a mission.
Another might be in military and commercial
situations in which vibration may cease, while a pilot,
for instance, must continue working or aiming for a further
short period of time.
Purpose and Scope
/ The purpose of this research was to study the
performance and recovery characteristics of man when subjected
to relatively long periods of whole-body, vertical
vibration.
The effects of a work/rest schedule and environment
on performance and recovery were investigated,
Analysis
of the performance in terms of the duration of vibration
exposure was also undertaken.

The vibration environment under investigation is
in the low-frequency range.
Specific points of interest
and investigation are:
1. Vertical vibration
2.
3.
Vibration frequency of 5 cps
Constant amplitude of 0.08 inches
displacement of 0,16 inches)
(total
4.
5.
Vibration intensity levei of about 0.20g
Vibration exposure time up to 60 minutes
The entire experiment was accomplished by simulating
a certain mission requiring that a vertical tracking
(performance
measuring) task be monitored at different times
during the mission, according to a specified work/rest
schedule. j Further discussion will occur later in this
study.
The task was performed in both a normal controlled
environment, and in a low-frequency vibration environment,
where man-task system was subjected to the vibrational
stress.
Review of Literature and Previous Research
A brief definition of vibration and of the terminology
employed in its study is believed pertinent, and
will be given; after which a review of the literature and
of previous research will be pursued.
The two common modes of vibration are sinusoidal
vibration and random vibration.
Sinusoidal vibration is

the motion of a body or system that is repeated after a
given interval of time known as the period (Church, 1964).
The number of cycles of motion per unit of time is called
the frequency.
The maximum displacement from equilibrium is
the "amplitude" of vibration.
The total travei is twice
the amplitude and is usually referred to in the literature
as double amplitude (DA).
Sinusoidal vibration may be
classified into two types and along various axes.
The
present study deals with a steady state vertical vibration.
Random vibration is one in which the frequency and amplitude
do not show except as a statistic, and only the probability
of future occurrence can be stated.
The acceleration
of random vibration is usually expressed in average
measures such as root-mean-square (RdflS) associated with
bands of frequency componente (Holland, 1967).
A vibratory
system may be comprised of a mass, a spring and/or
damper, or a number of these together.
Every system has
a natural frequency, and, in addition, a forced frequency
upon application of externai force.
If the frequency of
the externai excitation is the same as the natural frequency,
resonance takes place.
Resonance in this research
is taken to mean an amplification of the input acceleration
by the human body.
The number of natural frequencies
is equal to the number of coordinates, or degrees of freedom
specifying the system.
A system with one degree of
freedom is characterized by one resonant frequency.

The review of tine literature ojill proceed by tne
steps shown below:
1. Sources of vibration and typical frequencies
2. Subjective effects and tolerance limits
3. (Tílechanical representation of the human
body transmission
4. Effects of vibration upon performance
Sources of Vibration
and Typical Frequencies
Early man was not concerned, apparently, with
minor vibrational conditions existing in his environment.
Uj'hen he experienced oscillatory disturbances while walking,
running, riding on an animal, or even in a conveyance
after he invented the wheel, he was still unaware of the
effects of these oscillations upon his well-being and the
efficiency of his performance.
It was not until recent
technological advancements and increased variety in forms
of both transportation and stationery machinery that he
became aware of and concerned about the problem of vibration.
One aspect of this concern pertained to physiological
well-being, the relationship between this and vibration
becoming apparent when Clayberg
(1949) indicated the effects
of rough driving upon the spine and supporting structures.
Fishbein and Salter (1950) lent further credence to this
view.
Another aspect of concern was that of impairment
of man's performance upon his subjection to vibration, an

o
observation with which many investigators were in accord,
as will be elaborated upon later.
(ílorrow (1963) indicated that there was a general
agreement about the equipment situation in which shock and
vibration are to be expected.
He listed the most important
of these situations as follows:
1. Handling
2. Transportation by trucks or similar vehicles
3. Transportation by train
4. Transportation or use in piston-engined manned
aircraft
5. Transportation or use in manned jet aircraft
6. Use in missiles involving pulse jet, ram jet,
or rocket engines
7. Blasts from nuclear and other bombs, and from
guns
As applied to humans, such conditions and similar ones
during use of Space vehicles, and equipment developed for
exploration of Space, will arise.
Types of shock or vibration
may differ; consequently, effects upon man*s physiological
well-being and performance may be varied.
Investigators have tried to specify the different
frequencies and other vibrational conditions to which man
is subjected in the foregoing situations. Radke (1958),
in discussing vehicular vibration, maintained that only
vibration under 10 cps may be considered a ride problem.
The higher frequencies are easily and generally attenuated.
He classified commercial and military vehicle vibrations
into frequency ranges between 1 and 10 cps.
Figure 1 shows
the average acceleration of earth moving equipnent measured
under operational conditions.
Holland (1967) maintained

2.0
1.8
FREQUENCY RAr^uE
RUBBER TIRED
EARTH (Y10Vn\IG
EQUIPÍTIENT
1.6
1.4
FREQUEfgCV RANGE
FARnf TRACTORS &
HIGHUJAY TRUCKS
CJ
QC
LLJ
>J
UJ
LJ
et
1.2
1.0
5 0.8
HIGHUJAY
TRUCK
• RUBBER TIRED EARTH
ílflOVI'\IG EQUIPiyiENT
O TRACK TYPE TRACTORS
CI> FARíll
TRACTORS
tr
LJ
0.6
EQUENCY RANGE
ACK TYPE TRACTORS
0.4
0.2
O
4 5 6
FREQUENCY,
CPS
8 10
Fig. 1.—Average acceleration measured under
operational conditions for commercial and military
vehicles. (Adapted from Harris and Crede, 1961)

10
that the range of frequencies from 1 to 6 cps corresponus
roughly to frequency response characteristics of aircraft..
Seris and Auffret (1965) found that helicopter occupants
experience vibration of frequencies varying from 1 to 35
cps, with the range of frequency from 2 to 6 cps causing
the maximurr^ disturbance and the greatest difficulty in
devising
protection.
It is to be noted that in almost ali vehicles the
man functions in a seated position. This position, according
to Radke (1958), robs him of his own natural vibration
attenuators: his legs. The seat, moreover, may amplify
the vehicle motion. The degree to which this is true
depende, of course, upon many other factors, such as the
type of seat and its transmissibility
Subjective Reactions to Vibration
and Tolerance Limits
A number of investigations have been made in an
effort to determine subjectively either the tolerance frequency
and amplitude limits of vibration, or the threshold
of sensation. These investigations, although yielding
different results, are in reasonable agreement insofar as
general trends are concerned. A comprehensive review of this
subject will be given here because of its relevance to this
study and ali other studies relating to vibrational stress.
ÍKIallock (1902) found that an amplitude of one onethousandth
of an inch, and a frequency of 10 cps, comprises

11
a source of annoyance.
He maintained that an acceleratiof
as low as 0.01 g is noticeable, and becomes unpleasant
when it reaches 0.05 g.
lYlelville (1903) suggested a formula to be used in
calculating leveis of annoying vibration.
According to
his formula, a frequency of about 1.67 cps, and an amplitude
of one inch, is considered an annoying vibration.
Digby and Sankey (1911) suggested that susceptibility
to vibrations depends upon velocity.
Reiher and (Ifleister (1931) investigated several
amplitudes and frequencies and their relationship to degree
of human susceptibility, classifying their results into
the following zones:
1. Imperceptible
2. Just perceptible
3. Easily Noticeable
4. Annoying
5. Unpleasant
6. Painful
Subjects were tested while standing or lying down on the
vibrating platform.
Reiher and (Tleister maintained that vertical vibration
is more noticeable than horizontal vibration of the same
intensity, when the body was in the supine position.
Jacklin
(1933), in a pioneer study, used subjective
judgments as relating to seated people.
He confined his

12
observations to three easily distinguishable reactions:
1. Perceptible: one feels that he is moving, or
that distant objects are moving
slightly.
2. Disturbing: one notes that certain organs
or parts of his body vibrate more than his
total body, and tries to prevent
this condition
by tightening certain muscles.
3. Uncomfortable: one wishes very little of
this treatment.
Jacklin analyzed several thousand observations upon
the reaction of humans to vibrations, working with a platform
and soft automobile seats.
From the results, he
computed a combined index of "riding
comfort."
Helberg and Sperling
(1941) experimented with
leveis of frequency ranging from 1 to 2 cps and amplitudes
of up to one inch.
They constructed a chart of comfort
zones.
Janeway (1948) recommended these definite limits
of vertical vibration as comfortable for humans:
1. Low Frequency Range, f=l to 6 cps constant
maximum
"jerk" limits.
ar^- 2
i.e., the sensation of vibration depends upon the
rate of change of acceleration.
At the tolerable limit the
relationship between magnitude and acceleration
can be
expressed as the above formula.

13
2. (Yliddle Frequency Range, f « 6 to 20 cps, the
limit is dependent upon accalaration, and the relationship
between magnitude and frequency is
af^ = 1/3
3. High Frequency Range f « 20 to 60 cps, the
limit appearlng to be directly proportional to the velocity
of the movement. Constant maximum velocity limit
af » 1/60
UJhere f •« frequency in cycles per second, and a = displacement
(amplitude) in inches.
These limits were based upon subjective judgment of
vibration.
Goldman (1948) reportrd average peak accelerations
at various frequencies at which
1. Subjects perceive vibration
2. Find it unpleasant
3. Refuse to tolerate it further.
Exposure time was 5 to 20 minutes. The Goldman
studies were oriented toward the automotiva passenger.
Brock (1956) constructed a chart, similar to the
one given by Reiher and ITIeister, basing it upon vertical
sinusoidal vibrational excitation.
Gorrill and Snyder (1957) instructed five subjects
to judge the levei of vibration while performing a tracking
task. These subjects were air crewmen. They wore full flight
gear and were restrained by leg belts and shoulder harnesses.

14
They were asked to
define:
1. Probable detection of vibration
2. Probable maximum operating time.
They were exposed to vibrational severities varying
between
frequencies of from 3 to 30 cps,
Ziegenruecker and (Iflagid (1959) used
reinforced
seats, lap belts and shoulder harnesses to strap their
subjects. The latter were asked to define the limit of
sinusoidal acceleration at different frequencies to which
a rider would be willing to go before it seemed that actual
bodily harm might occur. Short time tolerance curves were
reported in this
study.
llflagid and Coermann
(1960) subjected 15 individuais
to vibrations to test for the acceleration
(in g units)
which they could tolerate in a seated position. Three
durations of tolerance were determined, namely, "short time,"
one minute, and three minutes.
The subject was restrained
Uiith seat belt and shoulder strap. Sinusoidal excitation
was applied at various frequencies with increasing amplitude
at 0.75 millimeters per second
(DA) until he reported his
belief that bodily harm would result from further increase.
The results indicated a minimal short time tolerance of
between four and eight cycles per second, at accelerations
of between 1.5 and 2.0 g.
Parks (1961) used 16 physically fit maie subjects to
derive subjective, standardized leveis of reaction to vibration

15
Four leveis were determined:
1. Definitely perceptible
2. lYlildly annoying
3. Extremely annoying
4. Alarming
The leveis were significantly different, and each
varied in acceleration as a function. of frequency.
Linder
(1962) studied personal threshold curves
for vertical vibration proposed by Lippert, Janeway, Gorrill,
Ziegenruecker and lYlagid and others.
He then composited a
curve which he recommended as an upper design limit for
not more than one hour's continuous exposure of an individual
under the most extreme predicted conditions.
This curve
is shown in Figure 2, which also summarizes the tolerance
curves extracted from some of the prevlously mentioned
studies.
Harris, et ai. (1965), taking a more conservative
attitude, suggested that LindBr's curve is too high for
one hour of continuous exposure, throughout the frequency
range.
They suggested a lower tolerance curve, which is
shown in Figure 2.
Chaney
(1964), utilized the descriptive leveis of
Parks to establish a series of subjective curves demonstrating
the response of seated subjects to sinusoidal vibration.
He
compared
the curves obtained in his studies with those

/
16
\
2.0
1.0
\
^
i
^ V \\
V ^ ^ ' ^^ Short Time
One r/lin. (íYlagid,et ai.)
\ \
/ •
Three ÍTlin.
\ \
/
\ ^ / /
\ ^^^^ I
\ f \\ I y^
Alarming
(Parks)
LD
.5
^./
o:
cr
Ul
CJ
(_)
C l
.2
\
\
\
\
\
^>^ ^—*. One Hour or Less (Linder)
et
UJ
a
^ One Hour or Less
"^ (Harris & Shoenberger)
.1
Long Time Tolerance Curve
For (Tlilitary Aircraft (UJADC)
.05
\
Prolonged Exposure (Janeway)
.02
^...^-
I I 1 1 1 1 1 i
O 6 8 10 12 14 16 18 20
FREQUENCY (CPS)
Fig. 2. — "Short-time"tolerance curves and "longtime"
recommended limits. (From Harris and Shoenberger,
1965)

17
obtained by Gold.T,ann, (yiagic, et ai. , Gorrill and Snyder, and
Parks and Snyder studies.
Studies on human subjective reaction to vibration
produced different results depending, of course, upon the
conditions and restrictions of the study.
Several investigators
tried to relate these studies and combined their own
results with them to reach a compromise levei of subjective
reactions.
It appears that in spite of the individual
efforts of many investigators in this field there is no
clear cut definition
for leveis of subjective response.
No serious attempts were made, either, for issuing standard
data upon the recommended leveis of vibration in different
situations and the limits that should be considered in
design situations.
The studies provided the means, however,
of selecting the vibration leveis and the safe exposure time
limits used in this experiment.
Simulation of Human Subjects
Experimenters cannot afford to subject their human
subjects to leveis of shock and vibration which might affect
their well-being.
In order to detect tolerance limits and
injury leveis, however, it is important to run experiments
possessing various degrees of potential hazard. Animais are
used first for detailed physiological studies, then with a
reasonable probability, safe limits for human subjects are
deduced from these studies, Goldman and Von Gierke (1961)
criticize this procedure for its obvious limitations:

18
The different structure, size, and weight of
most animais shift their response curves to mechanical
forces into other frequency ranges and to other
leveis than those observed on humans.
Of course, this is besides the known physiological
differences between species.
Still, if good scaling laws
are established, studies on animais can be useful.
Animais
as mice, pigs and dogs are commonly
used.
Anthropometric dummies
(sometimes called anthropomorphic)
are widely used in current research.
They are
especially useful in aviation and automotivo crash research
and for evaluating protective seats and harnesses.
The anthropometric dummies approximate the human
body in size, form, mobility, total weight, and weight distribution
in body segments (Hertzberg, 1958). Even resiliency
of the skin is simulated by some kind of padding.
Static and dynamic properties of the human body are simulated
by these dummies, but in somewhat crude fashion.
Specific parts of the body, such as the head, brain and
bonés, for example, may be and have been simulated.
Cadavers
were frequently used to study the static and dynamic strength
of ligaments and muscles, and breaking strength of bonés.
Generalized results obtained by use of dummies and other
artificial simulatory objects should be accepted with great
caution as applied to human subjects.

19
(Tlechanical Simulation of Human
Response to VibratidlT
The human body, by its very structure, has a
built-in vibration isolation capacity.
The bony skeleton,
the structure of the vertebral column and the tough,
flexible ligaments and pads connecting
the different parts
of the body possess the ability
to absorb a portion of
the vibrational energy.
The visceral organs contained
within the thoracic cage and abdominal cavity can slide
freely over each other.
The kidneys, the heart and the
brain are held in place by ligamentr surrounded by paddings,
tissues and liquids.
The human body, in short, is a very
complicated
system of masses, elasticities and viscous
dampeners, each connected to the other.
Using mechanical systems, attempts were made to
simulated human response,
The objective was to use the
resultant data to calculate quantitatively
the transmission
and dissipation of vibratory energy through human body
tissue; to estimate vibration amplitudes and pressures at
different locations in or on the body; and to predict the
effectiveness of various protective measures
(Goldman and
Von Gierke, 1961),
For cases in which detailed quantitativo
investigations are lacking, this Information may serve as
a guide in the explanation of observed phenomena, or the
predictlon of results.
In order to calculate the transmission and dissipation
of vibratory energy in the human body
quantitatively,

most of the experimenters assumed that the body is a linear,
20
passive mechanical system
(passivo elements uf a system
being inertia, stiffness and damping factors). Experimenters
concentrated their work upon, also, the effects
of low frequency externai vibrations of the order of
between
,5 to 20 cps, a levei at which man*s built-in
vibration isolation ability is least effective (ílflagid and
Coermann, 1963),
They approached the problem in two ways:
by analyzing
the factors involved in the transmission of
vibrations to various parts of the body, such as the hips,
shoulders and head; and by measuring the body*s mechanical
impedance,
The problem in determining the mechanodynamic
properties of the human body is to find how many degrees
of freedom it has, or how many lumped parameters must be
found in order to predict the response of the system to
the excitation applied. Coermann (1963), approaching the
problem from the impedance point of view, considered the
human body in both the sitting and standing positions, and
treated the forces acting upon the longitudinal axis of
the body.
He constructed a simple model of the human body
consisting of a one-mass spring system, with damping
(Figure 3).
In order to enhance understanding of this and
other models discussed, some basic definitions are set out.
Impedance is defined as the ratio of the transmitted
force to the velocity at the point where force is applied.

21
XI
...... ///A
Fig. 3.—The simplest model of the human body is
a one-mass-spring system with damping. (Adapted from
Coermann, 1963)
(Ylj^ + (YI2 = total body weight
^1
(YI2 =
Wnj^ « 5 Hz
u^ni '
.08 (yij^
1 •
0,30
5 Hz
"'n2 =
(«2 = frii
1 • 10,5 Hz
.318
Uin2 - 10 Hz
2 =
2 = 0,15
,129
^y/////////////y/y/
/ / / / / / / / /
(Kl
Fig, 4,—Parallel and series models for duplicating
the human body impedance, (Adapted from Phillips, 1969)

22
For the simplified model shown in Figure 3 (Coermann,
1963) the modulus of the impedance is given by
2 =-^
The phase angle of z is given by:
tan (f
(z)
1 - n^ (1 - 6^^)
n o
and the formula for the impedance is composed of the values
of the impedance of a purê mass:
mass = mx
X
_
mXgO)
x^w
= moj
where:
5: = rate of change of distance
P = Force transmitted
m = mass
k = spring elasticity
(stiffness)
c = damping constant
z = the mechanical
impedance
n = f = Frequency of the forced vibration
fg undamped national frequency
to
O)
o
Ô =
0) =
damping factor of the system =
27rf
ü)Qm
(O O = 27Tfo =/ k
m

23
Dieckman
(1957,1958) determined the maximum transmission
factor (ratio of movement of the body to the movement
of the platform) for subjects exposed to vibration in
both the vertical and transversa directions.
Head/table,
shoulder/table and hip/table transmissibility factors were
determined.
Von Bekesey (1939), Dieckman (1958), and Agate
(1963) studied the effects of and responses to vibration
transmitted from the hand tool to the arm and body.
Abrams
and Suggs (1968) applied known vibrational inputs to the
hands and measured the resulting vibrations at various
points on the skeletal structures. Latham (1957), also,
studied transmissibility to different parts of the body and
at different frequencies,
It was observed that below approximately 2 cps the
body acts as a unit mass.
Resonance for a standing man
occurred at about 5 and 12 cps and for a sitting
man at
between 4 and 6 cps.
Calculating the impedance values (modulus and phase)
allows the calculation of the transmitted energy.
Figures
5 and 6 show one of the impedance studies.
A solid mass has an impedance of mco (mass X angular
acceleration).
A simple mass spring system with viscous
damper has one resonant frequency.
The human being reacts
similarly to a complex mass spring system with two resonant
peaks at 5 and 11 cps (iDagid and Coermann, 1963).
From

24
6 .
( JD
1 O
-H
M
X
0}
U
c
(D
T3
fD
a
E
•—(
E
U
"V^
u
Q)
to
X
0)
c
>>
TD
2 ,
O
5 10
Frequency - cps
standing erect
sitting erect
one-mass—spr ing
system
^ y
sitting reiaxed
Fig. 5.—The modulus of the impedance of one subject
at varied body postures compared with the impedance of a purê
mass (moj) and of a one-mass-spring system with damping.
(Adapted from Coermann, 1953)
-30'
sitting erect
one-mass-sprinq system
yy
y>standin g erect
O
(D
—i
Dl
C
et
(D
(O
(O
JI
CL
30
60
\ sitting
V reiaxed
Subject R. C,
íyiass: 84,000 dyne x sec2/c m
90
5 10
Frequency - cps
15 20
Fig, 6,—Phase angle of the impedance for one subject
at varied body postures compared with phase angle of a oneass-spring
sytem with damping. (Adapted from Coermann, m
1963)

25
both resonant peaks the effective masses, elasticities,
and damping
factors for tí ese frequencies can be calculated.
Goldman and Von Gierke
(1961) noticed that the
impedance and the transmissibility
factors are changed
considerably by individual differences in the body, its
posture, and the type of support lent by a seat or back
rest to a sitting
subject.
Coermann, et ai., suggested that the human body
acts as a unit mass up to 2 cps.
From 2 to 100 cps it
acts as a lumped parameter system of masses, elasticities
and dampers,
They developed a mechanical analogue for the
human body that will represent the resonance frequency of
the torso, the thorax, abdômen and other parts of the body,
Coermann constructed, also, several curves for the mechanical
impedance of humans in sitting and standing positions,
and in different postures (Coermann, 1963),
Other investigators analyzed the human body in
different ways. Payne (1962) examined a two degrees of
freedom linear mass spring system, and Toth
(1966) proposed
an eight-degrees of freedom non-linear mass spring model.
Liu and llflurray (1956) studied a continuum model
consisting of a visco-elastic rod with a mass at the upper
end.
Coermann, in his attempt to relate impedance resonance
to body segment motion by using a two degrees of

26
freedom system, concluded that two mechnnical systems in
parallel could accurately duplicata the impedance modulus.
Phillips (1958) suggested, instead, duoücation of man' s
mechanical impedsnce by a series of mechanical systems.
UJittman
(1967), in another study, maintained that
a series representation of elements might be more applicable
in establishing an analogy between biological and
externai force responses (Figure 4, page 21).
Suggs and Abrams (1968) suggested a human simulator
which would possess the average dynamic properties of a
population of individuais, and could be exposed to vibration
inputs hazardous to man.
They suggested using such a device
to test various types of vehicle seats and other devices
with which acceleration may be hazardous.
The interesting
aspect of this simulator was its design, in which the heavier
mass represents the head and chest,
The masses are uncoupled,
in that they are each attached to a rigid frame analogous
to the relatively stiff spinal column, rather than to each
other.
As demonstrated in this review, conslderable interest
has developed in the simulation of human subjects by mechanical
or other analogs,
Although no efficient model has yet
been found, the results obtained from some research seem
reasonably encouraging,
If such an analog is found, it will
help to explain the response of the humans to different
leveis of vibration,
Transmissibility studies are of

27
comparable importance,
It is believed that extra work is
needed in order to correlate the transmissibility
factor
and human performance,
In this study the transmission of
the vibration
stress from the platform to the hip and the
head was recorded with platform vibration adjusted to 5 cps
(the body resonant frequency as indicated in the above
studies),
Effects of Vibration upon
Performance
The harmful effects of vibration have been studied
for years, researchers having investigated the effects of
vibration upon the various parts of the body as well as
its total physiological effect.
None of these studies
revealed the exact mechanical reasons for these effects.
To enable the design and system engineers to improve
the vibrational environment, the testing of human performance
in this environment was necessary,
Specific
answers had to be found as to how man is affected by vibration
in varying situations,
It was natural as a result of
advances in Space achievements and military equipment to
start experimenting with tasks of vital importance in these
sectors, such as tracking efficiency, reaction time and
visual abilities in reading displays and dials.
Extensive
research was done in these fields and others, and conflicting
results were obtained in some instances,
These are
believed to be mainly due to the different conditions of
control under which each experiment was run.

28
Following
is a brief review of some of the experiments
in these fields and the results obtained from each,
Tracking
Tasks
In studying tracking tasks under vibration, Gorrill
(1957) used frequencies of 3 to 30 cps with acceleration
up to 2.5 g in a vertical tracking task.
Durations of
exposure constituted a function of the tolerance levei.
He
found a significant decrement in performance at 15 cps and
1,5 g, (Tlozell and UJhite (1958), using frequencies from
8 to 25 cps and double amplitudes of 0.05, 0.1 and 0,16 inches,
found no significant differences for either frequency or
amplitude on vertical tracking performance,
Duration of
trials was two minutes, with four minute rests between trials,
Their results were unpredlctable, and contradict those obtained
from other studies mentioned herein for the same
leveis of frequency and amplitudes.
Analyzing the results
of their experiment, Harris
(1965) concluded that the task
was an easy one and not greatly affected by some loss of
visual acuity,
It would seem, therefore, to be quite easy
to explain
the difference between this study and most other
studies of tracking
performance,
Schmitz
(1959) reported decrement in performance in
a horizontal tracking task,
He used vertical vibration of
0,18 and 0,3 g at frequencies of 2,5 cps, Duration of
trials was 90 minutes.
In another study of 30 minutes
exposure and vertical vibration of 0.15, 0.25 and 0,35 at

29
frequencies from 0,9 to 6.5 cps, he found significant decrements
in tracking performance with increase in frequency
and amplitude,
Ashe
(1960) used frequencies of 2,4, 6, 8, 11, and
16 cps, and double amplitudes of 0,055 and 0,13 (0.03 to
1.4 g). For a 20 minute exposure he found significant
decrements at ali frequencies at 0.13 inch (DA).
Besco
(1961) found significant decrements in performance
under simulated turbulance, with "gusts" at 2 ft/
sec Rins and 4 ft/sec RdílS.
Frazer, et ai,, (1961) explored the effect of amplitude,
and of plane of vibration as well as the difference,
if any, between
tracking on a vibrating and a non-vibrating
display,
Two runs of 60 seconds duration were made under
each of four frequencies from 4 to 12 cps and four amplitudes
of 0,063, 0,125, 0.189 and 0,250 inches,
Decrement in performance
was observed, as related to plane and to function
of amplitude modified by a fractional exponent of frequency,
Greatest decrements were noted with vertical vibration and
a vertical tracking
component.
UJorking with the Bostrom Corporation, Hornick (1961)
reported some decrement in horizontal tracking
performance
under vibration,
In another study for the same corporation, Simons
and Hornick
(1961) performed an experiment in which the
subjects were required to perform a task for flfteen minutes

30
without vibration,
The shake table was then activated for
thirty minutes and performance measures were again taken,
A fifteen minute post-vibration period was employed to
determine recovery effects.
Frequencies ranged from 1,5 to
5,5 cps, with intensities of 0.15, 0,25, and 0.35 g. For
compensatory
tracking under vertical vibration they reported
that time is the only significant factor,
That is, vibration
generally causes a decrement in tracking ability, and
"the recovery of this ability is not complete
following
exposure" for the duration of the recovery period considered,
They reported, also, effects upon body sway, reaction time,
foot pressure and peripheral vision,
Parks
(1961), also, found significant decrement in
performance for vertical compensatory
tracking, but not for
horizontal,
Exposures were for four minutes, with five trials
per day.
Catterson, et ai,, (1962) ran a study to clarify the
relative importance of changes in frequency as opposed to
changes in amplitude, in determining the severity of wholebody
vibration in the veftical plane, The task was twodimensional
compensatory tracking, Operator performance was
compared, under six frequencies from 2 to 15 cps and two
amplitudes of 0,13 and 0,25 inches
(DA), as a measure of the
effective severity of a given levei of vibration,
Five
subjects performed the task in two five-minute intervals
of time, while experiencing vibration for twenty minutes.

31
Buckhout
(1964) studied tracking performance at
frequencies of 5,7 and 11 cps at peak acceleration
values
which were equal to 25^, 3Q',o and 35^o of the one minute
tolerance levei given by (Ylagid, Coermann and Ziegenrueker,
Decrements in vertical tracking performance ranged from
34^ to 74^ over the control trials, Harris, et ai,, (1964)
extended this experiment for the 5 cps levei under different
acceleration leveis (Figure 29, page 72).
Statistically significant results began at 0,20 g,
Another interesting
result was that subjects* performance
improved through each of five trial testing periods,
Trials
were three minutes in length and five trials were presented
during 20 minutes of continuous vibration,
Holland (1967) investigated the tracking performance
during long-term random vibration.
Performance measures
were taken during the first forty-five minutes of each hour,
of a six hour session, He found that tracking performance
over a period of time was remarkably stable,
Average tracking
performance differed slightly over the entire six hours,
Tracking on the horizontal axis was found to be superior to
•vertical tracking
performance,
Several other investigators studied tracking performance
under vibration. Among them are Dolkas (1965),
UJoods (1967), UJeisz (1965), Forbes (1960) and others.
The
majority of the experiments agreed in their evidence that
tracking performance is impaired by vibration.
The studies

32
variec in their explanations of the factors influencing the
tracking decrement, as well as in leveis of frequency,
amplitude, or time elapsed which miyht have caused this
decremen t.
As is evident in the above review numerous experiments
were performed to detect the effect of vibration on
tracking performance.
To draw a general conclusion from
these studies is very difficult.
The diverse purposes behind the many studies of
tracking performance gave rise to great differences in the
controlled variables tested; consequently, varying results.
rhe one conclusion that one might draw is that
vibration in general produces a decrement in tracking performance.
This decrement is more likely to occur if the
body resonates.
Tracking is utilized in this experiment as a performance
measuring task.
Vibration Effects upon Vision, Visual
Acuity and Reaction Time
Coermann (1939) investigated the effects of vibration
during several tasks.
Visual acuity was the only thing which
seemed to be adversely affected.
Dennis (1965) used a frequency range of 5 to 90 cps
with accelerations of 0.25 and 0.50 g in a visual performance
experiment which required twelve subjects to read two digit
numbers.
Error scores increased by an average of 55% for

33
the 0,50 g and 25% for the 0.25 g, compared with a novibration
control period.
Response time increased by 3^ for
the 0,25 g and 12^ under 0.50 g acceleration,
Hornick
(1961) performed a series of studies on
reaction time and visual acuity, In one study he investigated
the human response to red signal lights under vertical
sinusoidal vibration at a frequency of 3,5 cps and acceleration
of 0.30 g, Trials were twenty minutes long. He found
that response time was slower after vibration,
During a
second study in which he required his subjects to respond to
light signals and judge Landolt Rings, he found that reaction
time was slower after than during vibration, but that visual
acuity was not affected,
Hornick obtained almost the same
results in a third study, in which trials were fifteen
minutes long, but found that while peripheral vision was
affected, this was only by side-to-side vibration,
Loeb (1954) used frequencies of 15 cps at 0.068
and 0.112 inches DA 25 cps at 0.034 and 0.056 inches DA,
and 35 cps at .030 and .054 inches DA. For the exposure time
of two and one-half hours, while visual acuity
decreased
with vibration, frequencies did not effect a significant
difference. Aiming tremor increased significantly with
vibration.
Schmitz, in his experiments
(1959,1960), demonstrated
that frequencies did not affect reaction time. Reaction time
was longer following vibration, but visual acuity was not
affected.

34
íílozell (1958), in a reading digits experiment, found
no effect of amplitude, but significant effect of frequency.
Frequency ranges were from 8 to 50 cps, and amplitudes were
of 0.05 and .1 inch DA.
Trials were two minutes and seventeen
seconds long.
Lewis
(1943) concluded that vibration at a frequency
of 20 cps with 0.04 and Q.OOt amplitudes has no effect upon
response time during operation of a (Iflashburn apparatus, a
control stick and rudder bar device manipulated in response
to changes in three banks of lights,
Imposing noise upon
the vibration did not affect the response time,
Lange and Coermann
(1962), in a study of visual
acuity under vibration, concluded that maximum
decrements
occurred at body resonant frequencies,
(líleasurement after
the cessation of vibration showed residual effects of it up
to 12 cps,
Shoenberger
(1968) investigated dial reading performance
while wearing a NASA prototype Apollo helmet,
The
results showed significant decrement in dial reading performance
during vibration which were differentially
related
to the direction of vibration, frequency of vibration, and
to conditions of wearing the helmet with a liner or with
no liner,
Dudek and Clemens (1965), investigating the effect
of vibration upon certain psychomotor responses, concluded
that the reaction
time was not affected by vibration as

35
compared with the control period.
They used frequencies
of 4, 8 and 12 cps,
Johnston
(1969), in an experiment on the effects of
vibration on body orientation, however, measured the reaction
time for twenty maie subjects before, during and after
vibration, using frequencies of O, 2, 5 and 8 cps, and
concluded that the reaction time is significantly slower
within a vibratory environment, as compared with before
and after vibration time periods,
The frequency of the
vibration had no significant effect upon reaction time,
Research on the effects of vibration on visual acuity
showed that in general visual acuity is suffered by low
frequency vibration,
Greater decrements in visual acuity
occurs at frequency values near the body resonance,
These
general statements, although pointing to certain trends in
performance, have little practlcal value in specific design
situations,
Research concernlng human reaction time is not as
conclusivo as it is hoped to be in same,
This is probably
true because of the different situations at which reaction
time was measured and due to a lack of agreement between the
studies in defining the exact time periods of pre-vibration,
vibration and post vibration,
The research in visual acuity is pertinent to this
research because it has been argued that decrements in

36
tracking performance occurs due to a loss of visual acuity
under vibrational stress.
Vibration Effect upon íYlotor Performance
and Body Configuration
Dudek and Clemens (1965) investigated the influence
of vertical vibration, within the limits encountered under
typical industrial conditions, upon performance of various
elements of total cycle time in the cases of certain
types
of hand motions. Three different switches requiring three
distinct types of motion were to be operated by twentyseven
maie subjects,
Frequencies of 4, 8 and 12 cps, and
amplitudes of 0,15, 0.2 and 0,25 inches were used,
The
investigators concluded that the speed of flexion and extension
movements is not affected by vibration,
The variation
in frequency, apparently, affects the supination movement
to a greater degree than it affects either the extension
or the flexion movements,
Bush
(1966) investigated the effects of low-level,
vertical vibration upon the performance of a sensory
inputphysical
response task which requires a decision
factor,
He used frequencies of 3, 4, 5, 6 and 7 cps, and amplitudes
of ,05 and ,10 inches,
Three seating configurations were
used to investigate the effect of varied seating
configurations
in modifying any performance decrement
resulting
from the vibration,
Bush concluded that within the scope
of his experiment, relative decrement in performance

37
accuracy will result from low-level vibration with increments
in amplitude,
The seating configuration is a determinant
of performance in a low-level dynamic environment,
Chaney and Parks (1964) investigated the effect of
vibration on visual-motor performance in the cases of subjects
who had to operate several controls of different shapes and
sizes,
The type of control had no effect upon accuracy of
performance under vibration,
Using a motor performance task, Larue (1965) concluded
that man can achieve accuracy leveis under vibration,
equal to those he can achieve in a normal environment, as
long as the frequency is higher than 5 cps,
Johnston (1959) investigated the effects of lowfrequency
vibration upon whole body orientation in both
sitting and standing body configurations,
He subjected
twenty maie individuais to frequencies of O, 2, 5 and 8 cps,
and an amplitude of 0,09 inch,
Vibration exposure times
were twenty minutes,
and after vibration,
He measured performance before, during
Johnston concluded that time to orient
toward a target, which he called travei time, increases significantly
as frequency increases, and that accuracy is
affected.
Travei time is shorter in a clockwise direction,
and in a standing, rather than a sitting, body configuration.
The duration of vibration exposure had no significant effect
in Johnston's study.

38
Summary
The results of the above mentioned investigations
indicate that vibration caures a general feeling of discomfort
when applied for a long period of time or at a high
levei of acceleration (g levei). Vibration in general
causes decrement in performance, especially around the
frequencies of the whole-body resonance.
This review of the literature reveals several
points of interest which have yet to be investigated with
sufficient thoroughness,
The effects of prolonged vibration
were not extensively discussed in the literature; neither
the intersessional or intrasessional effects of vibration
are well investigated; and recovery from vibration, or
the post-vibration period, is an área which needs further
research,
Questions in these provinces constituted the
impetus for this research.

CHAPTER
II
EXPERIITIENTAL
EQUIPÍYIENT AND lYlEASUREÍYIENT
The equipment used in this experiment is divided
into three different categories, namely, the equipment for:
1, Íílaintaining the independent variables
2, lYleasuring the dependent variables
3, Supporting control of variables
Equipment for 'flain taining the
Independent Variables
Vibration
Platform
A vibration platform which was built by the Department
of Industrial Engineering at Texas Tech served as the
control over the vibrational environment, On the platform,
a mechanical shake table, four feet square and fourteen
inches above the floor, a performance console and a seat
were mounted, The upper portion of the table was mounted
on four springs, one at each corner, Restraints at each
corner limited the horizontal, or transversa, movement to
plus or minus 0,01 inch, thus providing for a single degree
of freedom (vertical) vibration, A simple cam mechanism
provided the forcing function, This mechanism is composed
of an inner eccentric pressed onto the drive shaft, and an
39

40
outer eccentric which, when rotated on the inner eccentric,
provides any amplitude from zero to approximately 0.2 inch.
The cam shaft and its drive train were mounted on the base
section of the platform. Force was transmitted to the
upper portion of the table (the moving section) through
ball-bearing pillow blocks, The cam shaft was driven by
a one-half horsepower variable speed electric motor through
a series of accurately turned pully and V-belts (Figure 7),
The forcing function operated directly through the midpoint
of the platform, with the seat mounted immediately over
this área in such a way that the force was transmitted
through the general location of the center of gravity of
the subject's upper trunk, Before experimentation with
each subject, the platform was balanced by adding lead
weights, A scribed wheel built into the platform permitted
speed adjustment of the motor, A generator tachometer was
installed within the platform and a voltmeter attached to
the outside, providing the means of adjusting motor speed,
consequently
frequency,
Accelerometers
The accelerometer is an electromechanical transducer
which produces at its output terminais an e,m,f. proportional
to the acceleration to which the transducer is subjected
Three accelerometers, manufactured by the Bruel and
Kjaer Company, in Copenhagen, Denmark, and marketed in

Fig. 7, — The vibration olatform drive mechanism
41

42
Clevelana, Ghio, i^-ere used, They are of the B & K 4332
piezoelectric compression type. Acceleration was monitored
at three different points: tne vibration platform; the
subject's hip; the subject's head. Figures 8, 9 and 10
give the positions in which the accelerometers were muunted,
Those for the hip and head were housed in small pieces of
styrofoam, to prevent any uncomfortable abrasion of the
subject's body, The head accelerometer was attached by
a variable length strap, Each accelerometer has its own
specifications, Table 15, Appendix B, shows those for the
accelerometers used in this study,
The output signals of the accelerometers were routed
through separate amplifiers, Those used were of the Bruel
and Kjaer 2624 chargé type, and each accelerometer had its
own amplifier, These were powered by a supply unit of the
Bruel and Kjaer type 2805, which operates on 28 volts, The
accelerometers, amplifiers and power supply unit are shown
in Figure 11,
The output of the amplifiers was controlled by a
switching knob which was connected to a dynograph recorder,
Through this arrangement, the output of any of the accelerometers
was recorded upon turning the switch to its particular
amplifier, The switch was used to reduce the number of
channels which the accelerometers would have used, had they
been connected directly to the recorder.

43
Fig. 8.—Accelerometer mounted on vibration platform
Fig. 9.—Accelerometer mounted on subject*s hio
and vibration stop button

44
Fig. 10.—Accelerometer mounted on subject's head
and earphones.
Fig, 11.—Accelerometers, amplifiers, power
and svuitch.
SUDDI/

45
Equipment for (Ifloasurinq the
Dependent Variables
Performance Console
A Performance Console was constructed according to
the human engineering standards given in the handbooks.
It
was mounted rigidly on the vibration platform in order to
provide a man-task vibration system,' In the middle of the
front panei of the console a cathode ray tube
(CRT) was
mounted,
It was snugged tightly into a soft synthetic
material which acted as a cushion protecting the tube from
any motion which might be caused by the vibration of the
whole device,
The CRT was secured, also, by belts and
supported by the front and back of the console panei,
A
horizontal black dotted line was displayed in the center
of the CRT, to serve as a target for the tracking task,
A protective plastic cover was installed in front of the
tube and on the face of the panei, to protect the subjects
should any damage occur to the tube,
Suitably located with respect to the field of
vision, and immediately above the tube, two colored signal
bulbs were mounted on the front panei,
One of the bulbs,
when lighted, was green indlcatlng that the subject should
start working,
The other, when lighted, gave a red signal
indicating that the subject should stop,
The two lights
were activated by a switch located in the experimenter's

46
station and within easy reach of the experimenter. Figure
12 shows the mounted cathode ray tube and the signal lights,
The signal switch is shown in Figure 13,
One side of the Performance Console was cut out,
to allow the subjects to get into the work station freely,
The other side had a slot in which the hand rest and control
assembly could slide freely upward and downward, for
adjustment to the subject's height, On the left side of
the console and within easy reach of the subject, a button
for the control of vibration was installed should the subject
have felt the need for stopping the experiment, Immediate
stoppage of vibration by this means is a wise precautionary
measure in such types of study, The emergency stop button
is shown in Figure 9 (page 43),
Hand Rest and Control Assembly Device
A hand rest and control assembly device was constructed,
providing several deslrable features: a control
lever necessary to the tracking task; a place to rest the
arm, thus preventing the subjects from leaning on the hand
controller when fatigued during tasks of long duration; a
design which forces ali subjects to assume the same posture
in controlling the task; a construction which allows extra
sensitivity and easier manipulation of the fingers in the
tripodo grasp, which the subjects were instructed to use,
The hand rest was a wood roller mounted on a
bolt and moving freely around its axis, It was covered

47
Fig. 12.^-Cathode ray tube ând signal
lights
Fig. 13.—Signal playing apparatjs, change of
polarity circuit, vibration platform activator and
signal switc^1,

with a foam rubber material, to provide a soft cushion for
48
the arm.
The hand controller was an aluminum lever with a
small rounded head, mounted on a potentiometer providing
the voltage for the subject's output signal.
Both hand
rest and control device were mounted on one board, which
slides freely on the side of the Performance Console and
may be secured in any position by bolts and wing nuts.
Figure 14 plctures the hand rest control assembly in position
for instrument's calibration.
Figure 15 shows the
tripodo grasp used by the subjects.
Seat
The seat used in this experiment was a standard
laboratory stool with a backrest.
It was adjustable in
height and position with respect to the work station.
The
seat was rigidly attached to the vibration platform and
centered directly above the forcing function of the platform.
No padding of any kind was used on it, nor were
straps or harnesses provided.
Figure 30 (page 85) shows
the stool mounted on the platform.
Signal Playing Apparatus
The signal playing apparatus shown in Figure 13 (page
47) consists of a regular sweep circuit for the horizontal
sweep on the cathode ray tube.
The vertical sweep on
the CRT is controlled by the voltage output of the tape

49
Fig. 14.—Hand rest and control assembly
device
Fig. 15.—Subject's controlling
position

50
recorder. This voltage output is rectified into a positive
DC voltage when passed to the input of a vertical amplifier
producing the vertical sweep on the CRT.
The hand controller operated by subjects is connected
to a potentiometer which produces a negative voltage counteracting
the positive voltage obtained from the recorded signal,
UJhen the signal was adjusted to the line in the middle of
the CRT and the hand controller was centered in the vertical
position, a negative voltage of 1.5 volts was introduced
by the potentiometer.
The voltage input from the recorded tape and the
voltage output from the hand controller were subtracted in
the circuit. The net voltage was then passed to the change
of polarity circuit. A schematic diagram of the CRT sweep
circuits and power supply is shown in Figure 16,
Change of Polarity Circuit
In order to convert the subject's error into an
absolute error score, a change of polarity was required
when the signal reversed its direction, This allowed integration
of the signal over time, A special circuit (shown
in Figure 17) was constructed for this purpose, A supply
unit powered the change of polarity unit, and the potentiometer,
as shown in the circuit.
Analog
Computer
An analog computer of type PAGE TR-10, manufactured
by Electronic Associates, Inc., New Jersey, was used to

52
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53
integrate the absolute error signal over a period of time
(Figure 19).
A schematic of the integrating circuit for the
analog computer as used in this study is shown below.
This
integrating circuit provided the means for having a direct
error score for the subject's performance.
Output
-0,5V, j^gj^
—o 'VNA^
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I—A'
(Absolute)
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Fig. 18.—Schematic of the analog computer integrating
circuit.
Dynagraph Recorder
A Beckman Type R Dynagraph Recorder was used in this
experiment to provide a means for measuring the error score.
It was used, also, to monitor the experiment variables
continuously.
The input signal from the tape was recorded
on channel number 2; the output signal from the hand controller,
on channel number 1.
The algebraic sum of the two
signals was recorded on channel number 3.
Channel 6 provided

Fig. 19.—View of the analog computer showing
integrating circuit connections.
54

55
the recording of the integrated signal, displayed on channel
3, over time. Channel 4 was connected to the accelerometer's
switch to give an indication of the acceleration of tne
switched on accelerometer.
Fig. 20,—Dynagraph recorder

56
Supporting Equipment for Controlling
the Wariable
Signal Recording Apparatus
The signal recording apparatus was designed to give
a complex profile signal which will repeat itself after a
constant period of time,
It consists of a cam which was
designed to give an oddly shaped and complex signal,
The
cam was mounted on a disc driven by a variable speed motor
through a pulley and V-belt,
A rod through a roller camfollower
connected it to a potentiometer,
Rotation of the
cam caused the follower to move in the same fashion as the
cam profile,
The potentiometer serves as a means of changing
the amplitude of the constant áudio signal generated
by the oscillator,
The signal recording apparatus and
supporting equipment are shown in Figures 21 and 22,
UJide Range Oscillator
A Hewlett-Packard Company ílflodel 2005D wide range
oscillator was used to provide a constant audio-frequency
of 1,000 cycles,
The oscillator has special circuitry which
ensures an output voltage of low distortion and high stability
with any output load impedance from zero ohms to
open circuits,
A voltmeter was connected to the oscillator
to provide a means of checking the quality of the signal input
to the tape recorder,
A voltage regulator was also used
to prevent any voltage drop in the power line (Figure 23),

Fig, 21
and voltometer. —Signal
recording apparatus, oscillator
Fig. 22.—Tape recorder, oscillator and voltmet er

zb
Fig. 23.—Voltage regulator
Fig, 24.—View of signal and vibration monitor-
equipment.
ma

ITIagnetic 'U'le
Recorder
59
A mcdei 761 Ampex magnetic tape recorder/producer
was used to record a complex signal for replaying on the CRT.
This recorder/player
features three heads; separate deep gap
play/monitor; hea : for direct-monitor from the tape; soundwith-sound;
sound-on-sound; and echo effect. A Scotch brand
magnetic taoe of Dynarang Series 201 was used for recording
and replaying the áudio signal, This type of tape was
selected because of its low noise features which help to
prevent áudio dropout,
Earphones
Standard type earphones were used to screen out
ambient noise as well as noise of operation of the vibration
platform,
They had certain features by which each subject
could adjust them correctly to his own head,
(Tleasuremen t
The most important measurement made in this investigation
was that of the subJBct's performance.
Vertical
tracking ability was used as the performance criterion,
The tracking apparatus, an assembly of the separate equipment
parts mentioned earlier in this text, consists of a
5-inch cathode ray tube
(CRT), mounted in the center of a
display console,
A horizontal line, displayed at the center
of the CRT, served as the target,
The controlled element

60
seen as a beam of light on the CRT (produced by the sweep
circuit), is driven upward and downward by a program prerecorded
on a magnetic tape,
The subject's task was to
maintain alignment of the controlled element with the horizontal
line displayed in the center of the CRT, using a
hand controller,
The deviation of the controlled element from the
center of the circle represents the sum of voltage inputs
from the programmed tape, and voltage outputs generated by
the subject's movement of the hand controller,
This sum
was then passed onto the change of polarity circuit to
unify the sign of the voltage output and allow for recording
an absolute error,
This absolute error was then integrated
by means of an analog computer over a period of 45
seconds,
The resulting value of integration was recorded
on a dynagraph recorder and read as an error score in terms
of the number of chart deviations on the recorder, i.e.,
arbitrary units served as an error score.
measure of performance in this experiment,
This was the
No effort was
made to calibrate the recorder to read the exact voltage
output because of its irrelevancy to the purpose of this
study,
The error score was continuously recorded while
the subject was performing the task.

CHAPTER
III
EXPERIÍYIENTAL DESIGN
This chapter is pertinent to three sectors of the
experiment: the task, the variables and the design, The
experimental routine is given, also,
The
Task
j A compensatory one-dimensional vertical tracking
task was utilized, The tracking system consisted of a
visual display with two indications, one fixed (target),
and the other moving (controlled element); a controlling
device; and an operator to perform the tracking function,
In compensatory tracking tasks, such as the one used in
this study, the subject must manipulate the controlling
device to superimpose the controlled element on the target,
Any difference represents an error and the function of the
operator is that of manipulating the controls to eliminate
or minimize this error, In this study the CRT served as
the visual display; the horizontal dotted black line in the
center, the target; and a beam of light on the CRT, the
controlled element, The latter was moved up and down by
means of the electronic equipment prevlously described.
61

62
The human tracking function is a man-machine closed
loop system,
A closed loop system is one in which the output,
or some result of the output is measured and fed back
for comparison with the input (Childs, 1965).
Figure 25 is
an illustration of a tracking system.
The signal input to
the system is usually characterized by a ramp, step, sinusoidal
or complex function, or a combination of these,
The input signal used in this study was one of
complex shape.
It was recorded on a tape by means of a
signal recording device and a cam possessing the same profile
as the desired signal.
Figure 26 shows the cam profile,
and Figure 27 shows the signal input as recorded on a dynagraph
recorder,
This relatively complicated profile was
selected for its ability to minimize the subject's adaptation
to the tracking signal, )
A tracking task was selected as a performance measuring
task for several reasons,
The tracking function is
important in many military, industrial and Space situations,
and tracking performance was shown to have suffered by
vibration in many studies,
In explaining the importance
of the vibration effect on tracking performance and its
incorporation in a system design situation, Chaney and
Parks (1964) say:
Definition of operator performance capability as
influenced by system environmental conditions, is
becoming incroasingly important as modern manned

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66
systems become more complex. To the system designer,
the accuracy and speed with which an operator can
accomplish assigned visual-motor and tracking tasks
within the operational environment is probably the
largest contributing factor to the salection of a
final display-control configuration,
Vibration was proved to be the cause of decrements in
tracking performance ability (Ashe, 1963; Gorrill and Snyder,
1957; Catterson, 1962; Frazer, et ai., 1961; and others),
In a paper
(1962), Hornick concluded:
UJhile there are discrepancies, there is general
agreement that compensatory tracking ability shows
a decrement which is related to amplitude and acceleration
increases and vibration duration in the range of
1 to 30 cps,
In some experiments (Buckhout, 1964), vertical tracking
error increased from 34 per cent to 74 per cent over the
control trials,
These are likely to be unsatisfactory leveis
of decrement in an operational situation (Harris, et ai,,
1965),
The importance of the task and the detrimental
effects of vibration upon its performance, made it challenging
to investigate the effect of the latter in a prolonged
period study, and totry to offer means of improving the
performance during such a period,
The Variables
Independent Variables
UJork/Rest Schedule, — The work/rest schedule as defined in
this experiment differs slightly from the classical

67
interpretation of it, although it conforms with the overall
objectives of its utilization,
íYlost of the research done
regarding the effect of a work/rest schedule upon performance
concentrated upon a duration of more than thirty hours,
In
their explanation of the importance of a work/rest schedule
and its effect upon performance, Adams and Chiles (1963) wrote
"A significant feature of many future man-machine systems is
the extent to which they will depend upon efficient utilization
of crew members."
Adams and Chiles focused their
efforts upon the long period missions.
The efficient utilization of crew members is of
great importance in ali criticai missions, especially those
during which performance is influenced by some environmental
factor, such as vibration, which imposes certain limitations
upon intensity and exposure time for the subject,
Scheduling
the phases of work and rest under vibration in a certain way
within the limits of safe exposure time may cause improvement
in mission performance,
Such questions, then, arise as these:
Is there an optimum work/rest schedule which may be
adapted for use under vibrational conditions?
Does an interchange of the phases of work and rest
allow.the subject to recover?
If there is recovery, what is its nature in this
case?
lyiany others may be triggered,
This study was designed
to obtain answers to some of these many questions.

68
j Four work/rest schedules were selected for testing
during this research, "work" here referring
to task performance,
and "rest" referring to no task performance. The
schedule's durations were selected within a simulated mission
of one hour, where crew members are subjected to vibration
throughout the mission period,
The work/rest schedules were
selected to provide the means to test if changing the schedule
will affect the crew performance and consequently, the
whole mission performance.
They were selected in such a way,
also, as to allow crew members
(in a practlcal situation) to
interchange work and rest periods if deslrable.
The four
work/rest schedules employed were divided into the following
work periods:
1. Thirty minutes of continuous work
2. Two fifteen-minute periods of work
3. Three ten-minute periods of work
4. Six five-minute periods of work
Figure 28 shows the work/rest schedules and their
distribution throughout the mission duration. The total work
and rest periods were determined in such a way that the work
periods fell within the safe durations of exposure to long
periods of vibration under the specified leveis of g in this
experimen t.
The Environment.—The experiment was performed under two
environmental
conditions:

69
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70
1. Normal (nil-vibration)
2. Vibration
Selection of these two environments allowed detection
of two performance effects:
the task effect; the vibration
effect.
Performance in a normal environment was taken as
a base levei with which performance under vibrational environment
can be compared.
Individuais were subjected to a levei
of vibration of approximately 0.20 g.
The frequency was set
at 5 cps (body resonance frequency), with an amplitude of
approximately 0.1 inch. )
These leveis of frequency and amplitude were selected
for several reasons.
The low levei of frequency was selected
because vibrational research is focused upon the low frequency
range.
(Ylagid, Ziegenruecker and Coermann (1960) employed a
low frequency range of 1 to 20 cps to investigate human tolerance
of vibration,
They justified this as follows:
(1) Buffeting and impact loads as they occur in
Space vehicles and in high speed, low altitude flights,
have a frequency spectrum of this range , , ,; (2)
resonances of important body áreas were found within
this range , , ,; (3) pilots can easily be protected
against frequencies above 20 cps by mechanical damping
systems.
Harris supported this view and wrote (1965):
In my opinion our greatest need for behavioral
studies is in the low g range within the frequencies
of 1 to 20 cps. I believe it is necessary that we
systematically study performance changes within this
range. From such studies we would hope to be able to
specify for a given frequency at what intensity levei
Gignificant decrement begins to occur.

71
Chaney and Parks (1964) write:
Low frequency vibration is an important part
of many operational environments and is perhaps the
most difficult to design out of the system. Consequently,
a criticai need exists for data concernlng
vibration effects on human ability to perform perceptual
and motor control activities, and design
features which will enhance their accomplishment.
(ílost vehicles, moreover, produce vibrations of
frequencies ranging from 1 to less than 10 cps (Figure 1,
page 9).
The higher frequencies are easily and generally
attenuated (Radke, 1957).
In a paper by Harris (1965), he compared the results
of several studies on tracking performance.
He then composited
the curve shown in Figure 29.
The geometric symbols
in the figure refer to the studies of particular authors.
Their locations on the figure indicate the leveis of vibration
at which the tracking function was tested.
The numbers
inside the symbols show the relative levei of decrement
obtained within any one study.
Number 1 denotes the greatest
decrement, with successive numbers indicating decrecsing
decrement.
The absence of a number means that no decrement
was obtained.
Although conditions under which the experiments were
pursued differed greatly, the leveis at which tracking decrements
begin are still apparent.
The greatest decrements in performance in general
appear to be around 4-5 cps (body resonance): P cps and 11
cps (resonance frequency of the facial musculature).
The

2.0
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Catterson, et ai.
Harris, et ai.
lYlozell and UJhi te
Schmitz
Buckhout
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FREQUENCY (CPS)
. n, ^^^' 29, —Performance on tracking tasks in relation
berge?"^g65) '^^°'^'^^^^^d limits, (From Harris anu Shoen-

decrements occurring under 8 and 11 cps, however, were under
73
a higher g levei than those occurring at 5 cps,
These
results, in addition to the helplessness of the body to
avoid the criticality of its resonant frequency
(5 cps),
made this 5 cps levei the most deslrable one with which to
work,
It has been argued that tracking decrement is related
primarily to amplitude of vibration, modified by a
fractional component of frequency, Catterson, et ai,, (1962)
suggested that regardless of differences in frequency, the
most significant deterioration occurred in association with
the change in amplitude of the vibration.
For this study
it was necessary to select a levei of acceleration failing
within the maximum tolerance levei of one hour or less of
vibration exposure,
It is to be noted that the levei of
g selected was 0,20, which does fali within the tolerance
limit to vibration of one hour or less given by Linder (1962),
and slightly higher than the one suggested by Harris (1965),
The frequency levei of 5 cps, and the amplitude of 0,08
inches, fali within the low frequency, high amplitude category,
and for most practlcal purposes, according to previous
tracking studies may produce certain performance effects,
allowing sensitive detection of variables.
Several investigators used comparative leveis of
frequency and amplitude,
Frazer, et ai., (1961) exposed
their subjects to frequencies of 2, 4, "^ and 12 CPS, and

74
double amplitudes of 0.063, 0.125, 0,189 and 0,250,
Simmons
and Hornick (1962), used frequencies of 1.5, 2,5, 3.5, 4,5
and 5,5 cps and g leveis of ,15, ,25 and 0.3,
their subjects to vibration for thirty minutes,
They exposed
Other
investigators used similar leveis,
Dennis (1960), in an experiment of visual acuity,
held a peak acceleration constant and varied the frequency,
The greatest error occurred at 5 cps at ,25 g, and at 14 cps
when acceleration was maintained at ,5 g,
The Period,—Each schedule was divided into nine performance
periods, six periods failing within the controlled environment
span of time (normal and vibrational), the other within
the recovery span, as illustrated in Figure 28 (page 69),
A performance period is five minutes long, with a performance
score based upon the integrated error of subject's tracking
over forty-five seconds of every one minute of work,
Subjects
worked in multiples of whole periods (multiples of five
minutes) during the controlled environment span, and for a
one minute interval,' only, at the beginning of each recovery
period.
Periods were selected as a variable because of the
importance of analyzing performance with respect to time of
exposure,
lYlost of the previous studies focused upon short
time periods (less than thirty minutes), and investigators
in general did not attempt to analyze performance with respect
to duration of time of exposure.

75
In support of this point, Harris and Chiles (1964)
wrote:
, , , several important variables have not received
sufficient attention in vibration research, One of
these variables is the duration of exposure period,
In the typical vibration study involving a tracking
task, a fixed duration is selected for investigation,
and the performance data are (perhaps) broken down
into first half versus second half.
ílflon itor ing performance continuously over the working
period, as was done in this experiment, was not a usual
procedure in preceding experiments.
Investigators used,
rather, only a sample of performance at different points
in their experiments (Catterson, et ai., 1962; Holland, 1967;
and others).
The one minute of work during recovery periods in
this study was required to ootain error scores during the
span of recovery specified.
The Dependent Variables
The dependent variables discusser! here include:
1. lYleasured dependent variables
2. Calculated dependent variables
ÍYleasured Dependent 'iariablíps. — The measurer! dependent
variable used in this study was the absolute error score
resulting from performance of a vertical r.rficking task and
integrated over the first forty-five secoids of eacn mmute
of
work.

76
The difference between the input signal from the
program displayed on the cathode ray tube, and the output
signal from the potentiometer connected to the hand controller
and monitored by the subjects, were added absolutely
(in terms of voltage), using the electronic equipment
described in Chapter II.
This value was then integrated
over time, by means of the analog computer.
A continuous
record of the input signal, the output signal, the difference
and the integrated value was obtained with the Dynagraph
recorder,
The error score was read in terms of chart divisions
on the recorder (arbitrary units).
Figure 27 (page
65) shows a complete chart recording of an actual run under
vibration,
Calculated Dependent Variables,—The two calculated dependent
variables used in this study were the difference in average
error score for perfoimance under vibration and performance
in a normal controlled environment for the particular work/
rest cycle used; and the average percentage of increase in
error for performance under vibration,
In order to test for only the effect of vibrational
stress on performance, it was assumed that the subject's
performance under normal environment represents his proficiency
in tracking at the levei of the work/rest schedule
used,
This is considered a rear^onable assumption, since
subjects' leveis of performance were raised before the

77
actual start of the experiment until they leveled off.
Ali
of the experiments were run in a random fashion.
The error
score readings over each five-minute work period were averaged
for normal and vibrational environments,
The difference
between average error reading of work under vibration, and
the average error during nil-vibration
(normal environment),
was then taken as the net average error score for the individual
subject at that particular levei of the work/rest
schedule,
This was used as a dependent variable in an
analysis of variance, as will be discussed later in this
chapter.
The second calculated dependent variable, the percentage
of increase in average error score, was calculated
as follows:
Percentage of increase in error =
Average vibration error -
average nil vibration error
Average nil vioration error
The average error is once more an average of the
five-minute readings taken within each period.
This dependent
variable was used in another ANOVA, as indicated later
in this chapter.
The Controlled Variables
Signal Input.—The signal input was in the form cf
a complex
function.
To record it, a cam possessinr, the sane profile
as the required signal was used, and procedures prevlously
discussed were followed.

78
The reason for chooslng a complex profile signal
was to decrease the amount of the subjects' ndaptation to
the signal pattern.
Repetition of the signal profile provided
the means of comparing performance directly from
output.
Normal Environment,—A normal environment was controlled
here by establishing a nil-vibration situation.
In the
meantime, subjects plugged their ears with cotton and
wore headphones to prevent the ambient noise from affecting
performance,
The room temperatura was kept constant at
72° F i 1° throughout the performance, The light intensity
was kept constant, as well.
Vibrational Environment,—A frequency of 5 cps was the
vibrational levei maintained for this experiment, uith an
amplitude of 0,08,
To calculate the g levei for this combination
of frequency and amplitude the following formula is
used (Harris, et ai,, 1965):
g = 0.0511 AF2
where:
g = peak acceleration in terms of the
acceleration of gravity
A = double amplitude (DA) in inches
f = frequency cps
The g levei selected for this experiment uas approximately
0,20 g at 5 cps, which gives the above values of
amplitude.

79
In the literature performance was shown to be
most affected by vertical vibration,
This was especially
true in the case of tracking performance, and was the reason
for selecting vertical vibration as the controlled environment
for this study,
Since most of the previous research
on vibration had been done under conditions of sinusoidal
vibration, it was decided to employ it for this research,
in order to make use of the findings of some other investigators.
It was argued, however, that there is no significant
difference in performance between random and sinusoidal
vibration (UJeisz, et ai,, 1965), and that in basic research
(such as this) sinusoidal vibration may be used and then
followed by other types of vibration in future research
(Johnston, 1969),
The leveis of frequency and acceleration used in
this study were monitored by means of accelerometers throughout
the experiment,
Appendix B gives the value of g
calculated from the accelerometer outputs.
Statistical Design of the Experiment
Three analyses of variance were run to allow testing
of the desired variables,
These analyses and the models for
each are given below:
First Analysis of Variance
The three independent variables, work/rest schediies.

80
environments and periods are analyzed in order to determine
their effects upon the dependent variable; namely, tracking
performance in terms of absolute error score during the
first minute of each period,
Analysis of variance was based upon mixed model
techniques,
A completely randomized factorial model 7 x 4 x
2x9 fits this experiment. To obtain the most Information
out of the design, however, the experiment was run in a
randomized block factorial design, with subjects (7 leveis)
serving as blocks,
This heips to block out individual differences
between subjects,
A summary of factors used is
given in Table 1,
TABLE 1
SUiniyiARY OF FACTORS
Factor Label Levei Type Effect
Subjects
S
UJork-rest schedule UJ
Environmen t
Period
V
P
1 30 min,
2 2/15 min,
3 3/10 min,
4 6/5 min.
1 normal
2 vibration
1
2
3
4
5
6
7
8
9
Random (Blocks)
Fixed
Fixed
Fixed

The model for the randomized block desjgn which
81
was used (Hicks, 1966) is as follows:
X = U 4- A(I) + B(J) + AB(IJ) + C(K) + AC(JK) -f-
BC(JK) + ABC(IJK) -f 5(L) -f E(lJKL)
where
X =
dependent variable observed at I, J, K, L
u
A(I)
B(J)
C(K)
S(L)
I
J
K
L
E(IJKL)
of their respective treatment
= common effect of the whole experiment
= first factor effect (work/rest schedule)
= second factor effect (environment)
= third factor effect (period)
= replications (blocks)
= 1, 2, 3, 4
= 1, 2
= 1, 2 , , , 9
= 1, 2 , , , 7
= error term (random)
A completely randomized order for the different leveis
within blocks was used,
Table 2 shows the design for the
estimated mean square (EíílS) and the ratios required for the
F-statistic.
Second Analysis of Variance
The two dependent variables, work/rest schedule and
period, are analyzed to determine their effects jpon perforf..anc'''
under a vibratioril environment,
The dependent

82
variable was the difference between the average error score
under vibration and that under nil-vibration (normal environment),
The model was a randomized block factorial design
embodying four leveis of work/rest schedules, periods and
seven replications (blocks), This model is given by
X = U -f A(I) + C(K) + AC(IK) + 5(L) +
E(IKL)
TABLE 2
EXPECTED (YIEAN SQUARE FOR EXPERIÍYIENTAL
DESIGN
df
(Ylodel
4
F
I
2
F
J
9
F
K
7
R
L
EIYIS
F
Test
3
1
3
8
24
A(I)
B(3)
AB(IJ)
C(K)
AC(IK)
0
4
0
4
0
2
0
0
2
2
9
9
9
0
0
7
7
7
7
7
2
^e
2
^e
2
^e
2
^e
4
+
+
+
+
+
126
252
63
56
14
2
^A
2
^B
^AB
^l
^AC
A/e
B/e
AB/e
C/e
AC/e
8
BC(JK)
4
0
0
7
<
-f-
28
^§C
BC/e
24
ABC(IJK)
0
0
0
7
a2
e
+
7
a2
ABC
ABC/e
4 26
E(IJKL)
1
1
1
1
a2
e
—
503

Third Analysis of Variance
83
The analysis of variance in the above experiment
covers both the performance and recovery periods.
In order
to analyze the results during the vibration period alone,
another analysis of variance was run.
The results covered
the first six periods of performance only, i.e., during the
vibration periods, but eliminating the recovery periods,
Again, the analysis for this experiment was a randomized
block factorial design, with 4x6
leveis and seven replications
(blocks),
The model is as given before in the
section regarding the second analysis of varirince experiment,
The dependent variables were the differences in average
error score under conditions of vibration and nil-vibration,
and the percentage of increase in average error score over
the five-minute period of work under conditions of vibration.
Subjects
Soven maie undergraduate students served as subjects
in this experiment,
Five of those had prevlously particlpated
in other intra-departmental studies of vibration.
To the other two, vibrational study was a new experience.
Ali subjects had undergone thorough physical examinations,
and had proved to be in excellent health,
Table 3 is a
summary of the Information obtained from subjects.

84
TABLE 3
SUBJECT
INFORÍYIATION
Subject
Number Age Classif ication Height UJeight Handedness
1, Valusek 21
Júnior
6'1"
165
Right
2, Stafford 24
Sênior
6'
170
Right
3, UJarren 22
Sen ^or
6»
165
Right
4, Ness 24
Sênior
6'
175
Right
5, UJideman 20
Júnior
6'
170
Right
6, Visser 21
Júnior
5'
165
Right
7, 0'Brien 21
Júnior
5'lOè"
151
Right
Experimental Routine
( The experiment was conducted in the new vibration
laboratory located on the first floor of the Industrial
Engineering Buildlng at Texas Tech,
The room is airconditioned
and basically a constant temperature was kept
.throughout the experiment,
Before beginning, the subjects were brought to the
laboratory for briefing,
They were informed of the exact
requirements from each, and given a description of the method
of conducting the experiment, as well as of the task to be
erformed.

85
Fig. 30.—Vibration
facility

86
After checking on the subjects' health, background
and the results of their physical examinatlon, an experiment
schedule was made for each to suit his convenience, «The
first sessions were devoted to acquainting the subjects with
the equipment and procedures, and in answering any questions
they might have,
The following two sessions were devoted
to raising the levei of performance of the subjects, until
it started leveling off,
This was done to avoid any learning
effect of the task. j UJhen it was believed that the performance
of a subject was still improving, an extra learning
session was added,
An example of a subject's learning curve
is given in Appendix C,
( After learning sessions were finished, actual experimentation
sessions began,
Upon arrival for each session,
the subject was seated in a comfortable chair for five to
ten minutes, a random draw for the experiment leveis at which
he was to perform was made,
The subject was informed of these
leveis, and instructed to proceed to the experimental apparatus,
where he was seated on the chair mounted on the platform.
The accelerometers were then attached to his hip and head, and
to the vibration platform.
He was given a small piece of
cotton with which to plug his ears as an extra defense against
noise, and was instructed to wear the earphones.
Ali of the
electronic equipment was then switched on, and the experiment
was continued as scheduled.)

87
The experiment began with calibration of the equipment.
The hand controller was brought to its central vertical
position, and a signal of constant value was played by
the recorder. The analog's potentiometers were then adjusted
to give a zero integration value as long as the line was
kept on
target.
For experimentation with a vibrational environment,
the vibration platform was activated. A check was made on
the accelerometer outputs. For performance of an experiment
in a normal environment, the accelerometer power supply was
not activated. UJhen it was the time scheduled for work to
begin, the recorder was started, and the green command
light in front of the subject*s eyes was switched on, signaling
that he should begin performing the task.
It is worthy of mention here that a short tone was
recorded on the second channel of the tape recorder as an
auditory signal to the experimenter at the beginning and
end of each performance measuring interval (based upon 45
seconds of integration for each one minute of work), UJhen
this tone is first heard by the experimenter, the analog
computer switch is turned to operate position and the integration
process started, UJhen the second tone is heard,
the computer switch is turned to hold, then reset, An
error score on the dynagraph recorder chart is then read
and
recorded.

88
r Continuous readings were taken for the subject's
performance.
A large clock was used to check the time
periods throughout the experiment,
At the end of each working
period, the red signal command switch was turned on,
and the tape recorder stopped,
This procedure was repeated
through ali work/rest schedules and with ali subjects,
At
the end of the one-hour period specified for the mission,
and following the last period of work, the subjects continued
working for one more minute and an error score was
recorded,
This reading was taken to check for recovery
immediately following exposure,
Two four-minute periods of
rest, followed by one minute of work, completed investigation
of the recovery period,
The schedule of work, rest
and recovery phases under normal and vibrational environments
according to which the experiments were conducted
is shown in Figure 28 (page 69),
After the cessation of vibration, the subject was
questioned regarding his reaction to its effect,
Following the work/rest schedule (performance period)
and the recovery period, the electronic equipment was
switched off, and the accelerometers were removed from
the subject's body and the vibration platform, the run
being complete for that particular date,'
Safety Precautions
Ali of the subjects were given a thorough physical
examinatlon
(the Tilaster Test) to ensure their physical

89
Fig. 31.—SubjBct's station
Fig. 32.—Experimenter station

90
ability to particlpate in the experiment.
The acceleration
levei used in this experiment was within the tolerance limits
established by Linder (1962) for exposure time of one hour
or less.
The acceleration intensity was monitored at the
platform, hip and head, to ensure reasonable leveis of
acceleration being transmitted to those locations,
The g
levei was calculated at each place,
Pieces of cotton and a
set of earphones were provided in order to screen out vibration
platform and ambient noise, to eliminate any side effects
these might have on performance,
An emergency button to stop
vibration was installed in the performance console, within
easy reach of the subjects,
They were instructed that should
they have any feeling of pain, extreme discomfort, náusea
or any other intolerable reaction, they should push the
emergency stop button at once,
They were under continuous
observation, also, by the experimenter, for the purpose of
keeping informed of their condition,
It is worthy of mention
that ali subjects completed ali experiments without having
to stop the vibration.

CHAPTER IV
FINDINGS AND IN TER PRETAT IONS
The statistical technique of analysis of variance
was employed in this study to determine the effects of
the independent variables, namely work/rest schedules,
environments and periods, upon the dependent variables
prevlously defined,
The F-statistic was used to test for
the significant effects,
Slgnificance was ascertained
at the conventional leveis of one and five per cent,
In order to thoroughly analyze the results obtained
from this experiment, it was necessary to run three different
analyses of variance,
A randomized block factorial
design was used in ali three ANOVA'S,
This design helps
block the subject effects, allowing a true testing of the
variables,
The ANOVA comoutations were performed on an
IBlYI-360 computer using the program QADTANOVA, written by
Charles Burdsal, Jr,
The Texas Tech Statistical Library
number of the program is TTS 050,
The program provided
the following Information:
1, Source table for each independent variable
2, F-ratio for ali main effects and interactions
3, Exact probability of occurrence by cnance
for ali main effects and interactions
91

92
4, Cell means for ali main eftects and interactions
and their leveis of slgnificance,
if any.
Following the analysis of data, Duncan (Ylultiple
Range Tests were performed to determine the significant
differences between treatments.
The results of the analyses
of variance experiments performed and those obtained
from the múltiplo range tests are discussed below,
First Analysis of Variance Results
The first analysis of variance was designed to
test the hypotheses that work/rest schedules, environment,
period, and their interactiuns, affect the absolute error
score measured at the first minute of each performance
period, The results of this ANOVA are shown in Table 4,
As indicated in Table 4 environment had a highly
significant effect upon human tracking performance.
Table
5 gives the cell means and standard deviation of this significant
factor:
environment,
An increase in vertical tracking error of approximately
43 per cent was noted when vertical vioration was
introduced,
This finding is in agreement with results
obtained in other investigations of performance during a
tracking task under a vibrational environment (Buckhout,
1964, Harris and Shoenberger, 1965, Chaney and Parks, 1964,
and others),
The controlled v.Lbrational environment leveis

93
in this experiment were selected in such a way as to fali
within the region of performance decrement specified by
other studies,
Obtaining such a high performance decrement
(43 per cent) indicates reasonable task difficulty, a
necessary condition for providing a good measure of other
variables of interest:
the work/rest schedule and oeriod,
A plot of means of absolute error score versus environment
is shown in Figure 33,
TABLE 4
ANOVA FOR ABSOLUTE ERROR SCORE (NORIYIAL
AND VIBRATIONAL EN V IRONÍÍlEN T)
Source
DF
Sum of
Squares
í^lean
Square
F-Ratio
Blocks
6
942.95
157.16
31.73*
A, UJork/Rest
3
16,04
5.35
1.08
B, Environment
1
1111,02
1111,02
224.67*
AB
3
15,66
5.22
1,06
C, Period
8
86,74
10.84
2,19*»
AC
24
186,46
7.77
1.57**
BC
8
50,93
6.37
1.29
ABC
24
43,32
1,80
0.37
Error
426
2105.60
4. 35
Total
503
4559.72
• Signifijant at 1^ levei
*• Significant at 5% levei

94
TABLE 5
lYlEANS AND STANDARD DEVIATION OF ENVIRONMENT
EFFECT ON ABSOLUTE ERROR SCORE (NOR.TÍIAL
AND VIBRATIONAL LN V IRONÍÍlEN TS )
Environment lYlean Standard Deviation
Normal
Vibration
6.93
9.90
2,39
2,93
The analysis of variance results also yielded a
significant effect at the five per cent levei, for the
period of performance,
Table 6 gives the cell means and
standard deviations of this significant factor. Figure 34
is a plot of these means against the absolute error score,
TABLE 6
(YlEANS AND STANDARD DEVIATIONS OF PERIOD EFFECT
ON ABSOLUTE ERROR SCORE (NORIYIAL
AND VIBRATIONAL EN V IRONIYIEN T)
Period
fYlean
Standard
Deviation
1
2
3
4
5
6
7
8
9
8,44
8,31
9.11
8.64
8,80
8,17
7.53
B.43
8.30
3.01
3.07
3,28
3,44
3,03
2,93
2.69
2,70
2,80

95
10
r
CD
M
O
U
(/)
U
o
u
LiJ
c
CO
CD
CO
-P
•H
C
ZD
>>
U
CO
Ul
-p
•H
n
u
cc
8
Normal
Vibration
Environmen t
^
Fig. 33,—Plot of means of absolute error score
versus environment.

96
10
CD
Ul
o
cn to
Ul
o
Ul
Ul
(D
-P
D
—t
o
CO
n
-p
•H
c
ZD
>>
Ul
CO
Ul
-p
•H
JD
Ul
8
j _
2 3 4 5 6 7 0 9
Performance
Recovery
Period
Fig, 34,—Plot of means of absolute error score
versus period.

97
The absolute error under controlled
environments
(normal and vibrational), stayed generally constant during
the first two periods of performance, increased during the
third, fourth and fifth periods, Significant improvement
in performance occurred immediately following exposure
(period 7).
The error score increased agiin in the second
and third periods of recovery (periods 8 .ri 9). An unexpected
improvomont
in performance duri ig :l e last period
of exposure (period 6) may be noted from t le curve. An
explanation o^ the behavior of this point ii delayed to a
later section in .his chapter.
The work/rest schedule, by period
interaction,
affected the absolute error score significantly at the five
per cent levei.
Figure 35 is a plot of the means of these
interactions against the absolute error score. Table 7
provides these means, as well as the standard devintions.
In order to clarify the trends in this figure, a
separate plot is given for each work/rest schedule uòed
(Figures 36, 37, 38, and 39).
UJhen the subjects performed
in accordance with the first work/rest schedule, their error
started high, tended to decrease in the middle of the run,
and fluctuated up and down toward the end.
UJhen the subjects performed in accordance with the
second work/rest schedule, their error started hjgh, improved
in the following period, then went up again in the third
period.
The same cycle was repeated in the second phase of
work.

98
1 I
1 i
(D (D CD
^ r-i r-i
TD
TD TD
(D (D CD
^ -C -C
u Ü u
cn CO cn
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TD
O
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Ul
(D
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Ul
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to
M
O
Ul
Ul
CD
CD
-P
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o
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JD
CD
(í_
O
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U
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Ul
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u
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CO c
(D O
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r-H C
CL H
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ro u.
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p
p
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3
/s.Tun
Aiemc^'^^)

99
TABLE 7
ÍYIEANS AND STANDARD DEVIATION OF UJORK/REST BY
PERIOD INTERACTION EFFECT ü^ ERROR SCORE
(NORIYIAL AND VIBRATIONAL EN V IRONIYIEN T)
UJork/Rest
Period
ÍYlean
Standard
Deviation
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
1
1
1
2
2
2
2
3
3
3
3
4
4
4
4
5
5
5
5
6
6
6
6
7
7
7
7
8
8
8
8
9
9
9
9
8.63
9,32
8.25
7.56
8,3
7,85
7,74
9,36
8,08
9.09
9,79
9,50
7,99
9,06
7,73
9,80
8,46
7,49
9,88
9,36
7,92
8,71
7.34
8,71
7,36
7,71
8.01
7.05
7.90
9.04
8,77
8,03
8,36
8.47
8.73
7.63
3,04
2,82
3,55
2,63
2,94
1,96
3,59
3.58
2.80
2,81
3.86
3.69
3.50
2.72
2.86
4.38
2.79
2.51
3.32
3.09
3.10
2.59
3.53
2.47
2.73
2,35
3,34
2.43
2,16
2,56
3,09
3.05
2,90
2.68
3.18
2.61

IGO
10
(D
M
Q
U
cn
p
o
p
p
UJ
CD
-p
D
O
CO
X)
CO
-p
•H
c
ZD
p
(O
p
-p
•H
JO
P
cr
8
z/
/
3 4 í
Performance
o - â — ^
Recovery
Period
Fig. 36.—Plot of means of absolute error score
versus period—UJ/R schedule I,

101
10
Q)
o to
u
cn
p
o
p
p
üj
CD
-p
D
r-i
O
CO
n
-p
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c
Z)
p
CO
p
-p
•H
JD
P
8
j . X X
2 3 4
Performance
Period
8 9
Recovery
Fig, 37.—Plot of means of absolute error score
versus period—UJ/R schedule II,

102
10 .
CD
P
o
u
cn
p
o
p
p
LU
CD
-P
rH
O
to
JD
CC
CO
H-i
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c
ZJ
>>
p
CO
p
-p
•H
JD
P
cac
8
2 3 4 5
PorformancB
Period
7 8 9
Recovery
Fig, 38,—Plot of means of absolute error score versus
period—UJ/R schedule III,

103
10
CD
P
O
U
cn
p
o
p
p
LU
(D
-P
D
—t
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cc
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JD
P
cr
8
o
o
o
O
o
o
o
o
o
J.
2 3 4 5
Performance
Period
8 9
Recovery
Fig. 39.—Plot of means of absolute error score versus
period—UJ/R schedule IV.

Ulhen the subjects performed in accordance with the
third work/rest schedule, their error score started high,
104
and improved in the following period,
The cycle repeated
itself twice more in this schedule for the second and third
phases of work,
UJhen the subjects performed in accordance with
the fourth work/rest schedule, their error score stayed
within a naorow range, except during the first period,
when the error score was low,
It is notable that under ali work/rest schedules,
subjects' error scores decreased significantly immediately
following the ends of the performance periods (start of
recovery), then went up in the following periods of recovery.
As shown from the above discussion, the results of
the first analysis of variance indicated the validity of
the hypothesis that environment, period and work/rest
schedule by period interaction have a significant effect
on the absolute error score,
The hypothesis concernlng
the remainder, however, was rejected according to the ATiOVA
results,
The values and plots made for the interaction of
period and work/rest phases discussed above are for the
combined effects of normc-:! and vibrational environments,
which does not give a clear idea of the effec: of the
environment upon each of these factors indiv i dL.al ly,
To
interpret the results obtainad in such a way a' to allow

full understanding of performance and recovery behavior
105
under normal and vibrational environments, performance in
terms of absolute error scores was averaged over each period
(average of five readings within the period), and plots of
period versus average error score for each work/rest
schedule under each environment, normal and vibrational,
were then made, as shown in Figures 40, 41, 42, and 43.
In the first work/rest schedule during which work
(tracking) was performed continuously, the average error
score under normal environment started high, improved
slightly in the second period, leveled off in the third,
then began to increase again,
This behavior is anticipated
in tracking tasks, in which the signal is time dependent
(i,e,, repeated after a constant period of time); the
subject starts at a certain levei of performance, observes
the signal pattern and rate of change, learns it and performs
in a more systematic way.
As time passes, the task
becomes boring to him and/or he becomes fatigued,
The score
then goes up once more,
The degree of the subject's adaptation
to the task, and the amount of time before he becomes
fatigued depend upon the difficulty of the task« duration
of exposure, arrangement of the work station, and many other
factors,
The decrease in error during the last of.riod of
performance under normal environment (period ü) was not
anticipated.
It is possible that subjects expectoc the end

106
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110
of the run and this motivated them toward improvement in
performance.
Similar behavior did not occur, however,
under vibratory conditions where subjects' performance
decreased further during this last period,
Since man
himself has a time sensing ability, it is tempting here
to argue that this ability could be impaired under vibratory
conditions since his body senses are disrupted by engaging
in vibration attenuation activity,
Further research, however,
is recommended before a full explanation of this behavior
could be ascertained,
The drop in error score at this last
period of performance under normal environment is believed
to have influenced the drop in error score appearlng in the
same period and shown in Figure 34 (page 96),
The first reading during recovery showed a continuation
of the trend of the last period of performance,
This
was anticipated because the task performance during the
first period of recovery was a continuation of the performance
from the last period of work.
The other two readings under recovery were taken
after the work was stopped for four minutes, and they appeared
to have a higher error score because of the interruption in
the pattern of work which brought the subjects back to their
initial levei of performance when starting a task.
The average error score under a vibrational environment
showed that subjects started at a certain levei of
performance, and kept this for about two periods.
Error

111
score increased toward the end of the run, and continued
to increase until vibration was stopped. Error score
dropped instantaneously following cessation of vibration,
then began rising again in the second and third periods
of recovery, Again the rise in these two last periods
is believed to be due to task interruption,
During the second work/rest schedule, with subjects
performing for two equally spaced fifteen-minute periods,
the average error score under normal environment started
high, improved after the first period probably because of
subjects' adaptation to the cyclic repetition of signal,
then went up again due, it is believed, to a combination
of boredom and fatigue, During the second phase of work
(also referred to as second trial) subjects reacted in the
same way, except that their leveis of performance were
even better than those during the first phase, This is
believed to be due to an immediate transferal of learning
from the first phase, The plotted points of the results
obtained during the recovery period do not seem to support
this transferal of learning explanation, It is, however,
important here to mention that these points were based on
one error score reading after a rest period and not an
average of five readings as is plotted for the performance
periods, It is therefore believed that the task interruption
effect was stronger than the transfer of learning effect at
these points, UJhen subjected to the vibratory environment.

112
subjects' behavior in performance of the task followed the
same general pattern of their performance behavior under
normal environment. This is cleariy shown in Figure 41
(page 107).
Error scores improved significantly during
the first minute of recovery immediately following vibration,
then went up again in the following two periods of recovery.
During the third work/rest schedule, in which subjects
performed for three equally spaced ten-minute trials,
the average error score under normal environment started
high during the first period, and improved during the second,
because of their adaptation to the task.
High error scores
were obtained during recovery because of the interruption of
work as prevlously explained.
UJhen subjected to vibratory environments, subjects'
behavior in performance of the task followed closely the
same trends of behavior that were noticed in performance
in a normally controlled environment as shown in Figure 42
(page 108).
Under vibrational environment, better performance
was displayed during the second and third phases of work
than during the first, a condition supporting the transfer
of learning explanation given earlier.
Error score decreased
immediately after the cessation of vibration, then increased
again in the following two periods.
During the fourth work/rest schddule, in which subjects
performed for six equally spaced five-mj.n^:te trials,
the average error score appeared to fluctuate within a narrow

113
range.
Performance through the last four trials under
vibration displayed a slight trend toward improvement,
with significant decrease in error score immediately
following the cessation of vibration.
Second Analysis of Variance Results
The second analysis of variance was designed to
test the hypotheses that work/rest schedule and period,
or their interaction, affect the difference between the
average error score obtained under vibration and novibration
conditions.
The periods under investigation
include both performance and recovery periods,
The
results of this ANOVA are shown in Table 8,
TABLE 8
ANOVA FOR DIFFERENCE IN AVERAGE ERROR SCORE
(VIBRATION AND RECOVERY)
Source
DF
Sum of
Squares
lYlean
Square
F-Ratio
••
Blocks
A, UJork/Rest
5
3
226,61
44,74
37,77
14,91
6.85*
2,70*»
C, Period
8
98,58
12.32
2,23*»
AC
24
38,72
1.61
0,29
Error
210
1158.48
5.52
Total
251
1567,13
* Significant at the i% levei
*• Significant at the 5% levei

114
The work/rest schedule had a significant effect
upon average performance, under vibration at the five per
cent levei,
lYleans and standard deviation of the significant
factor, the work/rest schedule, are given in Table 9,
TABLE 9
(YlEANS AND STANDARD DEVIATIONS OF 'JORK/REST SCHEDULE
EFFECT ON DIFFERENCE IN AVERAGE ERROR
SCORE (VIBRATION AND RECOVERY)
UJork/Rest Schedule (Ylean Standard Deviation
1
2
3
4
7,62
7,73
8,26
8,57
2,58
2.78
2.73
1.65
Plots of the means against the difference in average
error score are shown in Figure 44.
A Duncan Tilultiple
Range Test indicated a significant difference between the
fourth work/rest schedule (five-minute trials), and both the
first schedule (thirty minutes of continuous work) and the
second schedule (fifteen-minute trials).
The data point to
the fact that over performance and recovery peri^^ds, work/
rest schedule I produced the least decrement when performed
under vibration and allowed maintenance of a low average
error score throughout periods of vibration and recovery.

115
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error score versus work/rest schedule (vibration and
recovery),

Average difference in error score was significantly
116
affected by the period at the five per cent levei.
Table
10 gives the means and standard deviations of the effect
of the significant factor:
period,
TABLE 10
ÍYIEANS AND STANDARD DEVIATIONS OF PERIOD
EFFECT ON DIFFERENCE IN AVERAGE ERROR
SCORE (VIBRATION AND RECOVERY)
Period (Ylean Standard Deviation
1
2
3
4
5
6
7
8
9
8,13
8,73
8,28
8,62
8,41
8,79
7,07
7.09
7,61
1,95
2,32
2.60
2,09
1,98
2,33
2,92
2,80
2,89
A Duncan (Ylultiple Range Test indicated that performance
during vibrational exposure periods differed significantly
from performance during recovery periods (period 7 and 8,
Figure 45), No significant difference was detected during
the different periods of performance under vibration,
This
was as expected, since cessation of vibration removes stress,

117
thus allowing almost immediate recovery from its effect.
Recovery is not complete, however, within the period specified
for this study,
This was manifested by the behavior of the ninth
period (Figure 45) where difference in error score increased
again•
The results of the second analysis of variance
indicated the validity of the hypotheses that work/rest
schedule and period have a significant effect on the increase
in average error score due to vibration exposure,
Third Analysis of Variance Results
The third analysis of variance was designed to
test the hypotheses that work/rest schedule and period
affect the difference between average error score obtained
under vibrational environment and that obtained under normal
environment,
Their effect upon the percentage increase in
average error score was also tested.
The periods under
investigation were those performed during the vibrational
environment exposure only (first six periods). Table 11
shows the results of the ANOVA in terms of difference in
average error score.
Table 11 indicates that a significant effect at the
five per cent levei is attributable to the work/rest schedule.
(Yleans and standard deviations of the effect of this significant
factor are shown in Table 12,

118
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119
TABLE 11
ANOVA FOR DIFFERLNCE IN AVERAGE
ERROR SCORE (VIBRATION)
Source
DF
Sum of
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íflean
Square
F-Ratio
Blocks
6
84,55
14.09
3,03»
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3
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15.75
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TABLE 12
ÍYIEANS AND STANDARD DEVIATIONS OF
SCHEDULE EFFECT 01^ DIFFERENCE IN AVERAGE
ERROR SCORE (VIBRATION)
L'I0RK/REST
UJork/Rest Schedule (Ylean Standard Deviation
1
2
3
4
7.96
8,04
8,76
9.25
2,06
2.55
2,47
1,30

120
A Duncan ^ultiple Range Test indicated that work/
rest schedule I and II differed significantly from work/
rest schedule IV.
Subjects performed better according to
the first two schedules than they performed during the
fourth work/rest schedule, when they were subjected to
vibration. This effect is shown in Figure 46. Periods of
performance during vibration exposure did not show a significant
effect on difference in average error score.
This was
also seen prevlously in the results of the second ANOVA.
This finding was in agreement with the results obtained by
other experimenters in the field.
Johnston (1969) found
that duration of exposure does not affect performance under
vibration,
Results of the ANOVA in terms of percentage of
increase in average error score are shown in Table 13,
TABLE 13
ANOVA FOR PERCENTAGE INCREASE IN ERROR
SCORE (VIBRATION)
Source
DF
Sum of
Squares
íil e a n
Square
F-Ratio
Blocks
A, UJork/Rest
C, Period
AC
Error
6
3
5
15
138
37,80
7.57
1.79
1.59
66.69
6.30
2.52
0.34
0.11
0.48
13.03*
5.22**
0.70
0.22
Total
167
* S n ifican t
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Pt
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the 1% 1
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121
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error score versus work/rest schedule (vibration).

122
Table 13 demonstrates that the work/rest schedule
has significant effect (at the one per cent levei) upon
percentage of increase in average error during performance
under vibration,
Table 14 provides the means and standard
deviation of the work/rest schedule effect upon percentage
of increase in average error score due to vibration,
TABLE 14
(YlEANS AND STANDARD DEVIATIONS OF
SCHEDULE EFFECT ON PERCENTAGE INCREASE
IN ERROR SCORE (VIBRATION)
UJORK/REST
UJork/Rest Schedule lYlean Standard Deviation
1
2
3
4
1,28
1,32
1,69
1,76
0,48
0.68
1.07
0,89
UJhen subjects were to perform according to the
selected work/rest schedules under vibrational environment,
schedules I and II resulted in significantly less percentage
increase in average error score than schedules III and IV did.
Figure 47 is a plot showing this result.
This is believed
to be due to the fact that subjects experience disturbances
during vibration,
UJhen instructed to begin the task, it was
observed that they adjust their bodies in such a way as to
attenuate the effects of the vibrational energy being

123
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124
transmitted to their bodies at this stage.
During this
adjustment process, subjects' performances are poor,
After
the process is complete, performance tends to improve,
Since the task is cyclic, subjects adapted to it after a
period of time, An example of this is shown in Figure 48,
where the acceleration of the hips is attenuated gradually
after the task started.
The work periods not being very
long, it is assumed that subjects did not reach a stage of
fatigue while performing in accordance with the first two
schedules, which may have nullified the improvement due to
adaptation to both the task and vibration,
This finding
has important implications relating to the designing of
phases of work and rest in tasks performed under vibrational
environments.
One such implication is that there may be an
optimum work/rest schedule that is least affected by vibrational
environment,
The criteria affecting the detection of
this optimum work/rest schedule are subjects' adaptation to
the task, their body attenuation to vibration, and the length
of work period before which they get into the stage of boredom
and fatigue,
Further research, however, is required in this
point taking longer work period into consideration in the
work/rest chedule tested,
Subject effect (blocks) will not be discjsr.ed in this
experiment,
It may be noted that subject effect was always
highly significant in ali ANOVA'5 at the one per cent levei.

Fig. 48.—Recording of • subjecfs hip acceleration
before and during taak performance.
125

126
This was expected, of course, and even influenced selection
of the model of experimental design used.
This chapter has presented the results of this
experiment and the interpretation of the significant
findings obtained from the analysis of the data. The
following chapter gives the conclusions drawn from this
study and the recommendation for further research.

CHAPTER V
CONCLUSIONS AND RECOíYlFílEN DAT IONS
Conclusions drawn from results obtained in this
investigation and presented in Chapter IV, will be discussed
in this section.
A comparison with the results
obtained in other studies in the field will be made when
appropriate, and recommendations for further research will
be set out.
Conclusions
Environmental Effects
( Environment has a highly significant effect upon
human performance.
A vibratory environment causes an increase
in vertical tracking error.
An increase in absolute error
score of approximately forty-three per cent occurred under
the leveis specified in this study.'
This conclusion is supported in the literature by
the findings of other investigators.
Buckhout (1964) arrived
at a similar conclusion, indicating that decrement in vertical
tracking performance ranged from thirty-four to seventy-four
per cent.
Harris and Chiles (1964) found that at a frequency
of 5 cps, and an amplitude of 0.16 inches (the leveis used
in this study), a significant increase of error in the
127

128
vertical component of their tracking task occurred.
Other
investigators, also, arrived at conclusions which support
those learned in the present study (Frazer, et ai., 1961;
Hornick, 1961; Schmitz, 1958; and others).
Period Effect
i UJhen an average of performance was taken over each
period (five minutes), and under both normal and vibrational
environments, the difference in error score between the two
did not seem to be affected by duration of exposure to
vibration.^
It was found that error score drops immediately
in the first minute interval of the first period of recovery.
It goes up again in the first minute of performance of the
second and third periods of recovery, remaining, however,
below that obtained under vibrational conditions,
This
leads to the conclusion that duration of vibration has no
effect upon human performance, and that the functions of the
body are performed in very much the normal way, directly
after removal of stress,
This improved performance is not
maintained for long after recovery and the error score
presently increases,
This may have the sericus i^^olication
that vibration at the levei specified in this study has an
after exposure effect which prevents co-iplete recov.jry within
as short a period of time as that employed in this study.
Other investigators reached conclusions jnder the
special conditions of their studies, which, u.hen analyzed.

129
oxhibit decided similarities to those reached in this study,
Johnston
(1969), in an experiment on body orientation under
vibration, concluded that the duration of exposure to vibration
had no significant effect upon reaction time, travei
time, accuracy, or average velocity,
Faster reaction time
was obtained following exposure to vibration,
Simons and Hornick (1962) reported that vibration
usually causes a decrement in tracking ability, and the
recovery of this ability is not complete following exposure,
In these studies, no differentiation was shown between
performance immediately following exposure, and performance
after a certain period of recovery, as was done in this
experiment.
Effect of UJork/Rest Schedule
by Period Interaction
( From the results obtained in this study it was concluded
that different work/rest schedules yield different
performance effects.
A general performance pattern noted
in the tracking task. employed is given below.
This may be
generalized to apply to other time-dependent and/or monotonous
tasks.
A subject's performance starts at a certain levei.
After a warm-up period, performance improves, probably because
the subject starts learning the pattern of the repeated signal,
and adapts to it.
After a certain time-span, performance
declines again, probably because the subject becomes either

130
bored, fatigued, or both.
The length of this span of time
depends, of course, upon the difficulty of the task, the
work station, and other environmental factors.
These performance characteristics appear in schedules
I, II and III, in which work phases are of more than five
minutes.
In schedule IV, however, in which the work phase
is only five minutes, the average performance appeared as one
point only; therefore, these performance characteristics do
not appear. This is cleariy demonstrated in Figures 40, 41,
42 and 43, pages 106, 107, 108 and 109, respectively. /
In order to observe the performance characteristics
within each five minute period, continuous plots of performance
profiles for each work/rest schedule were made (Figures
51, 52, 53 and 54, Appendix D), These figures show the
error score for each minute of work, rather than an average
for five minutes of work, as prevlously shown in Figures 40,
41, 42 and 43, pages 106, 107, 108 and 109, respectively.
The profiles in these minute by minute figures Ied
to a very interesting conclusion:
that no matter how short
the period, subject's error score following a task interruption
starts high,
Once he is accustomed to the signal pattern,
his performance improves.
This is shown to be true under ali
work/rest schedules used, and explains the high error readings
obtained during the second and third periods of recovery
(periods 7 and 8), in which performance was measured directly
after a task interruption,
The continuous plot of the

eadings also showed trends similar to those prevlously
131
shown for the average error score over the periods.
(it was concluded, also, that a subject's tracking
performance follows essentially the same pattern under both
normal and vibrational environments, even though the levei
of performance is much better under normal environment,
Under both environm!:nts a subjecfs performance starts at a
certain levei, and passes through a warm-up stage, during
which performance improves,
It then declines in quality once
more, J
Another conclusion which may be drawn is that there
is a process of transfer of learning,
That is, if a subject
were to perform a tracking task in several trials, with short
periods of recovery between thfí^m, successive trials would be
expected to yield an average performance as good as, or better
than, that of the first trial,
This is particularly true
under a vibrational environment, because learning is transferred
not only as pertains to the task, but possibly, also,
to the subject's attenuating ability, or adjustment of his
body in such a way as to weaken the effect of vibration,
The process of transfer of learning was evident only
during periods of controlled environment (normal and vibrational)
and did not hold during periods of recovery.
It is to
be noted, however, that performance during the recovery period
was recorded for only one minute of work (task ner f o r-.an ce).
This was in comparison with an avnragc ocrforma^.-O over five

minutes of work during the controlled environment periods.
As indicated prevlously, subjects always performed poorly
132
at the beginning of a new phase of work.
It is believed,
therefore, that the effect of task interruption (during
recovery periods) was stronger than the effect of transfer
of learning.
Harris and Chiles (1964), in an experiment with
vibration, and using the same leveis of frequency used in
this study, reported that the performances of their subjects
improved significantly through each five-trial testing period.
They were tempted to postulate that the disruptive effects
of vibration are temporary, and that subjects adapt to them.
The results of this study support the hypothesis regarding
adaptation.
A temporary disruptive effect of vibration occurs
only at the beginning of work trials, when subjects' bodies
are not adjusted in such a way as to attenuate the vibrational
effect. Performance at this point is usually poor. A detailed
discussion of this point is given below.
Effect of UJork/Rest Schedule upon
Increase of Error Score due to Vibration
UJithin the scope of this study, when work/rest schedules
were put into effect under normal and vibratory environments,
those with longer work phases resulted in less decrement in
subject performance.
This conclusion is based upon the results
of two AN0VA'S.
One demonstrated that the work/rest schedules
used had a significant effect upon the difference in average

133
error score between performance under vibration and performance
in a normal controlled environment.
UJork/rest schedule
I, based upon thirty minutes of continuous work, affects
the average increase in error score less than do other
shorter work/rest schedules, especially work/rest schedule IV.
Another ANOVA showed that the percentage of increase
in error score under a vibrational environment, as contrasted
with that under a normal environment, was also significantly
affected by the work/rest schedule.
Schedules having longer
phases of work (schedules I and II) resulted in a lower
percentage of increase in error score than did schedules
having shorter work phases (schedules III and IV).
í Another interesting conclusion which may be derived
from the above mentioned findings is that when a subject is
required to perform a task under vibration, his body utilizes
its built-in vibration isolation capabilities, and adjusts
itself in such a way as to attenuate the effect of vibration
transmitted to it.
In short, his body adapts to vibration.
UJhen the task performance is stopped, the body relaxes and
a disruption in body adaptation occurs.
If the task is
performed again after a time, the same cycle will repeat
itself, and the body will attenuate vibration again.
During
the process of this adjustment, subject's performance is
poor. I
According to this study and the above hypotheses of
adaptation and disruption, a recommendation might be made to

134
schedule the subjects in such a way as to allow minimum
disruption effects, i.e., minimum task interruption.
Since, toward the end of the longer phases of work,
performance tended to show a decrement, it indicated that
subjects had reached a certain levei of boredom and/or
fatigue.
Entering this period of fatigue at a higher levei
may be the limiting factor in designing the length of the
work phase.
A suggestion which is very tempting to make
is that there might be such a thing as an optimum work/rest
schedule which would result in minimum decrement when
performed under a certain levei of vibration.
Future
research might pursue this question and try to ascertain
if such an optimum work/rest schedule does, indeed, exist.
Transmissibility of Vibration
The frequency of vibration used in this study (5 cps)
caused an amplification of acceleration transmitted from
the platform to both the hip and the head.
Transmissibility
factors of about 1,4 between the platform and the nip, and
of about 2 between the platform and the head, were noted for
one experimental subject,
Calculations of the accelerations
transmitted are shown in Table 17, Appendix H,
'his transmissibility
factor varied widely from subject to subject,
according to his anthropometric measurements ano ncdy structure,
These leveis of acceleration transmission were
observed to change when subjects moved from one posture to

135
another and from a working phase to a resting phase with
the body reiaxed.
No attempt, however, was made to measure
the amount of this change.
Subjective Reaction to Vibration
In this investigation, ali subjects completed ali
runs without experiencing any difficulty or having to stop
the vibration during any experiment.
They reported a sensation
of relaxation during the vibration, and immediately
following it, differing sensations.
Some felt dizzy; others
reported an indefinable feeling best described as "floating";
still others reported no effect,
These feelings ended shortly
after cessation of vibration and ali subjects agreed that no
serious difficulty was encountered—only boredom and mild
unpleasantness,
The conclusion is that under the leveis of
vibration used in this investigation, prolonged (one hour)
exposure was withstood very well by subjects,
Recommendations for Further Research
This investigation gave rise to many questions deserving
future research,
Since vibration frequency and intensity
were held constant in this study, further study extending to
different leveis of vibration is recommended.
As different subjects have different vibration attenuation
capabilities, it would oe of interest to study the
rolationship between performance and leveis of vibration

136
after attenuation by the subjects.
A comparison of results
based upon frequency and acceleration
changes is deslrable,
UJork/rest schedules investigated in this research
were but a few of the many which may be incorporated in a
mission of one hour under continuous vibration.
Future
research should undertake to find the optimum
work/rest
schedule to be used under each levei of vibration exposure.
Such an optimum may be obtained by including work/rest schedules
which have longer phases of work than those used in
this study, and establishing the span of work after which
fatigue may occur,
Research pertaining to one work/rest schedule and
a comparison of this effect with that of a pretask vibration
period, is also recommended,
This will allow detection of
the effects of vibration upon different crew members; in
fact, future studies are recommended during whiuh two or
more subjects are put under the same degree of vibration in
a mechanical simulator, with tests being made of their efficiency,
and of the mission efficiency, after changing
their
work/rest schedules,
In this study, subjects showed conslderable
improvement
in their tracking ability during the last period of performance
under normal environment,
It was suspected that this
might be due to subjects' time-sensing abilities which might
have caused anticipation of the end of the task, and acted as
a motivation to them, This behavior did not occur under

137
vibrational conditions, however, and performance during this
last period was poorer than during previous periods, It
is suggested that future research investigate the effect
of subjects' knowledge of the end of the task, and
the
effect of this upon performance, as well as the question of
vibration being responsible for impairment of their timesensing
abilities.
Investigations concernlng recovery from vibration
are still greatly needed. U/ithin the periods allowed
for recovery in this study, complete recovery did not
seem to occur. This could be attributable to physiological
or psychological factors, The factors leading to complete
recovery following vibration exposure are recommended as
áreas of investigation, Since the concept of transfer of
learning did not seem to hold during the recovery period,
further research is required to clarify this point,
Effects of vibration upon tracking behavior have
been studied by many investigators. Attempts to compare
their findings, however, have been very few, oractical
usage of the data available is limited, It is believed that
there is a great need for studies which will relate the
results to a one-factor effect (i.e,, only frequency, amplitude,
or acceleration),
Recent development of industrial and military equipment
and situations requires a great investigation of vibrational
effects during long periods, It is recommonded that

138
this be undertaken in future research,
Long period studies,
however, give rise to the question of availability of Information
about safe human exposure times and tolerance limits.
In spite of some excellent studies, detailed and precise
Information on this subject is still lacking,
Clear-cut
definitions and leveis are needed, with more samples from
different populations being used,
In the general field of vibration studies, more
interest was focused upon sinusoidal vibration,
In a multitude
of situations vibration is not of this kind,
Random
vibration is common in most vehicle operation situations,
Research is recommended, therefore, in the sector of the
effect of random vibration.
Data upon vibrational effects in the different
planes, vertical, horizontal and transverse, and combinations
of these, is still needed and research should be
oriented toward obtaining this Information,
As was reported in this study, an effort was made
to prevent ambient and vibration platform noise reaching
the subjects by using cotton pieces and earphones,
This
arrangement is not always practlcal, and a combination of
stresses might affect the subject.
Studies in the sector
of combined environmental stresses are needed and recommended.

139
Summary
This research was conducted for the purpose of studying
the performance and recovery characteristics of men when
subjected to relatively long periods of whole-body, vertical
vibration,
The effect of environment, and of a work/rest
schedule, upon performance and recovery were investigated,
Analysis of the performance in terms of duration of exposure
to vibration was also undertaken,
The significant
conclusions drawn from this research are listed below:
1, A vibratory environment causes significant
decrement in vertical tracking ability (performance
measuring task in this experiment).
2, Absolute tracking error scores increased as
much as 43;^ under vertical vibration,
3, Duration of exposure to vibration did not
affect the performance,
4, Performance improved significantly immediately
following the cessation of vibration,
5, Complete recovery after vibration did not occur
during the period allowed for recovery in this
study,
6, Performance of a tracking task follows a pattern
in which subjects start at a certain levei of
performance, experience a warm-up period during
which performance improves, then display a

decline in performance,
140
7, Performance of the subjects improved throughout
consecutive trials when they were given
periods of rest between trials.
8, For the work/rest schedules specified in this
study, those having longer working phases
showed less increase in average error scores
resulting from vibration than those having
shorter working phases,
9, For the work/rest schedules specified in this
study, those having longer working phases
showed a lower percentage of increase in error
score resulting from vibration,
10. Acceleration was amplified when transmitted
from the platform to the hip and head.
11, Subjects did not have serious complaints in
relation to the leveis of vibration used in
this study.

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APPENDIX
A, Natural Frequencies of the Body Parts
B, ÍYlonitoring Accelerometers Data and Calculations
C, Example of a Subject Learning Curve
D, Continuous Plots of Performance Profiles
155

APPENDIX A: NATURAL FREQUENCIES OF THE BODY PARTS
156
TABLE 15
NATURAL FREQUENCIES FOR VARIOUS PARTS OF THE BODY'
Body Position or (Ylember Natural Frequency
Whole body, vertical standing
Whole body, prone
Whole body, vertical seated
Body seated on cushion
Head with respect to body
Lower jaw with respect to skull
Shoulder and head, transverse
Eyeball
Hand
Skull
The resonance frequency of the
thoracic cavity
The resonance frequency of the
heart
The resonance frequency of the
facial musculatura
5 to 12 cps
3 to 4 cps
4 to 6 cps
2 to 3 cps
20 to 30 cps
100 to 200 cps
2 to 3 cps
60 to 90 cps
30 to 40 cps
300 to 400 cps
5 cps
7 cps
11 cps
Compôs
ited from Rathbone (1953) and Buckhout (1964)

157
APPENDIX B:
(YIONUORING ACCELEROÍYIE TERS DATA ANO CALCULATIONS
TABLE 16
SPECIFICATIONS OF THE ACCELEROíflETERS
USED IN THIS STUDY
Specifications
Platform
Hip
Head
Serial No.
230746
230726
230611
Reference Sensi*^i7ity
at 23°
50 Hz
50 Hz
50 Hz
able Capacity of
105 pF
104 pF
105 pF
'/oltage fensitivity
60.8 mV/g
60,7 mV/g
54.8 mV/g
Change Sensitivity
65,0 pC/g
69,1 pC/g
5G.0 pC/g
Capacity (incuding
cable)
(Tlaximum Transverse
Sensitivity at
30 Hz
Undamped N^jtural
Frequency jndüi
Tested ConditiohS
'linmum ResioCancc
at Room icmnerature
in l;legjhms
1070 pF 1138 pF
1,6^
1.-65^
46 kHz
4b kHz
2ü,0ÜÜ 20,000
lO^i pF
3,75^
46 kH7
20,000

158
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i
T
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-Y
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Fig. 49.—Typical response of monitoring
acclerometers.
TABLE 17
CALCULATION OF THE G LEVEL TR ANSíYl ITTED
Specifications
Platform
Accelerometer
Hip
Placement
Head
Vibration
frequency
Recorder
sensitivity
Chart
defIection
Output
voltage
Accelerometer
sensitivity
Acceleration
intensity
5 cps
200 mV/cm
0.5 cm
120 mV
65.0 pC/g
0.185 g
5 cps
200 mV/cm
0.8 cm
160 mV
69,1 pC/g
0,246 g
5 cps
200 mV/cm
1,2 cm
240 mV
58,0 pC/g
0,369 g
Example calculation
(0.6 cm) (200 mV/cm)
(65.0 pC/g; (10 mV/pc)
120 mV
650 mV/g
0,185 9

159
APPENDIX C: EXAÍYIPLE OF A SUBJECT LEARNING CURVE
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Fig. 50. — Example learning curve obtained "^or subject
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