Strain reduction (2438 kb) - Ergonomia.de

Contribution made by ergonomic investigation towards strainreducing
design of pruning shears
Abstract
J. Wakula and K. Landau
Institute of Ergonomics, Darmstadt University of Technology
Petersenstrasse 30, D-64287 Darmstadt, Germany
Objective and subjective ergonomical methods (EKG, EMG, questionnaires) were used in field and laboratory
studies to evaluate hand-operated pruning shears produced by different manufacturers. All of the tools investigated
were readily available on the market. The salient features and parameters (e.g. cutting principle, type and form of
grip, weight, centre of gravity, distance between hand and blade, span of grip) of all the shears were documented and
analyzed. Stress-strain levels were determined and analyzed. No significant differences in overall physical strain were
observed with any of the hand-operated shears tested in field studies. The laboratory study comprised three different
investigation cycles, in which four types of shears were analyzed. EMG measurements in two muscles m. flexor
digitorum superficialis and m. extensor digitorum were used to determine the local strains. These revealed that shears
using the knife-edge principle caused higher muscular strains than those using the shearing cut principle. Each
volunteer was asked to give a subjective assessment of each type of shears. Ergonomic requirements have been
worked out.
Keywords: pruning shears, stress-strain analysis, EMG, ergonomic design requirements
1. Introduction
There is an increasing awareness that good
ergonomic design of hand tools used by manual
workers is an important contributory factor in
alleviating physical stress and reducing the risk of
cumulative trauma disorders (CTD) in the upper limbs.
It is, for example, a recognised fact that cumulative
trauma disorders like carpal tunnel syndrome and
tenosynovitis are linked with work involving high
levels of hand activity [1]. The nature of the work task
and the work object and the design of the hand tools
being used are all factors which can influence how the
worker applies the tools and what postures will be
adopted.
Anthropometric and physiological design deficits in
some existing cutting hand tools, e.g. high actuation
forces and poor adaptation to the human hand-arm
system may lead to undesirable body postures and
consequently considerable worker strain and financial
losses from inefficient job performance.
Pruning grapevines causes high stresses on vineyard
workers. The stress factors include repetitive
movements combined with external forces acting on
the finger-hand-wrist system (by using of non-power
pruning shears) and static work in the upper armshoulder
system. A wide variety of prunes is used in
vineyards daily during the five months from November
to March for the manual pruning of grapevines.

2. Methods and subjects
The field studies analysed stress-strain-situation of
grapevines pruning with five non-power shears from
different manufacturers (Fe7, Fe2, WRS22, "Fi-Erg",
Lö; see Fig. 1) and two power tools (electrically and
pneumatically) operated by a total of eight workers [2].
Wolf RS 22
260 g
Löwe - old
280 g
Löwe - new
280 g
Felco 2
210 g
Felco 7
280 g
Fiskars
260 g
Figure 1: Analysed prunes in the field studies
Electrocoup
850 g
Table 1 presents technical data of non- power tools
analysed in lab.
Table 1: Technical data of non- power tools analysed in lab
Electrophysiological methods [3] were used to
evaluate the effects of the stresses on the vineyard
workers in field and lab studies. Effects on the
cardiovascular system were assessed by heart rate
measurement and the workload on selected muscles by
measurement of electrical activity (EMG) in the:
- m. flexor digitorum superficialis, m. bizeps
brachii and the m. deltoideus pars spinalis
while pruning grapevines in field study
- m. flexor digitorum superficialis and m.
extensor digitorum) in lab.
In order to make an assessment of the acceptance of
the analysed prunes using during cutting of grapevines,
a questionnaire was prepared as a rapid testing method.
The questionnaire includes 27 questions due
different characteristics of prune (e.g. handles, blades,
locking mechanism, functioning & operation, see Table
2).
Table 2: Structure of the questionnaire

3. Results
3.1 Main results from the field study
The field study revealed that the cutting grapevine
entails many working situations with postures that can
cause musculoskeletal disorders. The results differed
depending on the person and the conditions at the
vineyards, e.g. inclination of the yards, mode of
growing and fixing the grapevine, age of the
grapevines. Important stress factors are repetitive
movements combined with external forces in fingerhand-wrist
system, extreme positions in arm-shoulder
system and climatic conditions.
The overall strain can be rated as tolerable for
cutting grapevines used different shears because the
working heart rate remained below the upper limit for
strain [2].
For local strains (muscle activity) it was found that
the musculus biceps brachii provided the greater part
of the support to the musculus flexor digitorum
superficialis during the pruning work. The minor rises
in electrical activity indicate a very low degree of
muscular fatigue. In contrast, the electrical activity in
the musculus deltoideus pars spinalis showed less
strong fluctuations.
3.2 Results of stress-strain analysis of cutting
performances in laboratory study
A detailed stress-strain analysis was carried out in
laboratory study. In the analysis, four different types of
non-powered shears F7, Fi-Erg, Lö and W-RS22, were
used. Two hand-arm postures: hand-upper arm neutral
position and forearm in supination as well as different
cutting tasks and hand grips were analysed.
The strain was evaluated by means of the EMGs of
two muscles (m. flexor digitorum superficialis and m.
extensor digitorum) and through subjective ratings.
Eleven persons - winegrowers and students - took part
in the study.
3.2.1 Main results from strain analysis
- In general, shears based on the ”shearing”
principle (e.g. F7, Fi-Erg.; see Fig. 2 and Fig.
3a) resulted in lower strain situations
compared to those operating in an
"hammer/amboss" principle (e.g. Lö see Fig.
2 and Fig. 3b).
- Prunes operated in neutral wrist position
resulted less fatigue than those position if
wrist was supinated. These results are based
on the strain situation of two muscles, namely
the m. flexor digitorum superficialis and the
m. extensor digitorum.
% EA- proportion related to max.
contraction
35,0
30,0
25,0
20,0
15,0
10,0
5,0
0,0
9,5
prune operated in neutral w rist position cutting w ood
18,6
14,9
32,5
8,5 8,0 7,3
27,6
Fe 7 Fi-Erg Lö-neu W-RS22
prunes
Figure 2: Strain in m. flexor digitorum superficialis while
prunes operated in neutral position with / without wood
Figure 3a, b: Prunes with ”shearing” (left) and "hammer/
amboss" principles (right)
- Looking on strain of the m. flexor digitorum
superficialis while shears will close /open of
the (hand and arm neutral), the mean values
have the following ranking order: 1) type "W-
RS22", 2) type "Fi-Erg", 3) type "Lö" and 4)
type "F7".
- Based of EMG data the significances has
been proved related to strain by using
different prunes (see Table 3).

Table 3: Significances testing related to strain by working
with different prunes
* Signifikance level 0,95
* * Significance level 0,99
3.2.2 Result of subjective ratings
The results from subjective ratings by 11 people are
presented in table 4.
Table 4: Results from subjective ratings (n=11)
- Results show that in the closing/opening
process, the type "F7" was to some extent
preferred to the type "Fi- Erg". Cutting of
grapevines produced a different ranking. In
this case, the type "F7" was substantially
preferred against the type "W-RS22" and the
type "Lö".
- While looking into the subjective rating of
prunes, it was found that lay people took into
consideration the results of the experiment
only, whereas professional users included also
the working experience in their rating. The
brand of prunes currently used by the worker
in his or her job also influenced the subjective
rating.
3.2.3 Result of subjective questionnaire
19 vine growers took part in this study. Following
results /answers have been achieved:
Evaluation of the handles.
- For a half of the persons who took part in the
survey, the length of the “Fi-Erg”-handles was
not sufficient (too short), especially while
wearing gloves. Because it is not possible to
produce the force needed and therefore not
possible to take advantage of the strength of
all five fingers, too short a handle associated
with an even heavier strain on the fingers. The
effectiveness of the work is reduced also.
- Women preferred prunes with relatively short
handles, because they have small hands.
- The form of the upper handle some prunes is
extremely discomfort able because point of
pressure ia concentrated on a small area near
the thumbs. As a positive example, the testers
referred to the prune of the type “Fe7”. The
advantage of this prune is the shape of their
handles follows the palm on the whole length
of the tool.
- The surface of the handles was mostly
considered appropriate because guaranteeing
a firm grip.
- Operating the shears was regarded normal or
easy.
Evaluation of blades
- The quality of the blades was considered
excellent because they are sharp and allow
long cutting sessions without sharpening inbetween.
Because the blades are very thin,
only minor displacement of wood takes place
while cutting the branches. Of course this
facilitates the cutting process.
Correspondingly, the quality of the cut was
found very clean. However, more than half of
the testees complained about the gap between
the blades during cutting.
- Coating of the blades was appreciated by only
a few testees. All of them claimed that the
coating wears out very quickly.
- Adjusting gap between the blades of Fi-Erg
prune was regarded very easy, because it can
be done simply by turning a screw.
Furthermore, it is very positive that the
clearance seldom needs to be readjusted.
- The opening of blades was considered
satisfactory or too small by many testees.

Evaluation of the size adjustment:
- About one third of the testees were not
familiar with the hand size adjustment
function of Fi-Erg prune. Most of them had
only used this prune for a short time period.
- There were several different opinions about
the functionality of this feature.
- Weight and centre of gravity were considered
appropriate.
4. Conclusions
Comparisons of different types of prunes were only
performed intraindividually, i.e. in individual workers
using various types of prunes.
Primary design differences between the prunes had
been established. No differences were observed in the
total strain caused by the various types of non-power
prunes.
Design suggestions were formulated from the data
obtained on stresses and strains observed during the
field and laboratory studies and from reference to the
literature. The ergonomic improvements (e.g. low
weight, force transmission, handle with rotation,
handling) were implemented in the Fi-Erg.-prune (see
Fig. 4).
Figure 4: Fi-Erg. prune (improved)
Acknowledgement
Main parts of the studies were supported by the
European Community under the Industrial and
Materials Technologies Programme Project
"Eurohandtool" (Contract no BRPR-CT96-0350)
References
[1] Kuorinka, I.; Forcier, L. Work related musculoskeletal
disorders (WMSDs): A Reference Book for Prevention.
Taylor& Francis, 1995
[2] Wakula, J., Beckmann, Th., Hett M., K. Landau:
Ergonomic analysis of grapevine pruning and wine
harvesting to define work and hand tools design
requirements. Occupational Ergonomics 2(3)
(1999/2000), 151-161
[3] Rohmert, W.: Physische Beanspruchung. durch
muskuläre Belastung In: Schmidtke, H. (ed.):
Ergonomie, München: Carl Hanser 1981, 115-131