Anthropometrics/Body Segment Parameters

Anthropometrics
Important body segment and whole body
parameters (BSP)
Dimensions (e.g., length, density, and mass)
Segmental center of mass location
Whole-body center of mass location
Moment of inertia (angular inertia)
Radius of gyration
Assumptions
BSP data sources
Anthropometry
Anthropometry
A major branch of anthropology that deals
with human body measurement and
dimensions, primarily for use in
anthropological classification and
comparisons between populations
Focus in biomechanics is on the parameters
of the major segments of the body:
– head, neck, torso, pelvis
– arm, forearm, hand
– thigh, shank, foot
Body Segments
Segment Dimensions
The body is frequently
considered to consist
of multiple segments
or “links”, connected
by hinge or ball-andsocket
joints
We need to know the
values of the
parameters that
describe these links or
segments
Drillis & Contini (1966)
Segment length is
the most important
dimension
Segment girth and
breadth may also
provide useful
information
Segment Dimensions
Segment Density
The biomechanically
relevant length (joint
center to joint center) may
differ from the anatomical
length
In practice, segment
lengths are often based
on easily identifiable
landmarks (e.g., lateral
femoral condyle to lateral
malleolus)
• Body segments are made up of various
tissues (bone, muscle, fat, skin, etc) that
have different densities
• Individual body segments do not all have the
same density: distal segments have a higher
density than proximal segments
Densities: bone 1.15 – 1.35 kg/L
muscle 1.04 – 1.06 kg/L
adipose (fat) 0.95 – 0.99 kg/L
limb segments 1.05 − 1.20 kg/L
whole body 1.00 − 1.05 kg/L
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Segment Mass
• Segment mass is a
quantitative indicator of a
segment’s inertia
• Determine by segment
density and volume
• For many segments
density can be estimated
from literature data, and
volume can be measured
Segment Mass
• Tables also exist that allow one to predict
segment masses, usually as a percentage of
whole body mass
• Finally, the volume represented by different
tissues (fat, muscle, bone) can also be
determined using modern imaging/scanning
techniques (MRI, CT, DEXA)
• These volumes can be combined with more
accurate individual tissue densities to yield
good estimates of segment mass
Center of Mass (COM)
• Body segments are made up of an immense
number of distributed mass particles
• Mechanical analysis is facilitated by
identifying the COM: a single point about
which the mass is equally distributed (i.e.,
the balance point)
• Typically specified as a distance (% of
segment length) relative to some unique
point (e.g. proximal joint center)
Segmental COM
The location of the COM can be defined as
follows:
x 1
m 1 m 2 m 3
x 2
x 3
X
M
M X = Σ m i x i so X = Σ m i x i
M
Segmental COM
The COM of an object can be determined using
a “reaction board”
COM is found by knowing that all moments
about the knife edge must sum to zero
Segmental COM
cog = X
X
cog = Y
Y
prox
prox
+ L%(X
+ L%(Y
dist
dist
- X
- Y
prox
prox
)
)
The x and y coordinates for the
right shoulder of a female tennis
player are . The x and y
coordinates for the elbow are
. What is the location of
the center of gravity of the upper
arm?
should be at
knife edge
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Whole-body COM
Whole-body COM
Σ(w r) = (Σw) r com
Σ(w r) = (ΣW) r com
A bar (length = 2.2 m; mass = 20
kg) is loaded with two 20-kg
plates on its right side (locked in
place, 35 and 40 cm from the
right end) and two 20-kg plates
on its left side (locked in place,
30 and 20 cm from the left end).
Where is the COM of this bar and
the four plates, relative to the left
end of the bar?
chapter05_files/wholebodycom.xls
Moment of Inertia (I)
I is the sum of the products of the (1) mass
element of an object and (2) square of the
distance between the mass element and axis of
rotation
axis
I AXIS = Σm i r i
2
r 1
m 1 m 2
r 2
Radius of Gyration
A more practical approach:
I AXIS = (m BODY )(k 2 )
k indicates radius of gyration,
which is an experimentally
determined length that applies to
the whole object at once
k depends on the
location of the axis
and location of the
mass
I AXIS = m 1 r 12 + m 2 r 22 + m 3 r 32 + .... + m n r n
2
Radius of Gyration
Mass Moment of Inertia
• Radius of gyration is the distance a particle
with the same mass as the segment would
need to be from the axis of rotation to have
the same moment of inertia (angular inertia)
as the segment
• Radius of gyration is really just a convenient
way of “packaging” mass moment of inertia
information
• In anthropometric data sets, k is often
expressed relative to segment length
For a three dimensional
object there will be 3
moments of inertia,
typically expressed
about the 3 principle
axes of the segment
For human limb
segments, I X and I Z will
be similar, I Y will be
much smaller
Z SEG
X SEG
Y SEG
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Parallel Axis Theorem
• Moment of inertia values are commonly
specified relative to an axis through the
segment center of mass
• The moment of inertia about a different axis
(e.g., through the proximal joint) can be
determined using the parallel axis theorem
I A = I CM + md 2
Where I A is moment of inertia about the new axis, I CM is
moment of inertia about an axis through the CM, m is
segment mass, and d is distance between the two axes
Parallel Axis Theorem
A prosthetic lower leg has a mass of 3 kg and
a center of mass 20 cm from the knee joint.
The radius of gyration is 14.1 cm. What is the
moment of inertia about the knee joint (I KNEE )?
I CM = mk 2
I CM = (3 kg)(0.141 m) 2 = 0.06 kg⋅m 2
I KNEE = I CM + md 2
I KNEE = 0.06 kg⋅m 2 + (3 kg)(0.2 m) 2 = 0.18
kg⋅m 2
Parallel Axis Theorem
The combined moment of inertia
of several segments about a
remote axis can be calculated by
using the parallel axis theorem,
and summing across segments
I LEG(HIP) = I T(HIP) + I S(HIP) + I F(HIP)
where:
I T(HIP) = I T(CM) + m T d T
2
I S(HIP) = I S(CM) + m S d S
2
I F(HIP) = I F(CM) + m F d F
2
Hip
d F
d S
d T
m F
m T
m S
Body segment parameter
assumptions
During the period of data collection:
• The segments are perfectly rigid
• The segments are connect by frictionless
hinge or ball-and-socket joints
• The length of each segment remains constant
• The CM location of each segment remains
constant
• The mass moment of inertia of each segment
remains constant
BSP Studies/Data
Harless (1860)
– One of the earliest quantitative study of BSPs
– Dissected 2 cadavers (former prisoners who had
recently been decapitated)
– Determined segment mass & CM locations
Braun & Fischer (1889)
– Determined BSPs in 3 cadaver specimens
– Divided body into 14 segments
– Used data from individual segments to determine
center of gravity of whole body during locomotion
BSP Studies/Data
Dempster (1955)
– Did most detailed and extensive dissection to
date in 8 cadavers specimens
– Determined mass, CM location, & moment of
inertia values for all major body segments
– Expressed data as proportion of total body mass,
and relative to segment lengths
– Results have been used extensively in
biomechanics research
– However, data were from only 8 cadavers, who
were older (52-83 yrs), Caucasian males, most of
whom were reported to be emaciated
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BSP Studies/Data
Hanavan (1964)
– Used simple geometric shapes
to model 15 major segments
– Assumed uniform density within
each segment
– Used anthropometric data from
the subject to personalize BSP
estimates (still up to 10% error)
Hatze (1980)
– More advanced model; included
17, irregularly shaped segments
– Requires 242 anthropometric
measurements from the subject
BSP Studies/Data
Clauser et al. (1969)
– Determined mass and CM location in 13 cadaver
specimens
– Weakness: did not determine moments of inertia
– Segment endpoints adjusted by Hinrichs (1990)
Chandler et al. (1974)
– Determined 3-D moments of inertia in 6 cadaver
specimens
– Did not generate prediction equations
– Equations later developed by Hinrichs (1985)
BSP Studies/Data
Zatsiorsky & Seluyanov (1980-1990)
– Use a gamma-ray scanner to estimate segment
mass, CM, and moment of inertia
– Used 100 male and 15 female young Caucasian
adults
– Developed regression equation to predict BSPs
de Leva (1996)
– Made Zatsiorsky’s data more user-friendly
– Adjusted prediction equations to use more
relevant segment endpoints
– Used as the current standard by many (appropriate?)
BSPs Summary
Issues and Shortcomings:
• Prior to 1970’s fewer than 50 cadavers had
been studied
• Most were adult, Caucasian males
• Data on other populations (women, children,
different ethnic groups) are scant
• Can cadaver data safely be applied to living
humans?
• Techniques on living subjects often suffer
from difficulty in validating results
BSPs Summary
• BSPs arguably represent the greatest source
of error in biomechanical analyses (errors
can easily be ±10%)
• The influence of these errors depends
greatly on the nature of the movement being
studied (magnitude of the accelerations)
• Several investigators have used techniques
like CT, MRI, DEXA to get more accurate,
personalized BSP estimates
– This is still far from being routine (cost & time)
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