Anthropometrics ppt

Anthropometrics and Body
Segment Parameter
Major segment parameters
BSP data sources
Estimation methods
Weaknesses and limitations
Anthropometry
Anthropometry
A branch of anthropology that deals with
human body measurement and dimensions;
used classify and compare populations.
Also used in industrial design, clothing,
ergonomics and criminology.
Biomechanical focus is on the parameters
of the major segments of the body:
– head, neck, torso, pelvis
– arm, forearm, hand
– thigh, shank, foot
Body Segments
The body is often
considered to consist
of multiple segments
or “links”, connected
by hinge or ball-andsocket
joints
We need to quantify
the parameters that
describe these links
Body Segment Parameters
The body segment parameters that we are
most interested in are:
– length
– density
– mass
– center of mass location
– mass moment of inertia
– radius of gyration
Segment Dimensions
Drillis & Contini (1966)
Segment length is
perhaps the most
important dimension
Segment girth and
breadth may also
provide useful
information at times
Segment Dimensions
The biomechanically
relevant length may differ
from anatomical length
(e.g., the biomechanically
relevant length of the
shank is usually longer
than the anatomical shank
length)
In practice, segment
lengths are often based
on easily identifiable
landmarks (e.g., lateral
femoral condyle to lateral
malleolus)
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Segment Lengths, How
The Relationship Between Limb-length
Inequality and Able-bodied Gait Asymmetry
1.0
Knee Angle
Asymmetry Coefficient
0.5
A: r = -0.29, p = 0.155
1.0
1.0 2.0
Knee Power
Asymmetry Coefficient
0.5
0.0
B: r = -0.51, p = 0.008
0.0 1.0 2.0
Limb Length Inequality (cm)
Segment Density
• 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 1,150 – 1,350 kg/m 3
muscle 1,040 – 1,060 kg/m 3
adipose (fat) 950 – 990 kg/m 3
limb segments 1050 − 1200 kg/m 3
• A quantitative indicator of a
segment’s inertia
• Mass can be determined via
a typical scale
• Tables also exist that allow one to predict
segment masses, usually as a percentage
of whole body mass
whole body 1000 − 1050 kg/m 3 Segment Mass
Segment Volume
• Volume of 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
One way to
determine volume
Segment Center of Mass
• Body segments are made up of an immense
number of distributed mass particles
• Mechanical analysis is facilitated by
identifying the center of mass: 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)
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Segment Center of Mass,
How
• Typically specified as a distance (% of
segment length) relative to some unique
point (e.g. proximal joint center)
• Can be better understood d by balancing the
segment on a knife edge
• Can also be quantified using a couple of
different mathematical methods…
Segment Center of Mass
The CM of an object can be determined using a
“reaction board” or “moment table”
CM is found by knowing that all moments about
the knife edge must sum to zero
should be at
knife edge
Segment Center of Mass
The location of the center of mass of a segment
can be defined as follows:
x 1
m 1 m 2 m 3
x 2
Body Center of Mass
If you know the locations of
the CMs for individual body
segments, the whole body
CM can be calculated for any
m 1 (x 1 ,y 1 )
posture (see “Calculating
M (x 0 ,y 0 )
Whole-body Center of Mass” in
Part II on class website) m 2 (x 2 ,y 2 )
x 3
X
M
x 0 = m 1 x 1 + m 2 x 2 + m 3 x 3
M
m 3 (x 3 ,y 3 )
M X = Σ m i x i so X = Σ m i x i
M
y 0 = m 1 y 1 + m 2 y 2 + m 3 y 3
M with M = m 1 + m 2 + m 3
Center of Mass,
Why Care
Center of Mass/Gravity
What is the difference CM and center of
gravity are not the same, but for human
body segments the difference is negligable
Stability…other
th
performancerelated
factors are
related to the
location of the CM
Nigg & Herzog (1999)
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Mass Moment of Inertia
Mass Moment of Inertia
• Where mass is a quantitative indicator of a
segment’s linear inertia, moment of inertia
indicates a segment’s rotational inertia
• Depends on mass and mass distribution
m 3
r 3
I AXIS = Σm i r i
2
(units: kg⋅m 2 )
axis
r 1
m 1 m 2
r 2
I AXIS = m 1 r 12 + m 2 r 22 + m 3 r 32 + .... + m n r n
2
For a three dimensional
object there will be 3
moments of inertia,
typically expressed
about the 3 principle
i 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
Determining Moment of Inertia
Moment of inertia is the sum of the products of all
the mass elements of an object and the square of
the distances of the mass elements from the axis
of rotation:
I AXIS = Σm i r i
2
axis
r 1
m 1 m 2
r 2
I AXIS = m 1 r 12 + m 2 r 22 + m 3 r 32 + .... + m n r n
2
Radius of Gyration
A more practical approach:
I AXIS = (m SEG )(k 2 ), so k = √ I AXIS / m SEG
where “k” is an experimentally determined
d
length known as the Radius of Gyration, that
applies to the whole segment at once
–k is not the same as the distance to segmental CM
–magnitude of k is different for different axes of
rotation
Radius of Gyration
Interpretation
• 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 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
BSP Studies/Data
Moment of inertia of cadaver
segments can be determined
from the period of oscillation
A “torsional pendulum”
is used to determine
moment of inertia about
the segment long axis
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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 h 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 = 0.1796 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
2
T
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 T
d S
m F
m T
m S
BSP Assumptions
During the period of data collection:
• The segments are perfectly rigid
• The segments are connect by frictionless
hinge or ball-and-socket and 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, determining segment mass
and 3D CM location with scales and a balance
plate; he later studied 7 cadavers
Braun & Fischer (1889)
– Determined mass, volume, and CM in 3 cadavers
using scales and thin rods
– Also estimated BSPs, based on height and mass
– Divided body into 14 segments
– Used data from individual segments to determine
CM 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
– 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)
BSP Studies/Data
Hatze (1975)
– More advanced model; included 17, irregularly
shaped segments
– Can be used with live subjects
Zatsiorsky & Seluyanov (1983)
– Use a gamma-ray scanner to estimate segment
mass, CM, and moment of inertia
– Used 100 male Caucasian adults
– Developed regression equation to predict BSPs
– 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%, or sometimes greater)
• 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)
I wish to acknowledge these individuals for
major contributions to the preceding slides
• Brian Umberger, PhD
• Iain Hunter, PhD
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