Transcript Anthropometry

Anthropometry
(Chapter 3 – Body Segment Parameters)
Wednesday March 29th
Dr. Moran
Lecture Outline
• Review Midterm Exam
• Upcoming Weeks
• Anthropometry Notes
What is Anthropometry?
• Studies the physical
measurements of the human body
• Used to study differences
between groups
•Race
•Age
•Sex
•Body Type
• Professional Fields: ergonomics,
automotive, etc.
• Mostly care about the inertial
properties of the body and its
segments
Drillis & Contini 1966
Body Segment Parameters
• Length
• Mass
• Location of segemental center of gravity
(also known as center of mass COM)
• Segmental Mass Moment of Inertia
• Before a kinetic analysis can occur these
properties must either be measured OR
estimated
Some Important Assumptions
1.) Segments behave as RIGID bodies
» Not true as we know that segments are composed of
bones & soft tissues. All of which bend, stretch, etc.
2.) Some segments over-simplifed
» Ex: Foot – represented as one segment by many
researchers
Assumptions (con’t)
3.) Segmental mass distribution similar
among a population
 Allows researches to ESTIMATE an individual’s segment
parameters from the AVERAGE group values
 Important researcher chooses a good match
 For instance, if you wanted to estimate the segment
parameters for a pediatric subject, then you would want to
be sure that the average segment parameters come for a
similar group
Body Segment Parameters
1.) Length
2.) Mass
3.) Volume
4.) Center of Mass
4.) Center of Rotation
5.) Moment of Inertia
Question: How can we determine these BSP values for a participant in our study?
DIRECT MEASURE
Segment properties are determined directly
from the participant. Only possible with
a cadaver specimen because each segment
would need to be disconnected and
analyzed.
INDIRECT MEASURE
Estimation of parameters is necessary for
living participants. There are numerous
techniques to estimate these values
W.T. Dempster: Space Requirements of the Seated Operator
US Air Force (1955)
Outlined procedures for DIRECTLY measuring parameters from
cadavers (8) AND included tables for proportionally determining
parameters from cadaver values.
Coefficient Method
(Table of Proportions)
BSP Determination Methods
CADAVER STUDIES
MATHEMATICAL
MODELING
SCANNING/IMAGING
TECHNIQUES
A laser-aligned method for anthropometry of hands
(Highton et al., 2003)
KINEMATIC
BSP Parameters
• Segment Length: most basic body dimension
• Can be measured from joint to joint
• Dempster et al. (1955, 1959): summarized
estimates of segment lengths and joint center
locations relative to anatomical landmarks
» This allows one to ESTIMATE the location of a joint
by palpating and measuring the easily identifiable
bony landmarks
» For instance, the hip joint center can be
approximated from the location of the greater
trochanter
Whole Body Density
• Human body comprised of many types of
tissues of different densities
• Ex: cortical bone (specific gravity > 1.8)
muscle tissue (just over 1.0)
fat (< 1.0)
• Average density is a function of body type:
somatotype
Average Density (con’t)
• Drillis & Contini (1966)
• Pondural Index
• c = h/w1/3
» w = body weight (lbs)
» h = height (inches)
• D = 0.69 + 0.0297c kg/l
• Ex: Find the whole-body density of Dr. Moran (5’ 10”; 150lbs).
In general
(1) the density of distal segments is > than proximal segments
(2) Individual segments ↑ as the whole body density ↑
Segment Mass
•
Individual Segment Mass is proportional to Whole Body Mass
•
The total mass of the segment is:
M = ∑ mi
where mi is the mass of the ith segment
mi = diVi
» Ex: A tape measure is used to take thigh circumferences every 1 cm. For one measurement the
circumference is 23.9 cm. Assuming a circular cross-section, what is the mass of that segment if
the average density is 1.059 kg/l.
circumference = 2 π r
0.239 = 2 π r
r = 0.0381
Volume of Slice = (π r2) (thickness)
V = (π * 0.03812)(.01) = 0.0000456 m3 = 0.0456 l
Mass of Slice = (1.059)(0.0456) = 0.048 kg
•
Simply weigh subject and then multiply by the proportion that each segment
contributes to the total.
» Handout (Table 3.1 from supplemental text)
» Ex: What is the mass of the left leg of a person that weighs 167 Kg?
m = (0.0465) * (167)
m = 7.7655 Kg
Segmental Center of Mass
• How to determine the center of mass?
• Cadaver Studies: find the center of balance point
» Dempster (1955) calculated the COM as the distance
from the endpoints of the segment
» xcg = xproximal + Rproximal (xdistal – xproximal)
» ycg = yproximal + Rproximal (ydistal – yproximal)
• In Vivo Studies: the cross-sectional area and length of
segment are necessary to approximate the segmental COM
x = (1/M) ∑ mi xi
 Ex: From the cross-sectional slice of the thigh compute its contribution to
the center of mass of the thigh if the circumference was taken 12 cm from
the hip joint.
mi xi = (0.048kg * 0.12m)
Segmental Center of Mass (con’t)
 From Table 3.1 calculate the coordinates of the center of
mass of the foot given the following coordinates: lateral
malleolus (84.9, 11.0), head of the 2nd metatarsal (101.1,
1.3).
xcg = 84.9 + 0.5 (101.1 – 84.9) = 93
ycg = 11.0 + 0.5 (1.3 – 11.0) = 6.15
Limb and Total Body COM
• How can you compute the COM of a limb
or combination of segments?
• First compute the COM of each individual segment
• Use the mass proportional value for that segment
• Use these formulas:
xlimb =
ylimb =
∑ Ps xcg
∑ Ps
∑ Ps ycg
∑ Ps
Thus, the heavier a
segment the more it
affects the total COM
Reuleaux’s Method
Used for Center of Rotation Calculation
• Reuleaux’s Method (1876)
•
•
Determines the center of rotation for ONE segment
Can be a 2D, graphical technique
A1
Center of Rotation
B1
Mass Moment of Inertia
• Rotational Inertia: the resistance of a body to change in its
rotational motion. The angular or rotational equivalent of mass.
• Classically defined as the “second moment of mass”: it is the
summed distance of mass particles from an axis
• Any time a movement involves accelerations we
need to know the inertial resistance to these
movements. (F = ma; M = Iα)
• Consider the moment of inertial about the COM,
Io
Io = m(ρo2)
where ρ = the radius of gyration
Parallel-Axis Theorem
• Most segments do not rotate about their
COM, but about their joint on either end
• Relationship between moment of inertia
about the COM and moment of inertia
about the joint is given by:
• I = Io + mx2
Moment of Inertia (con’t)
• Ex: A prosthetic leg has a mass of 3 kg and a COM of 20 cm from
the knee joint. The radius of gyration is 14.1 cm. Calculate I about
the knee joint.
• Io = m(ρo2)
• Io = 3(0.141)2 = 0.06 kg ∙ m2
• I = Io + mx2
• I = 0.06 + 3(0.2)2 = 0.18 kg ∙ m2
• Ex: Calculate the moment of inertia of the leg about its distal end
(ankle joint) for an 80 kg man with a leg length of 0.435m.
• Mass of Leg = 0.0465 x 80 = 3.72 kg
• Io = m(ρo2)
• Io = 3.72(0.435 x 0.643)2 = 0.291 kg ∙ m2
A laser-aligned method for
anthropometry of hands
(Highton et al., 2003)
Recent Application
Maternal Anthropometry as Predictors of Low Birth Weight
• Journal of Tropical Pediatrics 52(1)24-29
Objective: The usefulness of maternal anthropometric parameters i.e. maternal weight (MWt),
maternal height (MHt), maternal mid-arm circumference (MMAC) and maternal body mass
index (MBMI) as predictors of low birth weight (LBW) was studied in 395 singleton
pregnancies. The maternal anthropometric parameters were measured in the first trimester of
pregnancy and were plotted against the birth weight of the newborns.
Results: Significant positive correlations were observed among MWt and birth weight
(r=0.38), MHt and birth weight (r=0.25), MMAC and birth weight (r=0.30) and MBMI and birth
weight (r=0.30). The most sensitive being MWt (t=7.796), followed by MMAC (t=5.759), MHt
(t=4.706) and MBMI (t=5.89). For prediction of LBW, the critical limits of MWt, MHt, MMAC
and MBMI were 45 kg, 152 cm, 22.5 cm, 20 kg/m2 respectively.
Web Data (Pediatric Anthropometry)
Muscle Anthropometry
• Prior to calculating muscular
forces we need to know some
muscle measurements
• Physiologic Cross-Sectional Area
(PCSA)
• Fiber Length
• Mass
• Pennation Angle
• Important values necessary for
computational modeling
http://www.duke.edu/~bsm/ovlpmris.GIF
Muscle Cross-Sectional Area
• A measure of the number of sarcomeres in parallel
with the angle of pull of the muscles.
• Def: Sarcomere: basic functional unit of a myofibril, contains a
specialized arrangement of actin and myosin filaments
necessary to produce muscle contraction
• With pennate muscles, the fibers act at an angle
from the long axis of the fiber
• Because of the off-angle, these muscles do not move their
tendons as far as parallel muscles do.
• Contain more muscle fibers--produce more tension than
parallel muscles of the same size.
PCA (con’t)
• Non-pennate muscles
• PCA = m/(dl) cm
» m = mass of muscle fibers, grams
» d = density of muscle = 1.056 g/cm3
» l = length of muscle fibers, cm
• Pennate Muscles
• PCA = (m cos Θ )/(dl) cm
»
»
»
»
m = mass of muscle fibers, grams
d = density of muscle = 1.056 g/cm3
l = length of muscle fibers, cm
Θ = pennation angle (increases as muscle
shortens)
Θ
Θ