ANTHROPOMETRY - Biomechanics and Motor Control of Human Movement - page 97

Figure 4.7 Balance board technique. ( a ) In vivo determination of mass of the location
of the anatomical center of mass of the body. ( b ) Mass of a distal segment. See text
for details.

4.2.3 Moment of Inertia of a Distal Segment
The equation for the moment of inertia, described in Section 4.1.5, can be
used to calculate I at a given joint center of rotation. I is the constant of
proportionality that relates the joint moment to the segment's angular accel-
eration, assuming that the proximal segment is fixed. A method called the
quick release experiment can be used to calculate I directly and requires the
arrangement pictured in Figure 4.8. We know that I = M /α , so if we can
measure the moment M that causes an angular acceleration α , we can calcu-
late I directly. A horizontal force F pulls on a convenient rope or cable at a
distance y 1 from the joint center and is restrained by an equal and opposite
force acting on a release mechanism. An accelerometer is attached to the leg
at a distance y 2 from the joint center. The tangential acceleration a is related
to the angular acceleration of the leg α by a = y 2 α .
With the forces in balance as shown, the leg is held in a neutral position and
no acceleration occurs. If the release mechanism is actuated, the restraining
force suddenly drops to zero and the net moment acting on the leg is Fy 1 ,
which causes an instantaneous acceleration α . F and a can be recorded on a
dual-beam storage oscilloscope; most pen recorders have too low a frequency
response to capture the acceleration impulse. The moment of inertia can now
be calculated,

M
α =

Fy 1 y 2
a

I = (4.15)