Biomedical Engineering Reference
In-Depth Information
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4.4 Moment of Inertia of the Human Body
The distribution of bones, fat, and muscles in the human body varies from subject to
subject, depending also on age. All of this, allied to the fact that parts of the body or
the whole body does not have regular geometric shapes, raises difficulties in
determining the moment of inertia by mathematical calculation. For each segment
and for the whole body of a standard man, the moment of inertia about a given
rotation axis was obtained using cadavers, mathematical model simulations, and via
photographic methods. This approach is very much exploited by sportsmen in
activities like running, changing the angle of the leg in relation to the thigh or in
ornamental jumping, by doubling up and clasping the knees to modify the moment
of inertia in order to obtain the desired performance.
To define the moment of inertia, it is necessary to specify first the axis of
rotation. The human body rotates, when it is free of support, about three axes,
called the principal axes. These are mutually perpendicular lines that pass through
the center of gravity corresponding to the posture of the body. These axes are called
transverse, anteroposterior, and longitudinal. Figure
4.9
shows these axes. The
moment of inertia related to each of these axes is called the principal moment of
inertia. Depending on the shape of the body, the moment of inertia acquires a
different value. The estimated values of the moment of inertia for a man of 70 kg
mass and 1.70 m height are as follows:
1.0-1.2 kg m
2
.
I
longitudinal with horizontally outstretched arms
¼
I
longitudinal with arms along the body
¼
2.0-2.5 kg m
2
.
