ANTHROPOMETRY - Biomechanics and Motor Control of Human Movement - page 98

Figure 4.8 Quick-release technique for the determination of the mass moment of
inertia of a distal segment. Force F applied horizontally results, after release of the
segment, in an initial acceleration a . Moment of inertia can then be calculated from
F , y 1 , y 2 .
Figure 4.8 shows the sudden burst of acceleration accompanied by a rapid
decrease in the applied force F . This force drops after the peak of acceleration
and does so because the forward displacement of the limb causes the tension
to drop in the pulling cable. A convenient release mechanism can be achieved
by suddenly cutting the cable or rope that holds back the leg. The sudden
accelerometer burst can also be used to trigger the oscilloscope sweep so that
the rapidly changing force and acceleration can be captured.
More sophisticated experiments have been devised to measure more than
one parameter simultaneously. Such techniques were developed by Hatze
(1975) and are capable of determining the moment of inertia, the location of
the center of mass, and the damping coefficient simultaneously.
4.2.4 Joint Axes of Rotation
Markers attached to the body are usually placed to represent our best estimate
of a joint center. However, because of anatomical constraints, our location
can be somewhat in error. The lateral malleolus, for example, is a common
location for ankle joint markers. However, the articulation of the tibial/talus
surfaces is such that the distal end of the tibia (and the fibula) move in a small
arc over the talus. The true axis of rotation is actually a few centimeters distal
of the lateral malleolus. Even more drastic differences are evident at some
other joints. The hip joint is often identified in the sagittal plane by a marker
on the upper border of the greater trochanter. However, it is quite evident that
the marker is somewhat more lateral than the center of the hip joint such that
internal and external rotations of the thigh relative to the pelvis may cause
considerable errors, as will abduction/adduction at that joint.
Thus, it is important that the true axes of rotation be identified relative
to anatomical markers that we have placed on the skin. Several techniques
have been developed to calculate the instantaneous axis of rotation of any
joint based on the displacement histories of markers on the two adjacent