KINETICS: FORCES AND MOMENTS OF FORCE - Biomechanics and Motor Control of Human Movement - page 109

Figure 5.2 Relationship between anatomical and link-segment models. Joints are
replaced by hinge (pin) joints and segments are replaced by masses and moments
of inertia located at each segment's center of mass.

segments and each segment's moment of inertia I 1 , I 2 , and I 3 about each
COM.

5.0.2 Forces Acting on the Link-Segment Model

1. Gravitational Forces . The forces of gravity act downward through the
COMs of each segment and are equal to the magnitude of the mass
times acceleration due to gravity (normally 9 . 8m / s 2 ).
2. Ground Reaction or External Forces . Any external forces must be mea-
sured by a force transducer. Such forces are distributed over an area of
the body (such as the ground reaction forces under the area of the foot).
In order to represent such forces as vectors, they must be considered
to act at a point that is usually called the center of pressure (COP). A
suitably constructed force plate, for example, yields signals from which
the COP can be calculated.
3. Muscle and Ligament Forces . The net effect of muscle activity at a joint
can be calculated in terms of net muscle moments. If a cocontraction
is taking place at a given joint, the analysis yields only the net effect
of both agonist and antagonistic muscles. Also, any friction effects at
the joints or within the muscle cannot be separated from this net value.
Increased friction merely reduces the effective “muscle” moment; the
muscle contractile elements themselves are actually creating moments
higher than that analyzed at the tendon. However, the error at low and
moderate speeds of movement is usually only a few percent. At the
extreme range of movement of any joint, passive structures such as
ligaments come into play to contain the range. The moments generated
by these tissues will add to or subtract from those generated by the