KINETICS: FORCES AND MOMENTS OF FORCE - Biomechanics and Motor Control of Human Movement - page 127

1. Numerically, the magnitude of the moment is incorrect because the
mass-acceleration products and the moment of inertia - angular accel-
eration products of the stance limb are not accounted for. As was
discussed in Section 5.2.2, the ground reaction vectors represent the
algebraic summation of the mass-acceleration products of all segments.
Using the correct technique, those effects must be subtracted as our
calculation moves from the ground upward. Thus, by the time we reach
the hip joint calculation, we have taken into account all inertial forces in
the thigh, leg, and foot. The FRFV approach does not do this. Numeri-
cally, the error has been shown to be negligible at the ankle, small but
significant at the knee, and large at the hip (Wells, 1981).
2. The second error in the FRFV technique is the polarity that is attributed
to the moment at the joint, and this reflects basic misconceptions about
action and reaction. The FRFV is a reaction to the gravitational (exter-
nal) and muscle (internal) forces acting on the body. As such, the
reaction force does not cause the moment of force; it merely reflects the
cause. An ankle plantarflexor moment, for example, causes a ground
action vector to move forward of the ankle joint, and this is reflected in
the magnitude and the forward location (COP) of the ground reaction
vector. If the ankle muscles were not generating any force, the FRFV
would remain under the ankle joint.
3. A final shortcoming of the FRFV approach is that it is not capable of
calculating moments during non-weight-bearing periods. Such a defi-
ciency is significant in the assessment of the swing phase of walking
and more so during running, and it is even more critical during athletic
jumping events.
5.2.8 Differences between Center of Mass and Center of Pressure
For many students and for many in the applied and clinical areas the terms
center of mass (COM) and center of pressure (COP) are often misinterpreted
or even interchanged. The COM of the body is the net location of the center
of mass in three-dimensional (3D) space and is the weighted average of the
COM of each segment as calculated in Section 4.1.4. The location of the COM
in the vertical direction is sometimes called the center of gravity (COG). The
trajectory of this vertical line from the COM to the ground allows us to
compare the trajectories of the COM and COP. The trajectory of the COP is
totally independent of the COM, and it is the location of the vertical ground
reaction force vector from a single force platform, assuming that all body
contact points are on that platform. The vertical ground reaction force is a
weighted average of the location of all downward (action) forces acting on
the force plate. These forces depend on the foot placement and the motor
control of the ankle muscles. Thus, the COP is the neuromuscular response
to the imbalances of the body's COM. The major misuse of the COP comes