KINETICS: FORCES AND MOMENTS OF FORCE - Biomechanics and Motor Control of Human Movement - page 128

Figure 5.16 A subject swaying back and forth while standing on a force platform.
Five different points in time are described, showing the COM and the COP locations
along with the associated angular accelerations and velocities of the body. See the text
for a detailed description.
from researchers who refer to COP as “sway,” thereby inferring it to be the
kinematic measure COM.
The difference between COM and COP is demonstrated in Figure 5.16.
Here, we see a subject swaying back and forth while standing erect on a
force plate. Each figure shows the changing situation at one of five different
points in time. Time 1 has the body's COM (shown by the vertical body
weight vector, W ) to be ahead of the COP (shown by the vertical ground
reaction vector, R ). This “parallelogram of forces” acts at distances g and
p , respectively, from the ankle joint. The magnitudes of W and R are equal
and constant during quiet standing. Assuming that the body to be pivoting
about the ankles and neglecting the small mass of the feet, a counterclock-
wise moment equal to Rp and a clockwise moment equal to Wg will be
acting. At Time 1, Wg > Rp and the body will experience a clockwise angu-
lar acceleration, α . It will also have a clockwise angular velocity, ω . In order
to correct this forward imbalance, the subject will increase his or her plan-
tarflexor activity, which will increase the COP such that at Time 2 the COP
will be anterior of the COM. Now Rp > Wg . Thus, α will reverse and will
start to decrease ω until, at Time 3, the time integral of α will result in a
reversal of ω . Now both ω and α are counterclockwise, and the body will be
experiencing a backward sway. The subject's response to the backward sway