KINETICS: FORCES AND MOMENTS OF FORCE - Biomechanics and Motor Control of Human Movement - page 119

Figure 5.10 Central support type force plate, showing the location of the center of
pressure of the foot and the forces and moments involved.

where: M z =

bending moment about axis of rotation of support

y 0 = distance from support axis to force plate surface

Since F x , F y , and M z continuously change with time, x can be calculated
to show how the COP moves across the force plate. Caution is required for
both types of force platforms when F y is very low ( < 2% of body weight).
During this time, a small error in F y represents a large percentage error in x
and z , as calculated by Equations (5.4), (5.5), and (5.6).
Typical force plate data are shown in Figure 5.11 plotted against time
for a subject walking at a normal speed. The vertical reaction force F y is
very characteristic in that it shows a rapid rise at heel contact to a value
in excess of body weight as full weight bearing takes place (i.e., the body
mass is being accelerated upward). Then as the knee flexes during midstance,
the plate is partially unloaded and F y drops below body weight. At pushoff
the plantarflexors are active in causing ankle plantarflexion. This causes a
second peak greater than body weight. Finally, the weight drops to zero as the
opposite limb takes up the body weight. The anterior-posterior reaction force
F x is the horizontal force exerted by the force plate on the foot. Immediately
after heel contact, it is negative, indicating a backward horizontal friction
force between the floor and the shoe. If this force were not present, the foot
would slide forward, as sometimes happens when walking on icy or slippery
surfaces. Near midstance, F x goes positive, indicating that the force plate
reaction is acting forward as the muscle forces (mainly plantarflexor) cause
the foot to push back against the plate.
The COP starts at the heel, assuming that initial contact is made by the
heel, and then progresses forward toward the ball and toe. The position of
the COP relative to the foot cannot be obtained from the force plate data