KINETICS: FORCES AND MOMENTS OF FORCE - Biomechanics and Motor Control of Human Movement - page 121

Similarly, in the vertical direction,

N

m i a yi + g F y = (5.8)

i = 1

where: a yi = acceleration of i th-segment COM in the y direction

g = acceleration due to gravity

With a person standing perfectly still on the platform, F y will be nothing
more than the sum of all segment masses times g , which is the body weight.
The interpretation of the ground reaction forces as far as what individual
segments are doing is virtually impossible. In the algebraic summation of
mass-acceleration products, there can be many cancellations. For example,
F Y could remain constant as one arm is accelerating upward, while the other
has an equal and opposite downward acceleration. Thus, to interpret the com-
plete meaning of any reaction force, we would be forced to determine the
accelerations of the COMs of all segments and carry out the summations seen
in Equations (5.7) and (5.8).

5.2.3 Special Pressure-Measuring Sensory Systems
The COP measured by the force plate is a weighted average of the distributed
COPs under the two feet if both are in contact with the ground or under the
one foot that is in contact. However, the COP signal tells us nothing about
the pressure at any of the contact points under the foot. For example, during
midstance in walking or running, there are two main pressure areas: the
ball of the foot and the heel, but the force plate records the COP as being
under the arch of the foot, where there may in fact be negligible pressure.
To get some insight into the distributed pressures around all contact points,
a number of special pressure measurement systems have been developed. A
typical pressure measurement system was introduced a number of years ago
by Tekscan and is schematically shown in Figure 5.12. It is a flexible sheet
of tactile force sensors that is cut in the shape of a shoe insole and that can
be trimmed down to any shoe size. Each sensel shown in the exploded view
is about 0.2 in. × 0.2 in. (5 mm × 5 mm), giving 25 sensels/sq. in. Two thin
flexible polyester sheets have electrically conductive electrodes deposited in
rows and columns. A thin semiconductive coating is applied between the
conductive rows and columns, and its electrical resistance changes with the
pressure applied. The matrix of sensels results in a matrix of voltage changes
that are computer displayed as different colors for different pressure levels.
Colors range from blue through green, yellow, orange, and red (1 - 125 psi).
Thus, the high- and low-pressure areas under the foot are visually evident as
the pressure moves from the heel to the toes over the stance period (Hsiao
et al., 2002). Such devices have been valuable in identifying high-pressure