Introduction to Sports Biomechanics: Analysing Human Movement ...

The load rate (Figure 5.25(h)) can be calculated as the rate of change with time
t (d/dt) of the contact force F (dF/dt). The load rate has often been linked to
injury. Other calculations that can be performed include whole body power (P = F.v)
(Figure 5.25(i)), which does not have x, y and z components, as power is a scalar (see
also Appendix 4.2).
All the above variables can be presented graphically as functions of time, as in
Figure 5.25, providing the qualitative analyst with a rich new set of movement patterns
and the quantitative analyst with further useful data. In addition, the forces acting on
the performer can be represented as instantaneous force vectors, arising from the
instantaneous centres of pressure (a side view is shown in Figure 5.26(a)). Front, top
and three-dimensional views of the force vectors are also possible. The centre of pressure
path can also be shown superimposed on the plate surface (Figure 5.26(b)).
MEASUREMENT OF PRESSURE
CAUSES OF MOVEMENT – FORCES AND TORQUES
As we noted in the previous section, force plates provide the position of the point
of application of the force, also called the centre of pressure, on the plate. This is the
point at which the force can be considered to act, although the pressure is distributed
over the plate and foot. Indeed, there may be no pressure acting at the centre of pressure
when, for example, it is below the arch of the foot or between the feet during double
stance. Information about the distribution of pressure over the contacting surface
would be required, for example, to examine the areas of the foot on which forces are
concentrated during the stance phase in running to improve running shoe design. In
such cases, a pressure platform or pressure pad must be used. These devices consist of a
set of force transducers with a small contact area over which the mean pressure for that
area of contact (pressure = force ÷ area) is calculated. For just a few selected regions of
the contact surface, pressures can be measured using individual sensors; problems with
this approach include choosing the appropriate locations, and movement of the sensors
during the activity being studied.
Various types of pressure plate are commercially available, and have been mostly used
for the measurement of pressure distributions between the foot or shoe and the ground.
They have been used only to a limited extent in sport, partly because their usually small
size causes targeting problems for the performer. Also, the distribution of pressure
is altered if the platform is covered with a sports surface. Pressure pads have been
developed for the measurement of contact pressures between parts of the body and the
surroundings. Specialist applications have included pads for the study of the dorsal
pressures on the foot within a shoe. Plantar pressure insoles are commercially available
(for example, Figure 5.27) and can be used to measure the plantar pressure distribution
between the foot and the shoe. This is generally more important, for the sports performer,
than the pressure between the shoe and the ground measured by pressure plates.
These insoles allow data collection for several foot strikes and do not cause problems of
targeting.
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