Introduction to Sports Biomechanics: Analysing Human Movement ...

systems that are responsive to changes in the displacement of a sensor or detecting
element. They incorporate a force transducer, which converts the force into an
electrical signal. The transducers are mounted on the supports of the rigid surface of
the force plate, usually one support at each of the four corners of a rectangle. One
transducer is used, at each support, to measure each of the three force components,
one of which is perpendicular to the plate and two tangential to it; for ground
contact forces, there are usually one vertical and two horizontal force components,
and we will use this example in what follows. The transducers are normally strain
gauges, as in the AMTI plates, or piezoelectric, as in the Kistler plates. The signals
from the transducers are amplified and may undergo other electrical modification.
The amplified and modified signals are converted to digital signals for computer processing.
The signal is then sampled at discrete time intervals, expressed as the sampling
rate or sampling frequency. The Nyquist sampling theorem (see Chapter 4) requires
a sampling frequency at least twice that of the highest signal frequency. It should
be remembered that, although the frequency content of much human movement
is low, many force plate applications involve impacts, which have a higher frequency
content. A sampling frequency as high as 500 Hz or 1 kHz may, therefore, be
appropriate.
Accurate (valid) and reliable force plate measurements depend on adequate system
sensitivity, a low force detection threshold, high linearity, low hysteresis, low crosstalk
and the elimination of cable interference, electrical inductance and temperature
and humidity variations. The plate must be sufficiently large to accommodate the
movement under investigation. Extraneous vibrations must be excluded. Mounting
instructions for force plates are specified by the manufacturers. The plate is normally
sited on the bottom floor of a building in a large concrete block. If mounted outdoors, a
large concrete block sited on pebbles or gravel is usually a suitable base and attention
must be given to problems of drainage. The main measurement characteristics of a force
plate are considered below. In addition, a good temperature range (−20 to 70°C) and a
relatively light weight may be important. Also, any variation in the recorded force with
the position on the plate surface at which it is applied should be less than 2–3% in the
worst case.
Force plate characteristics
Linearity
CAUSES OF MOVEMENT – FORCES AND TORQUES
Linearity is expressed as the maximum deviation from linearity as a percentage of
full-scale deflection. For example, in Figure 5.21(a), linearity would be expressed as
y/Y × 100%. Although good linearity is not essential for accurate measurements, as a
non-linear system can be calibrated, it is useful and does make calibration easier.
A suitable figure for a force plate for use in sports biomechanics would be 0.5% of
full-scale deflection or better.
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