Introduction to Sports Biomechanics: Analysing Human Movement ...

100 Hz in sports activities. The natural frequency must, therefore, be as large as
possible to record the frequencies of interest. The structure must be relatively light but
stiff, to give a high natural frequency; this consideration relates not only to the plate but
also to its mounting. A high natural frequency, of around 1000 Hz, would be desirable
for most applications in sports biomechanics. Among the highest natural frequencies
specified for commercial plates are: for one piezoelectric plate 850 Hz, all three
channels; for two strain gauge plates (a) 1000 Hz for the vertical channel, 550 Hz for
the two horizontal channels; and (b) 1500 Hz (vertical) and 320 Hz (horizontal). However,
the value specified for a particular plate may not always be found in practical
applications.
Experimental procedures
General
A force plate, if correctly installed and mounted and used with appropriate auxiliary
equipment, is generally simple to use. When used with an A–D converter and
computer, the timing of data collection is important. This can be achieved, for example,
by computer control of the collection time, by the use of photoelectric triggers or by
sampling only when the force exceeds a certain threshold. When using a force plate with
video recording, synchronisation of the two will be required. This can be done in several
ways, such as causing the triggering of the force plate data collection to illuminate a
light in the field of view of the cameras (see also Study task 5).
The sensitivity of the overall system will need to be adjusted to prevent saturation
while ensuring the largest possible use of the equipment’s range. This can often be done
by trial-and-error adjustments of amplifier gains; this should obviously be done before
the main data collection. If the manufacturers have recommended warm-up times for
the system amplifiers, then these must be carefully observed.
Care must be taken in the experimental protocol, if the performer moves on to
the plate, to ensure that foot contact occurs with little (preferably no), targeting of
the plate by the performer. This may, for example, require the plate to be concealed
by covering the surface with a material similar to that of the surroundings of the
plate. Obviously the external validity of an investigation will be compromised if
changes in movement patterns occur to ensure foot strike on the plate. Also important
for external – ecological – validity, particularly when recording impacts, is
matching the plate surface to that which normally exists in the sport being studied.
The aluminium surfaces normally used for force plates are unrepresentative of most
sports surfaces.
Calibration
CAUSES OF MOVEMENT – FORCES AND TORQUES
It is essential that the force plate system can be calibrated to minimise systematic errors.
The overall system will require regular calibration checks, even if this is not necessary
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