Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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Data processing
for the force plate itself. Calibration of the amplifier output as a function of force input
will usually be set by the manufacturers and may require periodic checking. The vertical
channel is easily calibrated under static loading conditions by use of known weights.
If these are applied at different points across the plate surface, the variability of the
recorded force with its point of application can also be checked. The horizontal
channels can also be statically calibrated, although not so easily. One method of doing
this involves attaching a cable to the plate surface, passing the cable over a frictionless
air pulley at the level of the plate surface, and adding weights to the free end of the
cable. Obviously this cannot be done while the plate is installed in the ground flush
with the surrounding surface. There appears to be little guidance provided to users on
the need for, or regularity of, dynamic calibration checks on force plates. The tendency
of piezoelectric transducers to drift may mean that zero corrections are required and
strain gauge plates may need more frequent calibration checks than do piezoelectric
ones.
Crosstalk can be checked by recording the outputs from the two horizontal channels
when only a vertical force, such as a weight, is applied to the plate. A similar procedure
can be used for assessing crosstalk on the vertical channel if horizontal forces can be
applied. Positions of the point of force application can be checked by placing weights
on the plate at various positions and comparing these with centre of pressure positions
calculated from the outputs from the individual vertical force transducers. As errors in
these calculations are problematic when small forces are being recorded, small as well as
large weights should be included in such checks. Finally, the natural frequency can
be checked by lightly striking the plate with a metal object and using an oscilloscope
to show the ringing of the plate at its natural frequency. This should be carried out, of
course, in the location in which the plate is to be used.
Processing of force plate signals is relatively simple and accurate, compared with
most data in sports biomechanics. The example data of Figure 5.25 were obtained from
a standing broad (long) jump. The three mutually perpendicular (orthogonal) components
of the ground contact force (Figures 5.20 and 5.25(a)) are easily obtained by
summing the outputs of individual transducers. As the plate provides whole body
measurements, these forces (F) can be easily converted to the three components
of centre of mass acceleration (a) simply by dividing by the mass of the performer
(F = m a, Figure 5.25(b)), after subtracting the performer’s weight from the vertical
force component. The coordinates of the point of application of the force, the centre
of pressure (Figure 5.25(c)), on the plate working surface can also be calculated.
The accuracy of the centre of pressure calculations in particular depends on careful
calibration of the force plate; this accuracy deteriorates at the beginning and end of any
contact phase, when the calculation of centre of pressure involves the division of small
forces by other small forces.
The moment of the ground contact force about the vertical axis perpendicular to the