Introduction to Sports Biomechanics: Analysing Human Movement ...

Hysteresis
Hysteresis exists when the input–output relationship depends on whether the input
force is increasing or decreasing, as in Figure 5.21(b). Hysteresis can be caused, for
example, by the presence of deforming mechanical elements in the force transducers. It
is expressed as the maximum difference between the output voltage for the same force,
increasing and decreasing, divided by the full-scale output voltage. It should be 0.5% of
full-scale deflection or less.
Range
The range of forces that can be measured must be adequate for the application and the
range should be adjustable. If the range is too small for the forces being measured, the
output voltage will saturate (reach a constant value) as shown in Figure 5.21(c). Suitable
maximum ranges for many sports biomechanics applications would be −10 to +10 kN
for the two horizontal axes and −10 to +20 kN for the vertical axis.
Sensitivity
Sensitivity is the change in the recorded signal for a unit change in the force input,
or the slope of the idealised linear voltage–force relationships of Figure 5.21. The
sensitivity decreases with increasing range. Good sensitivity is essential as it is a limiting
factor on the accuracy of the measurement. In most modern force plate systems, an
analog-to-digital (A–D) converter is used and is usually the main limitation on the
overall system resolution. An 8-bit A–D converter divides the input into an output
that can take one of 256 (2 8 ) discrete values. The resolution is then 100/255%,
approximately 0.4%. A 12-bit A–D converter will improve the resolution to about
0.025%. In a force plate system with adjustable range, it is essential to choose the range
that just avoids saturation, so as to achieve optimum sensitivity. For example, consider
that the maximum vertical force to be recorded is 4300 N, an 8-bit A–D converter is
used, and a choice of ranges of 10 000 N, 5000 N and 2500 N is available. The best
available range is 5000 N, and the force would be recorded approximately to the nearest
20 N (5000/255). The percentage error in the maximum force is then only about
0.5% (20 × 100/4300). A range of 10 000 N would double this error to about 1%. A
range of 2500 N would cause saturation, and the maximum force recorded would be
2500 N, an underestimate of over 40% (compare with Figure 5.21(c)).
Crosstalk
CAUSES OF MOVEMENT – FORCES AND TORQUES
Force plates are normally used to measure force components in more than one direction.
The possibility then exists of forces in one component direction affecting
the forces recorded by the transducers used for the other components. The term
crosstalk is used to express this interference between the recording channels for the
various force components. Crosstalk must be small, preferably less than 3% of full-scale
deflection.
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