Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
Dynamic response
Forces in sport almost always change rapidly as a function of time. The way in
which the measuring system responds to such rapidly changing forces is crucial to the
accuracy of the measurements and is called the ‘dynamic response’ of the system. The
considerations here relate mostly to the mechanical components of the system. A representation
of simple sinusoidally varying input and output signals of a single frequency
(ω) as a function of time is presented in Figure 5.22.
The ratio of the amplitudes (maximum values) of the output to the input signal is
called the amplitude ratio (A). The time by which the output signal lags the input signal
is called the time lag. This is often expressed as the phase angle or phase lag (�), which is
the time lag multiplied by the signal frequency.
In practice, the force signal will contain a range of frequency components, each of
which could have different phase lags and amplitude ratios. The more different these
are across the range of frequencies present in the signal, the greater will be the distortion
of the output signal. This will reduce the accuracy of the measurement. We therefore
require the following:
All the frequencies present in the force signal should be equally amplified; this
means that there should be a constant value of the amplitude ratio A (system
calibration can allow A to be considered as 1, as in Figure 5.23).
The phase lag (�) should be small.
Figure 5.22 Representation of force input and recorded output signals as a function of time.
206