Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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Average rectified EMG
The average rectified EMG is the average value of the full-wave rectified EMG, and
is easily computed from a digital signal by adding the individual EMG values for
each sample and dividing by the sample time. This is recommended by SENIAM for
amplitude estimation of the EMG in non-dynamic contractions. The average rectified
EMG is closely related to the integrated EMG (see below). As for other amplitude
estimators of the EMG, the average rectified EMG is affected by the number of active
motor units; the firing rates of motor units; the amount of signal cancellation by
superposition; and the waveform of the MUAP. The last of these depends upon
electrode position, muscle fibre conduction velocity, the geometry of the detecting
electrode surfaces and the detection volume.
Root mean square EMG
The RMS EMG is the square root of the average power (voltage squared) of the signal
in a given time. It is easily computed from a digital signal by adding the squares of the
individual EMG values for each sample, then taking the square root of the sum before
dividing by the sample time. It is considered to provide a measure of the number of
recruited motor units during voluntary contractions where there is little correlation
among motor units. This is also recommended by SENIAM for amplitude estimation
of the EMG in non-dynamic contractions.
Integrated EMG
This is simply the area under the rectified EMG signal. It is not recommended at all by
SENIAM and is mentioned here only because of its wide use – and abuse – in earlier
research.
Smoothed, rectified EMG
The standard way of obtained the smooth, rectified EMG is by the use of a linear
envelope detector (hence another name for this estimator, the ‘linear envelope’), comprising
a full-wave rectifier plus a low-pass filter. This is a simple method of quantifying
signal intensity and gives an output (Figure 6.23(c)) that, it is claimed, follows the trend
of the muscle tension curve with no high-frequency components. It is the only intensity
detector recommended by SENIAM for dynamic contractions – which predominate in
sport – because of the statistical properties of such contractions.
The main issue with this estimator is the choice of the epoch duration, which, for
this estimator, is related to the filter cut-off frequency (see also Chapter 4). A low cut-off
frequency gives a reliable estimate of the signal intensity for ‘stationary’ activation of the
muscle – this is a statistical term meaning, roughly, that the statistical properties of the
activation signal do not change with time, not that the activation is constant. However,
the amplitude estimates with a low cut-off frequency are inaccurate if the activation