Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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After the electrodes have been attached to the skin, the electrodes and cables usually
need to be fixed to prevent cable-movement artefacts and pulling of the cables;
SENIAM recommends the use of rubber (elastic) bands or double-sided tape or
rings for this purpose. They also recommend a clinical test for each individual
muscle, performed from the starting posture, to ensure satisfactory signals; a few
examples of these tests are shown in Box 6.5.
After use, the electrodes should be sterilised or disposed of.
EMG DATA PROCESSING
Much kinesiological, physiological and neurophysiological electromyography simply
uses and analyses the raw EMG. The amplitude of this signal, and all other EMG data,
should always be related back to the signal generated at the electrodes, not given
after amplification. Further EMG signal processing is often performed in sports biomechanics
in an attempt to make comparisons between studies. It can also assist in
correlating the EMG signal with mechanical actions of the muscles or other biological
signals.
Although EMG signal processing can provide additional information to that contained
in the raw signal, care is needed for several reasons. First, to distinguish between
artifacts and signal, it is essential that good recordings free from artifacts are obtained.
Secondly, the repeatability of the results needs consideration; non-exact repetition of
EMGs is likely even from a stereotyped activity such as treadmill running. The siting of
electrodes, skin preparation and other factors can all affect the results. Even the activity
or inactivity of one motor unit near the pick-up site can noticeably change the signal.
Thirdly, such experimental factors make it difficult to compare EMG results with those
of other studies. However, normalisation has been developed to facilitate such comparisons.
This involves the expression of the amplitude of the EMG signal as a ratio to
the amplitude of a contraction deemed to be maximal, usually a maximum voluntary
contraction (MVC), from the same site. No consensus at present exists as to how to
elicit an MVC and it is not always an appropriate maximum (see Burden, 2007, for
elaboration of this point; Further Reading, page 280).
Temporal processing and amplitude estimation (time domain analysis)
Temporal processing relates to the amplitude of the signal content or the ‘amount of
activity’. It is often referred to as amplitude estimation. Such quantification is usually
preceded by full-wave rectification as the raw EMG signal (Figure 6.23(a)) would have
a mean value of about zero, because of its positive and negative deviations. Full-wave
rectification (Figure 6.23(b)) simply involves making negative values positive; the
rectified signal is expressed as the modulus of the raw EMG. Various terms have been,