Introduction to Sports Biomechanics: Analysing Human Movement ...

calculating the internal forces in the human musculoskeletal system. It should, however,
be noted that the EMG cannot necessarily reveal what a muscle is doing, particularly in
fast multi-segment movements that predominate in sport.
Recording the myoelectric (EMG) signal
A full understanding of electromyography and its importance in sports biomechanics
requires knowledge from the sciences of anatomy and neuromuscular physiology as well
as consideration of many aspects of signal processing and recording, including instrumentation.
A schematic representation of the generation of the (unseen) physiological
EMG signal and its modification to the observed EMG is shown in Figure 6.18 – the
EMG varies over time, as can be seen in this figure. So, we have another ‘time series’,
which could be seen as another ‘movement’ pattern that could be useful to qualitative as
well as quantitative movement analysts. Because of the complexity of the signal, it has
rarely, if ever, been used in this way. Indeed, even quantitative biomechanical analysts
often use it as no more than an ‘on–off’ indication of whether a muscle is contracting.
As with kinematic data (Chapter 4), the EMG also consists of a range of frequencies,
which can be represented by the EMG frequency spectrum (see below).
The physiological EMG signal is not the one recorded, as its characteristics are
modified by the tissues through which it passes in reaching the electrodes used to detect
it. These tissues act as a low-pass filter (Appendix 4.1, Chapter 4), rejecting some of the
high-frequency components of the signal. The electrode-to-electrolyte interface acts
as a high-pass filter, discarding some of the lower frequencies in the signal. The twoelectrode,
or bipolar, configuration, which is normal for sports electromyography,
changes the two phases of the depolarisation–repolarisation wave to a three-phase one
and removes some low-frequency and some high-frequency signals – it acts as a ‘band
pass’ filter. Modern high-quality EMG amplifiers should not unduly affect the
frequency spectrum of the signal but the recording device may also act as a band pass
filter.
Many factors influence the recorded EMG signal. The intrinsic factors, over which
the electromyographer has little control, can be classified as in Box 6.4.
BOX 6.4 INTRINSIC FACTORS THAT INFLUENCE THE EMG
THE ANATOMY OF HUMAN MOVEMENT
Physiological factors, which include the firing rates of the motor units; the type of fibre;
the conduction velocity of the muscle fibres; and the characteristics of the volume of muscle
from which the electrodes detect a signal (the detection volume) – such as its shape and
electrical properties.
Anatomical factors, which include the muscle fibre diameters and the positions of the fibres
of a motor unit relative to the electrodes – the separation of individual MUAPs becomes
increasingly difficult as the distance to the electrode increases.
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