Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
Figure 6.21 EMG signals without mains hum; V a and V b are biphasic motor unit action potentials recorded from two
monopolar electrodes, V a–V b is the triphasic differential signal from using the two electrodes in a bipolar configuration.
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EMG and muscle tension
Obtaining a predictive relationship between muscle tension and the EMG could solve a
major problem for quantitative sports biomechanists. This problem arises because the
equations of motion at a joint cannot be solved because the number of unknown
muscle forces exceeds the number of equations available. The problem is often known
as muscle redundancy or muscle indeterminacy. If a solution could be found to this
problem, it would allow the calculation of forces in soft tissue structures and between
bones. It is not surprising, therefore, that the relationship between the EMG signal and
the tension developed by a muscle has attracted the attention of many researchers.
The EMG provides a measure of the excitation of a muscle. Therefore, if the force in
the muscle depends directly upon its excitation, a relationship should be expected
between this muscle tension and suitably quantified EMG. A muscle’s tension is
regulated by varying the number and the firing rate of the active fibres; the amplitude of
the EMG signal depends on the same two factors. It is, therefore, natural to speculate
that a relationship does exist between EMG and muscle tension. We might further
expect that this relationship would only apply to the active state of the contractile
component, and that the contributions to muscle tension made by the series and
parallel elastic elements would not be contained in the EMG (for a diagrammatical
model of these elements of skeletal muscle, see Figure 6.10).
However, discrepancies exist between research studies even for isometric contractions,
which should not lead us to expect a simple relationship between EMG and
muscle tension for the fast, voluntary contractions that are characteristic of sports
movements. For such movements, the relationship between EMG and muscle tension
still remains elusive although the search for it continues to be worthwhile.
Additionally, in complex multi-segmental movements, such as those we observe in
sport, muscles influence other joints as well as the ones they cross. This means that, in
sports movements, the EMG tells us when a muscle is active but not, generally, what
that muscle is doing. This is a very important limitation to the use of EMG.