Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
ELECTROMYOGRAPHY – WHAT MUSCLES DO
This and the next two sections are intended to provide you with an appreciation of
the applications of electromyography to the study of muscle activity in sports movements.
This includes the equipment and methods used and the processing of electromyographic
data. We will also touch on the important relationship between muscle
tension and the recorded signal, known as the electromyogram (abbreviation EMG,
which is also, somewhat loosely, used as an abbreviation for electromyography).
Electromyography is the technique for recording changes in the electrical potential of a
muscle when it is caused to contract by a motor nerve impulse. The neural stimulation
of the muscle fibre at the motor end-plate results in a reduction of the electrical
potential of the cell and a spread of the action potential through the muscle fibre. The
motor action potential (MAP), or muscle fibre action potential, is the name given to the
waveform resulting from this depolarisation wave. This propagates in both directions
along each muscle fibre from the motor end-plate before being followed by a repolarisation
wave. The summation in space and time of motor action potentials from the fibres
of a given motor unit is termed a motor unit action potential (MUAP, Figure 6.18).
A sequence of MUAPs, resulting from repeated neural stimulation, is referred to as a
motor unit action potential train (MUAPT). The physiological EMG signal is the sum,
over space and time, of the MUAPT from the various motor units (Figure 6.18).
Figure 6.18 Schematic representation of the generation of the EMG signal.
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Electromyography is the only method of objectively assessing when a muscle is
active. It has been used to establish the roles that muscles fulfil both individually and in
group actions. The EMG provides information on the timing, or sequencing, of the
activity of various muscles in sports movements. By studying the sequencing of muscle
activation, the sports biomechanist can focus on several factors that relate to skill, such
as any overlap of agonist and antagonist activity and the onset of antagonist activity at
the end of a movement. It also allows the sports biomechanist to study changes in
muscular activity during skill acquisition and as a result of training. Electromyography
can also be used to validate assumptions about muscle activity that are made when