MUSCLE FIBRE, MOTOR UNIT, MUSCLE

A single muscle fibre (cell) does not contract on its own, but rather in concert with other muscle fibres that are part of the same motor unit (i.e. innervated by the same motor neuron). Its axon reaches the muscle via a motor nerve. Within the muscle the motor axon tapers and then branches to innervate muscle fibres, which are scattered throughout the muscle. Fibres that are part of the same motor unit are not adjacent to one another. Bioelectrical activity generated by the concomitant activation of muscle fibres from one motor unit is ‘summated’ by the recording electrode as a ‘motor unit action potential’ (MUP). As many motor units are active within a contracting muscle and the recording surface of the EMG electrode is adjacent to muscle fibres from several motor units, several MUPs are recorded by the recording electrode. This produces an ‘interference pattern’ of MUPs in a given time interval of recording. If the activation of a normal muscle is strong, most motor units are activated and the interference pattern is ‘full’.