The propagation of single motor unit action potentials detected by a surface electrode array

Propagation of single motor unit action potentials (MUAPs) was detected by surface electromyography. Sixteen EMG signals were simultaneously recorded by a linear bipolar electrode array placed along the longitudinal axis of the biceps brachii. EMGs were obtained from 8 healthy adults at 10-40% of the maximum contraction. Single MUAPs were extracted by superimposing and averaging the EMG signals at the timing of potential peaks. Most MUAPs were triphasic and propagated symmetrically in opposite directions from the middle length of the muscle to the tendons. Some MUAPs showed more than 5 phases and asymmetrical wave forms on the proximal and distal recording sites. The asymmetrical wave forms were assumed to be caused by the scatter of myoneural junctions and by the time delay of the excitation at the junctions. The position of the myoneural junctions on muscle fibers was estimated from the source of propagation. The innervation zone of individual motor unit was found to spread up to 14 mm in the muscle fiber directions.