The propagation of motor unit action potential and the location of neuromuscular junction investigated by surface electrode arrays.

A myoelectric signal was recorded from the biceps brachii muscle using surface electrode arrays which consisted of 15 pieces of stainless steel wire placed parallel to each other with 5 mm interelectrode spacing. The electrode assembly was positioned parallel to the underlying muscle fibers and amplifiers were connected bipolarly to 11 pairs of adjacent electrodes. In the chart of recorded signals a propagation of the action potential was observed, originating from a neuromuscular junction and traveling both in the proximal and distal directions. In one recording under an isotonic and isometric contraction condition, two neuromuscular junctions were found to be active. One was located approximately at the center of the muscle and the other was 15 mm distal from it. It was found that the amplitude of the myoelectric potential originating from the distal neuromuscular junction increased gradually during a 1 min heavy muscular loading (50% maximum voluntary contraction).