Recovery of conduction velocity of muscle fiber action potential after strenuous isometric contraction.

The change of conduction velocity of muscle fiber action potentials of individual motor units after loading of 50% maximal voluntary contraction was examined in biceps brachii muscles of twenty healthy male subjects. A surface electrode array, consisting of fifteen electrodes spaced at 5 mm intervals, was used to detect EMG signals during minimal isometric contraction which is adjusted so as to generate action potentials in a single motor unit. Conduction velocity was measured from the recorded signals of 14 channels. Mean conduction velocity of all subjects was 3.33 +/- 0.28 m/s (n = 20). A significant decrease of the conduction velocity was observed after contraction maintaining 50% of maximal voluntary contraction torque as long as possible. From the results of conduction velocity decrease during 6 min minimal contraction following the contraction at 50% of maximal strength, subjects were divided into two groups, having endurance times exceeding 50 s and less than 50 s, respectively. The former showed significant recovery during minimal contraction after loading, while the latter showed no apparent recovery. It was demonstrated that the conduction velocity of muscle action potentials obtained with minimal contraction decreased after strenuous contraction and the extent of its recovery was related to the endurance time of contraction which the subjects could maintain 50% of maximal strength.