CHANGES IN THE FREQUENCY OF DISCHARGE OF SINGLE MOTOR UNITS DURING THE PROLONGED VOLUNTARY CONTRACTION IN MAN / 体力科学

ABSTRACT

The purpose of the present study is threefold first, to observe simultaneously the changes in both the frequency of motor unit (MU) discharges and the amplitude of surface electromyogram (EMG) during the prolonged and sustained voluntary efforts; and second, to demonstrate the effects of arterial occlusion on the frequency of MU discharges ; lastly, to discuss the mechanism of motor control in the light of the present results and others'. Two male adults participated in the experiment. They were with no nervous disorders, right-handed, aged 24 and 33 years. Each subject was seated with both eyes closed in a chair in an electrically shielded room. He held a weight of 1 kg on his wrist during periods of about 45 minutes. The angle of his elbow joint was maintained at 90 degrees during the course of the experiments. The impulses of MU and the surface EMG were led off from muscles biceps brachii of the left upper arm by using coaxial needle electrodes and surface ones respectively. Discharge intervals of MUs were recorded as ordinate by means of ‘Electro-tachograph’. The action voltages of single MU was monitored for differentiation from that of others with a cathode-ray oscillograph and with a loud speaker. The subject's complaints were noted during the course of the experiments. The following results were obtained.<BR>1. The patterns of changes in the frequency of discharges of MUs were divided into three types. The first pattern showed the steep decrease in the frequency of discharge for short period, and at last the discharge disappeared, the second the gradual prolongation of discharge intervals and the increase in both irregular fluctuation and slow undulation, the third constant frequency of discharge even immediately before the exhaustion of voluntary effort.<BR>2. The undulation and prolongation of discharge intervals appeared during the abolition of blood supply to the arm. It did not recover for about 1 minute even though the arterial occlusion was released.<BR>3. The gradual increase in the amplitude of direct EMG in muscles biceps brachii was observed during holding a weight of constant load.<BR>4. The rotational activity of MU was suggested to exist in the case of prolonged, sustained and voluntary muscular contraction.<BR>5. The gradual increase in prolongation and undulation of MU seems to come from the decrease in number of afferent impulses resulting from fatigue.