Surface Electromyography During Sustained Isometric Contractions

摘要

Ten male volunteers were asked to maintain isometric contractions involving elbow flexor muscles as long as possible at levels equal to 30%, 50%, and 70% of their macimum voluntary strength and concurrently to report when they experienced five successive levels of pain resulting from the contraction. Surface electromyographic recordings were made on the biceps muscle as well as three peripheral muscles. The results indicated that the macimum duration of the contraction could be reliably predicted from the reports of mild and moderate pain intensities. The EMG suggested a similarity in amplitude increase between increased force and increased duration. A frequency analysis of the EMG demonstrated that during a sustained contraction amplitude increase resulted from an increase in the activity within a narrow low frequency band. The frequency range of maximum EMG amplitude with fatigue onset was related to the level of contraction. The amplitude change during a contraction was proposed to result from loss of motor units with a high frequency rate of firing and a concomitant recruitment of units with a lower rate. The process was suggested to result in the typical synchronization of the EMG with accompanying onset of tremors associated with fatigue. (Author)