CHANGES IN TENSION OF SLOW MOTOR UNITS IN RAT MEDIAL GASTROCNEMIUS DURING CONSTANT-RATE STIMULATION AT DIFFERENT FREQUENCIES

Changes in tension of slow motor units in the medial gastrocnemius mus cle of the rat were measured and compared in three fatigue tests, each of 4 min duration. In one of these tests motor unit tetani were evoke d by repeated trains of stimuli of the standard frequency, 40 Hz, whil e in other tests frequencies of 20 and 60 Hz were used. Tetanic tensio n initially decreased in the majority of units during these three test s, then potentiated and, in the last part of the fatigue test, the ten sion was normally stable. Initial changes in tension were greatest in the fatigue tests with the lowest stimulation frequency (20 Hz) and sm allest or even absent during tests with the stimulation frequency of 6 0 Hz. Moreover, initial changes in tension occurred for the longest pe riod of time when the lowest frequency of stimulation (20 Hz) was used . Some of the motor units showed a slow decrease in tension in the las t part of tests, especially when frequencies of 60 and 40 Hz were used . These results show that the tension of repeated unfused tetani of a slow motor unit is not constant, even during constant frequency stimul ation, and changes in tension are greater in less fused tetani.