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 muscle 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 evoked by repeated trains of stimuli of the standard frequency, 40 Hz, while in other tests frequencies of 20 and 60 Hz were used. Tetanic tension initially decreased in the majority of units during these three tests, then potentiated and, in the last part of the fatigue test, the tension was normally stable. Initial changes in tension were greatest in the fatigue tests with the lowest stimulation frequency (20 Hz) and smallest or even absent during tests with the stimulation frequency of 60 Hz. Moreover, initial changes in tension occurred for the longest period 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 last 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 stimulation, and changes in tension are greater in less fused tetani.