FORCE OUTPUT IN MUSCLE FIBERS DURING REPETITIVE STIMULATION IN HUMANS

Abstract

The evoked force was observed during repetitive electrical stimulation for 3 min on m. vastus medialis. The stimulus frequency was 0.2 Hz, 10 Hz and 20 Hz. The time to peak of twitch was 90.8 ms at 0.2 Hz stimulation. The changes in the evoked force did not represent a constant or a monotonic pattern but were complex at 10 Hz and 20Hz stimulations. At 10 Hz the evoked force represented an initial transient increment (steep peak), then an abrupt decrement followed by a gradual increase (gentle peak) and then a gradual decrease. At 20 Hz the steep phase did not appeared. The magnitude of potentiation was not necessarily large at 20 Hz. These results suggest that a constant discharge rate of motor units cannot maintain constant force development, and “rate coding” is considered to be necessary for keeping a constant force.