Contractile property of muscle fibers during repetitive electrical stimulation in humans

The evoked force was observed during repetitive electrical stimulation for 3min on m. vastus medialis. The stimulus frequency was 5, 6.7 and 10Hz. The changes in the evoked force did not represent a constant or a monotonic pattern but were complex; that is, there was an initial transient increment (initial peak), then an abrupt decrement followed by a gradual increase (second peak) and then a gradual decrease. Although the evoked force increased with the stimulus intensity, the change pattern did not change according to the stimulus frequency. These results suggest that a constant discharge rate of motor units cannot maintain constant force development, and "rate coding" and recruitment/decruiitment of motor units is considered to be necessary for keeping a constant force.