Tests have shown that when frequencies higher than 20 Hz are used, muscle tension falls off after about 60 seconds. The muscle's neurophysiological requirements cannot be met, and the effect of training leaves rather a lot to be desired. The conclusion is that when 20 Hz is used, muscle tension is maintained over time, which allows for sustained efficient training. Ref:Moritani et al.Exp Neurol 88:471-483,1985
Moritani et al.Exp Neurol 88:471-483,1985