The electrophysiologic and mechanical responses of a synergistic muscle group composed of different muscle fiber types were studied with respect to excitation frequency and muscle fatigue in five men. The force and evoked action potentials obtained from surface and intramuscular fine wire electrodes were recorded continuously during stimulated contractions of the gastrocnemius and soleus at 20, 50, and 80 Hz. The stimulus voltage was adjusted so that the force generated by high-frequency tetani (50 and 80 Hz) could initially match the force of maximal voluntary contractions (MVCs). The surface and intramuscular EMG signals were digitized at a sampling rate of 10 kHz with 16-bit fast A/D converters and stored on a floppy disk. The digitized data were then processed for every 5 s to calculate evoked potential amplitude and conduction time using an HP 9836 computer. Results indicated that after 30 s of high-frequency stimulation, significantly less force was generated than after a similar period of MVC. During this period of high-frequency force fatigue, considerably greater force was developed at 20-Hz stimulation. The excessive force loss during high-frequency tetanic contractions was accompanied by a marked reduction in the evoked potential amplitude and conduction time (prolongation of the M wave). The recording of intramuscular evoked potentials showed the gastrocnemius muscle to have greater reductions in these parameters. Our results support the hypothesis that force fatigue during high-frequency stimulation results from failure of electrical propagation due to reduced muscle membrane excitability. The observed muscle-fiber-dependent electrophysiologic responses may suggest that the metabolic profile of muscle fibers plays an important role in regulating the muscle membrane excitability during high-frequency stimulation.