A, trains of EMG potentials evoked by 13 pulses at 40 Hz at the start of the fatigue test (0 s) and after 60 s and 120 s of stimulation. B, overlays of the first and last potentials of each train at 0 s, 60 s and 120 s. The arrow indicates the time of stimulation. C, corresponding 40 Hz forces. D, reduction in force during fatigue measured every 5 s to show that gradual changes occurred. E, initial, post-tetanic, and post-fatigue EMG potentials and twitch force of the same unit.