This study was designed to examine separately the changes in the first and second phases of the muscle compound action potential (M-wave) during and after a sustained 3-minutes maximal voluntary contraction (MVC). M-waves were evoked by supramaximal single shocks to the femoral nerve given at 10-seconds intervals throughout a sustained isometric 3-minutes MVC and also during six brief MVCs performed throughout a 30-minutes recovery period. The amplitude, duration, and area of the M-wave first and second phases, together with muscle conduction velocity and force, were measured. During the 3-minutes MVC, the amplitude of the first phase increased progressively for the first minute (33%-43%, P<.01) and remained stable thereafter, whereas the second phase initially increased for 25-35 seconds (30%-50%, P<.01), but subsequently decreased significantly before stabilizing. During the recovery period, the amplitude of the M-wave first phase showed a decreasing trend, returning to pre-fatigue values (P>.01) within 5-10 minutes, while the second phase increased progressively and remained higher than control (7%-20%, P<.01) after the 30-minutes recovery time. Maximal cross-correlations between the time course of the first phase amplitude and those of conduction velocity and force (0.9-0.93) occurred for a lag of 0 seconds, whereas maximal cross-correlations corresponding to the second-phase amplitude (0.6-0.7) occurred for a 50-seconds time lag. The present findings indicate that the potentiation of the first phase results from impaired muscle membrane excitability. The peak-to-peak amplitude and second-phase amplitude are not valid indicators of muscle excitability as they might be critically affected by muscle architectural features.