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.