The adductor pollicis of intact man was electrically stimulated via its motor nerve, with series of two supramaximal pulses delivered at various intervals. Muscle surface action potential, myogram and corresponding first derivative (dPt/dt) were recorded under isometric conditions. When two stimuli are delivered at intervals shorter than 150 ms a more-than-linear summation, or potentiation, is observed. This potentiation of force production by a second stimulus involves both increased duration and acceleration of contraction. The potentiation is larger in muscles with small twitch-to-tetanus ratio (Pt/Po), and is found to decrease or increase when Pt/Po is respectively augmented (for example in positive staircase and in contractile disuse) or reduced (for example in negative staircase and in dynamic training). No significant modification of muscle surface action potential is observed when these contractile changes are present. Combined prolongation and intensification of intracellular processes, associated with muscle excitation-contraction coupling in a twitch, could account for the potentiation observed in nonlinear summation of contractions.