The maximal force and median frequency (MF) of the electromyogram (EMG) power density spectrum (PDS) have been compared in disused (6 weeks' immobilization) and control (contralateral) human adductor pollicis muscles during fatigue induced by voluntary or electrically-triggered (30 Hz) contractions. The results indicated that after 6 weeks' immobilization, MF was not significantly different in disused and control muscles although the force and integrated EMG were drastically reduced during a maximal voluntary contraction (MVC; by 55% and 45%, respectively, n = 8). During sustained 60 s MVC, the force decreased at the same rate in immobilized and control muscles, but the shift of MF towards lower frequency values was smaller (P less than 0.05) in disused muscle as compared to control by (14% vs 28%, respectively). In electrically-induced fatigue, the force decrease and the MF shift were larger after inactivity (41% and 43% in one subject, and 50% and 54% in the other subject, respectively) as compared to control (29% and 34% in one subject, and 37% and 38% in the other subject, respectively). These results emphasize the caution that should be exercised when EMG signals are quantified by computing the power density spectrum. The different effects of fatigue during voluntary and electrically-imposed contractions in disused and control muscles indicated that immobilization induced changes in the neural command for the contraction which compensated, at least in part, for its decreased contractile efficiency and resistance to fatigue.