Healthy males (n = 14) performed three bouts of 32 unilateral, maximal voluntary concentric (CON) or eccentric (ECC) quadriceps muscle actions on separate days. Surface electromyography (EMG) of the m. vastus lateralis (VL) and m. rectus femoris (RF) and torque were measured. Integrated EMG (IEMG), mean (MPF) and median power frequencies and torque were averaged for seven separate blocks of four consecutive muscle actions. Torque was greater (P less than 0.05) for ECC than for CON muscle actions at the start of exercise. It did not decline throughout ECC exercise, but decreased (P less than 0.05) markedly for each bout and over bouts of CON exercise. Thus, torque overall was substantially greater (P less than 0.05) for ECC than for CON exercise. At the start of exercise IEMG of VL or RF was greater (P less than 0.05) for CON than for ECC muscle actions. This was also true for overall IEMG activity during exercise. The IEMG increased (P less than 0.05) modestly for both muscles during each bout of CON or ECC muscle actions, but did not change for the VL over bouts. The IEMG of RF decreased (P less than 0.05) modestly over CON but not ECC exercise bouts. At the beginning of the first bout of exercise the IEMG/torque ratio was twofold greater (P less than 0.05) for CON than ECC muscle actions. The ratio of IEMG/torque increased (P less than 0.05) markedly during CON but did not change during ECC exercise. Thus, by the end of the third bout there was a fivefold difference (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)