• localized muscular fatigue;
  • mean power frequency;
  • motor unit recruitment;
  • power density spectrum.

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 < 0.05) for ECC than for CON muscle actions at the start of exercise. It did not decline throughout ECC exercise, but decreased (P < 0.05) markedly for each bout and over bouts of CON exercise. Thus, torque overall was substantially greater (P < 0.05) for ECC than for CON exercise. At the start of exercise IEMG of VL or RF was greater (P < 0.05) for CON than for ECC muscle actions. This was also true for overall IEMG activity during exercise. The IEMG increased (P < 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 < 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 < 0.05) for CON than ECC muscle actions. The ratio of IEMG/torque increased (P < 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 < 0.05). The MPF decreased (P < 0.05) substantially in each CON bout with only a partial recovery between bouts. In contrast, MPF remained the same within each bout and over bouts of ECC exercise. These results indicate that the ability to maintain force during repeated bouts of maximal voluntary muscle actions at a relatively high angular velocity is remarkably greater for eccentric than for concentric exercise. It is suggested that the factors responsible for fatigue and for changes in the EMG signal pattern during concentric exercise do not materialize during, or are different for, eccentric exercise as performed here.