Effects of fatigue duration and muscle type on voluntary and evoked contractile properties.

The effects of fatigue duration and muscle type on voluntary and evoked contractile properties were investigated with an isometric, intermittent, submaximal fatigue protocol. Four groups performed contractions of the plantar flexors and quadriceps at various intensities to produce long (LDF; 19 min 30 s)- and short-duration fatigue (SDF; 4 min 17 s). The LDF group had a significantly greater decrease in muscle activation than did the SDF group (12 vs. 5.8%) during recovery, although there was no difference in the impairment of maximum voluntary contraction force beyond 30 s of recovery. The significant decrease in the compound muscle action potential of the LDF group (M-wave amplitude; 14.7%) contrasted with the M-wave potentiation of the SDF group (15.7%), suggesting changes in membrane excitation may affect LDF. The quadriceps group performing contractions at 50% MVC experienced a smaller decrease in agonist electromyograph activity than did other groups, indicating both muscle and fatigue duration specificity. Impairments in excitation-contraction coupling were indicated by changes in quadriceps peak twitch and time to peak twitch while decreases in PF M-wave amplitudes suggested a disruption of membrane potentials. Results suggest that fatigue mechanisms may be duration (activation, half relaxation time) or muscle specific (electromyograph, twitch torque) or a combination of both (M wave, time to peak twitch torque).