Self-Regulated Force and Neuromuscular Responses During Fatiguing Isometric Leg Extensions Anchored to a Rating of Perceived Exertion

The purpose of the study was to examine the fatigue-related patterns of responses for electromyography (EMG), mechanomyography (MMG), and force during a sustained isometric muscle action anchored to RPE=5. Ten men (22.9 +/- 2.0 year) performed maximal voluntary isometric contractions (MVIC) prior to and following an isometric leg extension muscle action, which was sustained for a maximal time-limit of 5 min or until it could not be maintained at RPE=5 (actual time-limit). EMG amplitude (AMP), EMG mean power-frequency (MPF), MMG AMP, MMG MPF, and force values were determined every 5% of the actual time-limit. Regression analyses were used to examine the neuromuscular parameters and force responses, and a t test was used to examine MVIC. The pretest MVIC (62.4 +/- 14.3 kg) was significantly (p < 0.001; d=1.07) greater than posttest (47.9 +/- 12.8 kg). The percent decline in force during the sustained isometric muscle action was 47.5 +/- 19.6%, and there was a significant, negative force versus time relationship (p < 0.001; R=-0.980). There was a significant, negative EMG AMP versus time relationship (p < 0.001; R=-0.789), but no significant (p > 0.05) relationships for EMG MPF, MMG AMP, or MMG MPF versus time. The findings indicated that it was necessary to reduce force and EMG AMP to maintain RPE=5. We hypothesize that the maintenance of RPE=5 was initially accomplished by an anticipatory feedforward mechanism and then continuous integrations of afferent feedback, which resulted in reductions of EMG AMP and force, due to reductions in neural drive, to attenuate the impact of metabolic byproducts.