BACKGROUND: The rating of perceived exertion (RPE)-Clamp Model has been used to examine the interaction between fatigue-induced changes in force and neuromuscular responses when exercise intensity is anchored to a fixed RPE. Neuromuscular responses are commonly described by changes in the amplitude of electromyographic signal (EMG AMP) and neuromuscular efficiency (NME; normalized force divided by normalized EMG AMP)], which provide information about muscle excitation and the level of muscle excitation required to produce a given amount of force, respectively. This study investigated the time course of changes in responses of force, EMG AMP, and NME during a sustained, isometric, handgrip hold to failure (HTF) using the RPE-Clamp Model. METHODS: Twelve men (Mean±SD: 28.2±3.8 yr) performed the handgrip HTF anchored to an RPE of 5 on the 10-point omnibus resistance scale. EMG signals were recorded from the brachioradialis throughout the HTF. Force, EMG AMP, and NME were calculated at standardized segments of 5% of time to task failure (Tlim) and normalized to the respective values from a pre-HTF maximum voluntary isometric contraction (MVIC). Analyses included 1(RPE=5) x 21 (time:0-100% Tlim) repeated measures ANOVAs and post-hoc t-tests with a Bonferroni corrected alpha level (p<0.0025). RESULTS: The Tlim was 512.4±245.9s and the initial normalized force was 25.9±14.3% MVIC. There were significant differences across time for force (F=24.989, p<0.001, η2=0.694), EMG AMP (F=8.416, p<0.001, η2=0.433), NME (F=22.368, p<0.001, η2=0.670). Relative to the initial time point, force decreased from 40% to 100% Tlim, EMG AMP decreased at 30%, 60%, and 100% Tlim, and NME decreased from 50% to 65%, and 80% to 100% Tlim. CONCLUSIONS: The subjects’ initial force selection may be explained by a feedforward mechanism, while a combination of corollary discharges and afferent feedback (i.e., sensory tolerance limit; STL) may explain the continuous decreases in force. Throughout the HTF, muscle excitation (EMG AMP) tracked the force decreases. However, the magnitude of force loss exceeded the magnitude of EMG AMP decreases (↓ NME) indicating greater levels of muscle excitation were required to compensate for the reductions in force generating capacity of fatigued muscle fibers. Task failure may be explained by the individual STL where central motor drive was continuously reduced to maintain the constant RPE.

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