Composite and inter-individual force and neuromuscular responses to a handgrip hold to failure anchored to a moderate perceptual intensity

BACKGROUND: The application of the rating of perceived exertion (RPE)-clamp model has allowed for the examination of unique physio-psychological fatigue mechanisms in humans. This study examined the time course of composite and inter-individual responses in force, electromyographic (EMG) and mechanomyographic (MMG) amplitude (AMP) and mean power frequency (MPF), and neuromuscular efficiency (NME) during a sustained, isometric handgrip hold to failure (HTF) anchored to RPE of 5 (RPE=5) on 0-10 omnibus-resistance RPE scale in females. METHODS: Twelve females performed a handgrip HTF at RPE=5 while force (N), EMG and MMG AMP and MPF, and NME (normalized force/normalized EMG AMP) were recorded on the brachioradialis of the dominant arm. Separate, polynomial regression analyses were used to determine the individual and composite relationships for the normalized force, EMG and MMG AMP and MPF, and NME versus normalized time during the HTF at RPE=5. RESULTS: The time to task failure for the handgrip HTF at RPE=5 was 585.3±445.6 sec. There were linear and quadratic decreases in force and neuromuscular parameters (P≤0.05) for the composite responses. However, there was inter-individual variability in the patterns of these responses. CONCLUSIONS: To maintain a moderate perceptual intensity over time, it was necessary to voluntarily decrease force, muscle excitation, motor unit recruitment and firing rates, but there were also likely fatigue-induced, intramuscular, metabolic perturbations. However, the variability in force and neuromuscular responses suggested that these responses should be examined on a subject-by-subject basis to provide better insight into fatigue mechanisms across different conditions of RPE-clamp exercise.