M-wave normalization of EMG signal to investigate heat stress and fatigue

Abstract

Objectives: We examined the extent to which peripheral changes affect EMG signal adjustments during repeated sprinting in temperate and hot conditions. Design: Randomised, crossover study. Methods: Ten males performed 10 × 6-s ‘all-out’ cycling sprints (recovery = 30 s) in either a temperate (24 °C/30%rH) or a hot (35 °C/40%rH) environment with concomitant surface EMG recordings of the vastus lateralis (VL) and rectus femoris (RF). In addition, peak-to-peak M-wave amplitudes were obtained for each muscle after each sprint (i.e., 15 s into recovery). Results: For both the VL and RF muscles RMS decreased across sprint repetitions (P < 0.01), while significantly lower values for the VL (P = 0.012), but not the RF (P = 0.096), occurred in hot vs. temperate conditions. M-wave-normalised RMS for VL muscle decreased across sprint repetitions (P = 0.030), with no condition or interaction effects (both P > 0.621). M-wave-normalised RMS for the RF muscle was lower in the heat (P < 0.034), with no significant sprint or interaction effects (both P > 0.240). Conclusions: Controlling for changes in maximal M-wave amplitude of the quadriceps muscles after each bout of a repeated cycling exercise in hot and temperate conditions allows researchers to account for fatigue- and/or heat-induced neural and peripheral adjustments.