Progressive hyperthermia elicits distinct responses in maximum and rapid torque production

Objectives: to investigate the effect of progressive whole-body hyperthermia on maximal, and rapid voluntary torque production, and their neuromuscular determinants.Design: repeated measures, randomised.Methods: nine participants performed sets of neuromuscular assessments in HOT conditions (∼50 °C, ∼35% relative humidity) at rectal temperatures (Tre) of 37, 38.5 and 39.5 °C and in CON conditions (∼22 °C, ∼35% relative humidity) at a Tre of ∼37 °C and pre-determined comparative time-points. Electrically evoked twitch (single impulse) and octet (8 impulses at 300 Hz) responses were measured at rest. Maximum voluntary torque (MVT), surface electromyography (EMG) normalised to maximal M-wave, and voluntary activation (VA) were measured during 3−5 s isometric maximal voluntary contractions. Rate of torque development (RTD) and normalised EMG were measured during rapid voluntary isometric contractions from rest.Results: all neuromuscular variables were unaffected by time in CON. In HOT, MVT, normalised EMG at MVT and VA were lower at 39.5 °C compared to 37 °C (p < 0.05). Early- (0−50 ms) and middle- (50−100 ms) phase voluntary RTD were unaffected by increased Tre (p > 0.05), despite lower normalised EMG at Tre 39.5 °C (p < 0.05) in rapid contractions. In contrast, late-phase (100−150 ms) voluntary RTD was lower at 38.5 °C and 39.5 °C compared to 37 °C (p < 0.05) in HOT. Evoked twitch and octet RTD increased with increased Tre (p < 0.05).Conclusions: hyperthermia reduced late-phase voluntary RTD, likely due to reduced neural drive and the reduction in MVT. In contrast, early- and middle-phase voluntary RTD were unaffected by hyperthermia, likely due to the conflicting effects of reduced neural drive but faster intrinsic contractile properties.