Progressive hyperthermia elicits distinct responses in maximum and rapid torque production

To investigate the effect of progressive whole-body hyperthermia on maximal, and rapid voluntary torque production, and their neuromuscular determinants.

Repeated measures, randomised.

Nine participants performed sets of neuromuscular assessments in HOT conditions (∼50°C, ∼35% relative humidity) at rectal temperatures (T_re) of 37, 38.5 and 39.5°C and in CON conditions (∼22°C, ∼35% relative humidity) at a T_re of ∼37°C and pre-determined comparative time-points. Electrically evoked twitch (single impulse) and octet (8 impulses at 300Hz) responses were measured at rest. Maximum voluntary torque (MVT), surface electromyography (EMG) normalised to maximal M-wave, and voluntary activation (VA) were measured during 3-5s isometric maximal voluntary contractions. Rate of torque development (RTD) and normalised EMG were measured during rapid voluntary isometric contractions from rest.

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- (0-50ms) and middle- (50-100ms) phase voluntary RTD were unaffected by increased T_re (p>0.05), despite lower normalised EMG at T_re 39.5°C (p<0.05) in rapid contractions. In contrast, late-phase (100-150ms) 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 T_re (p<0.05).

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.