Progressive hyperthermia elicits distinct responses in maximum and rapid torque production

Abstract

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 (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.

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- (0-50ms) 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).

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.

Keywords: Contractile properties; Excitation contraction coupling; Heat; Muscle contraction; Muscle strength; Rate of torque development.