Progressive hyperthermia elicits distinct responses in maximum and rapid torque production

TY - JOUR

T1 - Progressive hyperthermia elicits distinct responses in maximum and rapid torque production

AU - Gordon, Ralph

AU - Tyler, Christopher J.

AU - Castelli, Federico

AU - Diss, Ceri

AU - Tillin, Neale A.

PY - 2021/3/19

Y1 - 2021/3/19

N2 - ObjectivesTo investigate the effect of progressive whole-body hyperthermia on maximal, and rapid voluntary torque production, and their neuromuscular determinants.DesignRepeated measures, randomised.MethodsNine 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, ~5% 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.ResultsAll 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). ConclusionsHyperthermia 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.© 2021, Sports Medicine Australia. Published by Elsevier Ltd. The attached document (embargoed until 19/03/2023) is an author produced version of a paper published in JOURNAL OF SCIENCE AND MEDICINE IN SPORT uploaded in accordance with the publisher’s self-archiving policy. The final published version (version of record) is available online at the link. Some minor differences between this version and the final published version may remain. We suggest you refer to the final published version should you wish to cite from it.

AB - ObjectivesTo investigate the effect of progressive whole-body hyperthermia on maximal, and rapid voluntary torque production, and their neuromuscular determinants.DesignRepeated measures, randomised.MethodsNine 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, ~5% 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.ResultsAll 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). ConclusionsHyperthermia 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.© 2021, Sports Medicine Australia. Published by Elsevier Ltd. The attached document (embargoed until 19/03/2023) is an author produced version of a paper published in JOURNAL OF SCIENCE AND MEDICINE IN SPORT uploaded in accordance with the publisher’s self-archiving policy. The final published version (version of record) is available online at the link. Some minor differences between this version and the final published version may remain. We suggest you refer to the final published version should you wish to cite from it.

M3 - Article

SN - 1440-2440

JO - Journal of Science and Medicine in Sport

JF - Journal of Science and Medicine in Sport

ER -