The Influence of Hyperthermia, Thermal Sensation, and Heat Acclimation on Muscular Strength

Abstract

Neuromuscular strength is defined as maximum or explosive strength and is considered important for sporting performance, health and occupational settings. Hot environmental conditions can lead to the development of hyperthermia, which places the human body under greater thermoregulatory and physiological strain. Hyperthermia is known to impair central nervous input (neural drive) to the muscle, impairing maximum force capacity (maximum voluntary torque (MVT); however, it is unclear how hyperthermia may influence explosive force production (rate of torque development (RTD)). Furthermore, MVT has typically been used to measure neuromuscular function when assessing strength in the heat, but RTD may be a more functionally relevant measure when time to develop force is limited, e.g., during rapid forceful movements. This thesis examined the effects of progressive hyperthermia on maximal and explosive strength; specifically, MVT and RTD, and their neural and contractile determinants. Hyperthermia was found to reduce neural drive to the muscle, causing a decrease in MVT and voluntary RTD, during the later contraction phase. Hyperthermia reduced neural drive during rapid voluntary contractions; however, torque did not decline. This was due to a warmer muscle causing adjustments in muscle contractile function, which produced greater involuntary torques and faster contraction and relaxation times. The adjustments in contractile function compensated for reduced neural drive. This thesis also assessed the effect of manipulating local thermal sensation of the head and neck regions, independent of core body temperature. Cooling the head and neck during hyperthermia did not mitigate reductions in neural drive. Heating the same regions whilst normothermic did not cause a decline in neural drive, providing further evidence for the causal link between elevated core body temperature and a downregulation in neural input to the muscle. Finally, the influence of isothermal heat acclimation was assessed on MVT and RTD. These results provided evidence that repeated heat exposure may cause adjustments to the relaxation rate of the muscle, with potentially beneficial functional implications for explosive strength in the heat. This work provides an analysis of the effects of progressive whole-body hyperthermia, the association with changes in local thermal sensation, and the effect of heat acclimation on maximal and explosive strength, as well as their neural and contractile mechanisms.

Date of Award	10 May 2022
Original language	English
Awarding Institution	University of Roehampton
Supervisor	Ceri Diss (Director of Studies) & Neale Tillin (Co-Supervisor)