Influence of Aerobic Fitness on Motor Unit Activation and Isometric Maximum Voluntary Contraction during Hyperthermia

Abstract

High environmental temperatures and internal heat storage have both been demonstrated to accelerate fatigue during prolonged and voluntary submaximal exercise. While the endpoint core temperatures at voluntary exhaustion within a sample group during treadmill exercise in the heat have been consistent, a clear separation has been reported between highly and moderately-fit humans even while normalizing for body composition, supporting a benefit in heat tolerance through aerobic fitness. One potential mechanism for fatigue may be a centrally-mediated impairment in neuromuscular activation. PURPOSE: To determine the role of aerobic fitness on central neuromuscular activation and maximal voluntary contractile force during hyperthermia. METHODS: Thirty-seven healthy males in three distinct groups based on aerobic fitness and training history were passively heated using a liquid conditioning garment in a hot (35°C, 50% RH) environment with the intention of testing neuromuscular function with whole-body hyperthermia. Of these, 11/13 Highly Fit (HF, VO2max= 71.2 ± 5.9 ml.kg-1.min−1, body fat= 5.6 ± 1.9%,), 11/13 Moderately Fit (MF, 57.2 ± 4.2ml.kg−1.min−1, 11 ± 3.4%) and 4/11 Low Fit (LF, 49.6 ± 1.1 ml.kg-1.min−1, 19.4 ± 2.6%) individuals tolerated heating to 39.0°C, and maximal force output and voluntary activation were examined during a 10-s maximal isometric knee extension. RESULTS: Passive heating to a core temperature of 39.0°C attenuated force production (−61.7 ± 69.6 N change from initial values) and decreased voluntary activation (8.6 (12.6)%, 18.1 (12.4)% and 6.1 (3.1)% for HF, MF, and LF training groups, respectively) equally across all fitness groups in those individuals able to reach that level of hyperthermia. Cardiovascular strain moderately increased to 60 ± 14% (p<0.001) heart rate reserve across all subjects reaching 39.0°C, while HF and MF had significantly higher mean arterial pressure than LF at the end of heating (HF = 98 ± 15, MF = 99 ± 7, LF = 79 ± 5 mmHg, p<0.05). However, the ability to tolerate passive heating to 39.0°C (and above) differed greatly between the HF and MF compared with LF, despite no difference in their psycho physical rankings of thermal sensations and/or (dis)comfort. CONCLUSIONS: Low aerobic fitness and activity level are associated with a decreased tolerance to passive hyperthermia. However, at high body temperatures, maximum force production and voluntary activation were impaired to an equal level irregardless of training status.