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 initial participants, 11 of the 13 highly fit (HF; V˙O2max = 71.2 ± 5.9 mL·kg−1·min−1, body fat = 5.6 ± 1.9%), 11 of the 13 moderately fit (MF; 57.2 ± 4.2 mL·kg−1·min−1, 11 ± 3.4%), and 4 of the 11 lower-fit (LF; 49.6 ± 1.1 mL·kg−1·min−1, 19.4 ± 2.6%) individuals tolerated heating to 39.0°C, with the remainder terminating the experimental protocol early. Maximal force output and voluntary activation were examined during a 10-s maximal isometric knee extension.
Results: Passive heating 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). Cardiovascular strain moderately increased to 60 ± 14% (P < 0.001), whereas HF and MF had significantly higher MAP than LF at the end of heating (98 ± 15, 99 ± 7, and 79 ± 5 mm Hg for HF, MF, and LF, respectively; P < 0.05). However, the ability to tolerate passive heating to 39.0°C (and above) differed between the HF and MF compared with LF, despite no difference in their psychophysical rankings of thermal sensations and/or (dis)comfort.
Conclusion: 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 regardless of training status.