Aerobic influence on neuromuscular function and tolerance during passive hyperthermia

To determine the role of aerobic fitness on central neuromuscular activation and maximal voluntary contractile force during hyperthermia.

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 degrees 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; VO2max = 71.2 +/- 5.9 mL x kg(-1) x min(-1), body fat = 5.6 +/- 1.9%), 11 of the 13 moderately fit (MF; 57.2 +/- 4.2 mL x kg(-1) x min(-1), 11 +/- 3.4%), and 4 of the 11 lower-fit (LF; 49.6 +/- 1.1 mL x kg(-1) x min(-1), 19.4 +/- 2.6%) individuals tolerated heating to 39.0 degrees 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.

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 degrees 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.

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.