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.
1Human Performance Laboratory, Faculty of Kinesiology, University of New Brunswick, Fredericton, New Brunswick, CANADA; 2PacificSport Canadian Sport Centre Victoria, Victoria, British Columbia, CANADA; and 3Environmental Ergonomics Laboratory, School of Health and Human Performance, Dalhousie University, Halifax, Nova Scotia, CANADA
Address for correspondence: Stephen Cheung, Ph.D., Associate Professor, School of Health and Human Performance, Dalhousie University, 6230 South Street, Halifax, Nova Scotia, Canada, B3H-3J5; E-mail: firstname.lastname@example.org.
Submitted for publication November 2005.
Accepted for publication May 2006.