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Effect of Physical Load on Aerobic Exercise Performance during Heat Stress


Medicine & Science in Sports & Exercise: December 2017 - Volume 49 - Issue 12 - p 2570–2577
doi: 10.1249/MSS.0000000000001392
Applied Sciences

Purpose This study aimed to investigate the effect of increasing external loads on 5-km treadmill time trial (TT) performance in 20°C and 40°C environmental conditions and to construct an ecologically relevant performance prediction decision aid.

Methods Twenty-six male and four female volunteers (age, 23.5 ± 6.9 yr; weight, 76.0 ± 8.9 kg; height, 1.75 ± 0.07 m; V˙O2peak, 50.7 ± 4.5 mL·kg−1·min−1) participated in a counterbalanced, mixed-model design, with each subject assigned to a load group (20%, 30%, or 50% body mass (BM); n = 10 per group). Volunteers performed three, self-paced 5-km familiarization TT (treadmill) without external load. Each volunteer then performed a 5-km TT in each environment with loads of either 20% (n = 10), 30% (n = 10), or 50% (n = 10) of BM.

Results 1) Loads of (20%, 30%, and 50% of BM) impaired 5-km TT performance compared with that when unloaded (P < 0.05); 2) the time penalties of the 20% and 30% load were <50% load (P < 0.05); 3) in all trials, the addition of heat exposure reduced 5-km TT performance beyond the penalty of load itself (P < 0.05); and 4) the combination of heat and 50% load resulted in a substantial penalty such that continuous work was not sustainable for all of the volunteers.

Conclusions Relative to prediction models using fixed or constant workload exercise trials, an ecologically valid decision aid was developed from self-paced data, in which pace (km·h−1) can be predicted for individual levels of heat, load, or heat + load in combination.

Heavy external loads and heat exposure can negatively affect exercise performance. Independent effects of heat and load have been studied previously; however, the combined effects remain unknown.

1US Army Research Institute of Environmental Medicine, Natick, MA; and 2Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA

Address for correspondence: Robert W. Kenefick, Ph.D., Thermal and Mountain Medicine Division, US Army Research Institute of Environmental Medicine, Kansas Street, Natick, MA 01760; E-mail:

Submitted for publication April 2017.

Accepted for publication July 2017.

© 2017 American College of Sports Medicine