AUTHOR=Mekjavic Igor B. , Ciuha Ursa , Grönkvist Mikael , Eiken Ola 

TITLE=The Effect of Low Ambient Relative Humidity on Physical Performance and Perceptual Responses during Load Carriage

JOURNAL=Frontiers in Physiology

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2017.00451

DOI=10.3389/fphys.2017.00451

ISSN=1664-042X

ABSTRACT=<p><bold>Introduction:</bold> The study evaluated the effect of low ambient relative humidity on physical performance and perceptual responses during load carriage in a hot environment.</p><p><bold>Methods:</bold> Ten heat-unacclimatized male subjects participated in three 130-min trials, during which they walked on a treadmill, carrying a load of ~35 kg, at a speed of 3.2 km.h<sup>−1</sup>, with an incident wind at the same velocity and ambient temperature at 45°C. Each trial commenced with a 10-min baseline at 20°C and 50% relative humidity (RH), the subjects transferred to a climatic chamber and commenced their simulated hike, comprising two 50-min walks separated by a 20-min rest period. In two, full protective equipment (FP) trials, RH was 10% (partial pressure of water vapor, p<sub><italic>H</italic>2<italic>O</italic></sub> = 7.2 mmHg) in one (FP10), and 20% (p<sub><italic>H</italic>2<italic>O</italic></sub> = 14.4 mmHg; FP20) in the other. In the control trial, subjects were semi-nude (SN) and carried the equipment in their backpacks; RH was 20%. Measurements included oxygen uptake, ventilation, heart rate, rectal and skin temperatures, heat flux, temperature perception, and thermal comfort.</p><p><bold>Results:</bold> In FP20, four subjects terminated the trial prematurely due to signs of heat exhaustion; there were no such signs in FP10 or SN. Upon completion of the trials, pulmonary ventilation, heart rate, and rectal temperature were lower in FP10 (33 ± 5 l/min; 128 ± 21 bpm; 38.2 ± 0.4°C) and SN (34 ± 4 l/min; 113 ± 18 bpm; 38.1 ± 0.4°C than in FP20 (39 ± 8 l/min; 145 ± 12 bpm; 38.6 ± 0.4°C). Evaporation was significantly greater in the SN compared to FP10 and FP20 trials. FP10 was rated thermally more comfortable than FP20.</p><p><bold>Conclusion:</bold> A lower ambient partial pressure of water vapor, reflected in a lower ambient relative humidity, improved cardiorespiratory, thermoregulatory, and perceptual responses during load carriage.</p>