Effects of endurance training and heat acclimation on psychological strain in exercising men wearing protective clothing

Abstract

Two experiments examined the influences of endurance training and heat acclimation on ratings of perceived exertion (RPE) and thermal discomfort (RTD) during exercise in the heat while wearing two types of clothing. In experiment 1, young men underwent 8 weeks of physical training [60-80% of maximal aerobic power ([Vdot]O2max) for 30-45 min day−1, 3-4 days week−1 at 20-22°C dry bulb (db) temperature] followed by 6 days of heat acclimation [45-55% [Vdot]O2max for 60 min day−1 at 40°C db, 30% relative humidity (rh)] (n = 7) or 2max corresponding periods of control observation followed by heat acclimation (n = 9). In experiment 2, young men were heat-acclimated for 6 or 12 days (n = 8 each). Before and after each treatment, subjects completed bouts of treadmill exercise (1·34 m s−1, 2% grade in experiment 1 and 0% grade in experiment 2) in a climatic chamber (40°C db, 30% rh), wearing in turn normal light clothing (continuous exercise at 37-45% [Vdot]O2max for a tolerated exposure of 116-120 min in experiment 1 and at 31-34% [Vdot]O2max for 146-150 min in experiment 2) or clothing protective against nuclear, biological, and chemical agents (continuous exercise at 42-51% [Vdot]O2max for a tolerated exposure of 47-52 min in experiment 1 and intermittent exercise at 23% [Vdot]O2max for 97 - 120 min in experiment 2). In experiment 1, when wearing normal clothing, endurance training and/or heat acclimation significantly decreased RPE and/or RTD at a fixed power output. There were concomitant reductions in relative work intensity (%[Vdot]O2max) [an unchanged oxygen consumption ([Vdot]O2max) but an increased [Vdot]O2max, or a reduced [Vdot]O2 with no change of [Vdot]O2], rectal temperature (T re), mean skin temperature (T sk), and/or heart rate (HR). When wearing protective clothing, in contrast, there were no significant changes in RPE or RTD. Although training and/or acclimation reduced % [Vdot]O2 or T re, any added sweat that was secreted did not evaporate through the protective clothing, thus increasing discomfort after training or acclimation. Tolerance times were unchanged in either normal or protective clothing. In experiment 2, when wearing normal clothing, heat acclimation significantly decreased RPE and RTD at a fixed power output, with concomitant reductions in T re, T sk, and HR; the response was greater after 12 than after 6 days of acclimation, significantly so for RPE and HR. When wearing protective clothing, the subjects exercised at a lower intensity for a longer duration than in the moderate exercise trial. Given this tactic, either 6 or 12 days of heatacclimation induced significant reductions in RPE and/or RTD, accompanied by reductions in T re, T sk, and/or HR. Tolerance times in protective clothing were re sk also increased by 11 - 15% after acclimation, despite some increase of sweat accumulation in the protective clothing. The results suggest that (1) neither endurance training nor heat acclimation reduce psychological strain when protective clothing is worn during vigorous exercise, because increased sweat accumulation adds to discomfort, and (2) in contrast to the experience during more vigorous exercise, heat acclimation is beneficial to the subject wearing protective clothing if the intensity of effort is kept to a level that allows permeation of sweat through the clothing. This condition is likely to be met in most modern industrial applications.