Exercise Physiology: Energy, Nutrition and Human Performance

The purpose of the present study was to determine the separate and combined effects of aerobic fitness, short-term heat acclimation, and hypohydration on tolerance during light exercise while wearing nuclear, biological, and chemical protective clothing in the heat (40 degrees C, 30% relative humidity). Men who were moderately fit [(MF); 55 ml . kg-1 . min-1 maximal O2 consumption; n = 8] were tested while they were euhydrated or hypohydrated by approximately 2.5% of body mass through exercise and fluid restriction the day preceding the trials. Tests were conducted before and after 2 wk of daily heat acclimation (1-h treadmill exercise at 40 degrees C, 30% relative humidity, while wearing the nuclear, biological, and chemical protective clothing). Heat acclimation increased sweat rate and decreased skin temperature and rectal temperature (Tre) in HF subjects but had no effect on tolerance time (TT). MF subjects increased sweat rate but did not alter heart rate, Tre, or TT. In both MF and HF groups, hypohydration significantly increased Tre and heart rate and decreased the respiratory exchange ratio and the TT regardless of acclimation state. Overall, the rate of rise of skin temperature was less, while DeltaTre, the rate of rise of Tre, and the TT were greater in HF than in MF subjects. It was concluded that exercise-heat tolerance in this uncompensable heat-stress environment is not influenced by short-term heat acclimation but is significantly improved by long-term aerobic fitness.

Heat acclimation, aerobic fitness, and hydration effects on tolerance during uncompensable heat stress

This study examined the impact of heat acclimation on improving exercise performance in cool and hot environments. Twelve trained cyclists performed tests of maximal aerobic power (Vo2max), time-trial performance, and lactate threshold, in both cool [13°C, 30% relative humidity (RH)] and hot (38°C, 30% RH) environments before and after a 10-day heat acclimation (∼50% Vo2max in 40°C) program. The hot and cool condition Vo2max and lactate threshold tests were both preceded by either warm (41°C) water or thermoneutral (34°C) water immersion to induce hyperthermia (0.8–1.0°C) or sustain normothermia, respectively. Eight matched control subjects completed the same exercise tests in the same environments before and after 10 days of identical exercise in a cool (13°C) environment. Heat acclimation increased Vo2max by 5% in cool (66.8 ± 2.1 vs. 70.2 ± 2.3 ml·kg−1·min−1, P = 0.004) and by 8% in hot (55.1 ± 2.5 vs. 59.6 ± 2.0 ml·kg−1·min−1, P = 0.007) conditions. Heat acclimation improved time-trial performance by 6% in cool (879.8 ± 48.5 vs. 934.7 ± 50.9 kJ, P = 0.005) and by 8% in hot (718.7 ± 42.3 vs. 776.2 ± 50.9 kJ, P = 0.014) conditions. Heat acclimation increased power output at lactate threshold by 5% in cool (3.88 ± 0.82 vs. 4.09 ± 0.76 W/kg, P = 0.002) and by 5% in hot (3.45 ± 0.80 vs. 3.60 ± 0.79 W/kg, P < 0.001) conditions. Heat acclimation increased plasma volume (6.5 ± 1.5%) and maximal cardiac output in cool and hot conditions (9.1 ± 3.4% and 4.5 ± 4.6%, respectively). The control group had no changes in Vo2max, time-trial performance, lactate threshold, or any physiological parameters. These data demonstrate that heat acclimation improves aerobic exercise performance in temperate-cool conditions and provide the scientific basis for employing heat acclimation to augment physical training programs.

Heat acclimation improves exercise performance

In this overview, human morphological and functional adaptations during naturally and artificially induced heat adaptation are explored. Through discussions of adaptation theory and practice, a theoretical basis is constructed for evaluating heat adaptation. It will be argued that some adaptations are specific to the treatment used, while others are generalized. Regarding ethnic differences in heat tolerance, the case is put that reported differences in heat tolerance are not due to natural selection, but can be explained on the basis of variations in adaptation opportunity. These concepts are expanded to illustrate how traditional heat adaptation and acclimatization represent forms of habituation, and thermal clamping (controlled hyperthermia) is proposed as a superior model for mechanistic research. Indeed, this technique has led to questioning the perceived wisdom of body-fluid changes, such as the expansion and subsequent decay of plasma volume, and sudomotor function, including sweat habituation and redistribution. Throughout, this contribution was aimed at taking another step toward understanding the phenomenon of heat adaptation and stimulating future research. In this regard, research questions are posed concerning the influence that variations in morphological configuration may exert upon adaptation, the determinants of postexercise plasma volume recovery, and the physiological mechanisms that modify the cholinergic sensitivity of sweat glands, and changes in basal metabolic rate and body core temperature following adaptation.

Human heat adaptation

In many athletic and occupational settings, the wearing of protective clothing in warm or hot environments creates conditions of uncompensable heat stress where the body is unable to maintain a thermal steady state. Therefore, special precautions must be taken to minimise the threat of thermal injury. Assuming that manipulations known to reduce thermoregulatory strain during compensable heat stress would be equally effective in an uncompensable heat stress environment is not valid. In this review, we discuss the impact of hydration status, aerobic fitness, endurance training, heat acclimation, gender, menstrual cycle, oral contraceptive use, body composition and circadian rhythm on heat tolerance while wearing protective clothing in hot environments. The most effective countermeasure is ensuring that the individual is adequately hydrated both before and throughout the exercise or work session. In contrast, neither short term aerobic training or heat acclimation significantly improve exercise-heat tolerance during uncompensable heat stress. While short term aerobic training is relatively ineffective, long term improvements in physical fitness appear to provide some degree of protection. Individuals with higher proportions of body fat have a lower heat tolerance because of a reduced capacity to store heat. Women not using oral contraceptives are at a thermoregulatory disadvantage during the luteal phase of the menstrual cycle. The use of oral contraceptives eliminates any differences in heat tolerance throughout the menstrual cycle but tolerance is reduced during the quasi-follicular phase compared with non-users. Diurnal variations in resting core temperature do not appear to influence tolerance to uncompensable heat stress.

The thermophysiology of uncompensable heat stress. Physiological manipulations and individual characteristics.

This study examined the effectiveness of endurance training and heat acclimation in reducing the physiological strain imposed by exercising in the heat while wearing protective clothing. Seven young men underwent 8 weeks of physical training [60-80% maximal aerobic power (VO2max) for 30-45 min.day-1, 3-4 days.week-1 at or = 95% of the subject's observed maximum, maintained for 3 min; (3) unwillingness of the subject to continue; (4) the elapse of 120 min. The training regimen increased mean VO2max by 16% and mean plasma volume by 8%. When tested in normal combat clothing, the rates of increase in T(re) and fc were slower after training. However, when wearing NBC protective clothing, the only significant change induced by training was a higher mean skin temperature (Tsk) in the early part of the test. Heat acclimation increased the mean plasma volume of untrained subjects by 8%, but their VO2max remained unchanged. When tested in normal combat clothing, acclimation decreased their mean values of T(re), Tsk, fc, and metabolic rate. When wearing NBC protective clothing, the only significant decrease after acclimation was in overall T(re). In trained subjects, heat acclimation induced no further improvement in any physiological variable when wearing normal combat clothing, but reduced overall T(re) and Tsk when wearing NBC protective clothing. Training- or acclimation-induced increases of sweat secretion (an average increment of 0.14-0.23 kg.h-1) were not accompanied by any statistically significant increase in sweat evaporation when wearing NBC protective clothing. Moreover, tolerance times were unchanged in either normal combat (116-120 min) or NBC protective clothing (47-52 min). We conclude that neither endurance training nor heat acclimation do much to improve exercise tolerance when wearing NBC protective clothing in hot environments, because any added sweat secretion decreases blood volume and increases discomfort without augmenting body cooling.

Effects of training and acclimation on heat tolerance in exercising men wearing protective clothing.

Physical training and heat acclimation are both commonly adopted tactics to improve performance and/or tolerance times when individuals must compete or work in the heat. Potential benefits include: (i) improved aerobic fitness and thus a greater cardiovascular reserve (probably seen mainly after training); (ii) a lower resting body temperature that allows greater heat storage (probably seen mainly after acclimation); (iii) a decreased energy cost of a given intensity of exercise (seen after acclimation and also as the learning component of training); (iv) an enhanced sweating response at a given percentage of maximal effort (probably developed by both treatments); (v) a slower increase in body temperature owing to (iii) and/or (iv) [seen after both treatments]; (vi) a reduced cardiovascular stress because of changes in the autonomic nervous system (probably realised mainly by training), expansion of blood volume (seen after both treatments) and/or a decreased peripheral pooling of blood (probably found after both treatments); and (vii) improved subjective tolerance reflecting a decrease in the relative intensity of a given activity (probably seen mainly after training), a reduction in the physiological strain (found after both treatments) and/or habituation to heat-exercise stress (probably developed by both treatments). Factors affecting improvements in physiological and psychological responses to a given set of conditions include: (i) the individual’s initial fitness and acclimatisation to heat; (ii) age, gender, hydration, sleep deprivation, circadian rhythms and in women the menstrual cycle; (iii) use of ergogenic aids such as fluid ingestion, carbohydrate and/or electrolyte replacement and blood doping; (iv) event or test conditions such as the mode of exercise, the severity of environmental heat stress and the type of clothing worn; and (v) treatment conditions such as the intensity, duration and frequency of exercise and/or heat exposure, the length of any rest intervals and cumulative depletion of body water and minerals.

Interactions of physical training and heat acclimation. The thermophysiology of exercising in a hot climate.

This study investigated the influence of 6 versus 12 days of heat acclimation on the tolerance of low-intensity exercise in the heat while wearing protective clothing. Sixteen young men were acclimated by treadmill walking (50% of each subject's maximal aerobic power for 60 min -day-') in a climatic chamber [40°C dry bulb (db), 30% relative humidity] for either 6 consecutive days or two 6-day periods, separated by a 1-day rest. Before and after heat acclimation, the subjects performed a heat-exercise test (1.34m·s−1, 0% grade; 40°C db, 30% relative humidity), either under control conditions [wearing normal light combat clothing (continuous exercise;n = 5)] or when wearing protective clothing resistant against nuclear, biological, and chemical (NBC) agents (repeated bouts of 15-min walk + 15-min rest;n = 8). Criteria for halting the test exercise were a rectal temperature (Tre) of 39.3°C, a heart rate (fc) ≥ 95% of the subject's observed maximum, unwillingness of the subject to continue, or the elapse of 150 min. Heat acclimation decreased overall test values ofTre,fc, and mean skin temperature for both control and protective clothing conditions. When wearing normal combat clothing, acclimation responses were about twice as large after 12 than after 6 days, but the response was not increased by longer acclimation when wearing NBC protective clothing. Both 6 and 12 days of acclimation increased tolerance times in NBC protective clothing by about 15 min [from 97 (4) to 112 (6) min and from 108 (10) to 120 (10) min for 6 and 12 days, respectively]. We conclude that the physiological strain and limitation of heat-exercise tolerance imposed by wearing NBC protective clothing are not reduced if heat acclimation is prolonged from 6 to 12 days.

Effects of 6 versus 12 days of heat acclimation on heat tolerance in lightly exercising men wearing protective clothing.

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 (VO2max) for 30-45 min day-1, 3-4 days week-1 at 20-22 degrees C dry bulb (db) temperature] followed by 6 days of heat acclimation [45-55% VO2max for 60 min day-1 at 40 degrees C db, 30% relative humidity (rh)] (n = 7) or 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 degrees C db, 30% rh), wearing in turn normal light clothing (continuous exercise at 37-45% VO2max for a tolerated exposure of 116-120 min in experiment 1 and at 31-34% VO2max for 146-150 min in experiment 2) or clothing protective against nuclear, biological, and chemical agents (continuous exercise at 42-51% VO2max for a tolerated exposure of 47-52 min in experiment 1 and intermittent exercise at 23% VO2max 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 (% VO2max) [an unchanged oxygen consumption (VO2) but an increased VO2max, or a reduced VO2 with no change of VO2max], rectal temperature (Tre), mean skin temperature (Tsk), 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 %VO2max or Tre, 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 Tre, Tsk, 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 heat acclimation induces significant reductions RPE and/or RTD, accompanied by reductions in Tre, Tsk, and/or HR. Tolerance times in protective clothing were 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.

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

Body heat storage can be estimated by calorimetry (from heat gains and losses) or by thermometry [from changes (Δ) in mean body temperature (Tb) calculated as a weighted combination of rectal (Tre) and mean skin temperatures (Tsk)]. If an invariant weighting factor ofTre andTsk were to be used (for instance, ΔTb = 0.8 · ΔTre + 0.2 · ΔTsk under hot conditions), body heat storage could be over- or underestimated substantially relative to calorimetry, depending on whether the subject was wearing light or protective clothing. This study investigated whether discrepancies between calorimetry and thermometry arise from methodological errors in the calorimetric estimate of heat storage, from inappropriate weightings in the thermometric estimate, or from both. Residuals of calorimetry versus thermometric estimates were plotted against individual variables in the standard heat balance equation, applying various weighting factors toTre andTsk. Whether light or protective clothing was worn, the calorimetric approach generally gave appropriate estimates of heat exchange components and thus heat storage. One exception was in estimating latent heat loss from sweat evaporation. If sweat evaporation exceeded 650 g·h−1 when wearing normal clothing, evaporative heat loss was overestimated and thus body heat storage was underestimated. Nevertheless, if data beyond this ceiling were excluded from the analyses, the standard 4:1 weighting matched calorimetric heat storage estimates quite well. When wearing protective clothing, the same 4:1 weighting approximated calorimetric heat storage with errors of less than approximately 10%, but only if environmental conditions allowed a subject to exercise for more than 90 min. The best thermometric estimates of heat storage were provided by using two sets of relative weightings, based upon the individual's metabolic heat production (\(\dot M\) in kilojoules per metre squared per hour): {4 − [(\(\dot M - \dot x\))·\(\dot x\)] 2}:1 for an initial, thermoneutral environment and {4 + [(\(\dot M - \dot x\)) ·\(\dot x^{ - 1} \)] · 5}: 1 for a final, hot environment; the optimal value of\(\dot x\) lay between 450 and 500 kJ m−2 · h−1. We concluded that the accuracy of thermometric estimates of heat storage can be improved by modifying weighting factors ofTre andTsk according to the environment, type of clothing, and metabolic rate.

Roy J. Shephard

Papers

Effects of training and acclimation on heat tolerance in exercising men wearing protective clothing.

Interactions of physical training and heat acclimation. The thermophysiology of exercising in a hot climate.

Effects of 6 versus 12 days of heat acclimation on heat tolerance in lightly exercising men wearing protective clothing.

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

Residual analysis in the determination of factors affecting the estimates of body heat storage in clothed subjects