I appreciate the thoughtful comments of McLellan et al. on my article, “Influence of Aerobic Fitness on Thermoregulation during Exercise in the Heat” (5). I read their interesting remarks and appreciate the opportunity to respond. My article focused on “situations of submaximal exercise during environmental conditions where heat dissipation is dependent greatly on sweat evaporation.” McLellan and coworkers studied thermal responses during exercise in the heat while wearing nuclear, biological, and chemical (NBC) protective clothing. These protective ensembles have limited vapor permeability, and subjects exercised in an uncompensable heat stress situation. To allow prolonged exercise in the heat while wearing these suits, workload is necessarily light. Under these conditions, McLellan’s group nicely showed that aerobically fit individuals exercised longer than the untrained individuals and fatigued at higher rectal temperatures (i.e., enhanced thermotolerance (1,10–11)), contending previous work by Sawka et al. (9).
These suits veil the adaptations that permit enhanced heat dissipation after endurance training. Thus, at similar heat production (i.e., walk at 3–4 km·h−1), rectal temperature (Tre) increases at the same rate in trained and untrained individuals (1,10). In contrast, when the skin is free to dissipate heat (i.e., uncovered), core temperature increases less in fit individuals when matched for absolute work rate (3) because of their improved heat dissipation. Furthermore, our laboratory (6) and, more recently, Jay et al. (4) have reported higher gains in Tre in trained subjects when exercising at similar percent of V˙O2peak than untrained counterparts. Thus, core temperature response to exercise depends on the matching of exercise intensity (absolute or relative workload) and on the restrictions for dissipating heat (i.e., clothing, high temperature-humidity, or low airflow). Recently, Periard and coworkers (7) measured thermotolerance in trained and untrained subjects during 60% to 75% V˙O2peak in a hot environment without covering the skin. They reported similar Tre at exhaustion in trained and untrained subjects. When workload is matched for percent V˙O2peak, the metabolic and cardiovascular strains are similar between groups, and these factors may cause fatigue before heat accumulation. Thermotolerance, it seems to me, is difficult to separate from other causes of fatigue, and the authors are applauded for their work in this complex area.
In the review by Cheung et al. (2), the authors stated, “with exercise in protective clothing… the rate of heat storage and heat tolerance are governed primarily by the rate of heat production and not by environmental conditions.” In figure 2, they summarized their studies showing that, at the same metabolic rate (heat production), tolerance time could somewhat vary because of the environmental heat load. Our proposal (i.e., Figure 4 in (5)) varied from that of Cheung et al. in that we predicted Tre in compensable and uncompensable heat situations in trained and untrained individuals. Importantly, our uncompensable situation was reached by increasing heat production (i.e., exercise intensity as percent V˙O2peak) above the environment dissipation capacity, not by restricting heat dissipation with clothing. The figure is novel because it dispels the apparent contradiction between the classical data from Saltin and Hermansen (8) and the new findings from our work (6) and others (4) showing higher Tre in aerobically trained individuals during uncompensable exercise situations.
Exercise Physiology Laboratory
Sports Sciences Department
University of Castilla-La Mancha, Toledo, Spain
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