Current evidence does not support an anticipatory regulation of exercise intensity mediated by rate of body heat storage.

A GROWING BODY OF LITERATURE has offered purported evidence of a “central governor” mechanism providing an “anticipatory” control of exercise performance to prevent catastrophic failures of homeostasis in any physiological system (13, 14, 16). The notion that sensations such as “thirst” and “perceived exertion” cause a preemptive reduction in motor unit recruitment and therefore exercise performance to avoid otherwise damaging levels of dehydration and skeletal muscle energy, respectively, (17, 24) have initiated considerable debate (24, 28). However, despite being heavily cited as a progressive exercise model to evaluate performance in the heat (1–3, 5, 10, 13, 15), the claim that uncontrollable hyperthermia is prevented via a feedforward calculation of the rate of body heat storage (26) remains relatively uncontested. It is clear that hyperthermia causes fatigue and that performance degradation and subsequent exhaustion during exercise in a hot environment coincides with the attainment of a high core temperature (8); the critical values of which appear to be influenced by multiple factors including dehydration and skin temperature (25) as well as central fatigue (18). However, when considering the known time courses of thermogenesis and thermolysis during exercise, the evidence of an anticipatory feedforward mechanism regarding the rate of body heat storage (26) appears fundamentally flawed. The principal evidence is the report (26) that subjects selfregulating cycling power output at a fixed rating of perceived exertion under three separate air temperatures (Ta 15°C, 25°C, and 35°C) only showed significantly different rates of body heat storage during the initial 4-min of exercise despite an air temperature difference of 20°C between the hottest and coolest conditions. These supposed early heat storage differences were thought to be “fed forward” with power output subsequently regulated to elicit similar near-zero rates of body heat storage, irrespective of environmental conditions. By definition, rate of body heat storage is the instantaneous difference between the rates of metabolic heat production and net heat loss by combined evaporative and dry heat exchange. At the onset of exercise, heat production is instantly elevated due to the liberation of energy supplying the demands of working muscle groups. In contrast, rate of net heat loss responds much slower with a slight delay followed by an exponential increase with a time constant of 10 min (12, 27). This inertia gives an increase in body heat content and a concurrent rise in core temperature eliciting reflex heat loss mechanisms of sweating and cutaneous vasodilatation to eventually compensate for elevated heat production. A heat storage rate of zero only occurs after 20 to 40 min of constant exercise (12, 27), and this is only possible if heat production is within the thermolytic capacity of the person, primarily determined by acclimation and hydration status as well as environmental biophysics (i.e., ambient temperature, evaporative capacity, clothing). The crux of the feedforward mechanism was the early difference in rate of body heat storage between 15 and 35°C [Fig. 3 (26)]. The source of these differences was the negative values estimated at 15°C. After 1 min, heat storage was reportedly 85 kJ/min ( 1,420 W) and external work was 245 W [Fig. 2 (26)]. Cycling work efficiency is 29% (29). Therefore, heat production after 1 min would be 625 W. For body heat storage to be 1,420 W, a net heat loss of 2,045 W is required. Since minimal changes in sweating and vasodilatation occur after 1 min, heat loss must primarily be via dry heat exchange. Ambient air temperature was 15.1°C with 10 km/h (2.8 m/s) air movement. According to heat balance calculations (7) that employ a conservative estimation (0.2 clo) of dry clothing insulation (no clothing details were provided), rate of dry heat exchange after 1 min was 360 W. In fact, even if the participants were nude this value would only be 570 W. The reported values are therefore clearly inconceivable. Equally unlikely is despite power output only declining by 25 W after 4 min at 15°C [Fig. 2 (26)], rate of net heat loss reduced from 2,045 to 540 W (rate of heat storage was reportedly 0 W). Environmental conditions remained unchanged and sudomotor/vasomotor activity if anything would have increased slightly [core temperature increased; Fig. 5 (26)]. Rates of body heat storage were estimated by calculating minute-by-minute changes in body heat content; which itself was estimated using the product of body mass; an estimated average specific heat (Cp) of 3.47 kJ kg 1 °C ; and volumeweighted (mean) body temperature (Tbody) derived from the two-compartment thermometry model of “core” using rectal temperature (Trec) and “shell” using mean skin temperature (Tsk) (4). Relative contributions of each compartment were determined by fixed sum-to-one weighting coefficients (6). Since mass and Cp remained constant throughout, changes in the rate of body heat storage were solely dependent on minuteby-minute changes in Tbody, or more specifically Trec and/or Tsk. Only minor early changes in Trec occurred, so the single difference between the 15°C and 35°C conditions to be fedforward within the first 4 min of exercise were changes in Tsk [ 3°C at 15°C; 2°C at 35°C; Fig. 6 (26)]. Forty percent of Tsk was derived from the legs (21); therefore initial changes in Tsk were likely due to altered dry heat exchange of the legs when they began moving. Address for reprint requests and other correspondence: O. Jay, Univ. of Ottawa, School of Human Kinetics, 125 Univ., Montpetit Hall, Ottawa, Ontario, Canada K1N 6N5 (e-mail: [email protected]). J Appl Physiol 107: 630–631, 2009; doi:10.1152/japplphysiol.90632.2008.