Julien D. Pe ́riard • Corinne Caillaud • Martin W. Thompson
Received: 10 May 2011 / Accepted: 2 September 2011 / Published online: 23 September 2011
The aim of this study was to examine the influence of aerobic fitness and exercise intensity on the development of thermal and cardiovascular strain in uncompensable heat stress conditions. In three separate trials, eight aerobically trained and eight untrained subjects cycled to exhaustion at 60% (H60%) and 75% (H75%) of maximal oxygen uptake V_O2max in 40C conditions, and for 60 min at 60% V_O2max in 18C conditions (CON). Training status had no influence on time to exhaustion between trained (61 ± 10 and 31 ± 9 min) and untrained (58 ± 12 and 26 ± 10 min) subjects (H60% and H75%, respectively). Rectal temperature at exhaustion was also not significantly different between trained (39.8 ± 0.3, 39.3 ± 0.6 and 38.2 ± 0.3C) and untrained (39.4 ± 0.5, 38.8 ± 0.5 and 38.2 ± 0.4C) subjects, but was different between trials (H60%, H75% and CON, respectively; P\0.01). However, because exercise was terminated on reaching the ethics approved rectal temperature limit in four trained subjects in the H60% trial and two in the H75% trial, it is speculated that increased rectal temperature may have further occurred in this cohort. Nonetheless, exhaustion occurred[96% of maximum heart rate in both cohorts and was accompanied by significant declines in stroke volume (15–26%), cardiac output (5–10%) and mean arterial pressure (9–13%) (P\0.05). The increase in cardiovascular strain appears to represent the foremost factor precipitating fatigue during moderate and high intensity aerobic exercise in the heat in both trained and untrained subjects.
In uncompensable heat stress conditions, the required evaporative capacity of the environment (Ereq) exceeds its maximum evaporative potential (Emax) (Givoni and Gold- man 1972; Robinson et al. 1945). Under these conditions, a thermal steady state cannot be achieved in exercising humans as heat is continually stored within the body at a greater rate than is dissipated (Kraning and Gonzalez 1991). Consequently, endurance performance in both aerobically trained and untrained populations is markedly impaired by an early onset of exhaustion (Cheung and McLellan 1998; Sawka et al. 1992; Selkirk and McLellan 2001). It has been suggested that in trained subjects time to exhaustion is delayed because of an enhanced ability to tolerate high core temperatures (Cheung and McLellan 1998; Selkirk and McLellan 2001; Selkirk et al. 2008). This ability may be acquired with regular exposure to high core temperatures during exercise, and an attenuated cardiovascular response due to physiological adaptations similar to partial heat acclimatization (Allan 1965; Armstrong and Pandolf 1988). However, this response does not appear to be associated with aerobic fitness per se in euhydrated and heat-acclimated subjects (Sawka et al. 1992). Exercise performance under uncompensable heat stress conditions has been well documented at low to moderate intensities in athletic populations (Gonza ́lez-Alonso et al 1999; Nybo and Nielsen 2001a), in those wearing protective clothing (Cheung and McLellan 1998; Cheung et al. 2000; Montain et al. 1994; Sawka et al. 1992; Selkirk and McLellan 2001) and during intermittent exercise (Kraning and Gonzalez 1991; Sawka et al. 2001). However, a systematic comparison of the thermoregulatory and cardiovascular responses of aerobically trained and untrained subjects undertaking moderate to high intensity exercise in uncompensable heat stress conditions has not been performed. Such a comparison may provide insight into the physiological commonalities that mediate exhaustion in the heat, namely the fatigue and associated thermal and cardiovascular strain.