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Effects of precooling on thermoregulation during subsequent exercise. Med. Sci. Sports Exerc., Vol. 31, No. 2, pp. 251-257, 1999.The purpose of this study was to examine the effect of a decreased body core temperature before a simulated portion of a triathlon (swim, 15 min; bike, 45 min) and examine whether precooling could attenuate thermal strain and increase subjective exercise tolerance in a warm environment (26.6°C/60% relative humidity (rh)).Six endurance trained triathletes (28 ± 2 yr, 8.2 ± 1.7% body fat) completed two randomly assigned trials 1 wk apart. The precooling trial (PC) involved lowering body core temperature (−0.5°C rectal temperature, Tre) in water before swimming. The control trial (CON) was identical except no precooling was performed. Water temperature and environmental conditions were maintained at 25.6°C and 26.6°C/60% rh, respectively, throughout all testing.Mean time to precool was 31 ± 8 min and average time to reach baseline Tre during cycling was 9 ± 7 min. Oxygen uptake (O2), HR, skin temperature (Tsk), Tre, RPE, and thermal sensation (TS) were recorded following the swim segment and throughout cycling. No significant differences in mean body (Tb) or Tsk were noted between PC and CON, but a significant difference (P < 0.05) in Tre between treatments was noted through the early phases of cycling. No significant differences were reported in HR, O2, RPE, TS, or sweat rate (SR) between treatments. Body heat storage (S) was negative following swimming in both PC (−92 ± 6 W·m2) and CON (−66 ± 9 W·m2). A greater S occurred in PC (109 ± 6 W·m2) vs CON (79 ± 4 W·m2) during cycling (P < 0.05).Precooling attenuated the rise in Tre, but this effect was transient. Therefore, precooling is not recommended before a triathlon under similar environmental conditions.