PRE-COOLING REDUCES THERMOREGULATORY STRAIN AND SKIN BLOOD FLOW DURING EXERCISE

Abstract

1515 It has been suggested that fatigue during exercise in the heat occurs when rectal temperature approaches 39.7°C. Indeed, a number of studies have reported that rectal temperature increases less rapidly following pre-cooling and that as a consequence, time to exhaustion is prolonged. This investigation examined the thermoregulatory effector response (specifically skin blood flow) during exercise with and without prior pre-cooling. Six physically active male subjects cycled intermittental at 65% VO2max in an ambient temperature of 22°C for 45 min on two occassions (control and pre-cooling). The pre-cooling (PC) strategy consisted of shower immersion for a duration of 60 min at a water temperature of 28°C which was subsequently reduced by 2°C every 20 min. Rectal temperature (Tr) was measured 10cm beyond the external anal spincter and sweat production rate (SPR) estimated from nude body mass measurement before and after exercise. Forearm skin blood flow (Skbf) was measured by laser Doppler fluximetry and lactate determined in an arterialised capillary sample. There were no significant differences in baseline heart rate, Tr, Skbf or lactate between trials. However, skin temperatures (Tsk) were significantly reduced as a result of pre-cooling (P<0.05). During exercise, the increase in Tr was significantly greater in the control (0.89±0.11°C) compared with the pre-cooling trial (0.63±0.16°C) (P<0.05). This was associated with a significant reduction in Skbf (57.2±16.1 vs 88.3±21.2PU) and a trend towards reduced weight loss (0.68±0.12 vs 1.40±0.38kg) during the pre-cooling trial. These data indicate that pre-cooling was effective in reducing Skbf and suggest that following pre-cooling central blood volume may be augmented which may reduce any thermoregulatory competition for muscle blood flow.