Effect of daily versus intermittent exposure on heat acclimation.

In order to compare the effectiveness of a daily to an intermittent acclimation protocol, 14 competitive rowers (mean +/- SD VO2peak = 48 +/- 7 ml x kg x min(-1)) were randomly assigned to either a consecutive (10 consecutive days) or intermittent acclimation group (10 sessions over 3 weeks). For every heat exposure, subjects in each group exercised for 30 min at 70% VO2peak in an environmental chamber set at 38 degrees C and 70% relative humidity. Acclimation state was monitored by measuring heart rate (HR), rectal and skin temperature (Tre and Tsk), ratings of perceived exertion (RPE) and whole body sweat rate (SR) during each heat exposure. Final exercise Tre decreased significantly by 0.6 +/- 0.7 degrees C with intermittent heat exposure but the decrease was significantly larger (p < 0.05) with consecutive day heat exposure (1.0 +/- 0.1 degrees C). Final exercise HR also decreased significantly by 13 +/- 12 bpm (p < 0.05) in the consecutive group, and non-significantly by 5 +/- 13 bpm in the intermittent group. RPE decreased with daily (5 +/- 1, p < 0.05) but did not significantly decrease with intermittent heat exposure (1 +/- 3). Similarly, Tsk significantly decreased with consecutive (0.4 +/- 0.2 degrees C, p < 0.05) but not intermittent exposure (0.2 +/- 0.3 degrees C) and SR did not change in either group. Minimal adaptation occurs with intermittent heat exposure and it appears that daily heat exposure is the most effective acclimation strategy.