Heat Acclimation Improves Exercise Performance

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA530499&Location=U2&doc=GetTRDoc.pdf

S. Lorenzo, J.R. Halliwil, M.N. Sawka, C.T. Minson


Thermal and Mountain Medicine Division
U.S. Research Institute of Environmental Medicine
Natick, MA 01760-5007

This study examined the impact of heat acclimation on improving exercise performance in cool and hot environments. Twelve trained cyclists performed tests of maximal aerobic power (V˙ O2max), time-trial performance, and lactate threshold, in both cool [13°C, 30% relative humidity (RH)] and hot (38°C, 30% RH) environments before and after a 10-day heat acclimation ( 50%V ˙ O2max in 40°C) program. The hot and cool condition V ˙ O2max and lactate threshold tests were both preceded by either warm (41°C) water or thermoneutral (34°C) water immersion to induce hyperthermia (0.8 –1.0°C) or sustain normothermia, respectively. Eight matched control subjects completed the same exercise tests in the same environments before and after 10 days of identical exercise in a cool (13°C) environment. Heat acclimation
increased V ˙ O2max by 5% in cool (66.8 2.1 vs. 70.2 2.3 ml·kg 1·min 1, P 0.004) and by 8% in hot (55.1 2.5 vs. 59.6 2.0 ml·kg 1·min 1, P 0.007) conditions. Heat acclimation improved time-trial performance by 6% in cool (879.8 48.5 vs. 934.7 50.9 kJ, P 0.005) and by 8% in hot (718.7 42.3 vs. 776.2 50.9 kJ, P 0.014) conditions. Heat acclimation increased power output at lactate threshold by 5% in cool (3.88 0.82 vs. 4.09 0.76 W/kg, P 0.002) and by 5% in hot (3.45 0.80 vs. 3.60 0.79 W/kg, P 0.001) conditions. Heat acclimation increased plasma volume (6.5 1.5%) and maximal cardiac output in cool and hotconditions (9.1 3.4% and 4.5 4.6%, respectively). The control group had no changes in V ˙ O2max, time-trial performance, lactate threshold, or any physiological parameters. These data demonstrate that heat acclimation improves aerobic exercise performance in temperate- cool conditions and provide the scientific basis for employing heat acclimation to augment physical training programs.

maximal oxygen uptake; time-trial performance; lactate threshold;
plasma volume; cardiac output; hot environment; cool environment