Heat acclimation improves exercise performance

Abstract

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 (VO2max), 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% VO2max in 40°C) program. The hot and cool condition VO2max 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 VO2max 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 hot conditions (9.1 ± 3.4% and 4.5 ± 4.6%, respectively). The control group had no changes in VO2max, 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.

Figures

Fig. 1.

Individual data for relationship between pre- and postacclimation in performance variables of heat acclimation (left) and control (right) groups under hot and cool condition. A: maximal aerobic power (V̇o2max). B: time trial performance. C: lactate threshold. Straight line represents line of equality.

Fig. 2.

Effect of heat acclimation on V̇o2max (A), total work done during a 1-h time trial (B), and power output at lactate threshold (C) in a cool (13°C) and hot (38°C) environment. Values are means ± SE for 12 heat acclimation subjects and 8 controls. *P < 0.05 vs. preacclimation within environmental condition.

Fig. 3.

Heat acclimation effects on maximal cardiac output (A), and their corresponding stroke volume (B), and heart rate (C) during V̇o2max test in a cool (13°C) and hot (38°C) environment. Values are means ± SE for 12 heat acclimation subjects and 8 controls. bpm, beats/min. *P < 0.05 vs. preacclimation within environmental condition.

Fig. 4.

Cardiorespiratory and performance changes as a percent change from the preacclimation trials in both environmental conditions. *P < 0.05 vs. preacclimation within environmental condition. There was no difference in the variables displayed for the control group. Values are means ± SE for 12 heat acclimation subjects and 8 controls. Qcmax, maximal cardiac output.