Heat training increases exercise capacity in hot but not in temperate conditions: a mechanistic counter-balanced cross-over study

The aim was to determine the mechanisms facilitating exercise performance in hot conditions following heat training. In a counter-balanced order, seven males ((V) over dotO(2max) 61.2 +/- 4.4 ml.min(-1).kg(-1)) were assigned to either 10 days of 90-min exercise training in 18 or 38 degrees C ambient temperature (30% relative humidity) applying a cross-over design. Participants were tested for (V) over dotO(2max) and 30-min time trial performance in 18 (T18) and 38 degrees C (T38) before and after training. Blood volume parameters, sweat output, cardiac output ((Q) over dot), cerebral perfusion (i.e., middle cerebral artery velocity [MCAv(mean)]), and other variables were determined. Before one set of exercise tests in T38, blood volume was acutely expanded by 538 +/- 16 ml with an albumin solution (T38A) to determine the role of acclimatization induced hypervolemia on exercise performance. We furthermore hypothesized that heat training would restore MCAv(mean) and thereby limit centrally mediated fatigue. (V) over dotO(2max) and time trial performance were equally reduced in T38 and T38A (7.2 +/- 1.6 and 9.3 +/- 2.5% for (V) over dotO(2max); 12.8 +/- 2.8 and 12.9 +/- 2.8% for time trial). Following heat training both were increased in T38 (9.6 +/- 2.1 and 10.4 +/- 3.1%, respectively), whereas both (V) over dotO(2max) and time trial performance remained unchanged in T18. As expected, heat training augmented plasma volume (6 +/- 2%) and mean sweat output (26 +/- 6%), whereas sweat [Na+] became reduced by 19 +/- 7%. In T38 (Q) over dot(max) remained unchanged before (21.3 +/- 0.6 l/min) to after (21.7 +/- 0.5 l/min) training, whereas MCAv(mean) was increased by 13 +/- 10%. However, none of the observed adaptations correlated with the concomitant observed changes in exercise performance.