Exercise and heat orthostatism and the effect of heat acclimation and physical fitness

Twelve trained and 16 untrained young men were administered 5 orthostatic tests while leaning upright against a wall: for 20 min before exercise; after 60 min of exercise at a load of 40 W; after 15 min of exercise at a load of 80 W; after a V(o ₂) max test; and after 3 hr of exercise at a load of 40 W in heat (39.4°C DB, 30.3°C WB). Eight of the untrained subjects were retested in the 5 orthostatic tests after 8 d of heat acclimation. The number of fainters in the orthostatic tests administered before exercise, after exercise at 40 W and 80 W, and after exercise in heat, were 3, 4, 6, and 13, respectively. There were no fainting episodes after the V(o ₂) max tests. Orthostatic responses in the different conditions were partially related. The trained subjects showed substantially better responses than the untrained ones (10% vs. 25% of fainting episodes), and heat acclimation resulted in marked improvement in orthostatism (decrease to 5% faintings). The results show that V(o ₂) max and heat tolerance account for most of the variability which determines orthostatic tolerance.