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A classic, unresolved physiological question is whether central cardiorespiratory and/or local skeletal muscle circulatory factors limit maximal aerobic capacity (V̇o2max) in humans. Severe heat stress drastically reduces V̇o2max, but the mechanisms have never been studied.

To determine the main contributing factor that limits V̇o2max with and without heat stress, we measured hemodynamics in 8 healthy males performing intense upright cycling exercise until exhaustion starting with either high or normal skin and core temperatures (+ 10° C and+ 1° C). Heat stress reduced V̇o2max, 2-legged V̇o2, and time to fatigue by 0.4±0.1 L/min (8%), 0.5±0.2 L/min (11%), and 2.2±0.4 minutes (28%), respectively (all P< 0.05), despite heart rate and core temperature reaching similar peak values. However, before exhaustion in both heat stress and normal conditions, cardiac output …