Whole body hyperthermia, but not skin hyperthermia, accelerates brain and locomotor limb circulatory strain and impairs exercise capacity in humans

Abstract Cardiovascular strain and hyperthermia are thought to be important factors limiting exercise capacity in heat‐stressed humans, however, the contribution of elevations in skin ( T sk ) versus whole body temperatures on exercise capacity has not been characterized. To ascertain their relationships with exercise capacity, blood temperature ( T B ), oxygen uptake ( V̇ O 2 ), brain perfusion ( MCA V mean ), locomotor limb hemodynamics, and hematological parameters were assessed during incremental cycling exercise with elevated skin (mild hyperthermia; HYP mild ), combined core and skin temperatures (moderate hyperthermia; HYP mod ), and under control conditions. Both hyperthermic conditions increased T sk versus control (6.2 ± 0.2°C; P < 0.001), however, only HYP mod increased resting T B , leg blood flow and cardiac output ( Q̇ ), but not MCA V mean . Throughout exercise, T sk remained elevated in both hyperthermic conditions, whereas only T B was greater in HYP mod . At exhaustion, oxygen uptake and exercise capacity were reduced in HYP mod in association with lower leg blood flow, MCA V mean and mean arterial pressure ( MAP ), but similar maximal heart rate and T B . The attenuated brain and leg perfusion with hyperthermia was associated with a plateau in MCA and two‐legged vascular conductance ( VC ). Mechanistically, the falling MCA VC was coupled to reductions in P a CO 2 , whereas the plateau in leg vascular conductance was related to markedly elevated plasma NA and a plateau in plasma ATP . These findings reveal that whole‐body hyperthermia, but not skin hyperthermia, compromises exercise capacity in heat‐stressed humans through the early attenuation of brain and active muscle blood flow.