Heat stress exacerbates the reduction in middle cerebral artery blood velocity during prolonged self‐paced exercise

Abstract

This study examined the influence of hyperthermia on middle cerebral artery mean blood velocity (MCA Vmean). Eleven cyclists undertook a 750 kJ self-paced time trial in HOT (35°C) and COOL (20°C) conditions. Exercise time was longer in HOT (56 min) compared with COOL (49 min; P < 0.001). Power output in HOT was significantly lower from 40% of work completed onward (P < 0.01). Rectal temperature increased to 39.6 ± 0.6°C (HOT) and 38.8 ± 0.5°C (COOL; P < 0.01). Skin temperature, skin blood flow, and heart rate were higher throughout HOT compared with COOL (P < 0.05). A similar increase in ventilation (P < 0.05) and decrease in end-tidal partial pressure of CO 2 (PETCO2; P < 0.05) occurred in both conditions. Arterial blood pressure and oxygen uptake were lower from 50% of work completed onward in HOT compared with COOL (P < 0.01). MCA Vmean increased at 10% in both conditions (P < 0.01), decreasing thereafter (P < 0.01) and to a greater extent in HOT from 40% of work completed onward (P < 0.05). Therefore, despite a comparable ventilatory response and PETCO 2 in the HOT and COOL conditions, the greater level of thermal strain developing in the heat appears to have exacerbated the reduction in MCA Vmean, in part via increases in peripheral blood flow and a decrease in arterial blood pressure.

Self-paced exercise in hot and cool conditions is associated with the maintenance of %V̇o2peak within a narrow range

“…In agreement with previous observations (27,28), V O 2 and power output (Fig. 1) decreased throughout the time trials in the HOT condition, concomitant with the development of thermal (Fig.…”

“…The lower power output noted at the start of the time trial in the HOT condition was somewhat atypical and may stem from subjects consciously adopting a conservative approach, knowing that four separate time trials had to be performed. Indeed, power output in the heat usually matches that of cooler conditions for the first 10 -15 min of time trials (10,27,28,30,42,46), unless participants are inexperienced at performing such tasks (40), or the self-paced effort is preloaded with exercise in the heat (9). Hettinga et al (18) also showed that cycling in the heat decreases gross efficiency by 0.9% relative to cooler conditions.…”

“…When undertaken in the heat, prolonged self-paced exercise performance is significantly impaired relative to cool conditions (10,27,28,42,46). The impairment in performance is mediated by the complex interplay of multiple physiological systems (e.g., nervous, metabolic) (8,34,45), with the development of thermal strain potentially modulating a performance-limiting increase in cardiovascular strain (5,27,28,30). Indeed, a narrowing core-to-skin temperature gradient during exercise-heat stress increases the skin blood flow requirement for heat dissipation (5,35,36).…”

“…The protocol and environmental conditions were selected as they replicate the effort associated with a prolonged time trial (i.e., ϳ60 min or ϳ40 km) in which a decrease and clear difference in V O 2peak occurred between conditions on completing exercise (27,28). The protocol also allows for the measurement of a reference V O 2peak at regular intervals to determine the relative exercise intensity maintained during prolonged self-paced exercise.…”

“…Furthermore, peak oxygen uptake (V O 2peak ) measured during the final maximal end-spurt of a 45-to 60-min time trial is 11-18% lower in hot (35°C) than in cool (20°C) conditions (27,28). It has been suggested that this decrease in V O 2peak leads to a progressive increase in relative exercise intensity (i.e., %V O 2peak utilized) for any given power output (i.e., absolute workload) sustained throughout self-paced efforts (27).…”