Is peak oxygen uptake a determinant of moderate-duration self-paced exercise performance in the heat?

This study aimed to identify whether reductions in peak oxygen uptake (VO₂(peak)) dictate performance outcomes during 30 min of self-paced exercise in the heat, which is expected to induce minimal hyperthermia. On 4 occasions, 11 male subjects completed peak and self-paced exercise in both hot (HOT, 40.2 ± 0.3 °C) and moderate (MOD; 20.4 ± 0.7 °C) conditions. During peak exercise, submaximal oxygen uptake (VO₂) was ∼8% higher in HOT, but VO₂(peak) (MOD, 4.64 ± 0.83 L·min⁻¹; HOT, 4.54 ± 0.77 L·min⁻¹) and peak cardiac output (Q(peak)) were similar. Self-paced exercise performance was reduced by ∼21% in HOT. VO₂ was similar through 15 min, but lower in HOT thereafter. Relative to MOD, this represented a higher and lower %VO₂(peak) during the initial and latter stages. Cardiac output was similar in both trials (MOD, 31.6 ± 6.6 L·min⁻¹; HOT, 30.1 ± 6.0 L·min⁻¹), representing a similar percentage of Q(peak) throughout. Rectal temperature was similar in both conditions until 30 min (MOD, 38.5 ± 0.3 °C; HOT, 38.7 ± 0.3 °C), while skin temperature was higher throughout in HOT (mean: MOD, 32.4 ± 1.1 °C; HOT, 37.3 ± 0.4 °C). Perceived exertion rose similarly in both conditions, while thermal discomfort was higher in HOT. These data indicate that when only skin temperature is elevated, reductions in exercise performance during moderate-duration self-paced exercise are not associated with changes in VO₂(peak). Rather, increases in VO₂ at a given submaximal external workload and (or) thermal discomfort appear to play a larger role.