The impact of elevated body core temperature on critical power as determined by a 3-min all-out test

Abstract

Critical power (CP) delineates the heavy and severe exercise intensity domains, and sustained work rates above CP result in an inexorable progression of oxygen uptake to a maximal value and, subsequently, the limit of exercise tolerance. The finite work capacity above CP, W′, is defined by the curvature constant of the power-duration relationship. Heavy or severe exercise in a hot environment generates additional challenges related to the rise in body core temperature (Tc) that may impact CP and W′. The purpose of this study was to determine the effect of elevated Tc on CP and W′. CP and W′ were estimated by end-test power (EP; mean of final 30s) and work above end-test power (WEP), respectively, from 3-min "all-out" tests performed on a cycle ergometer. Volunteers (n = 8, 4 female) performed the 3-min tests during a familiarization visit and two experimental visits (Thermoneutral vs Hot, randomized crossover design). Before experimental 3-min tests, subjects were immersed in water (Thermoneutral: 36°C for 30 min; Hot: 40.5°C until Tc was ≥ 38.5°C). Mean Tc was significantly greater in Hot compared to Thermoneutral (38.5±0.0°C vs. 37.4±0.2°C; mean±SD, P<0.01). All 3-min tests were performed in an environmental chamber (Thermoneutral: 18°C, 45% RH; Hot: 38°C, 40% RH). EP was similar between Thermoneutral (239 ± 57W) and Hot (234 ± 66W; P = 0.55). WEP was similar between Thermoneutral (10.9 ± 3.0 kJ) and Hot (9.3 ± 3.6; P = 0.19). These results suggest that elevated Tc has no significant impact on EP or WEP.

Effect of acute heat exposure on the determination of critical power and W′ in women and men

“…On the other hand, we found a 12.4 ± 18.0% increase in W′ in HOT compared with TEMP. This was a novel and surprising finding, as neither Kuo et al (2021) nor Kaiser et al (2021) reported significant changes in W′ with acute heat exposure. Depletion of W′ is associated with the depletion of muscle high‐energy phosphates (ATP, PCr) and accumulation of fatigue‐inducing metabolites (H + , ADP, AMP, Pi) (Black et al, 2017; Chidnok et al, 2013; Jones et al, 2008).…”

“…More specifically related to the CP concept, two studies derived CP from a 3 min all‐out test (3MT), in a hot environment. Kuo et al (2021) reported a decrease of 4% in CP (228 vs.219 W), while Kaiser et al (2021) did not find a significant reduction in CP (239 vs. 234 W). Although, the decrease in PO at CP was rather small or not observed in these studies, the heat exposure time prior to the test in the study of Kuo et al (2021) was longer than in our study (45 min vs 15 min).…”

“…Although, the decrease in PO at CP was rather small or not observed in these studies, the heat exposure time prior to the test in the study of Kuo et al (2021) was longer than in our study (45 min vs 15 min). Furthermore, Kaiser et al (2021) applied a passive heat strategy (warm water immersion) to increase core temperature to 38.5°C prior to 3MT, as it has been shown that hyperthermia by means of an increased core temperature (> 38.5°C) is an important factor leading to a decrease in performance (Nybo, 2008). It is possible that, because of its short duration, a 3MT is not able to detect heat‐induced changes in CP.…”

“…Recently, two studies reported estimates of CP and W ′ using a 3 min all-out test (3MT) in a hot environment (Kaiser et al, 2021;Kuo et al, 2021). While Kaiser et al (2021) hypothesized that CP would decrease and W ′ would increase, they demonstrated that mild hyperthermia (core temperature 38.5°C) via preceding hot water immersion (41.5°C) was not sufficient to alter CP and W ′ during a 3MT in hot conditions (38°C). Conversely, Kuo et al (2021) found a reduction in CP, but not in W ′ , after a combination of passive (40 min) and active (i.e.…”