Impairment of Cycling Capacity in the Heat in Well-Trained Endurance Athletes After High-Intensity Short-Term Heat Acclimation

Abstract

Purpose: To investigate the effects of short-term, high-intensity interval-training (HIIT) heat acclimation (HA). Methods: Male cyclists/triathletes were assigned into either an HA (n = 13) or a comparison (COMP, n = 10) group. HA completed 3 cycling heat stress tests (HSTs) to exhaustion (60% Wmax; HST1, pre-HA; HST2, post-HA; HST3, 7 d post-HA). HA consisted of 30-min bouts of HIIT cycling (6 min at 50% Wmax, then 12 × 1-min 100%-Wmax bouts with 1-min rests between bouts) on 5 consecutive days. COMP completed HST1 and HST2 only. HST and HA trials were conducted in 35°C/50% relative humidity. Cycling capacity and physiological and perceptual data were recorded. Results: Cycling capacity was impaired after HIIT HA (77.2 [34.2] min vs 56.2 [24.4] min, P = .03) and did not return to baseline after 7 d of no HA (59.2 [37.4] min). Capacity in HST1 and HST2 was similar in COMP (43.5 [8.3] min vs 46.8 [15.7] min, P = .54). HIIT HA lowered resting rectal (37.0°C [0.3°C] vs 36.8°C [0.2°C], P = .05) and body temperature (36.0°C [0.3°C] vs 35.8°C [0.3°C], P = .03) in HST2 compared with HST1 and lowered mean skin temperature (35.4°C [0.5°C] vs 35.1°C [0.3°C], P = .02) and perceived strain on day 5 compared with day 1 of HA. All other data were unaffected. Conclusions: Cycling capacity was impaired in the heat after 5 d of consecutive HIIT HA despite some heat adaptation. Based on data, this approach is not recommended for athletes preparing to compete in the heat; however, it is possible that it may be beneficial if a state of overreaching is avoided.

Heat alleviation strategies for athletic performance: A review and practitioner guidelines

“…60 min), demonstrating effective heat adaptation [23,60,137]. Whilst HA sessions of 30 min continuous exercise have demonstrated adaptation [144], several studies have also demonstrated a minimal response for this duration (particularly over STHA) [52,145] and in this regard, even if exercise durations are minimized, the heat exposure duration should be prolonged, likely by implementing an extended maintenance phase post-exercise or combining exercise with a passive HA technique.…”

Short-term isothermic heat acclimation elicits beneficial adaptations but medium-term elicits a more complete adaptation

“…Another potential practical benefit of isothermic HA protocols is that thermal adaptations may be achieved with shorter exercise durations and lower exercise intensities than fixed-intensity HA protocols (Gibson et al 2015a) and, therefore, they may be appropriate during the tapering phase in training (Tyler et al 2016). Heat adaptations are lost at a rate of approximately ~ 2.5% per day when individuals are not exposed to heat and so HA should be undertaken as close to competition as possible to minimise de-acclimation (Daanen et al 2018); however, an overly exerting HA protocol may compromise subsequent exercise performance and health due to over-activation of the hypothalamic-pituitary-thyroid axis (Reeve et al 2019) (resulting in increased cortisol concentrations) and/or increased permeability of the gut (leading to the translocation of endotoxins such as lipopolysaccharide (LPS) in to the blood stream) (Lim et al 2009). Within the current literature, an isothermic HA approach has been investigated either with an absolute increase in core temperature (Magalhaes et al 2010) or the attainment of a set thermal strain (Garrett et al 2012;Gibson et al 2015a).…”

Short-Term Repeated-Sprint Training in Hot and Cool Conditions Similarly Benefits Performance in Team-Sport Athletes

“…An a priori power analysis for sample size estimation was conducted for changes in resting core temperature following short-term heat acclimation based on two studies with a partial eta-squared of 0.27 (Moss et al, 2019) and 0.10 ( Reeve et al, 2019). Power analyses were also performed for changes in repeated-sprint performance with a partial eta-squared of 0.32 (Duvnjak-Zaknich et al, 2019) and Yo-Yo IR2 performance with a partial eta-squared of 0.92 (Racinais et al, 2014).…”

“…In contrast, others have reported a failure to improve performance following short-term high-intensity training in the heat and ascribed the lack of improvement to the intense nature of such regimens. Reeve et al (2019) demonstrated that 30 min of high-intensity interval training in the heat for 5 consecutive days reduced physiological and perceptual strain, but impaired endurance capacity during a heat stress test. It was suggested that such a regimen may not be ideal for athletes preparing to compete in the heat, as it may lead to a state of overreaching.…”

“…Altogether, it appears that heat adaptations and performance improvements with short-to-medium-term maximal sprint heat acclimation protocols relate to both the magnitude and total duration of exercise-heat exposure, as well as the environmental conditions in which performance tests (e.g., aerobic capacity and repeated-sprint ability) are completed. The performance benefits may also be influenced by the fatiguing nature of the exercise and a lack of adequate recovery between training sessions and subsequent testing (Reeve et al, 2019).…”