Effectiveness of short-term isothermic-heat acclimation (4 days) on physical performance in moderately trained males

Abstract

Introduction: A typical heat acclimation (HA) protocol takes 5-7 d of 60-90 minutes of heat exposure. Identifying the minimum dose of HA required to elicit a heat adapted phenotype could reduce financial constraints on participants and aid in the tapering phase for competition in hot countries. Therefore, the aim of this study was to investigate a 4 d HA regimen on physical performance.

Methods: Twelve moderately trained males were heat acclimated using controlled hyperthermia (Tre>38.5°C), with no fluid intake for 90 min on 4 consecutive days, with a heat stress test (HST) being completed one week prior to (HST2), and within one-week post (HST3) HA. Eleven completed the control study of HST1 versus HST2, one week apart with no intervention. Heat stress tests comprised of cycling for 90 min @ 40% Peak Power Output (PPO); 35°C; 60%RH followed by 10 minutes of passive recovery before an incremental test to exhaustion. Physical performance outcomes time to exhaustion (TTE), PPO, end rectal temperature (Tre END), and heart rate (HREND) was measured during the incremental test to exhaustion.

Results: Physiological markers indicated no significant changes in the heat; however descriptive statistics indicated mean resting Tre lowered 0.24°C (-0.54 to 0.07°C; d = 2.35: very large) and end-exercise lowered by 0.32°C (-0.81 to 0.16; d = 2.39: very large). There were significant improvements across multiple timepoints following HA in perceptual measures; Rate of perceived exertion (RPE), Thermal Sensation (TS), and Thermal Comfort (TC) (P<0.05). Mean TTE in the HST increased by 142 s (323±333 to 465±235s; P = 0.04) and mean PPO by 76W (137±128 to 213±77 W; P = 0.03).

Conclusion: Short-term isothermic HA (4 d) was effective in enhancing performance capacity in hot and humid conditions. Regardless of the level of physiological adaptations, behavioural adaptations were sufficient to elicit improved performance and thermotolerance in hot conditions. Additional exposures may be requisite to ensure physiological adaptation.

Fig 1. Schematic model of the short-term isothermic heat acclimation protocol for moderately trained males.

Fig 2. Mean (SD) work output on days 1–4 of acclimation after 90 min heat exposure.

Fig 3. Mean (SD) Time to 38.5°C (min) and Mean (SD) Time above 38.5°C (min) from Day 1 to Day 4 of acclimation after 90 min heat exposure.

Fig 4

Mean±SD for T_re (upper), ${\overline{T}}_b$ (middle), and ${\overline{T}}_{sk}$ (lower) pre- to post short-term heat acclimation in hot conditions (35°C; 60%RH; n = 12).

Fig 5. Mean (SD) for HR pre- to post- short-term heat acclimation in hot conditions (35°C, 60%RH).

Fig 6

Mean (SD) for perceptual measures RPE (upper), TS (middle), and TC (bottom) pre- to post- short-term heat acclimation in hot conditions (35°C, 60%RH).

Fig 7. Completion (%) of the 90 min (s) and ramped protocol to exhaustion (s) pre- to post- acclimation in hot conditions (35°C, 60%RH).