Heat adaptation (HA) is a popular strategy to combat the negative effects of thermal stress. The HA literature has expanded since a 2016 meta-analysis, and we provide an updated meta-analysis, incorporating 39 additional studies and advanced analysis.
Following Pubmed searches, full-text original articles using human participants were reviewed using the four-stage PRISMA process. Data were extracted by at least two of the authors. Hedges’ g effect sizes, 95% confidence intervals, and prediction intervals were calculated. Correlations were run where appropriate.
One hundred and thirty-five total articles (96 previous, 39 new) were reviewed. Medium-term (8–14 days), active, constant work HA regimens remain the most common despite a recent focus on isothermal, passive, and short-term (≤ 7 days) alternatives. HA still improves subsequent exercise performance and capacity in the heat (g = 0.7), reduces resting core temperature (g = − 0.6) and heart rate (g = − 0.5), and increases sweat rate (g = 0.4) but the effect sizes are lower than previously reported. HA has a moderate or larger effect (g > 0.5) on lowering sweat onset temperature, mean heart rate, sweat sodium and chloride concentrations, resting thermal sensation, and thirst sensation, and increasing resting plasma volume. There is considerable heterogeneity within the data for most variables.
HA regimens can reduce physiological and perceptual strain and improve subsequent exercise performance and capacity in the heat. Longer regimens may be more effect than shorter ones, but the data are lacking. Passive HA is a practical, effective alternative to active HA.
Elite athletes will compete in extreme heat more frequently as global land and sea temperatures increase, alongside more intense, frequent and longer duration heatwaves. Best practices to protect athlete health and performance during competition include heat acclimation/acclimatisation [(HA); i.e., long-term pre-competition preparation] complemented by pre-planned and practised cooling and hydration strategies (i.e., short-term interventions immediately before or during competition). This review explores elite athletes’ current behaviours and practices when preparing for competition in the heat and assesses the level of knowledge that has been exhibited by athletes and their practitioners in this space. Recommendations for future research, discussions of current best practices, and methods to improve translation of research into practice are provided. Available research focuses on small samples of elite endurance athletes during a selection of World Championship/Olympic/Paralympic events (~6% of competing athletes). While generally an increase in the adoption of evidence-based HA is seen chronologically from 2015 onwards, universal adoption is not seen. HA adoption is lowest in those who live/train in cold/temperate environments with cost and access to facilities/equipment being the most commonly reported barriers. Further research is required across the sporting landscape to fully characterise elite athlete behaviours and practices in these spaces. International federations and national governing bodies should continue their efforts to educate athletes and focus on regularly updated and reinvigorated release of evidence-based guidelines (in multiple germane languages) for competing in the heat, to increase the adoption of HA and other heat related best practice.
Heat acclimation (HA) is regarded as the most important countermeasure to protect athlete health and performance when exercising in hot ambient conditions. HA involves passive or exercise heat stress applied intentionally to increase sweating, core temperature, and skin temperature. However, these responses can lead to significant physiological stress, increasing the risk of accumulated fatigue and overreaching. Post-exercise cooling is an effective strategy to restore neuromuscular function and perceptive recovery following hyperthermia-induced fatigue. However, the influence of post-exercise cooling on heat adaptation remains largely unexplored. This review discusses the potential impact of this recovery modality on heat adaptation. Studies investigating the interaction between hot and cold exposures in the context of thermal adaptation were reviewed. The examined literature collectively indicates: (1) no impairments in heat adaptation when cold exposures did not interfere with the physiological responses attained during the heat stress, (2) marginal compromises in thermal impulse during heat stress did not diminish the magnitude of heat adaptation, and may be compensated through enhanced absolute training intensity (3) while substantial cooling during heat stress can potentially impair sudomotor adaptations to HA, it is reasonable to expect no impairments in this context as recovery-based cooling does not influence the physiological responses garnered during heat stress. It is acknowledged that this conclusion is based on exploratory findings, as direct data on the effects of recovery cooling interventions on heat adaptations are currently lacking.
Assess the effectiveness of three cooling strategies during a 10-min break vs. no break or no cooling on internal body temperature responses during an intermittent treadmill exercise simulating the intensity of a tennis match.
Twelve physically active females (mean ± SD; age, 26 ± 3 years; height, 167.0 ± 4.8 cm; body mass, 58.2 ± 4.2 kg; VO2 peak 46.2 ± 2.5 mL/kg/min) completed five 90-min intermittent exercise trials in the heat (WBGT 30.9 ± 0.2 °C) with a 10-min break in different cooling groups: cold water immersion (CWI), ice towel with dampened towels (Towel), cooling vest (Vest), passive rest (Passive), and no break (None). Rectal temperature (Trec), skin temperature (Tsk), and heart rate (HR) were monitored throughout the trials.
There was a significant difference in ΔTrec between None and CWI (P < 0.001, d = 1.71). The effect size for ΔTrec indicated a moderate impact with a 10-min break using Towel, Vest and Passive, without a significant difference from None (P > 0.05, d = 0.52–0.67). CWI had a significantly greater ΔTrec during the first 15-min of subsequent exercise period compared to Vest and Passive while there was no significant difference during a 10-min break between the cooling groups. The effects of cooling on Tsk and HR were not maintained at the end of exercise period.
A 10-min break with CWI effectively attenuated the rise in internal body temperature following intermittent treadmill exercise. A 10-min CWI break may be a potential strategy to mitigate the risk of exertional heat illnesses during tennis tournaments in hot conditions, however the impact on physical performance and the logistical challenges associated with implementation remain unexplored.
Heat acclimation (HA) kinetics often necessitates that the intervention is conducted in the weeks immediately preceding athletic competitions, potentially interfering with a training taper. Therefore, we investigated the efficacy of a mixed-method HA protocol, superimposed over planned external training loads, during the 3-weeks prior to the 2022 U23 World Triathlon Championships.
Six international triathletes completed 8 pre-competition HA sessions (5 active: running/cycling, 3 passive: hot water immersion [HWI]), across 2-weeks. Outdoor high-intensity training sessions were followed by 30–60 min HWI, whilst low-intensity cycling/running sessions were completed in a hot, humid environmental chamber. To assess heat adaptations, participants completed three 25 min heat stress tests (HST) involving iso-speed treadmill running (session 1 = HST1, session 5 = HST2, and session 8 = HST3). Physiological, haematological and wellbeing monitoring were conducted throughout HA.
Reduced heart rate (~ − 6 beats/min) was observed within HST3 (P = 0.01, η p 2 = 0.64), versus HST1 and HST2. No changes in core temperature were observed across HSTs (P = 0.055, η p 2 = 0.44). Sweat sodium concentration was lower by HST2 at the arm (− 23 ± 16 mmol/L, P = 0.02) and back (− 27 ± 17 mmol/L, P = 0.01). White blood cell count reduced from baseline to the end of HA (P = 0.02, η p 2 = 0.27), but no changes were found in any other haematological markers (all P > 0.05). Perceptual wellbeing measures did not change across HA (all P > 0.05).
By HST3, seven prior mixed-method HA sessions improved markers of heat adaptation (exercising HR and sweat concentration) within international triathletes. Mixed-method HA may be implemented without modifying training load, with no apparent detrimental effects on athlete health or training stress markers.
Whilst modifications in thermoregulatory responses and plasma volume during heat acclimation (HA) are well researched, much less is known regarding hemoglobin mass. The aim of this study was to investigate the hematological adaptations associated with a long-term, progressive, work-matched controlled heart rate HA protocol.
Ten males (VO2peak: 4.50 ± 0.50 L/min) completed two three-week training interventions consisting of HA (36 °C and 59% RH) and exercise in temperate conditions (TEMP: 18 °C and 60% RH) in a counter-balanced crossover design. Weekly training included 5 consecutive laboratory-based sessions (i.e. 4 controlled heart rate training and 1 repeated sprint training) and 2 days off.
Hemoglobin mass decreased from day 4 of training in HA (−22 [−37, −8] g, P < 0.001) but not TEMP (+2 [−12, +17] g, P = 0.743), returning to baseline at the end of HA (−7 [−22, +7] g, P = 0.333). As compared to day 1, several other adaptations were present from day 5 onward in HA including a decrease in heart rate at rest (−4 [−8, −0] beats/min, P = 0.040) and at a given work rate (−6 [−10, −1] beats/min, P = 0.012), an increase in whole-body sweat rate (+0.3 [+0.1, +0.5] L/h, P = 0.015), and an increase in power output (+18 [+8, +28] W, P < 0.001); while there was no changes in TEMP (P ≥ 0.143). Plasma volume increased in both HA (+168 [+23, +314] mL) and TEMP (+166 [+20, +311] mL) by the 11th day of training (P ≤ 0.027).
While training in both hot or temperate conditions led to plasma volume increases, training in the heat lead to specific physiological adaptations, including a transient decrease in hemoglobin mass that was rapidly reversed within a few days of HA.
The aim of this study was to confirm the impact of heat acclimation on aerobic performance in hot conditions and elucidate the transfer of heat adaptations to cool and hypoxic environments.
Ten males (VO2peak: 4.50 ± 0.50 L/min) completed two three-week interventions consisting of heat acclimation (HA: 36°C and 59% RH) and temperate training (TEMP: 18°C and 60% RH) in a counter-balanced crossover design. Training weeks consisted of four work-matched controlled heart rate sessions interspersed with one intermittent sprint session, and two rest days. Before and after the interventions VO2peak and 20-min time trial performance were evaluated in COOL (18°C), HOT (35°C) and hypoxic (HYP: 18°C and FiO2: 15.4%) conditions.
Following HA, VO2peak increased significantly in HOT (0.24 L/min [0.01, 0.47], P = 0.040) but not COOL (P = 0.431) or HYP (P = 0.411), whereas TEMP had no influence on VO2peak (P ≥ 0.424). Mean time trial power output increased significantly in HOT (20 W [11, 28], P < 0.001) and COOL (12 W [4, 21], P = 0.004), but not HYP (7 W [−1, 16], P = 0.075) after HA, whereas TEMP had no influence on mean power output (P ≥ 0.110). Rectal (−0.13°C [−0.23, −0.03], P = 0.009) and skin (−0.7°C [−1.2, −0.3], P < 0.001) temperature were lower during the time trial in HOT after HA, whereas mean heart rate did not differ (P = 0.339).
HA improved aerobic performance in HOT in conjunction with lower thermal strain and enhanced cardiovascular stability (similar heart rate for higher workload), whereas the mechanistic pathways improving performance in COOL and HYP remain unclear.
Cities are applying reflective coatings on streets in an attempt to mitigate urban heat. These coatings are also being used to try to reduce heat stress during outdoor sports. This study models the progression of heat strain in elite marathon and race walk athletes competing on traditional dark asphalt, reflective pavement, or shaded asphalt in past and future Olympic Games [Tokyo (Sapporo), Paris, Los Angeles].
Observed weather (Sapporo) or expected climate conditions for each city, along with modeled mean radiant temperature (TMRT) differences across the three surface types, were fed into the joint system (JOS-3) thermoregulation model. Resultant changes to heat strain parameters of core temperature (Tcr) and mean skin temperatures ( ), as well as skin wettedness and cardiac output, were modeled.
Reflective pavement slightly increased the average TMRT (1.2–2.2 °C), which caused higher overall radiant heat loads on athletes and thus slightly higher (yet insignificant) Tcr and . These changes in simulated heat strain (worsening the situation) are the opposite of what is expected from a heat mitigation technology. Shading the athletes resulted in lower predicted Tcr (− 0.37 °C) and (− 0.68 °C) across events compared to sun-exposed asphalt, also decreasing cardiac output.
The minor increase in TMRT over reflective pavement transferred a negligible difference in simulated athlete heat strain over a 2–3 h intense competition. Overall, the large impact of solar radiation (even in the morning hours) should be decreased via design strategies that block the sun rather than strategies that increase radiant heat load.
We previously developed the FAME Lab PHS software (PHSFL), a free offline software to calculate the predicted heat strain for a group of individuals based on the ISO 7933. The objectives of this study were to: upgrade the PHSFL from an offline (desktop-version) tool to a web-based platform, as well as assess its validity in recreational athletes in different forms of exercise and across various temperature recording methodologies and environmental conditions. The web PHSFL was developed as browser-based software developed using HTML, CSS, and JavaScript, and included several updates from the previous offline version. Its validity was assessed in 83 healthy non-smoking males and females during rest, exercise, and post-exercise recovery in 165 trials (cycling: 97; running: 68). Trials were performed in an environmental chamber under varying environmental conditions: 19.1 to 40.6 °C air temperature, 30.0% to 60.0% relative humidity, 0.1 to 0.5 m/s wind speed, and 0 or 800 W/m2 solar radiation. Comparison of actual vs. predicted core body temperature showed 0.85 Willmott’s Index of Agreement, 0.76 (P < 0.001) correlation coefficient, and 95% limits of agreement of 0.16 ± 0.83 °C (mean difference ± 95% limits). Results for rectal temperature showed 0.79 Willmott’s Index of Agreement, 0.68 (P < 0.001) correlation coefficient, and 95% limits of agreement of 0.18 ± 0.76 °C. Results for skin temperature showed 0.77 Willmott’s Index of Agreement, 0.75 (P < 0.001) correlation coefficient, and 95% limits of agreement of − 0.24 ± 2.28 °C. We conclude that the web PHSFL provides acceptably accurate predictions of core body temperature and skin temperature to be used as indicators of physiological heat strain.