Julien D. Periard

Heat acclimation (artificial exposure) and acclimatization (natural exposure) are key strategies for elite and world-class athletes preparing to compete in hot outdoor environments, aiming to reduce performance impairments and risk of heat illness. Evidence-based guidelines, primarily derived from laboratory studies in non-elite cohorts, recommend 10–14 consecutive days of heat exposure (60–90 min per day), initiated at least 2 weeks before competition in the heat, or earlier if re-acclimation is possible closer to the event. Sessions should elicit sustained elevations in core and skin temperature, skin blood flow, and sweating. However, logistical constraints, training demands, and tapering often limit protocol implementation in elite and world-class athletes, prompting the adoption of alternative approaches. This review synthesises observational studies documenting heat training practices in these athletes and evaluates their alignment with current guidelines. Eleven of the 12 studies included sport-specific heat exposure within preparation for competition in hot environments, often in similar or more severe environmental conditions than competition, with five studies incorporating post-exercise passive heating. However, heat intervention design, session structure, and monitoring of thermal and cardiovascular strain varied widely, with many reports lacking sufficient detail to determine their effectiveness at inducing and retaining adaptations outside laboratory-controlled settings. Nonetheless, initial reports indicate that classical markers of heat adaptation such as reduced exercise heart rate and core temperature, and increased plasma volume, are commonly observed. Standardized documentation of heat training characteristics is needed to assess their effectiveness in high-performance sport and refine evidence-based guidelines.

Purpose: To examine the influence of hot-dry and warm-humid heat acclimation (HA) on endurance performance and the cross-acclimation benefit between environments. Methods: In a cross-over design, trained male cyclists completed a 30-min time trial in hot-dry (Dry-TT; 42°C, 25% RH) and warm-humid (Humid-TT; 33°C, 80% RH) conditions before and after 8 days of HA in dry (Dry-HA, n =10) and humid (Humid-HA, n =12) conditions matched for wet-bulb globe temperature (~31°C). Bayesian multilevel models were employed to estimate posterior means and 90% credible intervals. Results: During the Dry-TT, mean power output increased by a further 8 W (2, 15) after Dry-HA (14 W [7, 21]) than Humid-HA (6 W [-1, 13]). Mean rectal temperature during the Dry-TT was lower after both HA regimens (-0.1ºC [-0.2, 0.0]) whereas heart rate was unchanged following Dry-HA (-1 beats·min ^-1 [-3, 2]) and lower after Humid-HA (-3 beats·min ^-1 [-6, 0]). Whole-body sweat rate (WBSR) during the Dry-TT increased with both Dry-HA (0.28 L·h ^-1 [0.14, 0.41]) and Humid-HA (0.23 L·h ^-1 [0.10, 0.35]). During the Humid-TT, mean power output increased similarly following Humid-HA (9 W [1, 17]) and Dry-HA (12 W [3, 20]). Mean rectal temperature during the Humid-TT was unchanged (0.0°C [-0.1, 0.1]) after Humid-HA and decreased (-0.2°C [-0.3, -0.1]) after Dry-HA, whereas heart rate was unchanged after Humid-HA (2 beats·min ^-1 [-3, 5]) and Dry-HA (-2 beats·min ^-1 [-6, 2]). WBSR during the Humid-TT increased with both Humid-HA (0.42 L·h ^-1 [0.32, 0.52]) and Dry-HA (0.35 L·h ^-1 [0.23, 0.47]). Conclusions: Our data indicate that Dry-HA and Humid-HA improved self-paced endurance performance in each respective environment, and in the opposing environment. Moreover, Dry-HA further improved performance in dry conditions than did Humid-HA, while performance in humid conditions was similarly improved following both interventions.

Purpose: Passive heating has emerged as a potential non‑exercise method to support muscle strength and mass, but its application is limited by inconsistent protocols and unclear dosage and safety guidelines. This study used a Delphi methodology to establish expert consensus on key parameters for passive heating interventions targeting neuromuscular and muscle morphology outcomes. Methods: An international panel of academic experts (≥ 3 peer‑reviewed publications in passive heating and neuromuscular function) completed a two‑round Delphi process. Round 1 gathered open‑ended responses on passive heating strategies, intervention targets, and safety. In Round 2, experts ranked the importance of generated statements. Consensus was defined as ≥ 70% ranking a statement as either first or second in importance. Results: Eleven experts completed Round 1 and ten in Round 2. From 177 qualitative responses, 130 statements were evaluated, with 11 reaching consensus (80–100% agreement; interquartile range = 0–1). Hot‑water immersion was prioritised as the most optimal passive heating modality. Experts agreed on targeting muscle temperatures of 39–40 °C, ~ 60-minute session durations, and once‑daily exposure as key intervention parameters. Strong consensus was also reached for monitoring core temperature and cardiovascular strain (e.g., heart rate, blood pressure) to ensure participant safety. No consensus was reached on a single target population or the total intervention duration. Conclusion: This Delphi study provides a consensus‑based framework outlining key parameters and safety considerations for passive heating interventions. These findings do not establish efficacy but identify priority variables to inform future experimental research and cautious translation into applied and clinical contexts.

Advancing age is associated with progressive increases in the rise in core temperature during extreme heat exposure, and this trajectory is evident across the entire adult lifespan rather than emerging only in older adults. Thermoregulatory capacity declines steadily with each decade, leading to greater thermal strain even in mid-adulthood. Although higher cardiorespiratory fitness enhances sweating, it does not eliminate the age-related increase in core temperature, underscoring that heat vulnerability may develop gradually throughout adulthood.

Age and biological sex have been suggested to affect sweating during exercise, but studies that have compared the local sweating response between boys and girls of different ages are limited. We standardized exercise intensity to the rate of metabolic heat production per body surface area and compared local sweat rate and the sweating onset threshold between boys, girls, adult males and adult females using Bayesian hierarchical modelling. Sixty‐one children (29 females; 10–16 years old) and 20 adults (9 females; 19–37 years old) walked for 45 min at a fixed rate of metabolic heat production per body surface area (300 W·m ⁻² ) in WARM (30°C, 40% relative humidity) and/or HOT (40°C, 30% relative humidity) environments. Differences in local sweat rate between boys and girls were ≤0.03 mg·cm ⁻² ·min ⁻¹ (90% credible intervals: −0.14, 0.08) in WARM and ≤0.08 mg·cm ⁻² ·min ⁻¹ (−0.05, 0.23) in HOT, indicating no meaningful sex differences in children. In HOT, the local sweat rate of the back was 1.17 mg·cm ⁻² ·min ⁻¹ (1.05, 1.28) for boys, 1.09 mg·cm ⁻² ·min ⁻¹ (0.76, 1.41) for adult males, 1.09 mg·cm ⁻² ·min ⁻¹ (0.99, 1.19) for girls and 1.01 mg·cm ⁻² ·min ⁻¹ (0.72, 1.27) for adult females. Sweating onset threshold times differed by ≤0.6 min (−0.2, 1.4) between sexes at the back in WARM. An increase in age of 4 years was associated with a 1.1 min (0.1, 2.0) and 0.8 min (0.1, 1.5) greater sweating onset threshold time of boys and girls, respectively. In conclusion, age and biological sex did not meaningfully influence the local sweating response and onset threshold of children and adults exercising at 300 W·m ⁻² . image Key points: Age and biological sex have been suggested to influence sweating during exercise, but their effect might be confounded by exercise intensity prescription and the consequent sweating requirement between individuals of differing body size. By standardizing exercise intensity to the rate of metabolic heat production per body surface area (in metres squared), we elicited an equivalent evaporative requirement for heat balance and compared local sweat rate and the sweating onset threshold between boys, girls, adult males and adult females exercising in warm and hot conditions. Our findings demonstrate that biological sex did not meaningfully influence the local sweating response and onset threshold time of children exercising at 300 W·m ⁻² in warm and hot environments. In a hot environment, older children had a greater local sweat rate compared with younger children, but this was not evident when comparing children and adults.

This study compared adaptive responses and cross‐acclimation between dry and humid heat acclimation (HA). In a cross‐over design, trained males completed a heat stress test in hot‐dry (Dry‐HST: 42°C, 25% relative humidity [RH]) and warm‐humid (Humid‐HST: 33°C, 80% RH) conditions before and after 8 days of controlled heart rate (HR) HA in hot‐dry (Dry‐HA, n =  10) and warm‐humid (Humid‐HA, n =  12) environments (wet‐bulb globe temperature: ∼31°C). Bayesian multi‐level models were used to determine posterior means and 90% credible intervals. Mean power output increased similarly during Dry‐HA and Humid‐HA but was higher throughout Dry‐HA (13 W [6, 20]). Sweat rate increased during both regimens but was higher throughout Dry‐HA (100 mL·h ⁻¹ [20, 180]). Plasma volume expanded with Dry‐HA (201 mL [71, 324]) and Humid‐HA (113 mL [−5, 232]), as did haemoglobin mass (24 g [2, 46] and 26 g [6, 47]). During the Dry‐HST, end‐exercise rectal temperature changed by −0.2°C (−0.3, −0.1; Dry‐HA) and −0.3°C (−0.4, −0.2; Humid‐HA), and during the Humid‐HST by −0.3°C (−0.4, −0.2; Dry‐HA) and −0.1°C (−0.2, 0.0; Humid‐HA). HR changed by −2 beats · min ⁻¹ (−5, −1; Dry‐HA) and −5 beats · min ⁻¹ (−9, −2; Humid‐HA) during the Dry‐HST, and by −6 beats · min ⁻¹ (−9, −4; Dry‐HA) and −1 beats · min ⁻¹ (−4, −2; Humid‐HA) during the Humid‐HST. Our data indicate that Dry‐HA and Humid‐HA induced similar adaptations, despite work rate and sweat rate being higher during Dry‐HA. Cross‐acclimation benefits were also similar between interventions. The adaptive stimulus to heat stress, whether dry or humid, appears more impactful on the exercise‐HA phenotype than the specificity of the environment. image Key points: The adaptive response to dry and humid heat remains poorly understood, as are the cross‐acclimation benefits between these environments. A cross‐over design approach was used to compare the adaptive response to hot‐dry and warm‐humid heat acclimation (HA), and the cross‐acclimation provided by each environment. The negligible physiological differences observed between dry and humid HA regimens suggest that the specificity of the environmental characteristics was overshadowed by the overall adaptive stimulus (controlled heart rate [HR] HA). The greater work rate sustained for a similar HR, rectal temperature, rating of perceived exertion and thermal comfort during dry compared to humid HA highlights that drier conditions may better preserve physical training quality. Athletes, soldiers and workers may use dry heat to optimise preparation for competition and work in both dry and humid heat, with the possibility of habituating to humid heat in the final few acclimation sessions if such conditions are anticipated.

Purpose: This study examined hydration responses to sodium hyperhydration in female athletes at rest and during cycling across the early follicular and mid-luteal phases of the menstrual cycle. Methods: Twelve cyclists/triathletes consumed 30 mL·kg⁻¹ fat-free mass fluid with either sodium chloride (7.5 g·L⁻¹) or placebo (sucrose) 2 h before 75 min of steady-state cycling (60% VO_2peak) and a 200 kJ time trial (TT) in a hot environment (34 °C, 60% RH). Body mass was measured, and urine was collected every 30 min, whilst blood samples were taken hourly pre-exercise, post-steady-state, and post-TT. Results: During pre-exercise, sodium hyperhydration increased fluid retention (509.0 mL, 95% CI: [349.0, 669.0]; p < 0.001), while reducing urine volume (−107.4 [−146.7, −68.1] mL; p < 0.001). During exercise, body mass loss was lower with sodium during steady-state (−0.20%; p = 0.001) and TT (−0.15%; p = 0.037), but sweat rates were reduced with sodium only during steady-state (−0.08 L·h⁻¹; p = 0.001). Exploratory analyses showed greater effects in the early-follicular phase, with reductions in body mass loss (−0.26%; p = 0.004), sweat rate (−0.1 L·h⁻¹; p = 0.003), and post-TT arginine vasopressin (−10.8 [−19.2, −2.3] pg·mL⁻¹; p = 0.013). Conclusions: Sodium hyperhydration effectively enhanced fluid retention and reduced body mass loss during exercise in the heat. While no consistent main effects of menstrual cycle phase were observed, some phase-specific differences emerged, particularly in the early-follicular phase. These findings highlight the importance of monitoring hydration responses across the menstrual cycle and tailoring strategies to individual needs, rather than applying universal phase-specific recommendations.