CrossTalk proposal: Heat acclimatization does improve performance in a cool condition

Environmental challenges facing athletes, stakeholders and spectators at Paris 2024 Olympic and Paralympic Games: an evidence-based review of mitigation strategies and recommendations

The upcoming Paris 2024 Olympic and Paralympic Games could face environmental challenges related to heat, air quality and water quality. These challenges will pose potential threats to athletes and impact thousands of stakeholders and millions of spectators. Recognising the multifaceted nature of these challenges, a range of strategies will be essential for mitigating adverse effects on participants, stakeholders and spectators alike. From personalised interventions for athletes and attendees to comprehensive measures implemented by organisers, a holistic approach is crucial to address these challenges and the possible interplay of heat, air and water quality factors during the event. This evidence-based review highlights various environmental challenges anticipated at Paris 2024, offering strategies applicable to athletes, stakeholders and spectators. Additionally, it provides recommendations for Local Organising Committees and the International Olympic Committee that may be applicable to future Games. In summary, the review offers solutions for consideration by the stakeholders responsible for and affected by the anticipated environmental challenges at Paris 2024.

Combining Heat and Altitude Training to Enhance Temperate, Sea-Level Performance

BACKGROUND

Repeated exposure to heat (ie, plasma volume expansion) or altitude (ie, increase in total hemoglobin mass), in conjunction with exercise, induces hematological adaptations that enhance endurance performance in each respective environment. Recently, combining heat and altitude training has become increasingly common for athletes preparing to compete in temperate, sea-level conditions.

PURPOSE

To review the physiological adaptations to training interventions combining thermal and hypoxic stimuli and summarize the implications for temperate, sea-level performance. Current Evidence: To date, research on combining heat and hypoxia has employed 2 main approaches: simultaneously combining the stressors during training or concurrently training in the heat and sleeping at altitude, sometimes with additional training in hypoxia. When environmental stimuli are combined in a training session, improvements in aerobic fitness and time-trial performance in temperate, sea-level conditions are generally similar in magnitude to those observed with heat, or altitude, training alone. Similarly, training in the heat and sleeping at altitude does not appear to provide any additional hematological or nonhematological benefits for temperate; sea-level performance relative to training in hot, hypoxic, or control conditions.

CONCLUSIONS

Current research regarding combined heat and altitude interventions does not seem to indicate that it enhances temperate, sea-level performance to a greater extent than "traditional" (heat or hypoxia alone) training approaches. A major challenge in implementing combined-stressor approaches lies in the uncertainty surrounding the prescription of dosing regimens (ie, exercise and environmental stress). The potential benefits of conducting heat and altitude exposure sequentially (ie, one after the other) warrants further investigation.

Can ten days of heat acclimation training improve temperate-condition rowing performance in national-level rowers?

This study investigated whether heat acclimation (HA) could improve rowing performance in temperate conditions in national-level rowers. Using a parallel-group design, eleven rowers (3 female, 8 male, age: 21±3 years, height: 182.3±6.8cm, mass: 79.2±9.0kg, V˙O2peak: 61.4±5.1ml·kg·min^-1) completed either a HA intervention (HEAT, n = 5) or acted as controls (CON, n = 6). The intervention replaced usual cross-training sessions and consisted of an hour of submaximal cycling or rowing ergometry in either 34±0°C for HEAT or 14±1°C for CON daily over two five-day blocks (10 sessions total), separated by 72h. Participants performed the ‘10+4’ test that consists of 10-min submaximal rowing and a 4-min time-trial (TT) in temperate conditions (20±0°C) before and after the intervention. Heat acclimation following the 10-session intervention was evidenced by large significant (p<0.05) decreases in maximum tympanic temperature (d = -1.68) and rate of perceived exertion (RPE) (d = -2.26), and a large significant increase in sweat loss (d = 0.91). Large non-significant (p>0.05) decreases were seen in average tympanic temperature (d = -3.08) and average heart rate (d = -1.53) in HEAT from session 2 to session 10 of the intervention. Furthermore, a large significant increase was seen in plasma volume (d = 3.74), with large significant decreases in haemoglobin concentration (d = -1.78) and hematocrit (d = -12.9). Following the intervention, large non-significant increases in respiratory exchange ratio (d = 0.87) and blood lactate (d = 1.40) as well as a large non-significant decrease in RPE (d = -1.23) were seen in HEAT during the 10-min submaximal rowing. A large significant decrease in peak heart rate (d = -2.27), as well as a large non-significant decrease in relative V˙O2peak (d = -0.90) and large non-significant increases in respiratory exchange ratio (d = 1.18), blood lactate concentration (d = 1.25) and power output (d = 0.96) were seen in HEAT during the 4-min TT. This study suggests that a 10-session HA intervention may elicit HA in national-level rowers, with potential to improve 4-min TT performance in temperate conditions.

Sex differences in adaptation to intermittent post-exercise sauna bathing in trained middle-distance runners

Background

The purpose of this study was to investigate the effect of sex on the efficacy of intermittent post-exercise sauna bathing to induce heat acclimation and improve markers of temperate exercise performance in trained athletes.

Methods

Twenty-six trained runners (16 female; mean ± SD, age 19 ± 1 years, V̇O_2max F: 52.6 ± 6.9 mL⋅kg⁻¹⋅min⁻¹, M: 64.6 ± 2.4 mL⋅kg⁻¹⋅min⁻¹) performed a running heat tolerance test (30 min, 9 km⋅h⁻¹/2% gradient, 40 °C/40%RH; HTT) and temperate (18 °C) exercise tests (maximal aerobic capacity [V̇O_2max] and lactate profile) pre and post 3 weeks of normal exercise training plus 29 ± 1 min post-exercise sauna bathing (101–108 °C) 3 ± 1 times per week.

Results

Females and males exhibited similar reductions (interactions p > 0.05) in peak rectal temperature (− 0.3 °C; p < 0.001), skin temperature (− 0.9 °C; p < 0.001) and heart rate (− 9 beats·min⁻¹; p = 0.001) during the HTT at post- vs pre-intervention. Only females exhibited an increase in active sweat glands on the forearm (measured via modified iodine technique; F: + 57%, p < 0.001; M: + 1%, p = 0.47). Conversely, only males increased forearm blood flow (measured via venous occlusion plethysmography; F: + 31%, p = 0.61; M: + 123%; p < 0.001). Females and males showed similar (interactions p > 0.05) improvements in V̇O_2max (+ 5%; p = 0.02) and running speed at 4 mmol·L⁻¹ blood lactate concentration (+ 0.4 km·h⁻¹; p = 0.001).

Conclusions

Three weeks of post-exercise sauna bathing effectively induces heat acclimation in females and males, though possibly amid different thermoeffector adaptations. Post-exercise sauna bathing is also an effective ergogenic aid for both sexes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40798-021-00342-6.

Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies

A rise in body core temperature and loss of body water via sweating are natural consequences of prolonged exercise in the heat. This review provides a comprehensive and integrative overview of how the human body responds to exercise under heat stress and the countermeasures that can be adopted to enhance aerobic performance under such environmental conditions. The fundamental concepts and physiological processes associated with thermoregulation and fluid balance are initially described, followed by a summary of methods to determine thermal strain and hydration status. An outline is provided on how exercise-heat stress disrupts these homeostatic processes, leading to hyperthermia, hypohydration, sodium disturbances, and in some cases exertional heat illness. The impact of heat stress on human performance is also examined, including the underlying physiological mechanisms that mediate the impairment of exercise performance. Similarly, the influence of hydration status on performance in the heat and how systemic and peripheral hemodynamic adjustments contribute to fatigue development is elucidated. This review also discusses strategies to mitigate the effects of hyperthermia and hypohydration on exercise performance in the heat by examining the benefits of heat acclimation, cooling strategies, and hyperhydration. Finally, contemporary controversies are summarized and future research directions are provided.

Effects of three-exercise sessions in the heat on endurance cycling performance

PURPOSE

To investigate the effects of a very short-term acclimation protocol (VSTAP) on performance, physiological and perceptual responses to exercise in the heat.

METHODS

12 trained male cyclists (age 31.2 ± 7; weight 71.3 ± 7 kg, VO_2max: 58.4 ± 3.7 mL/kg/min) randomly performed two Time to Exhaustion Tests (TTE) at 75% of normothermic peak power output (PPO), one in normothermia (N,18°C-50% RH) and one in the heat (H,35°C-50% RH), before and after a VSTAP intervention, consisting of 3 days-90 min exercise (10min at 30% of PPO+80 min at 50% of PPO) in H (≈4.5h of heat exposure). Performance time of TTEs and physiological and perceptual variables of both TTEs and training sessions (T1, T2 and T3) were evaluated.

RESULTS

Magnitude Based Inferences (MBI) revealed 92/6/1% and 62/27/11% chances of positive/trivial/negative effects of VSTAP of improving performance in H (+17%) and in N (+9%), respectively. Heart Rate (HR) decreased from T1 to T3 (p < 0.001) and T2 to T3 (p < 0.001), whereas Tympanic Temperature (TyT) decreased from T1 to T2 (p = 0.047) and from T1 to T3 (p = 0.007). Furthermore, despite the increased tolerance to target Power Output (PO) throughout training sessions, RPE decreased from T1 to T3 (p = 0.032).

CONCLUSIONS

The VSTAP determined meaningful physiological (i.e. decreased HR and TyT) and perceptual (i.e. decreased RPE) adaptations to submaximal exercise. Furthermore, showing good chances to improve performance in the heat, it represents a valid acclimation strategy to be implemented when no longer acclimation period is possible. Finally, no cross-over effect of the VSTAP on performance in temperate conditions was detected.

Change in Exercise Performance and Markers of Acute Kidney Injury Following Heat Acclimation with Permissive Dehydration

Implementing permissive dehydration (DEH) during short-term heat acclimation (HA) may accelerate adaptations to the heat. However, HA with DEH may augment risk for acute kidney injury (AKI). This study investigated the effect of HA with permissive DEH on time-trial performance and markers of AKI. Fourteen moderately trained men (age and VO_2max = 25 ± 0.5 yr and 51.6 ± 1.8 mL·kg⁻¹·min⁻¹) were randomly assigned to DEH or euhydration (EUH). Time-trial performance and VO_2max were assessed in a temperate environment before and after 7 d of HA. Heat acclimation consisted of 90 min of cycling in an environmental chamber (40 °C, 35% RH). Neutrophil gelatinase-associated lipocalin (NGAL) and kidney injury molecule-1 (KIM-1) were assessed pre- and post-exercise on day 1 and day 7 of HA. Following HA, VO_2max did not change in either group (p = 0.099); however, time-trial performance significantly improved (3%, p < 0.01) with no difference between groups (p = 0.485). Compared to pre-exercise, NGAL was not significantly different following day 1 and 7 of HA (p = 0.113) with no difference between groups (p = 0.667). There was a significant increase in KIM-1 following day 1 and 7 of HA (p = 0.002) with no difference between groups (p = 0.307). Heat acclimation paired with permissive DEH does not amplify improvements in VO_2max or time-trial performance in a temperate environment versus EUH and does not increase markers of AKI.

The effect of medium-term heat acclimation on endurance performance in a temperate environment

We investigated whether an 11-day heat acclimation programme (HA) enhanced endurance performance in a temperate environment, and the mechanisms underpinning any ergogenic effect. Twenty-four males (V̇O_2max: 56.7 ± 7.5 mL·kg^-1·min^-1) completed either: (i) HA consisting of 11 consecutive daily exercise sessions (60-90 min·day^-1; n = 16) in a hot environment (40°C, 50% RH) or; (ii) duration and exertion matched exercise in cool conditions (CON; n = 8 [11°C, 60% RH]). Before and after each programme power at lactate threshold, mechanical efficiency, VO_2max, peak power output (PPO) and work done during a 30-minute cycle trial (T30) were determined under temperate conditions (22°C, 50% RH). HA reduced resting (-0.34 ± 0.30°C) and exercising (-0.43 ± 0.30°C) rectal temperature, and increased whole-body sweating (+0.37 ± 0.31 L·hr^-1) (all P≤0.001), with no change in CON. Plasma volume increased in HA (10.1 ± 7.2%, P < 0.001) and CON (7.2 ± 6.3%, P = 0.015) with no between-groups difference, whereas exercise heart rate reduced in both groups, but to a greater extent in HA (-20 ± 11 b·min^-1) than CON (-6 ± 4 b·min^-1). VO_2max, lactate threshold and mechanical efficiency were unaffected by HA. PPO increased in both groups (+14 ± 18W), but this was not related to alterations in any of the performance or thermal variables, and T30 performance was unchanged in either group (HA: Pre = 417 ± 90 vs. Post = 427 ± 83 kJ; CON: Pre = 418 ± 63 vs. Post = 423 ± 56 kJ). In conclusion, 11-days HA induces thermophysiological adaptations, but does not alter the key determinants of endurance performance. In trained males, the effect of HA on endurance performance in temperate conditions is no greater than that elicited by exertion and duration matched exercise training in cool conditions.

Heat Versus Altitude Training for Endurance Performance at Sea Level

Environmental stressors, such as heat or altitude, elicit dissimilar physiological adaptations to endurance training programs. Whether these differences (i.e., increased hemoglobin mass vs plasma volume) differentially influence performance is debated. We review data in support of our novel hypothesis, which proposes altitude as the preferred environmental training stimulus for elite endurance athletes preparing to compete in temperate, sea-level climates (5°C-18°C).

Intermittent post-exercise sauna bathing improves markers of exercise capacity in hot and temperate conditions in trained middle-distance runners

PURPOSE

This study investigated whether intermittent post-exercise sauna bathing across three-weeks endurance training improves exercise heat tolerance and exercise performance markers in temperate conditions, compared to endurance training alone. The subsidiary aim was to determine whether exercise-heat tolerance would further improve following 7-Weeks post-exercise sauna bathing.

METHODS

Twenty middle-distance runners (13 female; mean ± SD, age 20 ± 2 years, [Formula: see text]O_2max 56.1 ± 8.7 ml kg^-1 min^-1) performed a running heat tolerance test (30-min, 9 km h^-1/2% gradient, 40 °C/40%RH; HTT) and temperate (18 °C) exercise tests (maximal aerobic capacity [[Formula: see text]O_2max], speed at 4 mmol L^-1 blood lactate concentration ([La^-]) before (Pre) and following three-weeks (3-Weeks) normal training (CON; n = 8) or normal training with 28 ± 2 min post-exercise sauna bathing (101-108 °C, 5-10%RH) 3 ± 1 times per week (SAUNA; n = 12). Changes from Pre to 3-Weeks were compared between-groups using an analysis of co-variance. Six SAUNA participants continued the intervention for 7 weeks, completing an additional HTT (7-Weeks; data compared using a one-way repeated-measures analysis of variance).

RESULTS

During the HTT, SAUNA reduced peak rectal temperature (T_rec; - 0.2 °C), skin temperature (- 0.8 °C), and heart rate (- 11 beats min^-1) more than CON at 3-Weeks compared to Pre (all p < 0.05). SAUNA also improved [Formula: see text]O_2max (+ 0.27 L^-1 min^-1; p = 0.02) and speed at 4 mmol L^-1 [La^-] (+ 0.6 km h^-1; p = 0.01) more than CON at 3-Weeks compared to Pre. Only peak T_rec (- 0.1 °C; p = 0.03 decreased further from 3-Weeks to 7-Weeks in SAUNA (other physiological variables p > 0.05).

CONCLUSIONS

Three-weeks post-exercise sauna bathing is an effective and pragmatic method of heat acclimation, and an effective ergogenic aid. Extending the intervention to seven weeks only marginally improved T_rec.

Prolonged Heat Acclimation and Aerobic Performance in Endurance Trained Athletes

Heat acclimation (HA) involves physiological adaptations that directly promote exercise performance in hot environments. However, for endurance-athletes it is unclear if adaptations also improve aerobic capacity and performance in cool conditions, partly because previous randomized controlled trial (RCT) studies have been restricted to short intervention periods. Prolonged HA was therefore deployed in the present RCT study including 21 cyclists [38 ± 2 years, 184 ± 1 cm, 80.4 ± 1.7 kg, and maximal oxygen uptake (VO2max) of 58.1 ± 1.2 mL/min/kg; mean ± SE] allocated to either 5½ weeks of training in the heat [HEAT (n = 12)] or cool control [CON (n = 9)]. Training registration, familiarization to test procedures, determination of VO2max, blood volume and 15 km time trial (TT) performance were assessed in cool conditions (14°C) during a 2-week lead-in period, as well as immediately pre and post the intervention. Participants were instructed to maintain total training volume and complete habitual high intensity intervals in normal settings; but HEAT substituted part of cool training with 28 ± 2 sessions in the heat (1 h at 60% VO2max in 40°C; eliciting core temperatures above 39°C in all sessions), while CON completed all training in cool conditions. Acclimation for HEAT was verified by lower sweat sodium [Na+], reduced steady-state heart rate and improved submaximal exercise endurance in the heat. However, when tested in cool conditions both peak power output and VO2max remained unchanged for HEAT (pre 60.0 ± 1.5 vs. 59.8 ± 1.3 mL O2/min/kg). TT performance tested in 14°C was improved for HEAT and average power output increased from 298 ± 6 to 315 ± 6 W (P < 0.05), but a similar improvement was observed for CON (from 294 ± 11 to 311 ± 10 W). Based on the present findings, we conclude that training in the heat was not superior compared to normal (control) training for improving aerobic power or TT performance in cool conditions.

The time course of adaptations in thermoneutral maximal oxygen consumption following heat acclimation

PURPOSE

This study investigated the effects of a 10-day heat acclimation (HA) programme on the time course of changes in thermoneutral maximal oxygen uptake ([Formula: see text]O_2max) during and up to 10 days post-HA.

METHODS

Twenty-two male cyclists were assigned to a HA or control (Con) training group following baseline ramp tests of thermoneutral [Formula: see text]O_2max. Ten days of fixed-intensity (50% baseline [Formula: see text]O_2max) indoor cycling was performed in either ~ 38.0 °C (HA) or ~ 20 °C (Con). [Formula: see text]O_2max was re-tested on HA days 5, 10 and post-HA days 1, 2, 3, 4, 5 and 10.

RESULTS

[Formula: see text]O_2max initially declined across time in both groups during training (P < 0.05), before increasing in the post-HA period in both groups (P < 0.05). However, [Formula: see text]O_2max was higher than control by post-HA day 4 in the HA group (P = 0.046).

CONCLUSIONS

The non-linear time course of [Formula: see text]O_2max adaptation suggests that post-testing should be performed 96-h post-training to identify the maximal change for most individuals. In preparation for training or testing, athletes can augment their aerobic power in thermoneutral environments by performing 10 days HA, but the full effects will manifest at varying stages of the post-HA period.

Passive Heating: Reviewing Practical Heat Acclimation Strategies for Endurance Athletes

Heat acclimation protocols—both active and passive—have been employed by athletes in an effort to attenuate the detrimental effects of heat stress on physical capacities and sports performance. Active strategies have been extensively reviewed, but have various practical and economic limitations. The purpose of this review was therefore to provide an overview of the passive strategies that have received less attention, yet may be more practical or economically viable; recommendations for athletes are also provided. With a systematic search of the relevant databases ending in June 2018, 16 articles on passive heat acclimation that met the inclusion criteria were included in the review. The review highlighted that passive heat acclimation strategies can successfully induce heat adaptations, evident by reports of improved exercise performance, thermoregulatory, cardiovascular, and perceptual responses accompanying such interventions. Based on the review it is apparent that the use of sauna, hot-water immersion and environmental chambers may be used to provide heat stress under passive conditions, for the purpose of acclimation. To maximize the thermoregulatory-adaptive responses, exercise bouts should be employed prior to passive heat stress, rather than passive heating alone, with a minimal delay between exercise and the application of heat stress. Heating bouts should have a minimum duration of 30 min per session and be employed on consecutive days, when possible, with a minimum of 6–7 exposures to induce adaptation. This review identified that information regarding the magnitude of performance changes that can occur, as well as the perceptual responses to passive heating protocols is limited. Future research should investigate the use of passive heat exposures before and/or after repeated heat training sessions, to assess if a further boost to heat adaptation can be achieved with this strategy.

Once- and twice-daily heat acclimation confer similar heat adaptations, inflammatory responses and exercise tolerance improvements

This experiment aimed to investigate the efficacy of twice-daily, nonconsecutive heat acclimation (TDHA) in comparison to once-daily heat acclimation (ODHA) and work matched once- or twice-daily temperate exercise (ODTEMP, TDTEMP) for inducing heat adaptations, improved exercise tolerance, and cytokine (immune) responses. Forty males, matched biophysically and for aerobic capacity, were assigned to ODHA, TDHA, ODTEMP, or TDTEMP. Participants completed a cycling-graded exercise test, heat acclimation state test, and a time to task failure (TTTF) at 80% peak power output in temperate (TTTFTEMP : 22°C/40% RH) and hot conditions (TTTFHOT : 38°C/20% RH), before and after 10-sessions (60 min of cycling at ~2 W·kg-1 ) in 45°C/20% RH (ODHA and TDHA) or 22°C/40% RH (ODTEMP or TDTEMP). Plasma IL-6, TNF-α, and cortisol were measured pre- and postsessions 1, 5, and 10. ODHA and TDHA induced equivalent heat adaptations (P < 0.05) (resting rectal temperature [-0.28 ± 0.22, -0.28 ± 0.19°C], heart rate [-10 ± 3, -10 ± 4 b·min-1 ], and plasma volume expansion [+10.1 ± 5.6, +8.5 ± 3.1%]) and improved heat acclimation state (sweat set point [-0.22 ± 0.18, -0.22 ± 0.14°C] and gain [+0.14 ± 0.10, +0.15 ± 0.07 g·sec-1 ·°C-1 ]). TTTFHOT increased (P < 0.001) following ODHA (+25 ± 4%) and TDHA (+24 ± 10%), but not ODTEMP (+5 ± 14%) or TDTEMP (+5 ± 17%). TTTFTEMP did not improve (P > 0.05) following ODHA (+14 ± 4%), TDHA (14 ± 8%), ODTEMP (9 ± 10%) or TDTEMP (8 ± 13%). Acute (P < 0.05) but no chronic (P > 0.05) increases were observed in IL-6, TNF-α, or cortisol during ODHA and TDHA, or ODTEMP and TDTEMP. Once- and twice-daily heat acclimation conferred similar magnitudes of heat adaptation and exercise tolerance improvements, without differentially altering immune function, thus nonconsecutive TDHA provides an effective, logistically flexible method of HA, benefitting individuals preparing for exercise-heat stress.

Cross-Adaptation: Heat and Cold Adaptation to Improve Physiological and Cellular Responses to Hypoxia

To prepare for extremes of heat, cold or low partial pressures of oxygen (O₂), humans can undertake a period of acclimation or acclimatization to induce environment-specific adaptations, e.g. heat acclimation (HA), cold acclimation (CA), or altitude training. While these strategies are effective, they are not always feasible due to logistical impracticalities. Cross-adaptation is a term used to describe the phenomenon whereby alternative environmental interventions, e.g. HA or CA, may be a beneficial alternative to altitude interventions, providing physiological stress and inducing adaptations observable at altitude. HA can attenuate physiological strain at rest and during moderate-intensity exercise at altitude via adaptations allied to improved O₂ delivery to metabolically active tissue, likely following increases in plasma volume and reductions in body temperature. CA appears to improve physiological responses to altitude by attenuating the autonomic response to altitude. While no cross-acclimation-derived exercise performance/capacity data have been measured following CA, post-HA improvements in performance underpinned by aerobic metabolism, and therefore dependent on O₂ delivery at altitude, are likely. At a cellular level, heat shock protein responses to altitude are attenuated by prior HA, suggesting that an attenuation of the cellular stress response and therefore a reduced disruption to homeostasis at altitude has occurred. This process is known as cross-tolerance. The effects of CA on markers of cross-tolerance is an area requiring further investigation. Because much of the evidence relating to cross-adaptation to altitude has examined the benefits at moderate to high altitudes, future research examining responses at lower altitudes should be conducted, given that these environments are more frequently visited by athletes and workers. Mechanistic work to identify the specific physiological and cellular pathways responsible for cross-adaptation between heat and altitude, and between cold and altitude, is warranted, as is exploration of benefits across different populations and physical activity profiles.

Effects of acute or chronic heat exposure, exercise and dehydration on plasma cortisol, IL-6 and CRP levels in trained males

This study examined the acute and chronic effects of euhydrated and hypohydrated heat exposure, on biomarkers of stress and inflammation. Eight trained males [mean (SD) age: 21 (3) y; mass: 77.30 (4.88) kg; V̇O_2max: 56.9 (7.2) mL kg^-1 min^-1] undertook two heat acclimation programmes (balanced cross-over design), once drinking to maintain euhydration and once with restricted fluid-intake (permissive dehydration). Days 1, 6, and 11 were 60 min euhydrated exercise-heat stress tests (40 °C; 50% RH, 35% peak power output), days 2-5 and 7-10 were 90 min, isothermal-strain (target rectal temperature: 38.5 °C) exercise-heat sessions. Plasma was obtained pre- and post- exercise on day 1, 2, and 11 and analysed for cortisol, interleukin-6 (IL-6), and C-reactive protein (CRP). Cortisol and CRP were also assessed on day 6. IL-6 was elevated following the initial (acute) 90 min isothermal heat strain exercise-heat exposure (day 2) with permissive dehydration ((pre exercise: 1.0 pg mL^-1 [0.9], post-exercise: 1.8 pg mL^-1 [1.0], P = .032) and when euhydrated (pre-exercise: 1.0 pg mL^-1 [1.4], post-exercise: 1.6 pg mL^-1 [2.1], P = .048). Plasma cortisol levels were also elevated but only during permissive dehydration (P = .032). Body mass loss was strongly correlated with Δcortisol (r = -0.688, P = .003). Although there was a trend for post-exercise cortisol to be decreased following both heat acclimation programmes (chronic effects), there were no within or between intervention differences in IL-6 or CRP. In conclusion, acute exercise in the heat increased IL-6 and cortisol only when fluid-intake is restricted. There were no chronic effects of either intervention on biomarkers of inflammation as evidenced by IL-6 and CRP returning to basal level at the end of heat acclimation.

Maximizing Cellular Adaptation to Endurance Exercise in Skeletal Muscle

The application of molecular techniques to exercise biology has provided novel insight into the complexity and breadth of intracellular signaling networks involved in response to endurance-based exercise. Here we discuss several strategies that have high uptake by athletes and, on mechanistic grounds, have the potential to promote cellular adaptation to endurance training in skeletal muscle. Such approaches are based on the underlying premise that imposing a greater metabolic load and provoking extreme perturbations in cellular homeostasis will augment acute exercise responses that, when repeated over months and years, will amplify training adaptation.

Temperate Performance Benefits after Heat, but Not Combined Heat and Hypoxic Training

PURPOSE

Independent heat and hypoxic exposure can enhance temperate endurance performance in trained athletes, although their combined effects remain unknown. This study examined whether the addition of heat interval training during "live high, train low" (LHTL) hypoxic exposure would result in enhanced performance and physiological adaptations as compared with heat or temperate training.

METHODS

Twenty-six well-trained runners completed 3 wk of interval training assigned to one of three conditions: 1) LHTL hypoxic exposure plus heat training (H + H; 3000 m for 13 h·d, train at 33°C, 60% relative humidity [RH]), 2) heat training with no hypoxic exposure (HOT, live at <600 m and train at 33°C, 60% RH), or 3) temperate training with no hypoxic exposure (CONT; live at <600 m and train at 14°C, 55% RH). Performance 3-km time-trials (3-km TT), running economy, hemoglobin mass, and plasma volume were assessed using magnitude-based inferences statistical approach before (Baseline), after (Post), and 3 wk (3wkP) after exposure.

RESULTS

Compared with Baseline, 3-km TT performance was likely increased in HOT at 3wkP (-3.3% ± 1.3%; mean ± 90% confidence interval), with no performance improvement in either H + H or CONT. Hemoglobin mass increased by 3.8% ± 1.8% at Post in H + H only. Plasma volume in HOT was possibly elevated above H + H and CONT at Post but not at 3wkP. Correlations between changes in 3-km TT performance and physiological adaptations were unclear.

CONCLUSION

Incorporating heat-based training into a 3-wk training block can improve temperate performance at 3 wk after exposure, with athlete psychology, physiology, and environmental dose all important considerations. Despite hematological adaptations, the addition of LHTL to heat interval training has no greater 3-km TT performance benefit than temperate training alone.

Effects of 10 days of separate heat and hypoxic exposure on heat acclimation and temperate exercise performance

Adaptations to heat and hypoxia are typically studied in isolation but are often encountered in combination. Whether the adaptive response to multiple stressors affords the same response as when examined in isolation is unclear. We examined 1) the influence of overnight moderate normobaric hypoxia on the time course and magnitude of adaptation to daily heat exposure and 2) whether heat acclimation (HA) was ergogenic and whether this was influenced by an additional hypoxic stimulus. Eight males [V̇o2max = 58.5 (8.3) ml·kg−1·min−1] undertook two 11-day HA programs (balanced-crossover design), once with overnight normobaric hypoxia (HAHyp): 8 (1) h per night for 10 nights [[Formula: see text] = 0.156; SpO2 = 91 (2)%] and once without (HACon). Days 1, 6, and 11 were exercise-heat stress tests [HST (40°C, 50% relative humidity, RH)]; days 2–5 and 7–10 were isothermal strain [target rectal temperature (Tre) ~38.5°C], exercise-heat sessions. A graded exercise test and 30-min cycle trial were undertaken pre-, post-, and 14 days after HA in temperate normoxia (22°C, 55% RH; FIO2 = 0.209). HA was evident on day 6 (e.g., reduced Tre, mean skin temperature (T̄sk), heart rate, and sweat [Na+], P < 0.05) with additional adaptations on day 11 (further reduced T̄sk and heart rate). HA increased plasma volume [+5.9 (7.3)%] and erythropoietin concentration [+1.8 (2.4) mIU/ml]; total hemoglobin mass was unchanged. Peak power output [+12 (20) W], lactate threshold [+15 (18) W] and work done [+12 (20) kJ] increased following HA. The additional hypoxic stressor did not affect these adaptations. In conclusion, a separate moderate overnight normobaric hypoxic stimulus does not affect the time course or magnitude of HA. Performance may be improved in temperate normoxia following HA, but this is unaffected by an additional hypoxic stressor.

Sports and environmental temperature: From warming-up to heating-up

Most professional and recreational athletes perform pre-conditioning exercises, often collectively termed a 'warm-up' to prepare for a competitive task. The main objective of warming-up is to induce both temperature and non-temperature related responses to optimize performance. These responses include increasing muscle temperature, initiating metabolic and circulatory adjustments, and preparing psychologically for the upcoming task. However, warming-up in hot and/or humid ambient conditions increases thermal and circulatory strain. As a result, this may precipitate neuromuscular and cardiovascular impairments limiting endurance capacity. Preparations for competing in the heat should include an acclimatization regimen. Athletes should also consider cooling interventions to curtail heat gain during the warm-up and minimize dehydration. Indeed, although it forms an important part of the pre-competition preparation in all environmental conditions, the rise in whole-body temperature should be limited in hot environments. This review provides recommendations on how to build an effective warm-up following a 3 stage RAMP model (Raise, Activate and Mobilize, Potentiate), including general and context specific exercises, along with dynamic flexibility work. In addition, this review provides suggestion to manipulate the warm-up to suit the demands of competition in hot environments, along with other strategies to avoid heating-up.

Passive heat acclimation improves skeletal muscle contractility in humans

The aim of this study was to investigate the effect of repeated passive heat exposure (i.e., acclimation) on muscle contractility in humans. Fourteen nonheat-acclimated males completed two trials including electrically evoked twitches and voluntary contractions in thermoneutral conditions [Cool: 24°C, 40% relative humidity (RH)] and hot ambient conditions in the hyperthermic state (Hot: 44–50°C, 50% RH) on consecutive days in a counterbalanced order. Rectal temperature was ~36.5°C in Cool and was maintained at ~39°C throughout Hot. Both trials were repeated after 11 days of passive heat acclimation (1 h per day, 48–50°C, 50% RH). Heat acclimation decreased core temperature in Cool (−0.2°C, P < 0.05), increased the time required to reach 39°C in Hot (+9 min, P < 0.05) and increased sweat rate in Hot (+0.7 liter/h, P < 0.05). Moreover, passive heat acclimation improved skeletal muscle contractility as evidenced by an increase in evoked peak twitch amplitude both in Cool (20.5 ± 3.6 vs. 22.0 ± 4.0 N·m) and Hot (20.5 ± 4.7 vs. 22.0 ± 4.0 N·m) (+9%, P < 0.05). Maximal voluntary torque production was also increased both in Cool (145 ± 42 vs. 161 ± 36 N·m) and Hot (125 ± 36 vs. 145 ± 30 N·m) (+17%, P < 0.05), despite voluntary activation remaining unchanged. Furthermore, the slope of the relative torque/electromyographic linear relationship was improved postacclimation ( P < 0.05). These adjustments demonstrate that passive heat acclimation improves skeletal muscle contractile function during electrically evoked and voluntary muscle contractions of different intensities both in Cool and Hot. These results suggest that repeated heat exposure may have important implications to passively maintain or even improve muscle function in a variety of performance and clinical settings.

Effect of Permissive Dehydration on Induction and Decay of Heat Acclimation, and Temperate Exercise Performance

Purpose: It has been suggested that dehydration is an independent stimulus for heat acclimation (HA), possibly through influencing fluid-regulation mechanisms and increasing plasma volume (PV) expansion. There is also some evidence that HA may be ergogenic in temperate conditions and that this may be linked to PV expansion. We investigated: (i) the influence of dehydration on the time-course of acquisition and decay of HA; (ii) whether dehydration augmented any ergogenic benefits in temperate conditions, particularly those related to PV expansion.

Methods: Eight males [VO_2max: 56.9(7.2) mL·kg⁻¹·min⁻¹] undertook two HA programmes (balanced cross-over design), once drinking to maintain euhydration (HA_Eu) and once with restricted fluid-intake (HA_De). Days 1, 6, 11, and 18 were 60 min exercise-heat stress tests [HST (40°C; 50% RH)], days 2–5 and 7–10 were 90 min, isothermal-strain (T_re ~ 38.5°C), exercise-heat sessions. Performance parameters [VO_2max, lactate threshold, efficiency, peak power output (PPO)] were determined pre and post HA by graded exercise test (22°C; 55%RH).

Results: During isothermal-strain sessions hypohydration was achieved in HA_De and euhydration maintained in HA_Eu [average body mass loss −2.71(0.82)% vs. −0.56(0.73)%, P < 0.001], but aldosterone concentration, power output, and cardiovascular strain were unaffected by dehydration. HA was evident on day 6 {reduced end-exercise T_re [−0.30(0.27)°C] and exercise heart rate [−12(15) beats.min⁻¹], increased PV [+7.2(6.4)%] and sweat-loss [+0.25(0.22) L.h⁻¹], P < 0.05} with some further adaptations on day 11 {further reduced end-exercise T_re [−0.25(0.19)°C] and exercise heart rate [−3(9) beats.min⁻¹], P < 0.05}. These adaptations were not notably affected by dehydration and were generally maintained 7-days post HA. Performance parameters were unchanged, apart from increased PPO (+16(20) W, irrespective of condition).

Conclusions: When thermal-strain is matched, permissive dehydration which induces a mild, transient, hypohydration does not affect the acquisition and decay of HA, or endurance performance parameters. Irrespective of hydration, trained individuals require >5 days to optimize HA.

Prolonged self-paced exercise in the heat - environmental factors affecting performance

In this review we examine how self-paced performance is affected by environmental heat stress factors during cycling time trial performance as well as considering the effects of exercise mode and heat acclimatization. Mean power output during prolonged cycling time trials in the heat (≥30°C) was on average reduced by 15% in the 14 studies that fulfilled the inclusion criteria. Ambient temperature per se was a poor predictor of the integrated environmental heat stress and 2 of the prevailing heat stress indices (WBGT and UTCI) failed to predict the environmental influence on performance. The weighing of wind speed appears to be too low for predicting the effect for cycling in trained acclimatized subjects, where performance may be maintained in outdoor time trials at ambient temperatures as high as 36°C (36°C UTCI; 28°C WBGT). Power output during indoor trials may also be maintained with temperatures up to at least 27°C when humidity is modest and wind speed matches the movement speed generated during outdoor cycling, whereas marked reductions are observed when air movement is minimal. For running, representing an exercise mode with lower movement speed and higher heat production for a given metabolic rate, it appears that endurance is affected even at much lower ambient temperatures. On this basis we conclude that environmental heat stress impacts self-paced endurance performance. However, the effect is markedly modified by acclimatization status and exercise mode, as the wind generated by the exercise (movement speed) or the environment (natural or fan air movement) exerts a strong influence.

Heat stress and dehydration in adapting for performance: Good, bad, both, or neither?

Physiological systems respond acutely to stress to minimize homeostatic disturbance, and typically adapt to chronic stress to enhance tolerance to that or a related stressor. It is legitimate to ask whether dehydration is a valuable stressor in stimulating adaptation per se. While hypoxia has had long-standing interest by athletes and researchers as an ergogenic aid, heat and nutritional stressors have had little interest until the past decade. Heat and dehydration are highly interlinked in their causation and the physiological strain they induce, so their individual roles in adaptation are difficult to delineate. The effectiveness of heat acclimation as an ergogenic aid remains unclear for team sport and endurance athletes despite several recent studies on this topic. Very few studies have examined the potential ergogenic (or ergolytic) adaptations to ecologically-valid dehydration as a stressor in its own right, despite longstanding evidence of relevant fluid-regulatory adaptations from short-term hypohydration. Transient and self-limiting dehydration (e.g., as constrained by thirst), as with most forms of stress, might have a time and a place in physiological or behavioral adaptations independently or by exacerbating other stressors (esp. heat); it cannot be dismissed without the appropriate evidence. The present review did not identify such evidence. Future research should identify how the magnitude and timing of dehydration might augment or interfere with the adaptive processes in behaviorally constrained versus unconstrained humans.