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

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Olivier Girard School of Human Sciences (Exercise and Sport Science), University of Western Australia, Perth, WA, Australia

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Peter Peeling School of Human Sciences (Exercise and Sport Science), University of Western Australia, Perth, WA, Australia
Western Australian Institute of Sport, Mt Claremont, WA, Australia

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Sébastien Racinais Environmental Stress Unit, CREPS Montpellier–Font Romeu, Montpellier, France

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Julien D. Périard Research Institute for Sport and Exercise, University of Canberra, Canberra, ACT, Australia

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https://orcid.org/0000-0002-6266-4246
DOI:
https://doi.org/10.1123/ijspp.2023-0250
Keywords:
thermoregulation; heat acclimation; hypoxia; combined stressors
First Published Online:
18 Jan 2024
In Print:
Volume 19: Issue 3
Page Range:
322–327
Free access

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.

Varying the environmental conditions in which athletes train and/or reside is a popular strategy to induce specific physiological adaptations and enhance performance.1 Individual athletes and teams regularly incorporate heat2 or altitude3 into their training program, either during an early-season training camp or in leading up to a major competition.4 Thermal and hypoxic stimuli have primarily been studied for their independent effects to induce the physiological adaptations commensurate with enhanced performance in each respective environment.1

Heat exposure can be incorporated into training programs to induce heat acclimation or acclimatization through artificial (eg, inside a climatic chamber) or natural (eg, outdoor) means respectively, or a combination of both.2 A variety of active heat training protocols exit, ranging from exercising at a constant or self-selected work rate, to controlling core temperature and heart rate.5 Training in the heat leads to plasma volume (PV) expansion within 5 to 7 days of heat exposure, which is associated with attenuated thermal, cardiovascular, and perceptual strain.5 There is consensus regarding the usefulness of heat training when preparing for competing in warm/hot climates2; yet, whether it enhances6 or fails to improve7 endurance performance in temperate conditions remains contentious.

Residing at terrestrial (ie, mountains) or simulated altitude (ie, altitude hotel) for several weeks, through “living high, training high” or “living high, training low” protocols, can enhance the production of red blood cell and increase total hemoglobin mass,8 leading to improved aerobic fitness.9 Alternatively, repeated-sprint training under hypoxic conditions during separate sessions can elicit targeted muscular adaptations (eg, increased citrate synthase activity, mitochondrial density, muscle oxidative capacity) that are not activated to the same extent by training in normoxia, or passive hypoxic exposure.10 The panorama of altitude training methods continues to expand, particularly the “living low, training high” approaches,11 along with the combination of different approaches. For instance, repeated sprint training in hypoxia combined with sleeping at altitude has been explored.12 However, contrasting views exist in the literature regarding its effects on sea-level performance.13,14

Incorporating both heat and altitude training in preparation for major events is becoming increasingly common. This is notable for professional cyclists who face climbs exceeding 2000 m in elevation and air temperatures approaching 40 °C in prestigious stage races (eg, Tour de France, Vuelta a España).15 The physiological responses associated with mixed environmental stressors have been studied for over 50 years,16 but their combination is still more an art than a science. Indeed, some studies have reported negative interactions (ie, antagonistic effects) between heat and altitude adaptations,17 whereas others believe that adaptation to one environmental stressor may augment tolerance to another (ie, cross tolerance) through the activation of common protective pathways (eg, heat shock protein and hypoxia-inducible factors-1α).18 Our intention here is not to review the growing body of knowledge describing the chronic effects of cross-tolerance (for review, see Gibson et al19), but rather to focus on applied intervention-style studies examining the effects of combined thermal and hypoxic stimuli on physiological adaptations and the potential consequent improvement in temperate, sea-level performance.

Contemporary Approaches of Combining Heat and Altitude Training

To date, training studies that have examined the impact of combined heat and altitude for improving temperate, sea-level performance have largely relied on 2 main approaches: simultaneous and concurrent (Figure 1). The simultaneous approach mixes the 2 stressors during a given training session,20–23 whereas the concurrent approach consists of training in the heat and sleeping at altitude, sometimes with additional training in hypoxia.24–29

Figure 1
Figure 1

—Different combined heat and altitude training approaches for temperate, sea-level performance improvement. Note: While the potential benefits of conducting heat and altitude training either simultaneously (ie, in close proximity in a session) or concurrently (ie, within the same training block but not at the same time) have been explored (continuous lines), the sequential approach (ie, one after the other, to maximize adaptative responses over a sequential week or training block) remains to be investigated (dashed lines).

Citation: International Journal of Sports Physiology and Performance 19, 3; 10.1123/ijspp.2023-0250

Combined Heat and Hypoxia in the Same Training Session: Simultaneous Approach

Several studies have examined the chronic adaptive response to combined heat and hypoxia during the same training session. Takeno et al23 were the first to investigate the physiological adaptations to repeated simultaneous heat (30 °C, 50% relative humidity: RH) and hypoxia (2000 m). Following 10 days of cycling for 60 minutes at 60% of maximal oxygen uptake (V˙O2max), both blood volume (4%–5%) and V˙O2max (7%–11%) increased to a similar extent with combined stressors compared with when each stressor was applied individually. In a subsequent study, 3 weeks of moderate- to high-intensity interval training in combined heat (32 °C, 50% RH) and simulated altitude (∼2000 m) was compared to training in temperate (22 °C, 50% RH) or hot (32 °C, 50% RH) conditions.21 The results showed that simultaneous heat and hypoxia training led to improvements in 20-km cycling time-trial performance (∼19–28 s) 3 weeks later, similar to improvements observed with temperate and hot training alone. Recently, the addition of heat stress (38 °C) to 2 weeks of repeated-sprint training in hypoxia at ∼3000 m simulated altitude did not enhance the improvement in 500-m time-trial performance and may instead have partially masked muscle tissue adaptations in canoe/kayak sprinters.22 As such, current evidence suggests that when environmental stressors are combined within the same training session, improvements in aerobic fitness,23 and time-trial performance20–22 in temperate, sea-level conditions are comparable with those of heat or altitude training alone.

Of note, the physiological strain induced by heat and/or hypoxia was not evaluated during training in the aforementioned studies, as neither body temperature nor arterial oxygen saturation were assessed. Accordingly, it is difficult to quantify the consistency and/or intensity of the training interventions over time. Additionally, the hypobaric23 and normobaric21 hypoxic stimulus applied was rather modest, with simulated altitudes of 1850 and 2100 m, respectively. Another observation is that the study by McCleave et al21 failed to demonstrate clear heat (eg, sweat rate) and altitude (eg, hemoglobin concentration) adaptations for key variables. Therefore, with the limited current evidence, it is possible that other factors, such as training per se, may have contributed primarily to increasing exercise performance following the interventions, independent of heat and/or altitude exposure.21 A major challenge in implementing combined-stressor approaches lies in the uncertainty surrounding the prescription of dosing regimens (ie, exercise and environmental stress), which can influence the individual variability in the magnitude and timing of adaptations.

Separate Heat Training Sessions and Nocturnal Altitude Exposure—Concurrent Approach

A period of heat training with additional altitude residence has perhaps been the most widely researched combined-environments intervention.24–29 The rationale for implementing this approach appears to be to increase performance gains by inducing the hematological, cardiovascular, and skeletal muscle adaptations associated with each environmental stressor.24

In one pioneer study, professional team-sport athletes completed heat-based, sport-specific training during a 2-week off-season warm weather camp, with one group adding a hypoxic stimulus during sleep and some training sessions.24 This intervention led to increases in PV and decreases in sweat sodium concentration and exercising heart rate in both groups. Additionally, total hemoglobin mass “likely” increased only in the combined heat and hypoxia group, with total hemoglobin mass and PV remaining elevated 4 weeks later. High-intensity running performance was improved immediately postintervention, with maintenance 4 weeks later mainly seen in the combined stressors group.24 However, this study was limited by the inability to assess the independent effects of heat exposure, making it difficult to determine the true cause of performance gains in the absence of a control group. To overcome this limitation, a recent study, which also recruited team-sport athletes (>50 elite rugby players), tested the independent and combined effects of heat and altitude exposure on physical performance during a 2-week training camp.27 The athletes underwent 5 endurance and 5 repeated-sprint training sessions per week on alternate days, with sleeping in hypoxia for altitude groups. Adding heat and/or altitude during selective training sessions and sleeping at altitude did not enhance the gains observed in markers of aerobic fitness (eg, 4% increase in V˙O2max) and performance (eg, 12% and 16% increase in repeated sprint ability and high-intensity intermittent running, respectively) compared with control. Although PV increased following the camp, sleeping at altitude impeded the increase when training in the heat, and only marginally increased hemoglobin mass (unpublished observations). Using a cross-over study design, Rendell et al28 recruited trained cyclists who underwent controlled hyperthermia heat training while sleeping in either normoxia or moderate hypoxia (∼2400 m) for 11 days. The addition of simulated altitude (∼80 h) did not influence the magnitude of increase in thermophysiological and hematological adaptations. Following heat training, work done during a 30-minute cycling time-trial, peak power output and lactate threshold were higher, with no further benefits in those exposed to hypoxia. Unlike Buchheit et al,24 the addition of hypoxia did not allow for the maintenance of adaptations 14-day postheat training. Collectively, altitude residence when training in the heat may not enhance or negatively influence heat-induced adaptations and performance.

Other studies have suggested that adding normobaric hypoxic residence to a heat training protocol can blunt the expected thermoregulatory adaptations.25,26,29 For instance, participants recruited by Sotiridis et al29 underwent 10 days of continuous (22 h/d) normobaric hypoxic exposure, interspersed with daily 90-minute controlled hyperthermia heat training in normoxia. While the hypoxic acclimation resulted in increased hemoglobin concentration, hematocrit and extracellular HSP72, only minor thermoregulatory benefits, such as sweating efficiency, commonly observed in heat training studies,5 were reported. This intervention also failed to improve aerobic capacity (ie, ventilatory threshold and V˙O2max) in temperate, sea-level conditions. Recently, training in the heat for 12 consecutive days improved “race-simulated” time-trial performance by ∼5.5% following steady-state exercise in a hot-humid climate; however, overnight hypoxic exposure (14 h/d at 2500 m) did not further augment performance.25 Finally, McCleave et al26 recruited well-trained distance runners to complete 21 days of training with 3 weekly interval sessions either in the heat while sleeping in normoxia (33 °C, 60 RH; <600 m), in the heat while sleeping in hypoxia (33 °C; 14 h/d at 3000 m), or in cool conditions while sleeping in normoxia (14 °C, 55% RH; <600 m). The authors reported a ∼3.8% increase in total hemoglobin mass (but no change in PV) immediately after 3 weeks of heat training and overnight hypoxic exposure, which remained elevated 3 weeks after exposure. However, there were no hematological changes in either the heat or temperate training groups. Arguably, in the presence of hypoxia, applying an even greater heat dose may be necessary to overcome any negative influences of chronic hypoxia (ie, hemoconcentration secondary to decreased PV) on heat acclimation.26

The general consensus using a concurrent approach is that when athletes acclimate to the heat and train and/or sleep at low-to-moderate altitudes during training blocks <3 weeks, improvements in temperate, sea-level performance are similar to those reported after heat training alone.

Exploring Other Possibilities for Combined Environments: Sequential Approach

Individually, exposure to heat and hypoxia (ie, respiratory drive and mitochondrial biogenesis) can enhance temperate, sea-level performance through nonhematological adaptations specific to each stressor.1 However, debate exists regarding the effectiveness of training with an environmental stressor (eg, heat training) for improving30 or not improving31 performance in control (ie, temperate) conditions. While altitude acclimatization is known to be effective for increasing hemoglobin mass,8 recent evidence suggests that heat acclimation, given sufficient time for adaptation (ie, 5 wk), can also promote the red blood cell formation.32 Prolonged (∼5 wk) heat acclimation protocols, such as training in hot climatic chambers for at least 5 times per week for about 1 hour per day,32,33 or with increased clothing insulation,34 have shown systematic increases in hemoglobin mass, even in well-trained athletes. These findings highlight the value of heat acclimation in enhancing hemoglobin mass as an alternative of altitude training.15 Although gaining both the hypoxia-related stimulation of erythropoiesis and the PV expansion with associated hemoglobin mass benefit from heat training is attractive, these 2 stimuli if superimposed may fail to induce desired physiological adaptations. Sequential implementation of heat and altitude training as a potential solution has not been thoroughly investigated.

One viable sequential approach could involve training in the heat for a few weeks, followed by an altitude training block (or vice versa). However, the adaptative response to sequential implementation of thermal and hypoxic stimuli over long periods of time not well understood. Only one case study of a world-class male racewalker observed over a 20-month period, where 7 blocks of altitude training and 2 blocks of heat training (often separated by several weeks) were conducted, has been documented.35 While the participant won Olympic bronze and silver medals in the 20 and 50 km, respectively, this approach was primarily used to prepare major international competitions in the heat, which is not directly aligned with the scope of this review.

When employing a sequential approach, it is important to consider the time course of decay in hemoglobin mass that occurs after altitude acclimation. Hematological adaptations, particularly after a period of overnight hypoxic exposure, are only short lived due to neocytolisis upon return to sea level.1 Therefore, using heat training to mitigate the rapid and often difficult to prevent decay of hemoglobin mass experienced by athletes upon returning from altitude, may be more advantageous than reexposure to altitude if applied for a sufficient duration (few weeks).36 Research suggests that the return of hemoglobin mass to baseline can be delayed for a significant period through 3 weekly maintenance heat training sessions following a 5-week heat training intervention.37 In this scenario, heat training could be implemented after athletes have already adapted to altitude, requiring a larger than usual portion of the less intense training to be replaced or allocated for inducing heat stress. Indeed, performing “hard” sessions in the heat may have potential drawbacks in terms of excessive heat production and physiological strain, which could compromise training quality. Further investigation is needed to determine to which extent this approach preserves the increase in hemoglobin mass while PV expands from training in the heat.

When combining independent stimuli, if physiological adaptations are induced by one stressor (eg, altitude), the addition of another stressor (eg, heat) may have little and even detrimental impact. Understanding the pathways via which adaptations develop following the use of each environmental stressor may help practitioners identify synergistic interactions that benefit athletes in enhancing temperate, sea-level performance. For example, “living low, training high” altitude training, especially when incorporating maximal sprinting exercise, likely results in nonhematological adaptations and activates hypoxia-inducible factors-1α pathways, leading to increased glycolytic activity and/or mitochondrial biogenesis and metabolism.38 Alternatively, it has been speculated that adaptation to heat could potentially improve oxygen supply to the body and/or working muscles through the upregulation of the hypoxia-inducible factors-1α pathway.39 In situations where heat and altitude activate common protective pathways,40 well-controlled studies are required to clarify how these 2 environmental stimuli interact, especially during the decay in heat/hypoxic adaptations when exposed to a different environmental stressor.

Practical Applications and Conclusion

We have attempted to summarize the effects of training with combined heat and altitude stimuli on temperate, sea-level performance. Despite evidence supporting environment-specific physiological adaptations, a synthesis of the current literature suggests that the addition of simultaneous heat and hypoxia does not enhance performance more than training with either thermal or hypoxic stimuli or temperate training alone. It is noteworthy that these conclusions are drawn from a restricted number of intervention studies. Future studies are required to determine the potential benefits of conducting heat and altitude exposure either simultaneously (ie, in close proximity in a session), concurrently (ie, within the same training block but not at the same time), or sequentially (ie, one after the other, to maximize adaptative responses over a sequential week or training block), likely requiring careful periodization.4

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    Figure 1

    —Different combined heat and altitude training approaches for temperate, sea-level performance improvement. Note: While the potential benefits of conducting heat and altitude training either simultaneously (ie, in close proximity in a session) or concurrently (ie, within the same training block but not at the same time) have been explored (continuous lines), the sequential approach (ie, one after the other, to maximize adaptative responses over a sequential week or training block) remains to be investigated (dashed lines).

Figure 1

—Different combined heat and altitude training approaches for temperate, sea-level performance improvement. Note: While the potential benefits of conducting heat and altitude training either simultaneously (ie, in close proximity in a session) or concurrently (ie, within the same training block but not at the same time) have been explored (continuous lines), the sequential approach (ie, one after the other, to maximize adaptative responses over a sequential week or training block) remains to be investigated (dashed lines).
  • 1.

    Baranauskas MN, Constantini K, Paris HL, Wiggins CC, Schlader ZJ, Chapman RF. Heat versus altitude training for endurance performance at sea-level. Exerc Sport Sci Rev. 2021;49(1):50–58. doi:

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2.

    Racinais S, Alonso JM, Coutts AJ, et al. Consensus recommendations on training and competing in the heat. Scand J Med Sci Sports. 2015;25(1):6–19. doi:

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3.

    Girard O, Amann M, Aughey R, et al. Position statement—altitude training for improving team-sport players’ performance: current knowledge and unresolved issues. Br J Sports Med. 2013;47(suppl 1):i8–i16. doi:

    • Crossref
    • Search Google Scholar
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  • 4.

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