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Heat Versus Altitude Training for Endurance Performance at Sea Level

Baranauskas, Marissa N.1; Constantini, Keren2; Paris, Hunter L.3; Wiggins, Chad C.4; Schlader, Zachary J.1; Chapman, Robert F.1

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Exercise and Sport Sciences Reviews: January 2021 - Volume 49 - Issue 1 - p 50-58
doi: 10.1249/JES.0000000000000238
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Key Points

  • Various practices of altitude training have been used by athletes as a means to improve sea-level endurance performance.
  • With recent world stage competitions held in hot ambient climates, debate has developed over whether heat training provides equivalent benefits for endurance performance.
  • Altitude acclimatization primarily enhances oxygen carrying capacity, which has the potential to improve performance among endurance-trained athletes.
  • Rather, the primary adaptation elicited by heat training is plasma volume expansion, which has been associated with improvements in maximal aerobic capacity — one determinant of endurance exercise performance — in untrained, but not trained, individuals.
  • Of note, effective heat and altitude training practices both require careful planning from coaches and athletes and may present logistical and financial challenges that should be considered against the potential benefits for performance.

INTRODUCTION

The benefits of physical training on athletic performance have been acknowledged for thousands of years. Over time, advances in sport and exercise science have refined athletic training practices, and the drive for performance optimization has produced no shortage of opinions on topics ranging from proper warm-up protocol, to the intensity, frequency, and specificity of training, as well as recovery techniques (e.g., “Should I drink chocolate milk before or after wearing my compression garments in the cryotherapy chamber?”). With modern-day ease of travel and the availability of remote coaching options, one of the more salient questions facing endurance athletes today pertains to training environment before elite-level competition.

Since the 1968 Mexico City Olympic Games, held at more than 2000 m above sea level, endurance athletes have regularly practiced altitude training. Long-term exposure to altitude elicits physiological adaptations that may provide performance benefits for endurance competitions held at sea level (1–4). Recently, due in part to athletes preparing to compete in World Athletics Championships (Doha 2019) and Olympic Games (Rio 2016 and Tokyo 2020/21) scheduled for hot ambient climates, heat training has emerged as an attractive alternative to altitude training. Some physiologists have suggested that heat training can improve performance in temperate conditions with less financial burden and fewer maladaptive side effects than altitude training (5). Within sporting communities, heat training has even offhandedly been referred to as “the poor man’s altitude training” — suggesting that if an athlete lacks the financial means necessary to afford relocation to and accommodation at altitude, training in perhaps more easily accessible hot environments will accomplish similar performance end points. However, as described in this review, effective heat training practices require similar careful planning to circumvent the challenges associated with relocation to hot geographical locations, access to facilities or equipment capable of mimicking hot environments, and an additional time burden from athletes (6).

Although emergent research has investigated the additive ergogenic potential of altitude and heat stress on endurance performance (7), data directly comparing the independent effects of long-term training in these two environments on endurance performance are lacking, especially in well-trained athletes. Existing evidence suggests that the physiological adaptations acquired with heat versus altitude training are not equivalent (8,9). Physiological incongruencies, attributed to the specific training environment, may translate to disparate performance outcomes for endurance athletes. We present a review in support of the novel hypothesis that altitude is the preferred environmental training stimulus for endurance athletes who are preparing to compete in temperate, sea-level conditions (5°C–18°C). Figure 1 provides an illustration of our working hypothesis and a general overview of the mechanisms contributing to improved endurance performance after heat versus altitude training.

Figure 1
Figure 1:
Schematic of the novel hypothesis relating the primary physiological adaptations acquired with heat (left) versus altitude (right) training and their proposed effect on endurance performance. Solid lines indicate well-established relation; dashed lines, limited evidence supporting the association.

ALTITUDE TRAINING DEFINED

Athletes live and train at natural or artificial (e.g., hypobaric chamber, normobaric hypoxic tents) altitude with the intent of achieving physiological adaptations associated with long-term altitude residence. As such, minimum standards for how high to live and train, as well as how long to reside at altitude, have been established. Current best practices (Table) recommend that athletes spend equal to or more than 12 h·d−1 for 3 wk or more at an altitude of 2000–2500 m to achieve positive physiological adaptations that will ultimately translate to improvements in sea-level performance (1,2). Considering the dose-response association between the level and duration of exposure to hypoxia and hematopoiesis (17), it would be reasonable for athletes and coaches to assume that residence and training at higher elevations are more beneficial for athletic performance. However, maladaptive responses to higher elevations, such as worsened sleep quality, increased susceptibility to illness, and an excessive work of breathing during exercise, may impair recovery from training sessions and subsequent performance outcomes (2,10). As such, many endurance athletes who practice altitude training reside at an elevation of 2000–2500 m, which is high enough to elicit substantial hematopoietic adaptations while minimizing any negative effects of more severe hypoxic exposure (2,10). To further avoid detraining effects due to a reduced oxygen flux and lower relative training intensities that could occur with exclusively training at altitude (i.e., live high-train high [LHTH]), it is recommended that athletes perform high-intensity training sessions at elevations less than 1250 m or as close to sea level as possible (i.e., live high-train low [LHTL]) to maintain a sufficient training stimulus and evoke skeletal muscle adaptations (11,12).

TABLE - Optimal recommendations for altitude and heat training camps
Environmental/Training Conditions Natural vs Artificial Minimum Training Camp Duration
Altitude training Reside and perform usual training at an elevation of 2000–2500 m (≥12 h·d−1).
Perform high-intensity sessions at an elevation <1250 m.
Natural 3 wk
Heat training Perform usual training in temperate conditions (5°C–18°C).
In addition to usual training, exercise for ≥90 min at an intensity eliciting a core body temperature of ≥38.5°C, e.g., fixed workload at 50%–55% V̇O2max/HRmax with ambient conditions of 35°C–40°C and 30%–70% relative humidity.
Natural or artificial 10 d
Recommendations adapted by Rusko et al. (1), Chapman et al. (2,10), Stray-Gundersen et al. (11), Sharma et al. (12), Sinex and Chapman (3), Bonetti and Hopkins (4), Lorenzo et al. (14), Racinais et al. (23), Rendell et al. (7), McCleave et al. (9), Guy et al. (78), and Kesier et al. (21).
HRmax, maximal heart rate; V̇O2max, maximal O2 consumption.

Both LHTL and LHTH methods have been shown to improve sea-level endurance performance, although improvements are likely more appreciable with LHTL compared with LHTH (3,4). Meta-analyses data indicate a 4.0 ± 3.7% enhancement in maximal aerobic power output in elite male and female athletes after natural LHTL practices, whereas the effects of artificial LHTL and natural LHTH methods are unclear (3,4). Performance benefits can be maximized by monitoring factors such as an athlete’s iron status and systemic inflammation resulting from illness or injury before arrival at altitude (3). Although neglect of such factors may be responsible for the high degree of interindividual variability evidenced in performance outcomes after altitude training, the possibility of a placebo or training camp effect in some accounts should not be ignored (4). Of note, several studies that have used rigorous controls in an effort to mitigate these effects have not observed improvements in endurance performance exceeding normobaric training conditions (18,19).

Despite the potential performance benefits gained from the adaptations to hypoxic environments, training at altitude has challenges. Specifically, altitude training camps — at least when performed according to best-practice LHTL recommendations — require considerable logistical planning because of the limited number of sites that offer “train-low” options within driving distance of higher-elevation residence sites. The length of time requisite for complete altitude acclimatization may also be viewed as both a financially and emotionally stressful burden for athletes who require relocation. For these reasons, the possibility of alternative, more accommodating training environments that accomplish similar performance benefits for competition at sea level are attractive to endurance athletes and coaches.

HEAT TRAINING DEFINED

Although heat training has gained interest over recent years, it is important to note that, unlike altitude training, the equivalent best-practice recommendations (e.g., environmental temperature and humidity, duration of exposure) are not as clearly established. In the context of this review, heat training is defined as the practice of combining heat stress with an exercise training stimulus for the explicit purpose of enhancing adaptations benefitting exercise performance in temperate conditions (7,9,13,14). This definition is distinct from the traditional use of heat acclimatization or acclimation, which is to improve tolerance to and reduce the physiological strain of physical exertion in the heat and, subsequently, exercise performance in hot ambient conditions (32°C–38°C) (14–16,20–23).

Some physiological adaptations occur after as few as three or four exercise-heat training sessions; however, at least 2 wk of consecutive, daily exposures are recommended for clear performance benefit (23). As adequate thermal stress is needed for adaptive responses to take place, the efficacy of heat training is dependent on both the magnitude and duration of increase in core body temperature during the exercise session (23). Importantly, as athletes become acclimated to a given thermal stress, the physiological strain of a fixed-intensity exercise bout is reduced, and core body temperature during exercise is lower. As such, the intensity of exercise sessions performed in the heat may need to increase with consecutive exposures, such that core temperature remains at ≈38.5°C for the majority of the training session (14,24). Exercise–heat sessions completed at ambient temperatures of 33°C–40°C and 30%–60% relative humidity for 90 min·d−1 or longer have resulted in improved running and cycling time trial efforts lasting 10–60 min in temperate conditions (13°C–22°C) among trained male and female endurance athletes after 9–10 sessions (7,9,14). In contrast, similar heat training protocols have not elicited performance improvements in trained male endurance athletes (16,21). The Table provides optimal recommendations for heat training practices based on methods that have resulted in improved exercise performance for trained endurance athletes.

Similar to considerations for training at altitude, the ability to maintain the absolute workload of an exercise training session decreases with higher ambient temperatures (25). Evidence suggests that performing high-intensity exercise sessions in the heat causes a temporary reduction in aerobic capacity and performance (20,22). As such, optimal heat training sessions may require athletes to dedicate a considerable amount of time (≥90 min·d−1) outside of their regularly scheduled training to performing low-intensity exercise in the heat. Gibson et al. (6) has proposed an LHTL equivalent for heat training, termed “live cool, train cool, acclimate hot.” This practice, with further details provided by Heathcote et al. (26), advocates for the use of passive heating strategies (e.g., hot water immersion, water-perfused suits, sauna) to raise core body temperature to 38.5°C or higher for a minimum of 30 min immediately after exercise performed in a temperate environment (6,26). Although 3 wk of postexercise sauna bathing improved temperate running time to exhaustion at 5-km race pace along with plasma and red cell volume in trained male runners (27), additional studies are needed to determine whether passive heat training strategies are effective in improving thermoneutral exercise performance.

ARE ADAPTATIONS AFTER ALTITUDE AND HEAT TRAINING COMPARABLE?

To compare and contrast the effects of heat versus altitude training on endurance performance, it is first necessary to consider the proposed physiological basis of improvement with long-term exposure to each environmental condition. Maximal aerobic capacity (V̇O2max), lactate threshold, and exercise economy account for a substantial portion of the variance in endurance performance (28). Here, we outline the likely effects of heat training versus altitude training on hematological and nonhematological factors influencing the primary determinants of endurance performance. Figure 2 presents an overview of the proposed physiological mechanisms contributing to improved endurance performance after altitude training (Fig. 2A) and heat training (Fig. 2B).

Figure 2
Figure 2:
Overview of the anticipated physiological adaptations to altitude training (A) and heat training (B) that lead to performance benefits for competitions at sea level with temperate conditions. A. Reductions in arterial oxygen content with exposure to altitude cause an accumulation of HIF-α and increased expression of its target genes responsible for transcription of erythropoietin (EPO) and nitric oxide (NO). Increased circulating concentrations of EPO function to stimulate hematopoiesis, increasing red cell volume (RCV) and Hbmass. A greater oxygen carrying capacity of the peripheral blood widens the arteriovenous oxygen difference (a-vO2 diff), leading to an augmented maximal aerobic capacity (V̇O2max) and improved endurance performance. Activation of hypoxic response elements may also influence the lactate threshold and exercise economy, but mechanisms remain speculative. B. Increases in core body temperature with heat training effectuate a decline in central venous pressure and renal perfusion, which stimulates aldosterone secretion, subsequently causing rapid and large expansions in plasma volume. Plasma volume expansion is proposed to enhance V̇O2max by increasing stroke volume. Furthermore, emergent data suggest increases in core or skeletal muscle temperature promote the accumulation of HIF-α and its downstream effects through activation of heat shock proteins, which may influence V̇O2max and lactate threshold. Solid lines indicate well-established relation; dashed lines, limited evidence supporting the relation.

Hematological Mechanisms of Improvement

Although long-term altitude exposure stimulates a number of physiological responses, improvements in the sea-level performance of elite endurance athletes after altitude training primarily originate from hypoxia-induced erythropoiesis (29). Production of erythropoietin, which typically peaks 24–48 h after initial exposure to hypoxia (2,30), is upregulated by the oxygen sensing mechanism, hypoxia-inducible factor alpha (HIF-α) (31). The HIF complex stabilizes in hypoxia but degrades rapidly upon return to normoxic conditions, emphasizing the importance of adequate and continued exposure to hypoxia for a substantial rise in erythropoietin to occur (31). The primary downstream effect of sustained increases in erythropoietin with continued altitude exposure is an expansion of hemoglobin mass (Hbmass) (2,32,33). Total hours of hypoxia is positively associated with Hbmass, such that for every 100 h spent at altitude of 2100 m or higher, athletes can expect an approximate 1.1% increase in Hbmass; however, the rate of increase begins to deviate from a linear relationship after ≈500 h of exposure (i.e., 3 wk) (34,35).

Expansion of red blood cell mass increases the capacity to transport oxygen to skeletal muscle and, subsequently, the potential for V̇O2max and thus endurance performance to improve in trained athletes, yet this relation has been a point of controversy for over a decade (29,36). For example, data from a pooled meta-analyses demonstrate a positive association between V̇O2max and Hbmass for competitive endurance athletes participating in various altitude training practices (37), and athletes who experience appreciable performance improvements at sea level observably have larger increases in red cell volume after altitude training (30). On the contrary, others have demonstrated a 2%–3% improvement in submaximal and maximal swim performance in male and female swimmers after 3 wk of altitude training despite no change in Hbmass; however, this may be a finding unique to the mode of swimming (38). Although it is clear that a variety of mechanisms contribute to improved endurance performance at sea level with altitude training (e.g., enhanced skeletal muscle buffering capacity; see “Nonhematological Mechanisms of Improvement” section), a large body of evidence supports accelerated hematopoiesis as the primary adaptation responsible for this change (29,39).

In contrast to altitude training, the primary adaptation after heat training is a marked expansion of plasma volume. Whether this adaptation is responsible for improvements in temperate endurance performance remains a point of contention (40–42). Prolonged exercise in the heat substantially increases skeletal muscle and cutaneous vascular conductance, potentially mediated by enhanced nitric oxide synthase bioactivity (43). As core body temperature approaches ≈38°C, skin blood flow becomes limited by cardiopulmonary baroreflex-mediated cutaneous vasoconstriction as a means to defend against falling central venous pressure (44,45). Reductions in blood pressure and renal perfusion from the combined exercise-heat stress stimulates a large release of aldosterone that causes an expansion of plasma volume by ≈4% to 15% of preacclimation levels evident after the first few heat training sessions (7,9,14,21,46).

Thermal stress itself, even in the face of reduced central venous pressure, does not compromise skeletal muscle blood flow, as long as fluid losses are adequately replaced during exercise (44). Further evidence suggests hyperthermia is associated with a smaller reduction in tissue oxygen saturation during central hypovolemia compared with normothermic conditions (47). Accordingly, reductions in endurance performance in the heat are not presumably attributed to limitations in oxygen delivery to the skeletal muscle as with exercise in hypoxic conditions (44). Rather, the detrimental effects of increasing internal temperature on the central nervous system likely contribute to the earlier onset of fatigue and impaired performance with heat stress (44). Yet, whether heat training practices result in adaptations that augment oxygen carrying capacity via augmented Hbmass remains a point of controversy. A recent account found the Hbmass of elite-level cyclists increased by 4.6% after 5 wk of heat training (48), whereas others have demonstrated similar statistical “trends” after heat training practices of 14 d to 5.5 wk (P = 0.054–0.061) (16,49). However, several shorter heat training protocols lasting 5 to 10 d have observed no change in Hbmass (7,21,50,51), and underlying mechanisms for improvement are unclear.

One theory for increases in Hbmass after heat training postulates that the resultant hemodilution from acute plasma volume expansion stimulates the release of erythropoietin from the kidneys in an attempt to normalize hematocrit levels (48,49). Although reductions in arterial oxygen content have been shown to influence erythropoiesis independent of oxygen tension (52), the relative contribution of hemoglobin concentration to this relation is unknown, and reductions would likely need to be substantial to accelerate erythropoiesis to similar levels observed with altitude training (53). For example, to accomplish the increase in plasma erythropoietin concentrations from 12.5 to 19.0 mIU mL−1 seen during the first 30 h of exposure to an elevation of 2500 m in male and female athletes who have responded favorably to altitude training (i.e., improved 5000-m run performance) (30), a theoretical 14% reduction in hemoglobin concentration from 13.8 to 11.9 g dL−1 would be expected (53). Likewise, an estimated 8% reduction in hemoglobin concentration from 13.8 to 12.7 g dL−1 is anticipated to elicit sustained increases in erythropoietin measured after 14 d of altitude exposure (30,53). In contradiction, hemoglobin concentration and hematocrit have remained unchanged after 5.5 wk of heat training despite a positive trend in Hbmass (49). Another, perhaps more plausible, explanation proposes that heat training accelerates erythropoiesis due to an increased expression of heat shock proteins, which enhance HIF-α expression (48). Although emerging data in humans are promising (48,49), consensus with regard to whether Hbmass definitively increases with heat training is needed before elucidating potential mechanism(s) by which heat training may perpetuate increases in Hbmass.

Nonhematological Mechanisms of Improvement

In addition to altitude-induced changes in red cell volume and Hbmass, exposure to hypoxic environments may improve endurance performance through nonhematological mechanisms. Although increasing the oxygen carrying capacity of blood is one physiological method to circumvent the challenges of a low-oxygen environment, an alternative adaptation is to improve the energetic output for a given V̇O2 (i.e., exercise economy). Economy describes the metabolic cost for a given exercise intensity, and although not fully agreed upon (54,55), some data suggest that altitude exposure results in an improvement in exercise economy (56–58). Although the underlying mechanism is unclear, nitric oxide signaling may be involved. Activation of HIF-α stimulates the expression of nitric oxide synthase, increasing nitric oxide activity with altitude acclimatization (59). Among other effects, nitric oxide may reduce the oxygen cost of moderate and severe-intensity domain exercise by enhancing mitochondrial coupling efficiency (60,61), yet data evaluating this mechanism with altitude training are lacking.

In addition, long-term exposure to hypoxia may improve endurance performance through improvements in skeletal muscle buffering capacity and lactate kinetics (62,63). LHTL has been associated with reduced rates of lactate production and increased rates of clearance (64) for a given workload, although again contributing mechanisms are speculative. Whereas recommendations for eliciting hematological adaptations typically call for a minimum of 21 d of altitude exposure, nonhematological adaptations may have an abbreviated time course where 14 d of hypoxic exposure may be sufficient (58,65).

Although the effects of heat training on exercise economy seem to be negligible in trained athletes (16,66), other adaptations affecting lactate metabolism are apparent (14). An enhanced lactate threshold has been observed as the result of 10 d of heat training (14), and lower lactate accumulation with submaximal exercise after 8 d (67). Potential mechanisms explaining the improvement in lactate kinetics may result from increased splanchnic circulation facilitated by an expanded plasma volume (14), increased reliance on fat oxidation during exercise (67), and/or expression of genes regulating skeletal muscle perfusion (i.e., vascular endothelial growth factor [VEGF], nitric oxide [NO]) (43,68). Collectively, these data support the notion that long-term exposure to heat or altitude is a stimulus for the development of phenotypic adaptations beneficial for endurance performance. Although these results are promising, further human trials are needed to differentiate whether the thermal stress of heat training and hypoxic stress of altitude training independently elicit metabolic adaptations that are beneficial to temperate exercise performance beyond those attained with endurance training alone.

IS HEAT TRAINING AN ADEQUATE REPLACEMENT FOR ALTITUDE TRAINING?

As previously mentioned, the primary adaptations resulting from altitude training and heat training stem from physiological responses designed to accommodate for reductions in oxygen availability (e.g., greater hemoglobin mass, red cell volume) and/or central blood pressure (e.g., expanded plasma volume). In agreement with our novel hypothesis, the effects of volume loading versus an enhanced oxygen carrying capacity on the potential to improve performance for endurance-trained athletes differ and are considered below.

Hypervolemia theoretically enhances V̇O2max by augmenting ventricular filling pressures, leading to a subsequent increase in stroke volume and a lower heart rate response to a given workload (14,15,22,66), which could improve endurance exercise performance. Indeed, a 6.5% increase in plasma volume after 10 d of consecutive exercise sessions in the heat increased maximal stroke volume and V̇O2max by 5% in trained male and female cyclists during graded exercise performed in a temperate environment (14). However, despite a significant expansion in plasma volume, increases in V̇O2max and improvements in temperate endurance exercise performance have not been consistently observed after long-term heat training practices (21).

This lack of agreement within the heat training literature is confounded by a larger question of whether plasma volume expansion alone is an effective stimulus for enhancing V̇O2max in endurance-trained athletes. Long-term endurance training by itself is a stimulus for plasma volume expansion (69), and endurance-trained athletes characteristically have a greater blood volume compared with untrained individuals (70). Accordingly, although volume loading tends to improve V̇O2max and performance in untrained individuals, it is generally not associated with improvements in trained athletes (41). Excessive hemodilution due to large expansions in plasma volume without concomitant increases in hemoglobin mass may even impair V̇O2max and performance (42). Given the limited and conflicting evidence available, it remains unclear whether plasma volume expansion independently improves the endurance performance of trained athletes in temperate environments, or whether the expansion of plasma volume that occurs with heat training may concomitantly induce a level of hematopoiesis that exceeds the effects of endurance training alone.

In contrast, an abundance of evidence supports an average improvement of 1%–4% in sea-level endurance performance after altitude training (either LHTL or LHTH) (for meta-analyses and review, see references [3,4]), likely brought about by improvements in oxygen carrying capacity via elevations in Hbmass (29,55,71). For endurance-trained athletes practicing altitude training, each 1% increase in Hbmass translates to an approximate 0.6%–0.7% increase in V̇O2max (34). Although the Hbmass of elite endurance athletes is substantially greater compared with strength and power sport athletes and untrained individuals (70), it is unclear whether these differences are the result of a training effect or due to genetic predisposition (72). Furthermore, autologous blood transfusion (“blood doping”) studies acknowledge the independent effects of an augmented red cell mass on V̇O2max, lactate threshold, and endurance exercise performance (39,73). Despite no change in maximal cardiac output with even large reinfusion volumes (≈900–1350 mL), V̇O2max increases as a result of a widened a-vO2 diff, secondary to a greater arterial oxygen content (73).

Taken together, it is clear that although some overlap exists in the physiological adaptations after heat versus altitude training, the prevailing mechanisms contributing to beneficial changes in sea-level endurance performance in temperate conditions are dissimilar. Long-term exposure to a hypoxic environment has a direct effect on hypoxic response elements (i.e., HIF-α) and their downstream targets. Accordingly, increased Hbmass is the likely catalyst influencing aerobic capacity and performance in endurance-trained athletes after altitude training. In contrast, the purported mechanism contributing to improved temperate endurance performance with heat training is resultant volume loading from expanded plasma volume, which is unlikely to benefit trained athletes without a concomitant increase in Hbmass. Although data exploring alternative mechanisms contributing to improved endurance exercise performance in temperate conditions after heat training are promising (e.g., augmented skeletal muscle oxidative capacity), it is important to note that carefully planned, controlled studies are likely necessary to observe an effect in trained endurance athletes due to small anticipated effect sizes (34).

LOGISTICAL CONSIDERATIONS FOR ALTITUDE TRAINING AND HEAT TRAINING

With regard to the implementation of altitude and heat training to improve temperate, sea-level endurance performance, two points should be emphasized. First, the importance of executing a proper and optimally timed training camp, without taking any “shortcuts,” should not be overlooked. Notably, in many of the studies that have failed to show performance improvements after either heat or altitude training camps, athletes may not have been exposed to a sufficient hypoxic dose (i.e., the elevation, duration of daily exposure, or total duration of the camp) or heat stress (i.e., core body temperature unmeasured or not sufficiently elevated), or did not perform all high-intensity sessions at lower elevations or in temperate conditions (20,22,46). These are all factors that would limit adaptations to the environmental stressor and, consequently, any enhancement of endurance performance (74). The effectiveness of heat or altitude training camps also is influenced by the timing of their implementation before competition. Notably, the physiological adaptations attained with acclimatization/acclimation to either stressor are transient, and therefore, the peak volume acquired and subsequent decay rate of adaptations (both positive and negative) with return to sea-level/temperate conditions will likely have a strong influence on performance optimization (23,75,76).

Second, for elite athletes seeking to improve endurance performance, where changes as small as 0.5%–1.0% can dramatically affect race outcomes, average and “statistically significant” improvements are less meaningful than the individual athlete’s response to a certain training intervention. Thus, even when heat or altitude training is executed optimally, interindividual variations in hematological, physiological, and, subsequently, performance outcomes still exist (23,30,77) and should not be neglected. It is therefore crucial that coaches, athletes, and sports physiologists realize that although these methods could be beneficial for some athletes, others may not experience similar gains in performance.

Although at first glance training in hot ambient environments may seem more logistically feasible than training at altitude for most athletes, successful heat training practices may require the same meticulous execution as is required with altitude training. To our knowledge, only three studies have demonstrated improvements in temperate endurance exercise performance after heat training practices (7,9,14). The athletes in the three aforementioned studies (7,9,14) were instructed to maintain their normal training volume and intensities in addition to performing daily heat training sessions, which consisted of 90 min·d−1 of cycling or running exercise at ambient temperatures of 33°C–40°C. In contrast, training interventions that either have completely replaced athletes’ regular temperate training load with low-intensity (40%–50% of max) exercise sessions (21) or had athletes continue with high-intensity training sessions in the heat (16) have not effectively improved exercise performance in temperate conditions. Assuming the improvements in endurance exercise performance were due to the additional heat stimulus and not the result of unaccounted differences in training load, these studies highlight extensive exercise–heat training session durations as an important feature of successful heat training practices. Passive heating (e.g., hot water immersion, water-perfused suits, sauna) implemented after usual temperate training sessions may be an effective strategy to combat the additional time burden of exercise-heat sessions (6,26,27); however, further studies are needed to clarify the impact of such strategies on endurance performance.

CONTINUING THE DEBATE — FURTHER QUESTIONS FOR ALTITUDE VERSUS HEAT TRAINING RESEARCH

To definitively compare and contrast the utility of altitude and heat training practices for endurance athletes competing in temperate sea-level conditions, we recommend that the following are addressed in future research:

  • The development of “best-practice” recommendations for heat training (i.e., an LHTL equivalent) for eliciting beneficial changes in endurance performance in temperate environments.
  • Studies investigating the mechanism(s) responsible for improvements in temperate endurance performance with heat training (e.g., plasma volume expansion, increased Hbmass). We recommend these studies implement careful monitoring of training load throughout the intervention and control for group differences in training status (untrained vs trained).
  • Further examination of cellular signaling events in human subjects with heat training. Although preliminary evidence in vitro suggests that elevated skeletal muscle temperature is an independent stimulus for the expression of genes regulating skeletal muscle oxidative metabolism (e.g., VEGF) and erythropoiesis (HIF-α), it remains unclear whether exercise–heat training invokes a greater stimulus compared with exercise training alone.
  • Direct comparison of the effects of heat versus altitude training practices on temperate exercise performance within a singular randomized and controlled study. A crossover design including athletes of the same training level and in similar sporting events would minimize any potential confounding effects of individualized responses to either training environment.

SUMMARY

In this review, we provided a rationale for our novel hypothesis that, based on current scientific literature, the physiological adaptations acquired through conventional altitude training practices are superior for temperate sea-level endurance exercise performance of well-trained athletes compared to those gained with heat training. Traditionally, endurance athletes have used altitude as a stimulus to augment Hbmass, thereby eliciting improvements in oxygen carrying capacity and, subsequently, sea-level endurance exercise performance. Practices of altitude training have been refined (e.g., LHTL), such that recommendations for the optimal duration of exposure and level of hypoxia are designed to maximize the hematological adaptations associated with long-term altitude exposure while minimizing the detrimental physiological and (de)training effects of exposure to a low-oxygen environment. Perhaps because of the actual or perceived logistical burdens associated with optimal altitude training practices, athletes and coaches have begun to explore heat training as an alternative environmental impetus by which the physiological adaptations to endurance training practices can be augmented. Whether the genesis of this belief is from thinking that acclimatization to heat yields greater physiological benefit than acclimatization to altitude, or whether it is simply a matter of convenience — with hot environments being more readily accessible than altitude — is not known. Although several accounts have shown improvements in endurance performance in temperate conditions after heat training practices, this outcome is not universal and the mechanisms of improvement are unclear. Plausible mechanisms related to heat acclimatization/acclimation, such as an increase in plasma volume, may explain improvements in temperate exercise performance. However, there remains debate over whether the performance of endurance-trained athletes benefits from plasma volume expansion. Other proposed, yet not confirmed, mechanisms include an increase in Hbmass and oxidative capacity due to higher muscle temperatures. Nevertheless, while current evidence suggests that both altitude training and heat training hold the potential to improve temperate sea-level performance beyond traditional endurance training practices, work from our laboratories and others support altitude training as the choice with the clearest beneficial impact for endurance-trained athletes.

Acknowledgment

C.C.W. was supported by a National Institutes of Health training grant (5T32DK007352-39).

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    Keywords:

    acclimation; acclimatization; athletes; adaptations; competition; hematology

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