A Clinician Guide to Altitude Training for Optimal Endurance Exercise Performance at Sea Level

Abstract

Constantini, Keren, Daniel P. Wilhite, and Robert F. Chapman. A clinician guide to altitude training for optimal endurance exercise performance at sea level. High Alt Med Biol. 18:93–101, 2017.—For well over 50 years, endurance athletes have been utilizing altitude training in an effort to enhance performance in sea level competition. This brief review will offer the clinician a series of evidence-based best-practice guidelines on prealtitude and altitude training considerations, which can ultimately maximize performance improvement outcomes.

Introduction

It has become a common practice by professional/elite and novice endurance athletes to incorporate altitude training camps into the training schedule before important competitions at sea level. There is both a strong belief within the athletics community and emerging scientific evidence that physiological adaptations associated with chronic exposure to altitude are, at least partly, responsible for the increases in maximal O₂ uptake (VO₂max) and enhanced endurance exercise performance at sea level following altitude training.

There are a number of quality reviews in the literature (Levine, 2002; Levine and Stray-Gundersen, 2006; Gore et al., 2007; Wilber, 2007; Bonetti and Hopkins, 2009; Sinex and Chapman, 2015) that discuss specifically how and why altitude training may facilitate physiological adaptations to improve exercise performance at sea level. The aim of this review, however, is to present the current best-practice guidelines for optimizing those altitude-induced physiological adaptations to ultimately enhance sea level performance. This guide for the clinician will cover prealtitude considerations, planning altitude/hypoxic exposure in terms of elevation, duration of stay and return to sea level before competition, and individual variations to be considered.

In addition to classic terrestrial altitude training, many athletes have been utilizing nitrogen apartments/houses and hypoxic tents (in most cases, creating normobaric hypoxic conditions) to artificially simulate the hypoxic environment that exists at altitude, while staying at sea level, in their own homes. These methods are perhaps logistically and financially advantageous compared to relocating to altitude; however, the efficacy of living under conditions of normobaric hypoxia (NH) on endurance exercise performance at sea level is a widely disputed topic in altitude physiology. While not the main focus of this review, we will also briefly discuss some of the benefits and limitations of such practices, as well as how to optimize the use of NH for an enhanced sea level performance. Although we are aware that various other hypoxic methods to enhance athletic performance exist (e.g., intermittent hypoxic training and short-term hypoxic exposure), those techniques will not be covered in this review.

Advantages of the Live-High Train-Low Model for Altitude Training

Chronic altitude exposure, or more specifically exposure to environments with lower partial pressure of O₂ (PO₂), induces a number of physiological adaptations that are potentially beneficial for athletic performance. First, lower O₂ availability in either natural or simulated hypoxic environments stimulates an erythropoietin (EPO) response that, given adequate iron stores, leads to increased rate of red blood cell production and hemoglobin mass (Hb_mass) (Stray-Gundersen et al., 2001; Rusko et al., 2004; Heinicke et al., 2005; Clark et al., 2009; Chapman et al., 2014). These hematological changes improve O₂ carrying capacity and delivery to skeletal muscles, thus typically improving sea level VO₂max. In addition, a number of nonhematological factors such as angiogenesis and improved buffering capacity and muscle efficiency have also been proposed to contribute to enhanced sea level performance following altitude training [see review by Gore et al. (2007)].

While hypoxic exposure is the necessary stimulus for hematological and ventilatory adaptations, the lack of O₂ availability impairs training ability at altitude. Training in hypoxia typically forces the athlete to train at lower intensities, resulting in reduced O₂ requirements and thus lower rates of O₂ flux from the capillaries to the mitochondria. Thus, there may be a detraining effect associated with chronic altitude training, which likely accounts for the lack of improvement in VO₂max and performance following training camps where the athletes lived and trained at altitude (Buskirk et al., 1967; Levine and Stray-Gundersen, 1997). To overcome this problem, Levine and Stray-Gundersen (1997) have suggested the “live-high train-low” (LHTL) model, which has become popular among athletes utilizing altitude training to enhance sea level endurance performance (Chapman and Levine, 2007; Wilber, 2007; Stray-Gundersen and Levine, 2008).

The LHTL model, in which athletes live and sleep at moderate/high altitude, but perform their training as close to sea level as possible, allows for maximization of the positive effects of hypoxic acclimatization, while minimizing the negative effects of training in those conditions. The maintenance of O₂ flux with LHTL allows physiological adaptations typically associated with chronic endurance training to occur (Chapman and Levine, 2007). Thus, not only does VO₂max improve following 4 weeks of LHTL (Levine and Stray-Gundersen, 1997; Stray-Gundersen et al., 2001; Chapman et al., 2014) but improvements have also been shown in O₂ consumption at the ventilatory threshold and the maximal steady state (Levine and Stray-Gundersen, 1997; Stray-Gundersen et al., 2001; Schmitt et al., 2006). Most importantly, however, in well-controlled studies, mean group running performance at sea level has also been shown to improve only when “low” training sessions are incorporated (Levine and Stray-Gundersen, 1997; Stray-Gundersen et al., 2001).

Multiple well-controlled peer-reviewed investigations and meta-analyses, listed in the paragraphs below, support the sea level performance benefit of properly executed LHTL altitude training. However, while the LHTL model appears to be advantageous for enhanced sea level performance, there are not many places around the world where this training strategy is logistically and conveniently possible. For a list of commonly used locations, see Table 1. It should also be noted that although many athletes utilize hypoxic training with the belief that such practices will improve their sea level performance, the effectiveness of altitude training has been repeatedly challenged (Lundby et al., 2012; Siebenmann et al., 2012; Lundby and Robach, 2016; Siebenmann, 2016). Lundby's group has argued that there is insufficient evidence to conclude that hypoxic exposure is indeed beneficial to elicit hematological adaptation performance enhancement (Lundby et al., 2012; Siebenmann et al., 2012; Siebenmann, 2016). Nevertheless, other leading researchers in the field of altitude training have refuted this position (Saugy et al., 2014) and are skeptical about the conclusions drawn from those studies due to a number of methodological issues (e.g., confinement in a limited space for 16 h/day, which could greatly affect plasma volume and technical aspects of Hb_mass measurement).

Table 1.

Training Sites Where Live-High Train-Low Is Logistically Possible and Convenient

Location (city & surrounding area)	Living altitude (m)	Accessible low training altitude (m)	Commute time between high & low locations (minutes)
Park City, UT	1700–2800	1250	30–45
Flagstaff, AZ	1950–2100	950–1285	45–60
Mammoth Lakes, CA	2100–2400	1250	45
Big Bear Lake, CA	2050–2150	<300	65–75
Lake Tahoe, NV/CA	1900–2200	570–1450	45–95
Cloudcroft, NM	2400–2600	1300	30
Sierra Nevada, Spain	2320	690	40–50

Due to logistical difficulties associated with descending from altitude for each training session, at times twice a day, many athletes choose to follow a modified, more convenient, and arguably more advanced version of traditional LHTL, termed “live-high, train-high and low.” In this iteration, base and lower intensity training sessions are performed at moderate altitude and only the high-intensity workouts (typically two to three sessions a week) are performed “low” (Stray-Gundersen et al., 2001). This method has been shown to be beneficial for athletic performance at sea level in various sports, including running (Chapman et al., 1998, 2014; Stray-Gundersen et al., 2001), orienteering (Wehrlin et al., 2006), cycling (Martin et al., 2002), and swimming (Rodríguez et al., 2015). Since “live-high, train-high and low” is a natural progression of the traditional model, we will refer to this paradigm as LHTL for the purpose of this review.

An extensive meta-analysis by Bonetti and Hopkins has strengthened the superiority of the LHTL model over other hypoxic methods in terms of performance enhancement (Bonetti and Hopkins, 2009). Specifically, the meta-analysis suggests that LHTL using terrestrial altitude likely benefits maximal endurance power output in elite (improvement of 4.0% ± 3.0%) and subelite (improvement of 4.2% ± 2.9%) endurance athletes, with classic “live-high, train-high” methods having a smaller and statistically unclear effect on performance (Bonetti and Hopkins, 2009). Recently, a number of training models combining LHTL along with other methods such as repeated sprints in hypoxia and training in the heat have been proposed (Buchheit et al., 2013; Brocherie et al., 2015, 2017). These methods have been shown to further enhance altitude-related physiological adaptation and performance, and/or minimize logistical issues associated with altitude training, especially in team sports (Buchheit et al., 2013; Brocherie et al., 2015, 2017).

Although simulated altitude using NH at sea level may be more practical for most athletes (logistically and financially) than relocating to altitude for an extended period of time, these methods do not come without a cost. Current literature remains equivocal regarding the physiological and performance adaptations that follow such regimens (Millet, 2012; Saugy et al., 2014, 2016). One of the drawbacks of hypobaric hypoxia (HH) is a greater degree of altered breathing patterns during sleep and more frequent episodes of apnea (Heinzer et al., 2015). On the other hand, the main disadvantages of using NH methods, specifically hypoxic tents, are that sleeping and living in those conditions can be very uncomfortable and it is much more difficult to accumulate the minimum “hypoxic dose” required for adaptations to occur in NH compared to terrestrial altitude exposure (i.e., HH). Work by Schmidt and Prommer (2008) suggests that >14 h/day of hypoxic exposure may be needed to experience a significant increase in red cell or total Hb_mass. Recent evidence also suggests that improvements in sea level performance last for a longer period of time following HH exposure compared to NH (up to 21 days postaltitude) (Saugy et al., 2016).

Prealtitude Considerations

To maximize altitude-induced adaptations and performance enhancement, several hematological, health, and training related factors should be considered before traveling to altitude. First, because injury and/or illness (e.g., severe inflammation or viral infection) reduce Hb_mass at sea level (Schmidt et al., 2011), athletes who have recently experienced those conditions may not benefit from an altitude training camp. Similarly, illness or injury during altitude training might also offset the would-be increase in Hb_mass following hypoxic exposure (Wachsmuth et al., 2013), as would preexisting health conditions (e.g., orthopedic issues) that could be exacerbated at altitude and/or prevent altitude-related adaptations. With regard to training leading up to altitude exposure, there is anecdotal evidence to suggest that high-intensity training, which could lead to a fatigued state, should be avoided in the days leading up to altitude training; however, more objective research may be required on this matter.

For the otherwise healthy athlete, the most important prealtitude consideration may be iron status. It is well accepted that sufficient iron stores are a necessary component of hypoxia-mediated increases in Hb_mass with chronic altitude exposure (Berglund, 1992; Stray-Gundersen et al., 1992; Levine and Stray-Gundersen, 1997; Govus et al., 2015). However, it is also well established that endurance-trained athletes often experience latent iron deficiency, despite having normal absolute levels of Hb (Chatard et al., 1999), and a large number of review articles highlight the clear and common prevalence of reduced iron reserves in well-trained endurance athletes (Clement and Sawchuk, 1984; McDonald and Keen, 1988; Haymes and Lamanca, 1989; Szygula, 1990; Weight, 1993; Clarkson and Haymes, 1995; Chatard et al., 1999; Shaskey and Green, 2000). Therefore, a primary prealtitude consideration for athletes (and their clinicians) is normalization of iron stores before departure.

Of the 2–5 g of stored iron typically found in healthy individuals, approximately two-thirds is contained within Hb, myoglobin, and mitochondrial cytochromes, while approximately one-third is stored in the liver, spleen, and bone marrow. Because of the invasive nature of a bone marrow aspiration (considered the “gold standard” of iron status determination) or a liver biopsy, a measure of serum ferritin is the most commonly used indicator of iron stores. There is a strong correlation between serum ferritin and mobilizable iron stores (r = 0.83) (Walters et al., 1973), and a reduction in reticuloendothelial iron stores is the only common cause of a low serum ferritin concentration (Worwood, 1982). However, in cases of acute and chronic disease, serum ferritin measures are typically increased (World Health Organization, 2005). Similarly, serum ferritin measures are elevated and unreliable in the presence of inflammation or infection; therefore, if suspected, assessment of ferritin should be avoided or appropriate markers included (e.g., C-reactive protein, alpha-1-acid glycoprotein, and erythrocyte sedimentation rate) (World Health Organization, 2005).

A geographically broad survey of the U.S. population places the median value for men 18–64 years of age at 92 ng/mL and for women 18–44 years of age at 27 ng/mL, and most clinical laboratories will have a lower end of the range of normal serum ferritin at ∼10 ng/mL (Custer et al., 1995). However, for well-trained endurance athletes, there are guidelines to support oral iron supplementation for serum ferritin values <35 ng/mL (Nielsen and Nachtigall, 1998), even in sea level residents, even though this value falls within the normal clinical range for the population as a whole.

The strongest support for normalizing iron stores before departure for altitude comes from Stray-Gundersen et al. (1992), who demonstrated no increase in red cell mass (RCM) or VO₂max in nine iron-deficient distance runners (serum ferritin <30 ng/mL for men, <20 ng/mL for women, mean ± SD 15 ± 3 ng/mL before departure) after 4 weeks at 2500 m, while athletes with adequate ferritin levels prealtitude (69 ± 10 ng/mL) demonstrated significant increases in RCM and VO₂max postaltitude camp. Interestingly, the work by Stray-Gundersen et al. (1992) represents, to date, the only published data examining the erythropoietic response of low ferritin athletes to moderate altitude, and there is no published standardized set of recommendations for iron supplementation in the literature for athletes before or during an altitude exposure.

Regardless of iron status prealtitude, emerging data suggest that iron supplementation may be a necessary requirement for adequate erythropoiesis with altitude exposure. Pooled data from multiple investigations by researchers at the Australian Institute of Sport (AIS) indicate that after chronic moderate altitude exposure, 15 athletes who did not supplement with oral iron while at altitude failed to demonstrate a significant increase in total Hb_mass (ΔHb_mass 1.1%; 95% CI −0.4, 2.6; p = 0.14). Athletes who supplemented with either 105 mg (n = 144) or 210 mg (n = 19) of elemental iron daily showed significant increases in Hb_mass (mean ΔHb_mass 3.3% and 4.0%, respectively) (Govus et al., 2015). From recent unpublished work from our laboratory (Chapman, unpublished observations), of three elite distance runners who failed to supplement with oral iron during a 23–28 day altitude camp at 2000 m, none demonstrated an increase in Hb_mass of more than the typical error of the measurement procedure in our laboratory (1.9%).

Typical oral iron supplementation routines used in field-based altitude training studies conducted by the Levine, Stray-Gundersen, and Chapman group (Levine and Stray-Gundersen, 1997; Stray-Gundersen et al., 2001; Chapman et al., 2014, 2016) can be found in Table 2. These studies used a preparation of liquid ferrous sulfate (220 mg/5 mL, 44 mg of elemental iron) added to ∼250 mL of orange juice or taken with 500–1000 mg of vitamin C. The recommended daily dosages of elemental iron per day are also comparable to dosages used in multiple studies by the AIS laboratory (see Govus et al., 2015). While more aggressive than some clinical recommendations, these dosages appear to be necessary for adequate erythropoiesis at altitude, particularly in endurance athletes. However, stomach upset, constipation (with iron in pill form), or diarrhea (with iron in liquid form) are common side effects with oral iron supplementation. In addition, several food products such as coffee, tea, dairy, and foods containing phytate can decrease iron absorption in the gut (Sharpe et al., 1950; Morck et al., 1983; Hallberg et al., 1989, 1992). Therefore, it is recommended to ingest iron supplements at least 1 hour before or at least 2 hours after meals or drinking coffee or tea. Finally, it is worth noting that intravenous and intramuscular iron injections are becoming more widely utilized, particularly in treatment of iron-deficient athletes (Pasricha et al., 2010).

Table 2.

Common Iron Supplementation Regimen Used in Altitude Training Studies Published by Levine, Stray-Gundersen, and Chapman

Prealtitude serum ferritin measure
Men	Women	Iron supplementation regimen used
>100 ng/mL, but <200 ng/mL	>100 ng/mL, but <200 ng/mL	None, or 44 mg elemental iron 2–3 times per week
40–100 ng/mL	30–100 ng/mL	44 mg elemental iron/day (5 mL ferrous sulfate elixir/day)
30–39 ng/mL	20–29 ng/mL	88 mg elemental iron/day (10 mL ferrous sulfate elixir/day)
20–29 ng/mL	10–19 ng/mL	176 mg elemental iron/day (20 mL ferrous sulfate elixir/day)
<20 ng/mL	<10 ng/mL	264 mg elemental iron/day (30 mL ferrous sulfate elixir/day)

The studies referenced in this table (Levine and Stray-Gundersen, 1997; Stray-Gundersen et al., 2001; Chapman et al., 2014, 2016) used a preparation of liquid ferrous sulfate elixir (220 mg/5 mL), containing 44 mg elemental iron. Regimen was started up to 6 weeks before altitude exposure and continued daily while at altitude.

Choice of Appropriate Living and Training Altitudes

One may be led to assume that since ambient O₂ content falls with increasing elevation, higher altitudes would induce a greater hematological response, ultimately resulting in greater performance improvements. However, performance improvements with altitude appear to diminish at elevations exceeding ∼2500 m.

It has been shown that following LHTL altitude camps where athletes lived in one of four different elevations (1780, 2085, 2454, and 2800 m), the two middle altitude groups had the best improvements in a 3000 m running time trial immediately and 2 weeks after return to sea level even though EPO and VO₂max increases were not as large as in the group living at 2800 m (Chapman et al., 2014). The EPO response in the lowest elevation (1780 m) group was the lowest among the four groups and neither VO₂max nor running performance improved in the 1780 m group at any time point upon return to sea level (Chapman et al., 2014). These data suggest that living at 2000–2500 m (moderate altitude; equivalent to fraction of inspired O₂ (FiO₂) ≈16% at sea level) appears to be the optimal altitude for hematological, physiological, and performance enhancements. Living at lower altitudes likely does not provide a strong enough stimulus to appreciably increase RCM or improve performance in most subjects, while higher altitudes are associated with negative effects of acclimatization (e.g., poorer sleep quality and increased ventilatory acclimatization) and do not offer a considerably larger increase in RCM over moderate altitudes. In terms of an optimal training altitude for maximal performance improvements, high-intensity workouts such as fast intervals and training at or faster than threshold pace should be performed at lower altitudes (<1250 m), to maintain O₂ demand and utilization by skeletal muscles at near-maximal levels (Levine and Stray-Gundersen, 1997; Chapman et al., 1998; Stray-Gundersen et al., 2001; Chapman and Levine, 2007).

How Long to Stay

Following exposure to environments with low O₂ availability, plasma EPO levels peak within 24–48 hours at altitude and then gradually decline, reaching sea level baseline values typically after 7–21 days at altitude and often falling below baseline thereafter (Friedmann et al., 2005; Wehrlin et al., 2006; Chapman et al., 2014). Considering changes in Hb_mass during chronic hypoxic exposure, two theories exist. The first school of thought suggests that during the first 2 weeks of hypoxic exposure, there are trivial or unclear changes present in Hb_mass and ∼3 weeks is the minimum required duration for substantial increases in RCM and Hb_mass (Rusko et al., 2004; Levine and Stray-Gundersen, 2006; Wehrlin et al., 2006; Clark et al., 2009). This view point also holds that, although these hematological adaptations are sufficient to improve sea level performance, prolonging the exposure to hypoxia by another week (to ∼28 days) appears to be even more optimal as RCM may increase exponentially during those additional days at altitude (Levine and Stray-Gundersen, 2006). Conversely, a meta-analysis by Gore et al. (2013) demonstrated a linear 1.1% increase in Hb_mass for every 100 hours of hypoxic exposure. Accordingly, it was suggested that athletes could expect hematological benefits following 2 weeks only of moderate altitude exposure.

Recently, this well-respected group from the AIS presented a new metric for “hypoxic dose” where both elevation and hours of exposure (km × h) were used to predict changes in Hb_mass (Garvican-Lewis et al., 2016). Fitting an exponential model to the data, it was determined that the maximal possible increase in Hb_mass was 7.7%, regardless of the manner by which the accumulated elevation and time in hypoxia (i.e., the product of km × h) were achieved (Garvican-Lewis et al., 2016). Although this model presents an important stepping stone moving the field of altitude training forward, it does not consider a number of important factors such as population (elite athletes vs. lesser trained individuals), the nature of hypoxic exposure (hypobaric vs. normobaric), and the fact that a “minimum threshold” for both altitude and duration likely exists for hematological adaptations to take place. Most importantly, it should be emphasized that there is no “one size fits all” model when considering altitude training, and intersubject variability must be considered (see below).

With regard to duration of exposure, the number of hours per day spent in hypoxia is an essential consideration. There is no definite “magic number,” and optimal exposure time appears to be related to the nature (i.e., HH vs. NH) and possibly the extent (i.e., elevation or partial pressure of O₂) of hypoxic exposure. For terrestrial altitude, Levine and Stray-Gundersen have shown an 8% increase in RCM and a 1.5% improvement in running performance after 4 weeks of 20–22 h/day of moderate altitude exposure (Levine and Stray-Gundersen, 1997), and these findings have also been confirmed in other sports (Schmidt et al., 2002; Friedmann et al., 2005; Wehrlin et al., 2006). The minimum dose for stimulating RCM production and hematological benefits appears to be ∼12–16 h/day (Rusko et al., 2003; Brugniaux et al., 2006; Wilber, 2007; Schmidt and Prommer, 2008).

When NH methods are utilized, the daily hypoxic “dose” is often substantially lower than terrestrial altitude because it is more difficult and/or not practical for an athlete to stay confined in a small space for most of the day. In practice, the athlete only spends the night hours (∼10 h/day) in hypoxia and therefore NH has been referred to as “sleep high, train low” (Levine and Stray-Gundersen, 2006). It appears, although, that this hypoxic dose may not be sufficient for hematological adaptations to occur (Ashenden et al., 1999; Robach et al., 2006), and at least 12–16 h/day for 4 weeks at a simulated altitude equivalent to 2500–3000 m is required for RCM to substantially increase (Rusko et al., 1999; Brugniaux et al., 2004; Levine and Stray-Gundersen, 2006; Robach et al., 2006; Wilber, 2007).

Individual Variations

Individual variations in response to altitude/hypoxic exposure are an important factor that needs to be accounted for when planning altitude training and specific living/training elevations (Chapman et al., 1998; Levine and Stray-Gundersen, 2006; Chapman, 2013). Different responses between athletes have been reported for various parameters such as the EPO response to both short- and long-term exposure to hypoxia, ventilatory acclimatization, and training ability under hypoxic conditions (Chapman et al., 1998; Ri-Li Ge et al., 2002; Friedmann et al., 2005). Overall, the balance between those adaptations, or lack thereof, will determine whether the athlete will experience improvements in VO₂max and performance following chronic hypoxic exposure.

For example, improvements in 5 km running performance following altitude training have been demonstrated to be related to the extent of increase in EPO concentration, RCM, and blood volume (Chapman et al., 1998). Furthermore, EPO levels decayed at a slower rate from their peak while at altitude in athletes who “responded” to chronic altitude exposure with significantly faster 5 km running performance compared to “nonresponders,” and VO₂max and RCM improved only in the former group (Chapman et al., 1998). Thus, monitoring hematological responses during and following altitude exposure could provide insight as to whether improvements in performance upon return to sea level are expected.

The ability to maintain training volumes and intensities at altitude should also be considered before commencement of an altitude training camp. In general, well-trained athletes experience greater reductions in VO₂max and performance at altitude than lesser trained individuals (Lawler et al., 1988). Highly trained athletes often also experience a greater degree of arterial oxyhemoglobin desaturation (SpO₂) both at altitude and sea level, and it has been shown that considerable reductions in SpO₂ at sea level are associated with greater reductions in VO₂max and running performance at altitude (Gavin et al., 1998; Chapman et al., 2011). Measuring SpO₂ during heavy exercise at sea level could therefore be a useful tool for screening athletes before altitude training and making appropriate altitude training recommendations on an individual basis. For example, while training at an altitude of 1250 m is likely low enough to induce positive training adaptations in most athletes (Levine and Stray-Gundersen, 1997), those who show substantial reductions in SpO₂ and are more prone to experiencing declines in training abilities should likely train at an even lower altitude (Chapman et al., 2011). Thus, we recommend measuring SpO₂ during heavy exercise at sea level to detect for a reduction of more than 4% from resting values and/or a drop below 92% in SpO₂ (the shoulder of the oxyhemoglobin dissociation curve), which would indicate that the athlete is likely to have impaired workouts at altitude, especially during the first week of exposure (Chapman et al., 2011; Chapman, 2013).

When to Return to Sea Level

If the primary goal of an altitude training camp is to maximize sea level performance, then timing the return to sea level may be as important as the logistics of the training camp itself. Due to the various adaptations that persist postaltitude, mistiming of the return can potentially result in the athlete performing worse than prealtitude. There is scant evidence-based research on optimal timing of return for enhanced sea-level performance and most recommendations are based on anecdotal evidence from coaches. Regardless, three physiological adaptations should be considered when timing the return from altitude before competition at sea level: (1) hematological decay, (2) ventilatory acclimatization, and (3) biomechanical/neuromuscular adaptations.

Considering hematological changes, Hb_mass and blood volume are significantly reduced on day 14 upon return to sea level (Prommer et al., 2010) and therefore it would be advantageous to compete within 2 weeks upon return to sea level. In terms of ventilation, it remains elevated with acute return from altitude during both submaximal and maximal exercise (Wilhite et al., 2013), but likely decays faster than hematological parameters. Finally, incorporation of “low” training sessions, which contribute to the maintenance of neuromuscular function, appear to prevent alterations in biomechanical indices (e.g., stride length, stride frequency, ground contact, and aerial time) (Stickford et al., 2016). Thus, biomechanical and neuromuscular considerations should not influence the timing of return from altitude before competition at sea level, as long as some training session is performed at lower altitudes.

Combining the above-mentioned factors, along with anecdotal evidence, it appears that for optimal endurance performance, athletes should compete either within the first 48–72 hours upon return to sea level or ∼14 days or more after return (Chapman et al., 2014). Individual responses are also important to consider when timing return from altitude as it is safe to assume that responses following altitude camps would be variable between individuals (Chapman et al., 2014). Specifically, athletes who experience a fast decline in RCM should time their return to sea level as close to the competition as possible, while those who experience an exaggerated ventilatory acclimatization response might need a few days at sea level before competition. For a more thorough review on timing of return from altitude for optimal sea level performance, see Chapman et al. (2014).

Conclusion

Table 3 summarizes current best-practice altitude training guidelines to optimize sea level endurance performance. The information provided is based on evidence-based practices from multiple laboratories and anecdotal observations by the authors and others. For athletes looking to enhance endurance performance at sea level, the potential benefits associated with properly executed altitude training camps are clear. While the specific response to altitude is highly individualized and, of course, not guaranteed for every athlete, following the guidelines and recommendations outlined in this review will help improve the odds of a successful altitude training camp outcome.

Table 3.

Practical Recommendations for Altitude Training to Optimize Sea Level Performance

Consideration	Recommendations
Prealtitude considerations	Health status: in cases of preexisting injury/illness, athletes should reconsider altitude training as those conditions may prevent altitude-induced adaptations or even exacerbate the injury/illness.
	Illness during altitude exposure is also likely to interfere with potential hematological adaptation to altitude.
	Ferritin levels: for iron supplementation prealtitude and during altitude exposure, see Table 2.
	Anecdotal evidence suggests that high-intensity training and increased levels of fatigue should be avoided in the days leading up to an altitude training camp.
Living altitude	Data suggest that an altitude of 2000–2500 m (FiO₂ ≈ 16% at sea level) is optimal for hematological, physiological, and performance improvements.
	Lower altitudes (<2000 m) do not appear to sufficiently stimulate hematological and physiological changes that could enhance performance.
	Marginally larger increases in RCM occur at higher altitudes (>2500 m); however, those altitudes are also associated with negative side effects (e.g., poorer sleep quality and increased ventilatory acclimatization) and no sea level performance improvements.
Training altitude	An abundance of data suggests that an altitude of 1250 m is low enough to maintain an appropriate training stimulus and maximize training adaptations.
	Athletes who experience significant arterial desaturation during heavy exercise (even at sea level) might need to go lower than 1250 m.
Considerations for initial ascent	Poor sleep quality and dehydration during the first few days at altitude may lead to inadequate recovery unless training loads are adjusted accordingly.
	Increased ventilation and perception of breathing effort during altitude/hypoxic exposure could impair training ability.
Workout selection	Low-intensity training sessions can be performed at altitude (i.e., the living altitude).
	Higher intensity workouts (e.g., fast intervals and training at or faster than threshold pace) should be performed at lower altitudes.
	Due to reduced aerobic capacity at altitude, training intensities and volumes should be adjusted (reduced) to avoid overtraining and allow better acclimatization.
How long to stay	Data on optimal duration of hypoxia to maximize hematological adaptations and performance are less clear and remain equivocal.
	It has been suggested that a minimum of 21 days at altitude are required for substantial increases in hematological parameters, but further increases have been shown with 28 days of exposure.
	In contrast, others have proposed that 14 days of hypoxic exposure are sufficient for beneficial hematological adaptations and Hb_mass increases by 1.1% for every 100 hours of hypoxic exposure with an upper limit of 7.7% increase in Hb_mass (or ∼700 hours).
When to return	Objective, scientific evidence is lacking on the question of optimal timing of return to sea level before competition.
	Based on anecdotal evidence, to maximize endurance performance, athletes should plan to compete within the first 48–72 hours or after ∼14 days after return to sea level.
	Individual responses should be taken into consideration when timing return to sea level with rate of RCM decay and ventilatory acclimatization responses being key factors.

RCM, red cell mass.

Footnotes

Acknowledgments

The U.S. Olympic Committee, USA Track & Field, Presbyterian Hospital of Dallas, and Indiana University have supported work by R.F. Chapman. K. Constantini and D.P. Wilhite were funded by institutional support from Indiana University.

Authors Disclosure Statement

No competing financial interests exist.

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