- It remains unclear whether “Live High-Train Low” (LHTL) can improve sport-specific exercise performance. However, it seems clear that the possible effects on performance are small (<2%).
- The best controlled LHTL studies do not find improvements in sport-specific exercise performance.
- Very few studies have evaluated whether hypo- or normobaric LHTL strategies elicit similar effects on sport-specific exercise performance. However, the available studies suggest that the exposure methods yield similar effects.
Since the 1970s, altitude training has become an integrated training modality among athletes and is applied worldwide with the single purpose of enhancing athletic performance.
One model frequently used by professional athletes is “Live High-Train Low” (LHTL) (1) where athletes adapt to hypoxia by living and sleeping at high altitude but train at sea level to avoid the hypoxia-induced reduction of maximal training intensity (2,3). The LHTL model was proposed in the early ‘90s (2) and reported to be an effective approach in 1997 because of an increase in red cell volume (4). Since then, LHTL has been widely studied and the underlying physiological mechanisms of LHTL have been reviewed extensively (5–10). Athletes engage in LHTL exclusively to improve sport-specific exercise performance. Thus, the primary outcome from the athletes’ perspective is performance defined as time to complete a certain distance (i.e., 5000-m running performance), which can also be termed a closed-end performance test with high ecological validity. In contrast, studies in exercise science often apply a performance test evaluating time to exhaustion (i.e., an open-end performance test). Albeit this type of test allows solid physiological evaluations such as determining metabolism at a certain exercise intensity (11), the outcome is of limited relevance in a competitive setting.
To date, no existing reviews have provided a strict focus on whether LHTL can be considered to improve sport-specific exercise performance. In recent years, two double-blinded placebo-controlled studies have demonstrated that 4–6 wk of LHTL do not improve time trial performance in highly trained (12) and elite athletes (13), which induced the novel hypothesis of the present review that LHTL does not improve sport-specific exercise performance (Fig. 1). Thus, the primary aim of the present review is a thorough discussion of whether hypobaric or normobaric LHTL can be claimed to improve sport-specific exercise performance when evaluated under strict scientific criteria.
A systematic search was performed at www.pubmed.com using the following key words and Boolean connectors: (altitude OR hypoxia) AND performance AND (live high train low OR living high training low). In addition, the reference list of identified studies evaluating closed-end exercise performance after LHTL was investigated for unidentified studies. The final literature search was conducted in August 2017. In the present review, all LHTL studies investigating closed-end exercise performance measures were included, except for studies investigating all-out exercise performance measures (14–16) due to the limited ecological validity. Furthermore, it is well known that altitude acclimatization improves performance at moderate and high altitudes (17). Thus, in the present review, we only included studies evaluating sport-specific exercise performance at sea level or low altitudes. Notably, a single study on normobaric LHTL (18) was excluded because of inconsistency in the reported performance data (see Table 1 of the mentioned study).
It is of high relevance to evaluate the efficacy of LHTL for improving performance because residing at altitude, in normobaric hypoxic rooms or tents, is mentally stressful and costly in both time and money for organizations, athletes, and staff.
In the present review, there will be a strict focus on the possible impact of study design and methodology on the conclusions drawn from previous studies. One major concern when evaluating the impact of an intervention on human exercise performance is the placebo and nocebo effect (28,29). The placebo effect has been shown to improve cycling time trial performance by 4% (29), whereas the nocebo effect may negatively influence performance, for example, due to loss of motivation by being allocated to a control group. Furthermore, a beneficial effect on performance of an LHTL intervention may be induced by training camp effects, because it is well-known that athletes often improve performance after a period with high-quality training as for example observed after an initial sea-level training camp before exposure to LHTL (4). Strong study designs may account for possible confounders by including a placebo group and use blinded designs when possible.
In addition, the method of hypoxic exposure may be of importance. It has been argued that hypobaric hypoxia induces different physiological responses compared with normobaric hypoxia (30), and it is evident that substantial physical differences exist in the methodologies applied to reduce arterial oxygen saturation (31). Thus, we will also briefly address whether a hypo- versus normobaric LHTL approach yields a different impact on sport-specific exercise performance.
EFFECT OF HYPOBARIC LHTL ON SPORT-SPECIFIC EXERCISE PERFORMANCE
In the present section, the effect of hypobaric LHTL on sport-specific exercise performance is discussed. Despite that numerous studies have investigated the physiological effects of LHTL using hypobaric hypoxia, only few of the hypobaric LHTL studies investigated the effect on sport-specific exercise performance even though this is the key outcome.
Several studies of athletes have been conducted without inclusion of a control group. In these studies, 18–28 d of hypobaric LHTL at altitudes of 2085–2500 m have been demonstrated to improve 3000-m and 5000-m running performance in elite middle- and long-distance runners (32), elite orienteering runners (33), highly trained triathletes (27), and highly trained college track and cross-country runners (34) by 1%–6% (Table 1) when compared with baseline. These improvements are present immediately (27,34) 3 (32), 7 (27), 8 (33), 14 (34), and 21 (27) d subsequent to the cessation of LHTL exposure. Thus, the indication of a performance enhancing effect seems overwhelming even though a single report of unchanged 3000-m running performance when compared with baseline in highly trained college track and cross-country runners immediately and 14 d after 28 d of hypobaric LHTL at 1780 or 2800 m (34) also exist. Importantly, the lack of a control group in the aforementioned studies makes it impossible to evaluate if the observed performance enhancing effect is a result of the hypoxic exposure or simply due to a training camp effect.
To date, only two studies of hypobaric LHTL evaluating sport-specific exercise performance have included a control group. Curiously, 18–28 d of hypobaric LHTL at an altitude of 2250–2500 m did not result in detectable improvements in 3000- or 5000-m running performance immediately (4,22) or 1, 2, and 3 wk after the intervention neither in highly trained college runners (4) nor in well-trained triathletes (22) when compared with control. Notably, time trial performance was improved within the LHTL group in both studies, which clearly highlights the importance of a control group. Furthermore, the importance of sufficient statistical power must be highlighted. For instance, studies demonstrating an improved performance within the intervention group but not between the control and intervention group may indicate insufficient statistical power to detect subtle differences between groups. In addition, the possible undetected, but in terms of elite performance, relevant effect of LHTL is highlighted by the observed significant interaction between groups, but with no time-specific difference in one of the studies, which may be interpreted as an increased performance in the LHTL group (4). These results clearly demonstrate the need for well-controlled and sufficiently powered studies of hypobaric LHTL evaluating sport-specific exercise performance. It is obviously not possible to conceal the hypoxic exposure while residing at altitude, but it is possible to include a sea-level control group and to do a cross-over trial that would increase the statistical power substantially. Furthermore, application of single blinded testing in well-controlled environments could improve the validity of the results. Thus, future strictly controlled hypobaric LHTL studies evaluating the effect on sport-specific exercise performance are warranted.
Another important aspect of an LHTL intervention is the applied hypoxic dose. However, the hypobaric LHTL interventions evaluating sport-specific exercise performance have been within the recommended (5,8,35,36) hypoxic dose except a single intervention group in the study of Chapman et al. (34). In addition, training volume and intensity as well as iron availability has generally been well controlled.
Finally, timing of performance evaluation seems to be important in relation to LHTL interventions and a recent review concluded that the optimal timing for peak performance after altitude training is undocumented from a physiological standpoint (37). It has been suggested that more than a few days (21) or even at least 2 wk (19) should elapse before exercise performance peaks after an LHTL intervention to overcome potential stress of altitude and exercise training and for the increase in performance to manifest. However, some hypobaric LHTL studies report similar 3000- and 5000-m running performance 1, 3, 7, 14, and 21 d after the intervention (4,22,34), whereas others have observed improved 3000- and 5000-m running performance 1, 3, 7, 8, 14, and 21 d after the intervention (27,32–34). Thus, the timing of performance evaluation cannot explain the apparent discrepancy between hypobaric LHTL studies in relation to the unclear performance effect. Ideally, future studies investigating whether LHTL affects sport-specific exercise performance should perform measurements at several time points after the intervention to explore a potential optimal timing for peak performance.
In summary, the evidence for a performance-enhancing effect of hypobaric LHTL is lacking strong scientific support. Despite most studies lacking a control group demonstrate improvements in sport-specific exercise performance immediately and up to 3 wk after hypobaric LHTL, it has to be acknowledged that studies including a control group have been unable to detect a sport-specific exercise performance improvement immediately and up to 3 wk after hypobaric LHTL. Furthermore, important methodological issues, such as the applied hypoxic dose and timing of performance evaluation, do not seem to explain the lack of improvement. In a future perspective, it is recommended that hypobaric LHTL studies focus on a strong study design where a high number of participants seem to be of utmost importance as well as inclusion of performance measures that resemble real-life competition performances.
EFFECT OF NORMOBARIC LHTL ON SPORT-SPECIFIC EXERCISE PERFORMANCE
Normobaric hypoxic exposure is used as a practical circumvention for the need to travel to high altitude and relocate daily to a sea-level training site when applying LHTL. Normobaric hypoxic exposure can be established by staying in a room or tent partly flushed with nitrogen or with an inlet of atmospheric air from which oxygen has been partly removed. Specialized hypoxic rooms can be a part of a hotel training facility that has been used for experimental purposes (13) and can even be established at the athletes’ private residence. Tents that can be flushed with air of varying oxygen content are commercially available and have the advantage that athletes are able to expose themselves to a hypoxic environment at training and competition sites without the need for travel to remote destinations (12). Importantly, the use of hypoxic rooms and tents makes it possible to apply a double-blind and placebo-controlled design to study the physiological- and performance-related effects. In the present section, the evidence of whether normobaric LHTL improves sport-specific exercise performance is discussed.
Two normobaric LHTL studies, without placebo or sea-level control groups, have been conducted. When comparing performance with before the intervention, 14–26 d of normobaric LHTL corresponding to residing at 2250–3000 m of altitude for 9–16 h·d−1 did not affect 2000-m swimming performance in elite swimmers (26) but improved 3000-m running performance in highly trained triathletes (27). However, due to the lack of a control group, these results should be interpreted with caution.
Several studies report no significant changes in sport-specific exercise performance after 13–23 d of normobaric LHTL at 2250–3000 m for 9.5–16 h·d−1 when compared with sea-level exposure in a parallel group design. Performance evaluations have included 100- or 200-m (25) as well as 2000-m swimming time trials in elite swimmers (19), 10-min maximal walking distance in elite race walkers (21), and 3000-m time trial running in well-trained triathletes (22). In contrast, an investigation reports that normobaric LHTL improves sport-specific exercise performance concomitant with an unchanged performance in sea-level controls (24). Specifically, a group of well-trained runners improved 400-m running performance after 10 d of hypoxic exposure corresponding to 2200 m for 16.5 h·d−1, but the performance improvement was not reported to be different from the control group (24).
Interestingly, 4500-m time trial running in elite middle- and long-distance runners is substantially improved (>75% likely to have a positive effect and <5% likely to have a negative effect) (20) and so is 1000-m running (effect size = 0.56) and possibly 2000-m running (25%–75% likelihood, effect size = 0.22) in semi-professional footballers (23) after 19–21 d of normobaric LHTL at 3000 m for 12–14 h·d−1. Albeit these results are reported relative to a control group, the results were interpreted by a magnitude-based inferences approach (38). Thus, there is a risk of positive interpretations of results that in a traditional statistical approach would not meet the P < 0.05 level and thus not be considered significant. Moreover, when Robertson et al. (20) adjusted for training volume, the performance effect was unclear, and when the athletes repeated the intervention after a wash-out period of 5 wk, the LHTL group performance was possibly slower compared with the sea-level control group. The apparent performance enhancing effect of LHTL when not being compared or adjusted with a control setting is in agreement with the improvement of time trial performance within, but not between groups after normobaric LHTL (21,22). Thus, the importance of strong study designs that can rule out potential biases such as training volume, training camp effects, and placebo or nocebo effects also becomes clear based on the available LHTL studies applying normobaric hypoxia.
Arguably, the strongest study design applied to date when investigating the effects of normobaric LHTL on sport-specific exercise performance is a double-blinded placebo-controlled cross-over design (12), whereas two other studies have applied a double-blinded placebo-controlled design (13,21). Unfortunately, the blinding failed in the study by Saunders et al. (21), why the placebo group would only serve as a control group. The two successful double-blinded and placebo-controlled studies demonstrate unchanged power output during a 26-km cycling time trial in highly trained triathletes (12) and elite cyclists (13) compared with placebo, after 42 d at 2500–3500 m for 8 h·d−1 using altitude tents and 28 d at 3000 m for 16 h·d−1 using hypoxic rooms. It has to be noted that only seven athletes completed the cross-over-designed study and a coefficient of variance of 3% on time trial performance was reported. Such variance may be too high for detecting sport-specific exercise performance changes as most studies report a 1%–3% change in time trial performance (Tables 1 and 2). Furthermore, the iron status of the athletes was not controlled and iron was not supplemented, which may in some cases (39), but not all (22,40), be important as iron is important for an effective erythropoiesis (41). Moreover, it should be noted that large individual variations in time trial performance were present in the study by Siebenmann et al. (13). For instance, one athlete of the LHTL group dropped 50 W in mean power output (corresponding to ~20%) in the first week after the LHTL intervention compared with the final days of the intervention but regained 50 W or more in the second week after LHTL. Fluctuations of such magnitude are unlikely to be due to the LHTL exposure and more likely to be caused by other factors (e.g., motivation or fatigue), which may increase the risk of type II errors.
As previously mentioned, timing of performance evaluation subsequent to LHTL may be of importance. Multiple time points subsequent to normobaric LHTL have been investigated, and the lack of a performance beneficial effect of normobaric LHTL has been observed after a few days (19,21,22) as well as 1–3 wk after normobaric LHTL (12,13,19) when compared with a control group. Thus, the lack of improvement in performance after normobaric LHTL does not seem to be related to the timing of performance evaluation.
The described studies evaluating sport-specific exercise performance after normobaric LHTL applied a range of 10–42 d with 8–16 h·d−1 of exposure at an altitude of 2200–3500 m. Of the best controlled studies (13,21), the hypoxic doses are well within the recommendation, whereas the study by Bejder et al. (12) applied a daily exposure of 8 h·d−1, which is slightly lower than recommended. However, more than 8 h·d−1 in an altitude tent seem unrealistic for most athletes, and the short daily exposure may be counterbalanced by the long duration of the intervention as indicated by the total hypoxic dose of 336 h at 2500–3500 m, which is well within the recommendations. Thus, it seems that neither the timing of performance evaluation nor the applied hypoxic dose can explain the lack of improvement in sport-specific exercise performance in the most well-controlled conducted normobaric LHTL studies. Furthermore, registration of training volume and intensity as well as ensuring sufficient iron availability is well documented in the majority of the existing normobaric LHTL studies evaluating sport-specific exercise performance.
In summary, the effect of normobaric LHTL on sport-specific exercise performance seems unclear, as studies applying a statistical approach of magnitude-based inferences using effect sizes report an improvement compared with control, whereas the remaining studies applying a traditional statistical approach demonstrate no improvements in sport-specific exercise performance compared with control. Furthermore, few of the conducted normobaric LHTL studies have applied a strong study design and conclusions have to be drawn cautiously. Finally, the applied hypoxic dose or timing of performance evaluations does not seem to explain the lack of improved performance. Future studies investigating whether normobaric LHTL affects sport-specific exercise performance should ideally perform measurements that are close to real-life competition performances and apply strong study designs.
DOES PERFORMANCE DURATION AFFECT LHTL EFFICIENCY?
Because of the red blood cell volume expansion with altitude acclimatization, LHTL is hypothesized to induce improved performance in disciplines with a high aerobic energy demand. However, as the red blood cells account for approximately 70% of the blood buffer capacity (42), an LHTL-induced increase in red blood cell volume may increase the systemic buffering capacity (43,44) with possible implications for performance demanding a high anaerobic energy metabolism. Thus, it may be expected that brief intense exercise performance could also be improved by LHTL. Albeit the impact of exercise performance duration has not been systematically investigated in existing LHTL studies, the difference in performance duration in existing studies allows for an initial evaluation. Figure 2 illustrates the impact of LHTL on sport-specific exercise performance dependent on the performance duration. It seems evident that there is no clear effect of the exercise duration. Thus, the relative importance of aerobic and anaerobic energy production appears of little relevance for the outcome of LHTL at least when durations are higher than approximately 1 min. Importantly, the differences in duration of the sport-specific exercise performance measured in both hypo- and normobaric LHTL studies do not seem to explain the lack of strong scientific evidence supporting that LHTL improves exercise performance. Notably, the figure also visually highlights the importance of a control group, as the performance improvement often appears similar in the control and intervention group.
HYPOBARIC VERSUS NORMOBARIC LHTL
The possibility of a different physiological response to normobaric and hypobaric hypoxic exposure exists (31). In addition, sea-level performance after hypo- or normobaric LHTL may also differ as previously indicated (45). Two studies with the specific aim to investigate the potential difference between hypo- and normobaric hypoxic exposure have been conducted, one including a cross-over design (27) and another including a separate control group (22). Interestingly, both studies demonstrated that 18 d of LHTL at 2250 m of hypo- or normobaric hypoxia elicited a similar 1%–6% improvement in 3000-m time trial performance (22,27). However, as highlighted by the authors, the hypoxic dose of the hypobaric hypoxia group was in both studies approximately 80 hours or approximately 4.4 h·d−1 larger compared with the exposure of the normobaric hypoxia group, which potentially blurs the true difference between treatments. Furthermore, in the study by Saugy et al. (27), the statistical power for the group effect was reported as rather low (power = 0.47), which introduces the risk of false-negative results. Thus, to conclusively determine whether hypobaric and normobaric hypoxia per se causes the same exercise performance adaptations, studies exposing the same amount of hypoxia to both groups with high statistical power are necessary. Nevertheless, the difference in hypoxic exposure time between the normobaric and hypobaric hypoxia groups may be justified, as a higher exposure time is expected during hypobaric LHTL compared with normobaric LHTL, primarily due to limitations in the time an athlete willingly resides in normobaric hypoxic exposure. Thus, hypobaric and normobaric LHTL interventions can be assumed to have similar effects on performance.
Hypo- and normobaric LHTL is widely used with the purpose of enhancing sport-specific exercise performance. However, the best controlled studies within each method of exposure cannot demonstrate a performance enhancement compared with a placebo or control group. Thus, the scientific evidence for both hypo- and normobaric LHTL to improve sport-specific exercise performance seems to lack strong scientific support. The majority of studies demonstrating an improved sport-specific exercise performance has applied study designs with no control group and cannot account for the potential bias of placebo or training camp effects. Furthermore, recent evidence suggests that LHTL established via hypo- or normobaric exposure results in equal performance enhancements. However, well-designed and controlled LHTL studies with an equal exposure time between hypo- or normobaric hypoxia are warranted to clarify the potential difference of the different exposure method on performance per se.
In addition, from the present review, it is clear that LHTL does not impose a negative effect on sport-specific exercise performance and thus is a training strategy that can be applied by elite athletes without having detrimental effects on performance. However, considering the cost of LHTL in time and money, athletes, coaches, and staff may choose alternative strategies for optimal performance enhancement. Nonetheless, it cannot be ruled out that a protocol investigating the effects of a novel combination of altitude level, altitude duration, training intensity, recovery, and the like may yield an improvement in sport-specific exercise performance, but we argue that if any it is most likely to be minor.
No funding was received for the present review.
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