Hypoxic Training Is Beneficial in Elite Athletes

Abstract

There is no doubt that many hypoxic conditions or prolonged exposures to altitude result in “biological costs of hypoxic adaptations that outweigh their benefits” (1), particularly in endurance athletes exposed to (a) exercise-induced arterial hypoxemia leading to a larger decrease in V˙O2max and aerobic endurance, (b) increased sympathetic activity and decreased baroreflex sensitivity, and (c) increased pulmonary arterial pressure. There is also no doubt that sleeping in moderate altitude (2000–3000 m) as performed by the athletes using either live high-train high (LHTH) or live high-train low (LHTL) methods leads to periodic breathing, intermittent hypoxia (IH), and increase in desaturation periods; for example, 3% oxygen desaturation index, but to a larger extent in hypobaric hypoxia (HH) (real altitude) than in normobaric hypoxia (NH) (simulated altitude), as shown at 2250 m (2). The three main questions debated in the present contrasting perspective are, however, different: Are there any evidences showing if the “counteracting maladaptation” (reported above) outweigh the benefits of the different hypoxic methods at short- or long-term in elite athletes? Are there any robust data supporting that hypoxic training is beneficial in elite athletes? Contradictory, are there robust data showing that hypoxic training is not beneficial in elite athletes? EVIDENCE OF “COUNTERACTING MALADAPTATION” OUTWEIGHING THE BENEFITS OF ALTITUDE TRAINING? Answering the first question is easy: as stated by Dempsey and Morgan (1), the “available evidence that predicts that the maladaptive responses (to hypoxic training/exposure or cyclical IH) would oppose or even erode the key adaptive performance-enhancing mechanisms elicited by physical training … are not verified yet.” These concerns are, therefore, of interest but remain purely theoretical and speculative. Moreover, the severity of altitude recommended for both LHTL and LHTH methods (i.e., 2200–2500 m) (3,4) is too low for inducing high-altitude illnesses (acute mountain sickness, high altitude pulmonary, or cerebral edemas) (5). Finally, there is a growing literature showing that adequate monitoring of the responses/behavior of the athletes may limit some hypoxic-induced detrimental effects; illness, dehydration, or sympathetic-induced “fatigue” (3). For example, heart rate variability guided training is effective for limiting perceived fatigue during LHTL (6). A vast majority of elite endurance athletes and support staff utilized altitude training and considered hypoxia as “very important” (7). Would you trust your doctor not asking you how you feel after a treatment? Or worse, maintaining that the medication was harmful or ineffective if you feel better? STUDIES SUPPORTING THAT HYPOXIC TRAINING IS BENEFICIAL IN ELITE ATHLETES? From the original review by Wilber (8) that defined three models (LHTH, LHTL, and live low-train high [LLTH]) used in the resting state (IH exposure [IHE]) or during training sessions (intermittent hypoxic training [IHT]), the panorama of the hypoxic training methods utilized in sport has been largely updated in two directions: first, the possibility to combine different methods; for example, LHTL + IHT; second the development of new methods at high-intensity, potentially useful for intermittent (team-, racket- or combat sports) athletes (9,10). There are many articles (11) supporting the positive hematological effects of LHTH or LHTL as long the hypoxic dose is high enough. The increase in total hemoglobin mass (Hbmass) is estimated at a mean rate of 1.0% to 1.1% per 100 h of exposure in both NH and HH conditions (12,13). In a critical review (4), some relevant weaknesses or methodological limitations have been emphasized as the needs of better controlling the placebo effects with elite athletes; but the main conclusion was “LHTH and LHTL may increase exercise performance in some but certainly not in all athletes” and did not throw the baby out with the bathwater. Several confounding factors that may limit the Hbmass increase, such as the health (illness/injuries) status of the athlete (14), insufficient iron store/supplementation (15), insufficient hydration for compensating the increased respiratory water loss (hyperventilation) and diuresis (16), and insufficient energy (particularly carbohydrate) availability (16), are now clarified and thus better monitored on the field by the servicing physiologists who support athletes during altitude training camps. Importantly, hypoxic training is not limited to LHTH and LHTL anymore, and the recent implementation of innovative methods, such as repeated-sprint training in hypoxia (RSH), is an important step forward (17,18). Despite its novelty, RSH is of high interest in exercise physiology: with 25 experimental studies published in the 5-yr period (19) after the pionneer RSH article in 2013 (18), RSH is shown to be effective for improving repeated-sprint ability in intermittent (team—rugby, football, field hockey; racket (tennis), as well as endurance (cycling, cross-country ski) sports (for an updated review: [19]). Moreover, from a mechanistic point of view, RSH questions the nonhematological responses to hypoxia: The underlying mechanisms are specific to RSH and not observed neither with passive exposure to hypoxia nor in the other hypoxic methods utilizing lower training intensities. The transcriptional and vascular responses lead to improved behavior of fast-twitch fibers, notably via compensatory vasodilatation and faster rate of phosphocreatine resynthesis (18,20). To our knowledge, there is no maladaptation to RSH (e.g., impaired immune function) identified yet. STUDIES SUPPORTING THAT HYPOXIC TRAINING IS NOT BENEFICIAL IN ELITE ATHLETES? An interesting point of view that “altitude training does not convincingly increase exercise performance and should not be recommended to elite (endurance) athletes” (21) is not based on the “counteracting maladaptation” discussed above (point 1) but on the assumption that athletes with a high initial Hbmass value are close to a “ceiling” level and would, therefore, not increase Hbmass and maximal oxygen consumption (22). We had already the opportunity to state that some of the previous studies supporting this noneffectiveness of LHTL may come from inaccurate data coming from “noisy” (poor “signal-to-noise ratio”) data with a relatively high typical error of Hbmass measurement (23). Furthermore, additional findings (24) confirmed that even athletes with high initial Hbmass value did benefit from a substantial increase (3%–4%) as long as the hypoxic dose was high enough (200–230 h at 2250 m). Recently, an additional study (25) on elite cross-country skiers performing LHTL with 26 nights at 2207 m (terrestrial altitude) did not show any additional effect on running economy, performance, oxidative muscle capacities, or lung diffusive capacity when compared with a LLTL group that trained up to 1500 m and slept at 1035 m. Unfortunately, there was no sea-level control group. Overall, it remains unclear if these authors are discussing the noneffectiveness of all hypoxic/altitude methods or if they restrict their concerns only to LHTL. If so, we concede that we could find an agreement because the superiority of LHTL over LHTH remains questionable, unclear, and probably overestimated. Regarding the effectiveness of RSH, only two studies of 25 did not report some positive outcomes (19). Although further work is requested on the underyling mechanisms and the optimal parameters specific to each sport, there is little doubt that this innovative method brings improvement in repeated-sprint ability (19), as now admitted by Prof. Lundby (http://www.worldrowing.com/photos-videos/videos/2018-world-rowing-coaches-conference-thursday 4:09:13 to 4:11:10). In conclusion, there are some maladaptative mechanisms related to altitude exposure but most are not relevant to the conditions (altitude severity, duration of exposure, …) and methods recommended and used by elite athletes. The altitude-induced erythropoeitic effects and improvement in oxygen transport capacity are observed in most athletes as long as the hypoxic dose is important enough. There are new effective hypoxic methods in intermittent sports. The robustness of most contradicting studies that reported a noneffectiveness of altitude training (LHTL only?) methods is questionable. RESPONSE TO SIEBENMANN AND DEMPSEY Siebenmann and Dempsey (26) argued that “the available evidence does not justify recommending any of the existing hypoxic training methods (LHTH, LHTL, or LLTH).” Despite our high respect for their work, our opponents’ points are often confused, due to the use of many references not directly related to the topic and erroneous statements. LHTH We agree that there are not enough well-controlled studies on LHTH likely due to logistical difficulties and the impossibility of “double-blinding” the protocol. Our opponents mentioned the “only controlled” LHTH study by Rodriguez et al. (27) where the two groups of swimmers who lived for 4 wk and train permanently (Hi-Hi) or partly (Hi-HiLo) at 2320 m improved their aerobic performance (400-m freestyle) to a larger extent that the control group (3.3% and 4.7% vs 1.6%). However, at least two other controlled studies on LHTH have been published: Bonne et al. (28) reported an Hbmass increase (by 6.2%) in elite swimmers who lived and trained 3 to 4 wk between 2130 and 3094 m. This LHTH group tended to improve aerobic performance (3000-m freestyle) to a larger extent than a sea-level (SL) control group. Similarly, Mellerowicz et al. (29) showed that moderately trained athletes who performed 4 wk at 2020 m had a larger improvement in V˙O2max and aerobic performance (3000-m running) than a SL group. Cherry picking? It is known that hypoxic exposure induces an increase in hemoglobin mass (Hbmass) (14,27,28,30–33). This was also shown in a study by Siebenmann et al. (34) who reported a 5.3% increase after 4 wk at 3450 m of altitude. Interestingly, their findings are in line with the most common recommendations and practice for LHTH duration (2 to 4 wk, (35)) because the increase started after 12 d with a plateau after 20 to 24 d. Regarding the potential adverse impacts—called “maladaptation”—that altitude can exert (e.g., chemoreflex activation, pulmonary vasoconstriction, impaired sleep quality, and recovery), none has been shown in elite athletes, is therefore only speculative, and highlights areas for future research. LHTL In this section, our opponents reported only “three controlled LHTL studies using natural altitude.” In the first study (32), despite a nonsignificant difference in training loads between groups, Hbmass increased by 4.4% and 4.1% in two LHTL groups who spent 18 d at 2250 m either in HH or in NH but remained unchanged in a control group. Moreover, the aerobic performance (3000-m running) was improved by 3.9% and 3.3% in the two LHTL groups and only by 2.1% in the control group. The second study (27) protocol was misunderstood by our opponents because the Hi-HiLo group trained only occasionally at low altitude. Finally, one may question the relevance of the third study (25) because there was no SL control group (36). One may question why our opponents did not extend their search to all LHTL studies? Cherry picking again? For example, in team-sport players who spent 14 d of LHTL at 3000 m, aerobic performance (YoYoR2) and Hbmass were significantly improved, whereas no change occurred in the SL control group (33). Our opponents do not recommend altitude/hypoxic training in elite athletes. Fine! But did they provide any evidence of deleterious effects of LHTH or LHTL reported in elite athletes? Or is their position purely theoretical? To our knowledge, there are no controlled studies where LHTH or LHTL led to performance impairment, when compared with a SL group. We concede that one of the confounding factor that remains debated is the definition of the optimal hypoxic dose and optimal altitude for erythropoietic responses: most of the LHTH studies conducted at low altitude (<1900 m) (37,38)—but not all (39)—did not lead to higher change in V˙O2max or performance than the control group. With other experts in the field, we do recommend an altitude between 2200 and 2500 m (3,35,40). Overall, we believe that the skepticism about the effectiveness of LHTL strategies comes originally from inaccurate Hbmass measurement (41–43), as stated by eminent researchers (13,44), working daily with elite athletes (23). However, we concede that the sometimes claimed superiority of LHTL over LHTH deserves further investigation. From a practical point of view, the optimal strategy remains likely to combine the different hypoxic training methods depending of the training phases and athlete/sport characteristics (33,35) to optimize the benefits and minimize the risks associated to altitude training. LLTH Among the various LLTH methods, RSH has gained in popularity with >26 articles published by various research groups in the last 5 yr (19), confirming its putative benefits displayed in this meta-analysis (45). Regarding the point raised by our opponents on the use of SE from one study (46), Comprehensive Meta-Analysis Software (Biostat, Inc., Englewood, NJ) allows user to create multidata entry formats, permitting for example to report SD, odds ratios, or 95% confidence intervals (in this case, SE is most useful as a means of calculating a confidence interval than SD (47)). Of note, recalculating SD remains possible using single unknown equation (SE = SD/√[sample size]) and might be more relevant than estimation of variance “from sample size and SD” or “from SD alone” (48,49). It is evident that detecting any publication bias is of paramount importance to avoid incorrect conclusions. In this view, Begg and Mazumdar’s rank correlation, Egger’s regression tests, and asymmetry examination of funnel plots were reported in (45). It is also obviously interesting to consider studies that did not report any RSH benefits to better define protocols. Unfortunately, the outcome of the only crossover study reporting no additional performance benefits of RSH (50) is likely due to methodological shortcomings, with too many tests after the intervention, precluding subjects to perform maximally. CONCLUDING STATEMENT The accumulating body of scientific evidences and the increasing interest/utilization by elite athletes for the different methods of altitude training are coherent. Because hypoxic training is beneficial if adequately performed and monitored, we do recommend its use by elite athletes. Expertise in this field requires understanding the pros and cons of each method (LHTH, LHTL, or LLTH) and how to combine them.

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