Repeated Sprint Training in Hypoxia – An Innovative Method

The year 2018 marked the 50th anniversary of the Mexico Olympic games, which represents the starting point of scientific research on hypoxic training. Since the original “Live High – Train High”, many altitude/hypoxic training methods have been developed. The aim of the present review is to present the most recent method called “Repeated Sprint training in Hypoxia” (RSH). RSH is of unprecedented interest in the altitude training area with 25 studies published in the 5-year period following the pioneer article in 2013, and with only two studies that did not report any beneficial effects.

Potential mechanisms include transcriptional factors involved in oxygen-signaling and oxygen-carrying capacity and mitochondrial metabolism enzymes, improved behavior of fasttwitch fibers notably via compensatory vasodilatation, improved vascular relaxation and greater microvascular oxygen delivery as well as faster rate of phosphocreatine resynthesis.

In general, RSH leads to superior repeated-sprint ability (i.e., faster mean sprint times or higher power outputs associated witha better resistance to fatigue during a repeated-sprint test) in normoxic conditions. RSH where hypoxia is induced by voluntary hypoventilation at low lung volume (named VHL) may also improve repeated-sprint performance more than in normoxia.

Practically, RSH benefits have been demonstrated for a large range of team- (rugby, football, LaCrosse, Australian Football, field hockey), endurance (cycling, track and field, cross-country ski), racket (tennis) or combat (Jiu-Jitsu) sports.

KEY WORDS: Altitude, High-Intensity, Team-Sports, Performance, Hypoxia

Das Jahr 2018 markierte den 50. Jahrestag der Olympischen Spiele in Mexico City, dem Beginn wissenschaftlicher Untersuchungen von Hypoxie-Trainingsmaßnahmen. Seit dem ursprünglichen Konzept des „Live High – Train High“ ist eine Vielzahl von Höhen- und hypoxischen Trainingsformen entwickelt worden. Das Ziel dieses Reviews ist es, die neueste Methode, d. h. „Wiederholtes Sprinttraining in Hypoxie“ (Repeated Sprinttraining in Hypoxia, RSH) zu präsentieren. RSH trifft aktuell auf ein äußerst großes Interesse im Bereich des Höhentrainings, was anhand von 25 internationalen Studien in den vergangenen fünf Jahren deutlich wird, von denen nur zwei keine Leistungssteigernden Effekte fanden.

Mögliche Mechanismenkönnten veränderte Transkriptionsfaktoren sein, die in den Sauerstoff-Signalweg, die Sauerstoff-Transportkapazität, enzymatische Aktivität des mitochondrialen Stoffwechsels, eine stärkere kompensatorische Dilatation insbesondere in den fast-twitch Fasern, eine verbesserte vaskuläre Relaxation, eine größere microvaskuläre Sauerstoffabgabe und eine schnellere Kreatinphosphat Re-Synthese involviert sind.

Generell führt RSH zu einer verbesserten Leistung bei Sprintwiederholungen (d.h., schnellere Sprintzeiten oder geringere Leistungsminderung bei wiederholten Sprints) unter normoxischen Bedingungen. Desgleichen wird, wenn die Hypoxie durch gewollte Hypoventilation bei geringem Lungenvolumen (VHL) erzeugt wird, die wiederholte Leistungsfähigkeit stärker verbessert als bei Training in Normoxie.

In der Praxis profitieren unterschiedlichste Athleten von RSH, d. h. Teamsportler (Rugby, Fußball, LaCrosse, Austalian Football, Feldhockey) Ausdauersportler (Rennradfahrer, Leichtathleten, Skilangläufer) Tennisspieler und Kampfsportler.

SCHLÜSSELWÖRTER: Höhe, hohe Intensität, Team-Sport, Leistung, Hypoxie

The year 2018 was the 50th anniversary of the Mexico Olympic games (2340 m, barometric pressure 580 mmHg) that was a pivotal point in sport history. Following the dominance of altitude acclimatized athletes during this event, altitude/hypoxic training has become increasingly popular among individual endurance athletes.

Therefore, the early 1970s was the starting point of the scientific investigation on the effectiveness of altitude training. At this time, several altitude training centres (e.g., Font-Romeu in France, Saint-Moritz in Switzerland, Colorado Spring in USA, Kunming in China) were developped for the exclusive purpose of “Live High – Train High” (LHTH) and its associated hematological benefits in endurance aerobic sports – i.e., an increase in hemoglobin mass (Hbmass) – which most often results in an improvement in maximal oxygen consumption (VO₂max) (44). It has been speculated recently that the effect of altitude/hypoxic training on Hbmasswould depend on baseline level; i.e., athletes with already high initial values would not increase their Hbmassmarkedly following the intervention; conversely, athletes with a low initial Hbmassvalue would demonstrate greater Hbmassgains (41). However, this analysis is based on inaccurate data (19, 48) and this theory has therefore been refuted (35). Hence, 3-4% Hbmassgains have been observed in highly-trained athletes with already high initial Hbmassvalues (>14g.kg-1) before the intervention (27, 44).

Since early 1970s’ and the popularity of LHTH, many altitude/training methods have been developed such as “Live High-Train Low” (LHTL) or more recently “Live Low-Train High” (LLTH) (Table 1) (23, 38). The 50thanniversary of Mexico Olympic Games was the opportunity to scrutiny the major evolution of altitude training methods during these last 50 years. For this purpose, an international symposium was organized in Font-Romeu on 7-8thJuly 2018, where Ben Levine stated that “three inputs have been paramount over this period: 1. The introduction of Living High – Training Low (LHTL) in the 1990s; 2. The development of different normobaric hypoxic (NH) facilities that make easier for athletes to use hypoxic training everywhere; and 3. The development of RSH in 2010s”.

The aim of the present review is therefore to present updated information on the most recent hypoxic method, the so-called “Repeated Sprint training in Hypoxia” (RSH).

The RSH paradigm requires the completion of maximal, short duration (typically ≤30s) efforts interspersed with incomplete recovery periods (≤60s) in hypoxic environment (6, 23). A fundamental difference with “Intermittent Hypoxic Training” (IHT; i.e., interval-training performed in hypoxia) is the “allout” effort required by RSH, which demands a very high recruitment of fast-twitch fibers. Single sprint performance is well preserved up to altitudes even higher than 3500 m (23) and for maximal efforts up to 60 s. However, repeating sprints in hypoxia results in performance decrement (2) with its magnitude being dependent on both the exercise duration and the exercise:recovery ratio, which in turn determines the oxidative and glycolytic contribution (13, 37).

Since 2007, it has been demonstrated that voluntary hypoventilation at low lung volume (VHL) could induce severe arterial oxygen desaturation (51, 52) leading to muscle (52) or cerebral (50) deoxygenation. In these conditions, the hypoxic state has been shown to be similar to what is observed at an altitude of about 2400m (49). The VHL approach is now recognized as one altitude training method potentially useful for a wide range of sporting activities (38). Noteworthy, two recent repeated-sprint studies have reported that training under hypoxic stress that is created through VHL produces larger improvement in repeated sprint ability (RSA) than training near sea level with an unrestricted breathing pattern (16, 45).

Acute hypoxia induces an increased skeletal muscle sympathetic discharge leading to an enhanced vasoconstrictor activity during exercise. Despite this mechanism, contracting skeletal muscle blood flow and coronary sinus blood flow increase, contributing to the maintenance of cardiac and peripheral O2delivery (26). This increase in blood flow is the consequence of a compensatory vasodilatation that aims at maintaining constant the total O2delivery to the muscle (10). At submaximal exercise intensity in hypoxia, this “compensatory” vasodilatation is well-established (10) and aims to ensure an augmented blood flow and maintenance (or limit the alteration) of oxygen delivery to the active muscles. Nitric oxide (NO) produced by endothelium seems the primary vasodilatory candidate since significant blunting of the augmented vasodilatation was reported with nitric oxide synthase (NOS) inhibition during hypoxic exercise (11). However, the source of NO contributing to compensatory dilatation seems less dependent on β-adrenergic mechanisms as exercise intensity increases. There are other candidates for stimulating NO release during higher intensity hypoxic exercise, such as ATP released from erythrocytes and/or endothelial derived prostaglandins. Overall, this enhanced hypoxic exercise hyperemia is proportional to the hypoxia-induced drop in arterial O2content and therefore its magnitude is larger at high than at low exercise intensity.

RSH is a recent training method mainly based on the above-described mechanisms and differs from interval-training in hypoxia performed at or near maximal aerobic power (13). Indeed, RSH performed at maximal intensity likely leads to a greater muscle perfusion and oxygenation (6) and specific muscle transcriptional responses (7, 14). Several mechanisms have been proposed to explain the effectiveness of RSH: during sprints in hypoxia, the compensatory vasodilation and associated higher blood flow would benefit more to the fasttwitch fibers than the slow-twitch fibers (13). Consequently, RSH efficiency is likely to be fiber-type selective and intensity dependent; i.e. requiring maximal intensity and short exercise bouts (13, 37). With repeated maximal-intensity hypoxic efforts, specific skeletal muscle tissue adaptations may arise through the oxygen-sensing pathway (i.e., capillary-to-fiber ratio, fiber cross-section area, myoglobin content and oxidative enzyme activity such as citrate synthase) that do not occur in normoxia or to a lesser degree if they do occur (28, 53). Potential mechanisms involve transcriptional factors involved in oxygen-signaling and oxygen-carrying capacity and mitochondrial metabolism enzymes, improved behavior of fasttwitch fibers (34), notably via compensatory vasodilatation and faster rate of phosphocreatine resynthesis (7, 14, 28, 53). Based on preliminary results on mice performing supramaximal exercise in hypoxia, we speculate that RSH likely also improves responsiveness of the vascular bed (32).

To date, the increasing popularity of the RSH method is shown by the publication of 26 papers on this topic in the last five years and its effective use by many professional sports (e.g., Welsh rugby, Roland Garros tennis training centre, Aspire football Academy, Australian football squads, cross-country sprint skiers). Table 2 presents the updated review of these 25 studies + 1 meta-analysis on RSH (up to 1st January 2019) in a chronological order.

Firstly, despite its novelty, RSH is of high interest in applied exercise physiology: with 25 experimental studies published in the 5-years period following the pionneer RSH article in 2013 (14). This represents more studies than for LHTL (18 studies between 1997 (33) and 2002).

Secondly, this method is of interest in a wide range of team- (rugby league, rugby union, football, LaCrosse, Australian Football, field hockey), endurance (cycling, track and field, cross-country ski), racket (tennis) or combat (Jiu-Jitsu) sports.

Thirdly, in general, superior performance outcomes, in particular for RSA, in normoxic conditions have been associated with RSH vs. RSN (repeated sprint training in normoxia) studies. For instance, RSH produces faster mean sprint times and/or smaller speed decrements compared to RSN (See Table 2 and (6)), likely resulting from an improved fatigue resistance. Following 6-8 RSH sessions, the number of sprints completed before task failure increased in well-trained cyclists (14), cross-country skiers (15) or rugby players (16) during an “openloop” protocol. Improvement in RSA has also been reported in “close-loop” protocol (1, 16, 21, 30, 31, 40, 47) meaning an increased velocity or power output during a repeated-sprint test. Recently, larger improvement in time to exhaustion during a tennis-specific incremental test was also reported in tennis players following RSH vs RSN (16). Negligible enhancement in glycolytic capacity is generally reported with RSH. However, RSH-VHL might be more effective for improving glycolysis due the hypercapnic effect of the apnea phase (45). Similarly, VO₂max improvement caused by RSH are also likely minimal (6, 14).

Fourthly, only two studies (24, 39) of 25 did not report any additional beneficial effects of RSH: the completion of 12–15 cycling RSH sessions over 4-5 weeks did not lead to further improvement in RSA, compared to similar normoxic training (24, 39). Arguably, the use of non-specific cycling RSH training in team- or racket- sport athletes performing predominantly run-based activities (24) and methodological shortcomings (i.e., absence of protective pacing measures leading to submaximal intensity, especially over long repeated-sprints sets) may potentially explain the absence of additional effect of RSH vs. RSN.

More than 50 years after the first scientific publications on altitude/hypoxic training and the launch of prestigious altitude training centers (e.g., Font-Romeu in France; Saint Moritz in Switzerland) for the preparation of the Mexico 1968 Olympic Games, major progress has been made in improving athletic performance by hypoxic training measures and understanding its underlying mechanisms. RSH is undoubtedly a promising and well-tolerated training model (6, 8) although its underlying mechanisms remain partly hypothetical at this stage. Further researches are required in order to confirm if the effectiveness of RSH comes from an improved muscle blood perfusion, which in turn would benefit from optimized oxygen extraction by fast-twitch fibers.

Recently, positive adaptive mechanisms on endothelial function leading to larger vascular relaxation have been described in mice performing supramaximal exercise in hypoxia, when compared to similar exercise in normoxia (32), paving the way for further investigation on the therapeutic use of high-intensity hypoxic exercise in patients (36).

Conflict of Interest
The authors have no conflict of interest.