Adam Beard

John Ashby

Mark Kilgallon

Franck Brocherie

P Millet

ISSUL Gregoire P Millet
email: gregoire.millet@unil.ch

Upper-body repeated-sprint training in hypoxia in international rugby union players

Keywords: Repeated-sprint training in hypoxia, repeated-sprint ability, team sports, competition, upper limbs, rugby union Highlights

This study investigated the effects of upper-body repeated-sprint training in hypoxia vs. in normoxia on world-level male rugby union players' repeated-sprint ability (RSA) during an international competition period. Thirty-six players belonging to an international rugby union male national team performed over a 2-week period four sessions of double poling repeated-sprints (consisting of 3 × eight 10-s sprints with 20-s passive recovery) either in normobaric hypoxia (RSH, simulated altitude 3000 m, n = 18) or in normoxia (RSN, 300 m; n = 18). At pre-and post-training intervention, RSA was evaluated using a doublepoling repeated-sprint test (6 × 10-s maximal sprint with 20-s passive recovery) performed in normoxia. Significant interaction effects (P < 0.05) between condition and time were found for RSA-related parameters. Compared to Pre-, peak power significantly improved at post-in RSH (423 ± 52 vs. 465 ± 69 W, P = 0.002, η²=0.12) but not in RSN (395 ± 65 vs. 397 ± 57 W). Averaged mean power was also significantly enhanced from pre-to post-intervention in RSH (351 ± 41 vs. 388 ± 53 W, P < 0.001, η²=0.15), while it remained unchanged in RSN (327 ± 49 vs. 327 ± 43 W). No significant change in sprint decrement (P = 0.151, η² = 0.02) was observed in RSH (-17 ± 2% vs. -16 ± 3%) nor RSN (-17 ± 2% vs. -18 ± 4%). This study showed that only four upper-body RSH sessions were beneficial in enhancing repeated power production in international rugby union players. Although the improvement from RSA to game behaviour remains unclear, this finding appears of practical relevance since only a short preparation window is available prior to international games.

Introduction

Rugby union is a game that requires high levels of fitness and technical abilities to be successful [START_REF] Deutsch | Timemotion analysis of professional rugby union players during match-play[END_REF][START_REF] Duthie | Applied physiology and game analysis of rugby union[END_REF]. The physical requirements of rugby union are dependent on playing position and whether players are playing in attack or defense throughout the game [START_REF] Duthie | Applied physiology and game analysis of rugby union[END_REF], which demand players to complete upper-or lower-body dominant movements or combination of both. Upper-body movements include wrapping the arms around the player when tackling, grappling and wrestling in rucks, mauls and collisions and fending players off in attack. Lower-body movements include leg drives in tackling, jumping in line-outs, pushing in scrums, rucks and mauls and all running activities such as change of directions, accelerations, decelerations and sprinting [START_REF] Deutsch | Timemotion analysis of professional rugby union players during match-play[END_REF][START_REF] Duthie | Applied physiology and game analysis of rugby union[END_REF]. The intermittent nature of rugby union has shown this sport requires a high number of repeated bouts of maximal-intensity efforts involving the aforementioned upper-and/or lower-body activities to be performed throughout the game [START_REF] Duthie | Applied physiology and game analysis of rugby union[END_REF][START_REF] Quarrie | Positional demands of international rugby union: Evaluation of player actions and movements[END_REF].

High-intensity repeated efforts and sprints have been identified as a physical fitness quality potentially connected to performance within intermittent team sports such as rugby union and league, basketball, field hockey, soccer, and Australian rules football [START_REF] Austin | The physical demands of Super 14 rugby union[END_REF][START_REF] Gabbett | Predictors of repeated high-intensity-effort ability in rugby league players[END_REF]. Repeated-sprint ability (RSA) is a complex component of fitness and may have many factors that influence its level of performance such as neural and/or metabolic mechanisms (Bishop, Girard, & Mendez-Villanueva, 2011;Girard, Mendez-Villanueva, & Bishop, 2011;[START_REF] Glaister | Multiple sprint work: Physiological responses, mechanisms of fatigue and the influence of aerobic fitness[END_REF]. The ability to repeatedly produce high amounts of force or speeds (i.e. power) comes from the utilization of the fast-twitch fibres more so than their slow-twitch counterparts [START_REF] Girard | Repeated-sprint ability -part I: Factors contributing to fatigue[END_REF]. It may be therefore interesting to enhance the fatigue resistance of the fast-twitch fibres through specific RSA-type training. To date, the repeatedsprint training literature has mainly concentrated on the lower-body (i.e. ergocycling, treadmill and overground running), whereas a number of studies examining the upper-body responses to RSA have recently shown that there may be some differences between how the upper-and lower-body react to RSA training protocols [START_REF] Halperin | Upper and lower body responses to repeated cyclical sprints[END_REF][START_REF] Koppo | Oxygen uptake kinetics during high-intensity arm and leg exercise[END_REF][START_REF] Sandbakk | The physiological responses to repeated upper-body sprint exercise in highly trained athletes[END_REF][START_REF] Zinner | The physiological mechanisms of performance enhancement with sprint interval training differ between the upper and lower extremities in humans[END_REF]. For instance, [START_REF] Koppo | Oxygen uptake kinetics during high-intensity arm and leg exercise[END_REF] found that the fast-twitch fibres are recruited earlier and to a greater magnitude in upper-body high-intensity exercise (arm cranking) than in lower-body high-intensity exercise (cycling). This could be due to the higher concentration of fast-twitch fibres in the upper-body compared to the lower-body [START_REF] Terzis | Upper body training and the triceps brachii muscle of elite cross country skiers[END_REF], resulting in an increased reliance on anaerobic pathways [START_REF] Zinner | The physiological mechanisms of performance enhancement with sprint interval training differ between the upper and lower extremities in humans[END_REF].

Recently, it has been shown that combining lowerbody repeated-sprint training with hypoxia (RSH) led to greater RSA enhancement than the same training in normoxia [START_REF] Brocherie | Highintensity intermittent training in hypoxia: A double-blinded, placebo-controlled field study in youth football players[END_REF], p. 2017;Faiss, Girard, & Millet, 2013;[START_REF] Galvin | Repeated sprint training in normobaric hypoxia[END_REF]Girard, Brocherie, & Millet, 2017;[START_REF] Hamlin | Hypoxic repeat sprint training improves rugby player's repeated sprint but not endurance performance[END_REF][START_REF] Millet | Hypoxic training and team sports: A challenge to traditional methods[END_REF]. RSH is a method derived from the 'live low-train high' (LLTH) branch of the altitude/hypoxic panorama [START_REF] Girard | Effects of altitude/hypoxia on single-and multiple-sprint performance: A comprehensive review[END_REF][START_REF] Millet | Hypoxic training and team sports: A challenge to traditional methods[END_REF]. In 2013, when [START_REF] Faiss | Significant molecular and systemic adaptations after repeated sprint training in hypoxia[END_REF] reported better RSA improvements in well-trained cyclists after 8 RSH sessions in comparison to similar training in normoxia (RSN), they postulated that the maximal nature of the RSH paradigm favors the usage of fasttwitch fibres [START_REF] Faiss | Advancing hypoxic training in team sports: From intermittent hypoxic training to repeated sprint training in hypoxia[END_REF]. The drop in arterial oxygen content accompanied with any hypoxic exposure is then matched by the increased perfusion of blood to the active muscles which matches this drop in oxygen [START_REF] Casey | Compensatory vasodilatation during hypoxic exercise: Mechanisms responsible for matching oxygen supply to demand[END_REF]. RSH methodology being made up of maximal "all-out" efforts of short duration (<30 s) with incomplete recoveries [START_REF] Brocherie | Effects of repeated-sprint training in hypoxia on sea-level performance: A meta-analysis[END_REF][START_REF] Girard | Effects of altitude/hypoxia on single-and multiple-sprint performance: A comprehensive review[END_REF], would therefore impact favorably the fast-twitch fibres due to the "compensatory vasodilation" and subsequent hypoxia-induced enhanced exercise hyperaemia [START_REF] Casey | Compensatory vasodilatation during hypoxic exercise: Mechanisms responsible for matching oxygen supply to demand[END_REF]. This mechanism is known to be mediated by the nitric oxide synthase pathway and is intensity-dependent; being larger at higher intensity [START_REF] Casey | Nitric oxide-mediated vasodilation becomes independent of beta-adrenergic receptor activation with increased intensity of hypoxic exercise[END_REF]. RSH differs from intermittent hypoxic training (IHT) that utilizes intervals of either the aerobic or anaerobic systems and is completed at a sub-maximal level therefore not always employing preferential vasodilation of the vessels to the fast-twitch fibres [START_REF] Faiss | Significant molecular and systemic adaptations after repeated sprint training in hypoxia[END_REF].

As previously mentioned, the upper-body movements (e.g. tackling, rucking and mauling) are frequent repeated skills in rugby union [START_REF] Duthie | Applied physiology and game analysis of rugby union[END_REF][START_REF] Quarrie | Positional demands of international rugby union: Evaluation of player actions and movements[END_REF]. Therefore, it may be of interest to investigate the upper-body's responses to RSH. The upper-body muscles having a greater concentration of fast-twitch fibres than the lower-body muscles [START_REF] Terzis | Upper body training and the triceps brachii muscle of elite cross country skiers[END_REF], it was postulated that RSH using the upper-body musculature could potentially have larger performance gains. To date, [START_REF] Faiss | Repeated doublepoling sprint training in hypoxia by competitive cross-country skiers[END_REF] have been the sole study that has investigated RSH on the upper-body among cross-country skiers. While double poling may not be considered movement pattern specific to many team sports, it is regularly used as a training exercise in rugby union. Therefore, it was of interest to us to see if there were similar results in power-based athletes who utilized repeated upper-body movements such as rugby. This study showed significant benefits for RSH vs. RSN after six sessions over a 2-week period, with a larger ability to reduce the increasing muscle fatigue throughout the double-poling RSA set (i.e. 58% more sprints at post-in the RSH group) while no significant difference in power output was reported between the RSH and RSN groups. The increase in the number of sprints completed but not in power output between RSH and RSN groups may be explained with the participants (cross-country skiers) being predominately involved in endurance sport. With such positive results shown by [START_REF] Faiss | Repeated doublepoling sprint training in hypoxia by competitive cross-country skiers[END_REF], the number of sessions, length of hypoxic exposure and the duration of the training period is still to be optimally determined. Recently, it has been suggested that a short block of RSH in team [START_REF] Brocherie | Effects of repeated-sprint training in hypoxia on sea-level performance: A meta-analysis[END_REF]Brocherie, Girard, et al., 2015) and racket [START_REF] Brechbuhl | Shock microcycle of repeated-sprint training in hypoxia and tennis performance: Case study in a rookie professional player[END_REF] sports may be of interest to further determine the minimal dose especially in competition when RSH sessions are combined with other normoxic conditioning training. This would be of practical interest in many professional sports where the window for physical preparation between games is extremely short.

Therefore, the aim of the present study was to investigate the effects of upper-body RSH vs. RSN on RSA in world-level male rugby union players' during an international competition period. We hypothesized that completing only 4 sessions of upper-body RSH would potentially induce greater enhancement in RSA performance than RSN during an international competition period in senior International rugby union players.

Methods

Participants

Thirty-six world-level players (24.1 ± 2.7 years, 186.4 ± 6.3 cm and 103.9 ± 12.2 kg) belonging to a senior international rugby union male national team (at the time of the study, the team was the current Six Nations champions) participated in the study as part of their normal national squad training schedule in preparation for the Six Nations rugby union tournament. The experiment was conducted according to the Declaration of Helsinki and the study was approved by the local Ethical Committee (Commission Cantonale Valaisanne d'Ethique Médicale, CCVEM).

Experimental design

The study consisted of two testing sessions before (Pre-) and after (Post-) a 2-week "in-season" training period including 4 RSH vs. RSN sessions. Players were matched regarding their normal units (backs and forwards) and randomized in either normobaric hypoxia (RSH, 3000 m, F I O 2 , 13.8%; n = 18) or normoxia (RSN, 300 m; F I O 2 , 20.9%; n = 18) (Table I). The 2-week period consisted of 2 small sided games sessions, 8 specific rugby sessions, 2 captains run sessions, 4 weight training sessions and 4 recovery sessions. The specific repeated-sprint training intervention was inserted in the aforementioned program of the team. No additional tapering period was added (Table II).

Specific repeated-sprint training

As part of their normal national squad training, in a double blinded fashion players performed four additional double-poling ergometer (SkiErg: Concept 2, Morrisville, VT, USA) sessions over a 2-week period. Players' trained in either hypoxic or normoxic conditions for two sessions per week that were completed in a normobaric hypoxic chamber (POWERbreathe, Southam, England, UK). This consisted of three sets of eight 10-s sprints with 20s intervals of passive recovery with 2 min between each set, with players being encouraged to complete all repetitions maximally while trying to produce the highest power output possible (Figure 1). Overview of the training sessions consisted of a 5-min warm-up at 1 W kg -1 , followed by a subsequent 1-min block with a 10-s submaximal sprint and 50-s passive recovery and a second 1-min block repeating block 1 but with greater emphasis on maximal sprint. After 1 min of rest, the training exercise was initiated.

Players were given a count down and encouraged energetically to complete every sprint maximally. Total hypoxic exposure per session was 24 min for RSH while cool-down was completed in normoxic conditions for both RSH and RSN groups. To ensure the participants isolated the arm and shoulder musculature while minimizing the potential bias of the trunk and legs' contribution, players were instructed to stand with straight legs, to look up at the screen and not to flex at the torso. The SkiErg double-poling ergocycle was calibrated according to the manufacturer guideline prior to each session.

Repeated-sprint ability test

The RSA performance tests were both conducted at the same time of the day (between 10:00am and 12:00pm), and day of the week (Wednesday), while players were explained the test several days prior to testing and again the day before the test was completed. RSA was evaluated in normoxia using a double-poling ergometer (same as mentioned in training protocol) set at a fixed drag resistance (highest level -10) and consisted of 6 repeated double-poling 10-s maximal sprints with 20 s of passive recovery.

The test started with a 5-min warm-up at 1 W kg -1 , followed by a subsequent 1-min block with an isolated 10-s maximal sprint and 50-s passive recovery followed by a second isolated maximal sprint. After 3 min of rest, the test was initiated. Players were given a count down and encouraged energetically to complete each of the 6 sprints maximally. To avoid any protective pacing, average power during the first two sprints was controlled to reach at least 90% of the best performance (peak power output, PPO) of the two isolated sprints, which was the case in all subjects. PPO was recorded for each of the six repetitions and averaged mean power output (MPO) was reported for the entire 6 × 10-s effort). PPO was used to calculate the sprint decrement score (S dec , %) as follows: [1 -(total power/ideal power)] × 100; where total power is the cumulated PPO across repetitions, while ideal power refers to the highest PPO during one single 10-s sprint multiplied by the number of repetitions [START_REF] Glaister | The reliability and validity of fatigue measures during multiplesprint work: An issue revisited[END_REF]. The double poling technique was utilized in an upper-body fashion as mentioned in the training protocol.

Statistical analysis

Data are presented as mean (±SD) or relative changes (%) unless otherwise stated. Normal distribution of the data was tested using the Shapiro-Wilk test. Two-way repeated-measures analysis of variance (ANOVA) with 1 between factor (condition; RSH vs. RSN) and 1 within factor (time; Pre-vs. Post-) were used to compare RSA-related performance variables. Multiple comparisons were made using Holm-Sidak post hoc test. For each ANOVA, partial eta-squared (η²) was calculated as measure of effect size [START_REF] Levine | Eta squared, partial eta squared, and misreporting of effect size in communication research[END_REF]. Values of 0.01, 0.06, and 0.14 were considered as small, medium, and large, respectively. All analyses were made using Sigmaplot 11.0 software (Systat Software, Inc., San Jose, CA, USA). Null hypothesis was rejected at P < 0.05.

Results

Training exposure

Players using RSH were exposed to 24 min of normobaric hypoxia (3000 m, F I O 2 13.8%) per training session, thereby totalizing 96 min over the entire intervention. Conversely, RSN spent 24 min in normoxia per training session for a total of 96 min (300 m, F I O 2 20.9%).

Repeated-sprint ability

Figure 2(A,B) presents the time-course changes in the power output during the RSA test (6 × 10-s sprints -20-s passive recovery) before and after the intervention period. While no significant differences (P = 0.145) between groups were observed in any RSA-related variables at Pre-, significant condition × time interactions were found in PPO (P < 0.05, η² = 0.02) and averaged MPO (P < 0.01, η² = 0.03) performed during the RSA test. Post hoc tests revealed the beneficial effect of adding hypoxia to the repeated-sprint training (RSH) in PPO (P = 0.002, η² = 0.12) and averaged MPO (P < 0.001, η² = 0.15), compared to similar training in normoxia (RSN).

Relative to Pre-, significant increases in PPO were observed at Post-in RSH (423 ± 52 vs. 465 ± 69 W, P = 0.002, η² = 0.11), but not in RSN (395 ± 65 vs. 397 ± 57 W, P = 0.855, η² = 0.00) (Figure 2(A,B)). PPO significantly increased in all sprint repetitions in RSH (+13 ± 16%, all P < 0.001-0.01, η² ranging 0.08-0.28) but not in RSN (+3 ± 11%, all P > 0.156, η² ranging 0.00-0.03) (Figure 2). The improvement in PPO was significantly higher in RSH vs. RSN (all P < 0.01, η² ranging 0.11-0.20) in all sprint repetitions except the last one (P = 0.928). Likewise, averaged MPO significantly improved from Pre to Post intervention in RSH (351 ± 41 vs. 389 ± 53 W, P < 0.001, η² = 0.14), while it remained unchanged in RSN (328 ± 49 vs. 326 ± 41 W, P = 0.836, η² = 0.00) (Figure 2). No significant change in S dec (P = 0.151, η² = 0.02) was observed in RSH (-17 ± 2% vs. -16 ± 3%) nor RSN (-17 ± 2% vs. -18 ± 4%). There was an interaction (F = 4.485, P < 0.05) on the isolated sprint with RSH improving from Pre-to Post-(408 ± 72 vs. 457 ± 69 W, P = 0.002) but not RSN (386 ± 76 vs. 392 ± 67 W).

Discussion

In the present study, we investigated the addition of four upper-body repeated-sprint sessions either performed in hypoxia (RSH) or normoxia (RSN) to the normal squad training schedule during the 2week preparation period of the 6-nations international men's rugby union competition. The main findings were that completing repeated double-poling RSH showed a greater improvement in both PPO and MPO in upper-body RSA than RSN in world-level rugby union players. These results provide novel insights into utilizing physical development strategies in competition for high-level team-sport athletes, especially in sports that utilize upper-body frequently such as ice-hockey, field hockey, Australian rules football, lacrosse, American football, combat sports and both rugby union and league.

This demonstrates support for the growing literature on the utilization of RSH as an effective hypoxic training method for enhancing repeatedsprint parameters at sea-level [START_REF] Brocherie | Effects of repeated-sprint training in hypoxia on sea-level performance: A meta-analysis[END_REF]. Recently, it has shown benefits in rugby players [START_REF] Galvin | Repeated sprint training in normobaric hypoxia[END_REF][START_REF] Hamlin | Hypoxic repeat sprint training improves rugby player's repeated sprint but not endurance performance[END_REF] soccer players (Brocherie, Girard, et al., 2015), cyclists [START_REF] Faiss | Significant molecular and systemic adaptations after repeated sprint training in hypoxia[END_REF], lacrosse players [START_REF] Kasai | Effect of training in hypoxia on repeated sprint performance in female athletes[END_REF], tennis players [START_REF] Brechbuhl | Shock microcycle of repeated-sprint training in hypoxia and tennis performance: Case study in a rookie professional player[END_REF] and cross-country skiers [START_REF] Faiss | Repeated doublepoling sprint training in hypoxia by competitive cross-country skiers[END_REF], while it may be noted that many factors (e.g. sport/ level, type of athlete and protocol) may contribute to the outcome results [START_REF] Brocherie | Effects of repeated-sprint training in hypoxia on sea-level performance: A meta-analysis[END_REF]. In rugby, [START_REF] Galvin | Repeated sprint training in normobaric hypoxia[END_REF] showed a 33% improvement on the Yo-Yo intermittent recovery test level 1 (endurance capacity) with academy level rugby union and league players after 12 sessions of RSH compared to a 14% improvement for RSN completed over 4 weeks, whereas [START_REF] Hamlin | Hypoxic repeat sprint training improves rugby player's repeated sprint but not endurance performance[END_REF] found positive changes in repeated-sprint parameters but not in endurance with well-trained rugby union players after 6 sessions. This may be due to several factors such as the amount of sessions (12 vs. 6), the level of players (academy vs. well-trained) and also the training protocol may not be sufficient load (1 set of 10 × 6, 30 s recovery vs. 4 sets of 5 × 5, 25 s recovery) [START_REF] Brocherie | Effects of repeated-sprint training in hypoxia on sea-level performance: A meta-analysis[END_REF]. Our study supported the results reported by [START_REF] Hamlin | Hypoxic repeat sprint training improves rugby player's repeated sprint but not endurance performance[END_REF] with improvements in RSA parameters when using RSH vs. RSN, which used similar multiple-set protocol (3-4 sets) compared to the single set completed in the study from [START_REF] Galvin | Repeated sprint training in normobaric hypoxia[END_REF]. One set of repeated sprints may not be sufficient load to improve RSA parameters or elicit appropriate adaptations to anaerobic glycolysis. From the pioneer RSH study [START_REF] Faiss | Significant molecular and systemic adaptations after repeated sprint training in hypoxia[END_REF], it seems that a multiple-set protocol (3-4) may be recommended to enhance RSA parameters [START_REF] Brocherie | Effects of repeated-sprint training in hypoxia on sea-level performance: A meta-analysis[END_REF]. Further, another important reason that may explain the lack of additional benefits of RSH over RSN is the non-specificity of training relatively to the test implemented (e.g. differences in exercise mode, motion direction, sprint duration or exerciseto-rest ratio) as pointed by Fornasier-Santos, [START_REF] Fornasier-Santos | Repeated-sprint training in hypoxia induced by voluntary hypoventilation improves running repeated-sprint ability in rugby players[END_REF]. Considering that [START_REF] Baker | Comparison of upper-body strength and power between professional and college-aged rugby league players[END_REF] has also shown that professional rugby league players were significantly stronger and more powerful in the upper-body than lower level (semiprofessional and collegiate) players, the world-level players from an international team (6-nations champion at the time of study) involved here may differ to that of the non-professional players in previous studies [START_REF] Galvin | Repeated sprint training in normobaric hypoxia[END_REF][START_REF] Hamlin | Hypoxic repeat sprint training improves rugby player's repeated sprint but not endurance performance[END_REF].

While putative benefits have been shown in the majority of RSH studies, to date, there are some that have reported no differences between RSH vs. RSN [START_REF] Goods | No additional benefit of repeat-sprint training in hypoxia than in normoxia on sea-level repeat-sprint ability[END_REF][START_REF] Montero | No improved performance with repeated-sprint training in hypoxia versus normoxia: A double-blind and crossover study[END_REF]. Athletes' backgrounds, type of test (double-poling, ergocycle, treadmill or over-ground running), number of sessions, hypoxic dose and training protocols (sets × repetitions) could be reasons for inconsistencies in the RSH literature. A major strength of the present study is to show that world-level professional players benefited of this innovative hypoxic training method. Since published studies investigating world-class athletes are lacking [START_REF] Mujika | Sharpshooting in sport science and elite sports training[END_REF], one would emphasize that the present results are directly transferable for practical application in real setting at such level. More importantly, with significant changes observed in already well-developed athletes, this is indicative of the effectiveness of the RSH paradigm for putative performance benefits in all playing level standards.

However, the suggested mechanisms for the performance changes are still under debate. It has been hypothesized that, because acute exposures to hypoxia increase compensatory vasodilation to the vascular beds of the skeletal muscle [START_REF] Casey | Compensatory vasodilatation during hypoxic exercise: Mechanisms responsible for matching oxygen supply to demand[END_REF], RSH may induce improvements of oxygen utilization by the fast-twitch fibres during 'all-out' maximal repeated sprints performed in hypoxia [START_REF] Faiss | Advancing hypoxic training in team sports: From intermittent hypoxic training to repeated sprint training in hypoxia[END_REF]. The fast-twitch fibres are the principal driver of maximal power in "all out' efforts such as sprinting activities, in particular in upper-body RSA-related exercise [START_REF] Kamandulis | Sport-specific repeated sprint training improves punching ability and upper-body aerobic power in experienced amateur boxers[END_REF][START_REF] Sandbakk | The physiological responses to repeated upper-body sprint exercise in highly trained athletes[END_REF], while their ability to reproduce high levels of power are limited to the supply of high energy phosphates which is dictated by the intensity of the sprint effort [START_REF] Hirvonen | Breakdown of high-energy phosphate compounds and lactate accumulation during short supramaximal exercise[END_REF]. Further, it cannot be ruled out that RSH may also have a positive impact on anaerobic and glycolytic pathway as shown previously [START_REF] Girard | Effects of altitude/hypoxia on single-and multiple-sprint performance: A comprehensive review[END_REF][START_REF] Puype | Sprint interval training in hypoxia stimulates glycolytic enzyme activity[END_REF]. However, in the present study, S dec remains fairly constant, despite increase in both PPO and averaged MPO. While caution is therefore requested in interpreting fatigue using such index, identifying the previously reported mechanisms associated to RSH may be difficult here.

In the present study, it has been demonstrated that there was an increase in world-level rugby union players' PPO and MPO seen after as little as four RSH sessions, whereas no difference was observed in power increase between RSH and RSN following eight sessions in competitive cross-country skiers [START_REF] Faiss | Repeated doublepoling sprint training in hypoxia by competitive cross-country skiers[END_REF]. It may be postulated that the level of athletes (world vs. competitive), training type (intermittent/power vs. endurance), and the actual physical size of the athletes (103.9 ± 12.2 vs. 74.7 ± 7.8 kg) could directly affect the different results seen between the studies. This warrants further examination while also investigating strength levels of the athletes as mentioned previously which has been shown to impact power outputs [START_REF] Baker | Comparison of upper-body strength and power between professional and college-aged rugby league players[END_REF]. This could be an area that is currently being overlooked to postulate the RSH mechanisms, especially between different levels and types of athletes. This could also be possibly different between upper-and lower-body due to the physiological and anatomical differences (upper-body has lower muscle mass and higher fast-twitch fibres, different metabolic and cardiovascular responses [START_REF] Calbet | Why do arms extract less oxygen than legs during exercise?[END_REF][START_REF] Koppo | Oxygen uptake kinetics during high-intensity arm and leg exercise[END_REF].

Double-poling involves several muscles groups such as the core muscles (i.e. rectus abdominus), the upper thighs (quadriceps) as well as the latissimus dorsi and pectoralis major which are responsible for shoulder extension and the triceps brachii which is a powerful elbow extensor [START_REF] Holmberg | Biomechanical analysis of double poling in elite cross-country skiers[END_REF]. These latter muscles are predominately made up of fast-twitch fibres [START_REF] Mygind | Fibre characteristics and enzyme levels of arm and leg muscles in elite cross-country skiers[END_REF]. Rugby union has been shown to utilize the upper-body in collisions, tackling, rucking and mauling [START_REF] Duthie | Applied physiology and game analysis of rugby union[END_REF][START_REF] Quarrie | Positional demands of international rugby union: Evaluation of player actions and movements[END_REF]. Tackling in rugby union has been demonstrated to differentiate winning and losing teams in the 6-nations rugby union tournament, where winning teams tend to complete a higher percentage of tackles than losing teams [START_REF] Ortega | Differences in game statistics between winning and losing rugby teams in the six nations tournament[END_REF]. This could indicate a higher sustainability of power or lower fatigability to the upper-body musculature to complete more tackles for winning over losing teams [START_REF] Ortega | Differences in game statistics between winning and losing rugby teams in the six nations tournament[END_REF]. In rugby league, fatigue development has been shown to reduce the effectiveness of tackling [START_REF] Gabbett | Influence of fatigue on tackling technique in rugby league players[END_REF]. Therefore, it could be advantageous to increase anaerobic power (minimum of 4 sessions as per our findings) and fatigue resistance (minimum of 6 sessions as per [START_REF] Faiss | Repeated doublepoling sprint training in hypoxia by competitive cross-country skiers[END_REF] for the upper-body through RSH in rugby union players. Apart from the performance enhancement and benefits of RSH seen in our study, investigations into how improving fatigue resistance in upper-body activities could also be an area to look at in the future. Research has shown that fatigue may be a risk factor for game and training injuries in the upper-body region in rugby union [START_REF] Williams | A meta-analysis of injuries in senior men's professional rugby union[END_REF].

As possible limit, one may question the translation to the field of the present results. Double-poling exercise is commonly used for conditioning in rugby union, despite being not specific to rugby and differing to the upper-body activities of the players during games. However, since, to our knowledge, there is no reliable and valid test using only upper-body and replicating any rugby action, the choice of an upper-body ergometer for measuring the pre-topost changes seems relevant.

Conclusion

In summary, the present study shows for the first time greater improvement in the upper-body repeatedsprint ability of world-level team-sport athletes during a competition period when training repeated double-poling sprints in RSH vs. RSN. This adds to the growing evidence of RSH as a putative hypoxic method in improving repeated-sprint ability and its associated parameters while also importantly showing that RSH can be used positively with world-level athletes during short-term competition periods. While conclusion for the effects of RSH on upper-body and lower-body performance enhancements may need to be evaluated independently so more accurate comparisons and protocols can be devised, it is worth mentioning that to date the majority of RSH studies have been lower-body dependent so more studies are required to add to the upper-body RSH literature. Further investigation is also required to improve the understanding on how best to program RSH, what is the most efficient number of sessions, sets and/or repetitions as well as the type of exercise mode (ergocycle, doublepoling, treadmill, ground-based sprints or combination).

Practical applications

The usefulness of repeated-sprint training in hypoxia to improve physical performance in world-level rugby union players has wider implications to other elite to high-level team-sport athletes that utilize the upperbody. Prescribing physical fitness training parameters in competition can be difficult as the focus on game performance is paramount. Therefore, when sports have increases in the level of competition (club to international) and small transition periods (1-2 weeks), strength and conditioning coaches and performance experts will always be looking for best practice for peaking performance. Reducing adaptation times in these small preparation windows without increasing loads unnecessarily to become further risk factors for competition safely is the aim of any good performance optimization program. This study has shown that RSH can be implemented in an international competition preparation with only four training sessions in hypoxia with significant larger change in RSA power outputs than the same training in normoxia.

Figure 1 .

 1 Figure 1. Description of one repeated-sprint training session.

Figure 2 .

 2 Figure 2. Time-course changes in power output (panel A) and percentage changes in the peak (PPO) and mean power output (MPO) (panel B) during the double-poling repeated-sprint test Pre-and Post-repeated-sprint training in hypoxia (RSH) or in normoxia (RSN). Values are mean ± SD. C, condition effects; I, interaction effects; T, time effects. Significant differences from Pre-test, * * P < 0.01 and * * * P < 0.001; Significant difference from RSN, # P < 0.05 ## P < 0.01.

Table I .

 I World-level Rugby Union players' characteristics in repeated-sprint training in hypoxia (RSH) and repeated-sprint training in normoxia (RSN) group.

		RSN (n = 18)	RSH (n = 18)
	Age (y)	22.7 ± 1.5	25.6 ± 2.8
	Body mass (kg)	103.3 ± 12.6	104.5 ± 11.7
	Height (cm)	185.1 ± 6.6	187.8 ± 5.6

Table II .

 II Training, RSH and testing schedule.

		Monday	Tuesday	Wednesday	Thursday	Friday	Saturday	Sunday
	Pre-test
	am	Recovery	Light practice	RSA Test	Practice	Walk-thru		Day-off
	pm				weights		Game
	Week 1
	am	Weights	Practice	Day-off	Practice	Walk-thru		Day-off
	pm	Recovery	RSH-1		RSH-2		Game
	Week 2
		Weights	Practice	Day-off	Practice	Walk-thru		Day-off
		Recovery	RSH-3		RSH-4		Game
	Post-test
		Recovery	Light Practice	RSA-Test	Practice	Walk-thru		Day-off
					Weights		Game

Testing was conducted between 10:00am and 12:00pm, in the National indoor gym that was set at a consistent 22 °C. Bold values are RSA and RSH sessions.

Acknowledgments

The authors are indebted to the National team players who participated and the support staff for their help.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Franck Brocherie http://orcid.org/0000-0002-0808-7986 Grégoire Millet http://orcid.org/0000-0001-8081-4423