Important performance characteristics of soccer players are excellent repeated sprint ability (RSA) and performance in the Yo-Yo intermittent recovery test (YYIR) (Krustrup et al., 2006; Rampinini et al., 2009). Both are associated with in-match physical performance (Krustrup et al., 2006; Rampinini et al., 2007) and influenced by a fast V[O.sub.2] kinetic and a high buffer capacity (Dupont et al., 2010; Girard et al., 2011). Furthermore, RSA was shown to depend on maximal aerobic power ([VO.sub.2max]), muscle glycogen content, concentration of aerobic enzymes, mitochondrial size and number and capillary density (Dupont et al., 2010; Girard et al., 2011; Rampinini et al., 2009). Based on the observations that high intensity training under hypoxia likely improves buffer capacity (Faiss et al., 2013a; 2013b; Girard et al., 2013a), lactate exchange and removal, tissue [O.sub.2] extraction (Faiss et al., 2013a; 2013b; Girard et al., 2013a), glycolytic enzyme activity (Hamlin et al., 2010; Puype et al., 2013), economy (Hamlin et al., 2010), citrate synthase activity and myoglobin content (Terrados et al., 1990), it could be speculated that this type of training might improve RSA and YYIR and thus influence physical soccer performance. Accordingly, Faiss et al. (2013a) found greater improvements in RSA (i.e., increased number of sprints until exhaustion) and Galvin et al. (2013) greater improvement in the YYIR test performance after repeated sprint training in hypoxia (RSH) compared to normoxic training (RSN). However, the training intervention was performed on cycle ergometers (Faiss et al., 2013 a) which represents an unusual activity for soccer players or on a non-motorised treadmill (Galvin et al., 2013). For obvious reasons soccer coaches generally might be reluctant in adopting these procedures. Therefore new training regimes and/or new technologies are requested that overcome this restriction. The mobile inflatable hypoxic marquees as described in detail by Girard et al. (2013b) can be considered such a new and promising technology. However many research and training centres are equipped with much smaller hypoxic chambers not allowing performing long distance sprints. Therefore, the present study aimed at investigating if performing shuttle-run sprint training--presumably reflecting a more common soccer training practice as cycling or running on a non-motorised treadmill--is feasible in a normobaric hypoxia chamber of limited size (4.75 x 2.25 m) and if such training improves RSA and YYIR test outcomes. We hypothesized that the proposed shuttle-run sprint training can be performed in small hypoxic chambers without restricting training intensity and that RSH improves RSA and YYIR test outcomes to a greater extent than RSN.
Sixteen healthy male youth soccer players (outfield players, age 15.3 [+ or -] 0.5 years, body height 1.73 [+ or -] 0.07 m and body mass 62.6 [+ or -] 6.6 kg) of an elite Tyrolean soccer training centre were informed about the training protocol and gave written informed consent to participate in the study. Players were randomly assigned to a normoxia or a hypoxia training group according to their RSA and YYIR performance. Three players of the control group and 3 players of the hypoxia group dropped out because of longer lasting illnesses and injuries during the training period. Additionally one player skipped the second YYIR test due to the feeling of discomfort. Players were not familiar with the training modalities described in detail below. The study was carried out in conformity with the ethical standards laid down in the 1975 declaration of Helsinki and has been approved by the Institutional Review Board of the Department of Sport Science, University of Innsbruck.
The shuttle-run sprint training started in the third week of the winter preparation phase and was arranged in agreement with the coaches. Besides the normal training regime, which was the same for both groups, 8 sessions (participants performing only 7 sessions were also included into analyses; reasons: colds, school engagement, minor injury) of a shuttle-run sprint training program was performed during a 5 week period. The training was performed each week on Tuesday and Thursday. Because of the 5 week training period participants were allowed to be absent not more than 3 times, with the request that at least one training session had to be completed during the last week. The training was performed in an air-conditioned (21[degrees]C) normobaric hypoxic chamber (size 4.75 x 2.25m, LowOxygen, Germany). For the hypoxia group the Fi[O.sub.2] was set at 14.8%, equivalent to approximately 3300m, shown not to reduce peak power output in repeated sprint efforts (Goods et al., 2014). The normoxia group trained in a separate training room under the same conditions in terms of floor conditions and cone distance at a Fi[O.sub.2] of 20.95%. Group assignment was blinded to the...