Intermittent hypoxic training (IHT) has emerged as a popular training method for team-sport athletes and aims to evoke greater adaptations than performance of similar training in normoxia. More recently, a new training method has been investigated by several researchers (Faiss et al., 2013, Galvin et al., 2013, Puype et al., 2013, Brocherie et al., 2015, Goods et al., 2015) that includes the performance of repeated sprints (RS) in hypoxic conditions (RSH). Previous RSH studies have used a variety of protocols, with sprint durations ranging from 5 to 30 s and recovery periods ranging from 15 s to 5 min (Galvin et al., 2013, Faiss et al., 2013, Puype et al., 2013, Goods et al., 2015). While three of these studies demonstrated RSH to evoke superior adaptations in muscle perfusion (Faiss et al., 2013), glycolytic enzyme activity (Puype et al., 2013) and the ability to perform repeated bouts of high-intensity aerobic work, i.e., improved Yo-Yo Intermittent Recovery 1 test performance (Galvin et al., 2013), one study demonstrated no additional benefit (Goods et al., 2015).
The concept of replicating the movements performed during team-sport match play (e.g. number/duration of sprints/recovery periods) is appealing when designing a RS training protocol to improve the RS ability (RSA) of team-sport athletes, however, further research is required regarding the efficacy of utilizing specific RS training for this purpose (Buchheit, 2012). A recent RS training study, which was limited by the lack of a control group, utilized a protocol that was characterized by clusters (i.e., multiple sets) of sprints which were separated by short (20 s) recovery periods, with longer recovery periods (4.5 min) between clusters (Serpiello et al., 2011). The authors suggested that single-set repeatedsprint protocols poorly reflect the demands of team-sports and that multiple sets provide for more accurate assessment of team-sport performance (Serpiello et al., 2011). This is supported by time-motion analysis studies that have described the repeated-sprint activity of team-sports (Spencer et al., 2004, Gabbett and Mulvey, 2008)
Combining the two ideas of clustered repeatedsprint training and RSH, Goods and colleagues (2014) demonstrated that peak power output (PPO) can be maintained at a simulated altitude of 3000 m, but not 4000 m, during three sets of 9 x 4-s sprint efforts, when compared with sea-level (RSN). The impairment of PPO at a simulated altitude of 4000 m would be an important factor to consider when designing a similar RSH training protocol, given the importance of specific muscle contraction speeds when training for sprint speed (Kristensen et al., 2006).
In addition to peak speed/power output and total work (e.g., mean speed or total distance), the rate of acceleration should be considered as an important performance outcome when prescribing a repeated-sprint training program for team-sport athletes (Lockie et al., 2011). Previous studies that have defined and described repeated-sprint activity during international-level team-sport competition, have reported average sprint times during repeated-sprinting in elite men's hockey (Spencer et al., 2004) and elite women's soccer (Gabbett and Mulvey, 2008) to be 1.8 s and 2.1 s, respectively. Meanwhile, average sprint times in elite Australian-rules football have been reported to be
In the present study, we sought to design a protocol that not only replicates repeated-sprint patterns in team-sports (i.e. clusters), but also allows for adequate recovery so as to attenuate reliance on aerobic metabolism and impairment in speed, acceleration, and total work (i.e., distance). Our aim was to examine performance (peak speed, acceleration and total work) and physiological responses (blood lactate and [S.sub.p][O.sub.2]) during performance of a RS running (RSR) protocol in order to provide information to be used in designing a RSH training protocol for team-sport athletes.
Ten amateur team-sport athletes (four Australian rules footballers, four rugby union players and two soccer players) volunteered to participate in the present study and gave their written informed consent. All procedures used in the study were approved by the Griffith University Human Research Ethics Committee. The physical characteristics of the group were (mean [+ or -] SD): age 22.6 [+ or -] 4.7 y, body mass 88.8 [+ or -] 7.3 kg, and height 1.83 [+ or -] 0.06 m. All participants had competed for a minimum of three consecutive years in their respective sport, as well as completed a minimum of two months (2-3 times per week) of intense training immediately prior to involvement in the study. All participants performed a repeated-sprint running (RSR) test consisting of sixteen (four sets of four) 4s sprints separated by 26 s (and 2 min 26 s between sets) of passive recovery in a standing position (i.e., [RSR.sub.444]) on two occasions in a commercial normobaric hypoxic chamber (Pro diving Services, Sydney, Australia) while breathing either room air (Fi[O.sub.2] = 0.209) or Fi[O.sub.2] = 0.140 (i.e., hypoxia). The hypoxic environment was created via the extraction of oxygen from air which was subsequently pumped into the chamber. Oxygen concentration was monitored using a gas detector (KB-501, Kingsby Electronics,) which utilizes an electrochemical sensor. The Fi[O.sub.2] (0.140) was selected due to the ability of highlytrained team-sport athletes to maintain performance during a similar study in our lab (Morrison et al., 2015) involving 10 x 6-s sprints in hypoxic (Fi[O.sub.2] = 0.140) conditions. Relative humidity and temperature were maintained between 45-50% and 19-21[degrees]C, respectively. While we acknowledge that a passive recovery does not replicate team-sport movement demands, the protocol was de signed to allow the maintenance of speed, acceleration and total work (i.e., distance) as well as the rotation of up to four athletes on one treadmill (thus improving teamsport training efficiency). The study followed a cross-over design with one group (n = 5) performing the RSR444 in normoxia first and then in hypoxia between 48 h and 72 h later; the other group (n = 5) performed the RSR tests in the reverse order. Testing sessions were performed immediately prior to participants' training sessions for their respective sports, and replaced specific repeated-sprint training activity for those training sessions. While the authors acknowledge that the second session may have been impacted by the short washout period, the investigation is counterbalanced and the participants would have been performing repeated-sprints regardless of if they completed the testing sessions or not. In addition, all participants indicated prior to commencing each session, via pre-participation questionnaires, that they were not suffering from any soreness. Participants performed the tests at the same time of day and were asked to 1) refrain from consuming alcohol/performing strenuous exercise in the 24 h prior to the tests, 2) refrain from consuming caffeine on the day of the tests, and 3) be consistent with food and fluid intake for both tests. All participants indicated in the pre-participation questionnaires that they were not taking any...