'Top-up' training is a term used to describe training sessions undertaken by athletes in addition to their normal training load, usually comprising 1-3 short duration workouts per week designed to target performance enhancement in a specific aspect of the sport (i.e., repeat-sprint ability). Using 'top-up' training in addition to a regular team sport training program first gained scientific attention and support over a decade ago, with soccer players reported to have improved their aerobic capacity by ~11% and in-game running output by ~20% after completing two extra weekly aerobic conditioning sessions over 8 weeks (Helgerud et al., 2001). Since then, methods of 'top-up' training in team sports have evolved to now include specific repeat-sprint training, which improves repeat-sprint ability (RSA) (Bravo et al., 2008; Hunter et al., 2011). In a recent review, RSA was defined as 'the ability to perform repeated short (~3-4 s, 20-30 m) sprints with only brief (~10-30 s) recovery between bouts', and is described as an integral component of team sport performance (Dawson, 2012). It has also recently become popular to use hypoxic environments for team sport performance enhancement, with recent research (McLean et al., 2013) showing that team sport athletes living at altitude (19 days at ~2130 m) demonstrate similar physiological and performance improvements to those reported in studies using endurance athletes (Robertson et al., 2010).
Altitude training has also been proposed to improve physiological determinants of RSA, including phosphocreatine resynthesis rate and [VO.sub.2max] (Bishop and Girard, 2013). Encouragingly, a recent investigation in moderately trained male cyclists found that 4 weeks of cycling repeat-sprint training in hypoxia (RSH) (8 sessions of 3 x 5 x 10 s sprints) significantly improved cycling RSA at sea-level compared to performing a matched training load in normoxia (~40% increase in number of sprints performed vs. ~5% decrease following sea-level training) (Faiss et al., 2013b). Additionally, Puype et al. (2013) investigated much longer sprint durations, finding that 6 weeks of cycling RSH (18 sessions of 4-9 x 30 s sprints) training in healthy males improved muscle phosphofructokinase activity and anaerobic threshold to a greater level than a matched training load performed in normoxia. Regardless, only three known studies have investigated the effect of RSH training in team sport athletes. Galvin et al. (2013) found that 4 weeks of a single set of running RSH (12 sessions of 10 x 6 s sprints on a non-motorised treadmill) in well-trained academy rugby players improved endurance performance, but not RSA compared to the same training performed in normoxia. Brocherie et al. (2015b) recently found that the addition of hypoxia to repeat-sprint training in highly trained youth footballers improved repeated agility performance compared to normoxic training (4.4% vs. 2.0%, respectively) but not RSA. Conversely, Brocherie et al. (2015a) added 6 sessions of RSH (4 x 5 x 5 s overground running sprints) to a live-high / train-low protocol and found that performance in an 8 x 20 m RSA test was significantly enhanced compared to living in altitude and performing the same training in normoxia (3.7 vs. 1.9%, respectively). In support of this multi-set training approach, Goods et al. (2014) demonstrated that oxygen saturation (measured in capillary blood) in team sport athletes performing multiple sets of RSH continued to decrease after the first set of sprints, indicating a more powerful hypoxic stimulus after the first set, which may further enhance the peripheral adaptations expected following RSH, as proposed by Bishop and Girard (2013).
Therefore, this study aimed to assess the impact of 5 weeks of multi-set cycling RSH 'top-up' training on sea-level cycling RSA, running RSA and endurance performance. Cycling was selected as a non-weight bearing training mode, which is an established method of 'top-up' training in elite Australian football during the preparatory phase (Buchheit et al., 2013), intended to increase internal training load without unduly increasing running loads. Whether a cycling protocol can improve sports specific running RSA and running endurance performance was therefore also of interest. It was hypothesised that 'top-up' RSH training would provide greater sea-level performance benefits for all outcomes than the same 'top-up' training performed in normoxia.
Thirty semi-elite male Australian football athletes from one Western Australian football club were recruited for this investigation; mean [95% Confidence Intervals (CI)] age 20.3 [19.7, 20.9] y, stature 1.84 [1.81, 1.86] m, body mass 81.7 [78.7, 84.7] kg. Two participants failed to complete the study due to injuries unrelated to the 'top-up' training intervention; one each from the control and hypoxic intervention groups. Prior to commencement, all participants provided written informed consent. Ethical approval was granted by the Human Research Ethics Committee of the University of Western Australia (RA/4/1/5703).
A single blind, randomised control trial design, incorporating 5 weeks of cycling repeat-sprint 'top-up' training, in either a hypoxic or normoxic environment, was used to assess the effects on RSA at sea-level. Participants first completed a cycling RSA test, a running RSA test and a 20 m shuttle run test within 7 d, in a randomised order. All participants were instructed to avoid caffeine intake on the day of each test and to keep food intake consistent before each test. Subsequently, participants were evenly allocated into a control (CON), normoxic training (NORM), or hypoxic training (HYP) group via simple randomisation. There were no significant performance differences between groups for any test prior to commencing training.
Subsequently, the NORM and HYP groups trained three times per week for 5 weeks in an environmental chamber, performing cycling repeat-sprint training at either sea-level (NORM) or 3000 m of simulated altitude (HYP). Training was planned in a progressive overload manner as a 'top-up' to the regular team sport training. Both groups were blinded to their treatment throughout the trial. The CON group continued to train as usual near sea-level (the altitude of Perth, Western Australia is ~30 m), with no 'top-up' sessions completed.
Regular team sport training was completed together by all groups, and comprised two 60 min sports specific training sessions per week (small sided games, skill and strategy drills), two 45 min strength training sessions, one circuit-based conditioning training session (15 min each of boxing, aerobic cycling intervals and gym circuit) and one 30 min interval running session performed on sand. As the intervention was performed in the pre-competition phase, no matches were played throughout the training period. After completing the 5 weeks of training, all performance tests were repeated to assess the effect of 'topup' training alone, and 'top-up' training in hypoxia on performance.
Running Repeat-Sprint Ability Test
This test was performed indoors on a sprung wooden floor. Participants were fitted with a heart rate strap (T31, Polar Electro, Finland), and watch receiver (F1+, Polar Electro, Finland), before they commenced a standardised warm-up, comprising a 2 min jog followed by 3 min of running drills (e.g. high knees, heel flicks) and finally 2 x 20 m submaximal sprints on the test track as a familiarisation. A post-warm-up capillary blood sample was then taken for analysis of blood lactic acid concentration before the test commenced.
The running RSA test comprised 3 sets of 6 x 20 m maximal sprints departing every 25 s, each performed from a standing start. Standardised verbal commands were used during each sprint, with times recorded via electronic timing gates (Swift Speed Light Timing System, Swift Performance Equipment, Australia). Upon completing each sprint, participants jogged back to the starting line to commence the next sprint from the common starting end. This test has been used previously (Sim et al., 2009), with excellent test-retest reliability (typical error...