Repeated-sprint training in hypoxia (RSH) is a promising intervention aimed at evoking greater metabolic (Puype et al., 2013) and performance (Brocherie et al., 2015; Faiss et al., 2013; Galvin et al., 2013; Kasai et al., 2015) adaptations compared with performing repeated-sprint training in normoxia (RSN). Greater increases in muscle enzyme activity (Puype et al., 2013), repeated-sprint ability (RSA; Faiss et al., 2013), Yo-Yo intermittent recovery test level 1 performance (Galvin et al., 2013) and peak and mean power output (Kasai et al., 2015) after 4-6 weeks of RSH compared with RSN have been demonstrated. However, these findings have not been universal (Goods et al., 2015).
In comparison to a single bout of RSN, a single bout of RSH results in higher heart rate, minute ventilation, blood lactate concentration, and muscle deoxygenation; as well as lower blood oxygen saturation, pulmonary oxygen uptake, integrated EMG (Bowtell et al., 2013) and cerebral oxygenation (Smith and Billaut, 2010). While altering the physiological responses is an essential outcome of RSH compared with RSN, it is not known if there are acute adverse effects of performing repeated-sprints in a hypoxic environment that should be considered in the broader context of exercise programming.
Exposure to normobaric hypoxia (Fi[O.sub.2]: 0.125 - 0.150) can elicit reductions in cerebral oxygenation (Komiyama et al., 2015; Seo et al., 2015) and may be related to impaired performance of modified Stroop, i.e., colour-word interference (Leon-Carrion et al., 2008) and Running Memory Continuous Performance (percentage correct/throughput score; Seo et al, 2015) tasks at rest. Nonetheless, prolonged submaximal exercise performed below ~75% peak oxygen uptake has been demonstrated to not only restore (throughput score; Seo et al, 2015), but also improve (reaction time during Go/No-Go task) aspects of cognitive function during hypoxic (Fi[O.sub.2]: 0.18, 0.15) exposure despite further reductions in peripheral oxygen saturation and cerebral oxygenation during exercise (Ando et al., 2013). Therefore, submaximal exercise acts as a countermeasure to restore cognitive function during exposure to hypoxic training environments. This may provide a safeguard from accidents or injuries related to acute cognitive impairment, given that individuals exposed to a hypoxic environment that are not aware of their acute cognitive dysfunction, may be at a higher risk of deleterious outcomes such as accidents (Seo et al., 2015). This is particularly important for team-sport athletes who participate in successive training sessions, including exercise in hypoxic environments, who may therefore be at a greater risk of injury.
Despite the restorative ability of prolonged submaximal cycling on cognitive function in hypoxia, it is unknown if short-duration, repeated treadmill sprinting can attenuate the risk of impaired cognitive function that occurs in hypoxic conditions. Lambourne and Tomporowski (2010) highlighted that cognitive function is impaired in the first 20 min of exercise before an improvement is observed. Therefore, repeated-sprinting protocols used by team-sport athletes may not be long enough to evoke improvements in cognitive function. Furthermore, as exercise intensity increases, arousal levels are thought to increase to the point that cognitive function is compromised (Lambourne and Tomporowski, 2010). This suggests that maximal-intensity, short-duration exercise (i.e., sprinting) may not have the same restorative ability as prolonged submaximal exercise on cognitive function in hypoxia, although the research remains unclear. Also, while it is apparent that cycling is associated with improved cognitive function, treadmill running has been demonstrated to impair cognitive function - possibly due to the greater attentional demands required to maintain a fixed running speed - compared with seated cycling (Lambourne and Tomporowski, 2010). Nevertheless, deleterious effects of treadmill exercise on cognitive function are reversed to values not different from baseline or slightly improved when measured 20 min after exercise (Lambourne and Tomporowski, 2010).
Acute exposure to a hypoxic environment will result in impaired cognitive function in young, healthy men (Seo et al., 2015). Several considerations are important to determine if exercise will restore this deleterious effect, including: i. The exercise mode, i.e., cycle ergometry versus treadmill running (Lambourne and Tomporowski, 2010); ii. The intensity of the exercise, i.e., below or above anaerobic threshold (Brisswalter et al., 1997; Chmura et al., 1994; Kamijo et al., 2004); iii. The type of cognitive tasks selected, i.e., simple versus complex (Lambourne and Tomporowski, 2010); and iv. The timing of the administration of cognitive tasks (Endo et al., 2013). The present study sought to determine if the ability to perform simple and complex cognitive tasks is compromized in individuals 20 min after performance of a single bout of RSN or RSH on a non-motorized treadmill. We predicted that blood oxygen saturation and prefrontal cortex oxygenation would be lower and RPE would be higher during a bout of RSH compared with RSN. We hypothesized that a bout of RSN would not result in any change to baseline cognitive function measured 20 min after exercise due to the inability of the short-duration, repeated treadmill sprinting protocol to enhance mental processes. However, we hypothesized that the performance of simple and complex cognitive tasks would decline from baseline 20 min after a bout of RSH.
Eleven amateur team-sport athletes (Australian football = 7, soccer = 3, touch-football = 1; age 22.8 [+ or -] 3.6 y, body mass 78.3 [+ or -] 5.9 kg, and height 1.81 [+ or -] 0.03 m) provided their written informed consent before participating in the present study. All procedures were approved by Griffith University Human Research Ethics Committee (AHS/72/14/HREC). All participants had competed for a minimum of two consecutive years in their sport at the local club level immediately prior to participating in the study, trained for a mean ([+ or -] SD) of 147 [+ or -] 31 min x [week.sup.-1], and competed in their sport for a mean ([+ or -] SD) of 106 [+ or -] 16 min x [week.sup.-1]. Participants performed the RSR test comprising sixteen (four sets of four) 4-s 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 normobaric hypoxic chamber (Synergy Physical Conditioning Systems, Yatala, Australia). The creation of the hypoxic environment was achieved via the extraction of oxygen from air which was then pumped throughout the chamber. The test was performed once under normoxic (Fi[O.sub.2] 0.209) and once under hypoxic (Fi[O.sub.2] 0.145) conditions. The study followed a single-blind, crossover design with 5 participants performing the [RSR.sub.444] in normoxia first and then in hypoxia 7 to 10 d later at the same time of day. The remaining 6 participants performed the RSR tests in the reverse order. Additional information regarding the testing and participant requirements can be viewed elsewhere (Morrison et al., 2015).
Participants completed four standardized familiarization sessions on the treadmill (Curve 3.0, Woodway, Waukesha, Wisconsin, USA) within a 3-wk period, which were designed to: i. Allow participants to acquire the technique required to accelerate/sprint maximally; and ii. Reduce any acute lower-limb soreness during the experimental-trial period due to unaccustomed...