In recent years, new hypoxia training concepts for team sport athletes have emerged. These concepts, originally based on classical normoxic sprint training, include repeated-sprint training (RSH) and sprint interval training (SIH) performed in hypoxia. Repeated-sprint training is characterized by repeated maximal exercise bouts of short duration ([less than or equal to] 10 s) interspersed with brief recovery periods (usually [less than or equal to]60 s) (Girard et al., 2011a; 2017). Sprint interval training, in contrast, includes repeated bouts with a duration of approximately 30 s interspersed with 2-4 min of passive recovery (Buchheit and Laursen, 2013b). Similar to training in normoxia (Buchheit and Laursen, 2013a; Buchheit and Laursen, 2013b), slightly different anaerobic and aerobic energy contributions during RSH and SIH might be expected depending on bout and relief duration (Faiss et al., 2013a). Consequently, the physiological adaptations may differ to some extent as well. As comparative studies investigating different sprint training protocols in hypoxia are scarce, identification of possible divergent adaptations is difficult. For instance, RSH was found to enhance muscle perfusion (Faiss et al., 2015; Montero and Lundby, 2017) and changes in the proportion of type IIx muscle fibers have been described after SIH (De Smet et al., 2016). Yet, it is unknown if SIH is equally able to enhance perfusion and RSH will change fiber type. Additionally, an enhanced anaerobic metabolism (Faiss et al., 2013b; Puype et al., 2013) and increased muscle buffer capacity (Faiss et al., 2013a) have been reported for both, RSH and SIH, yet the magnitude of differences is hard to assess. Furthermore, Faiss et al. (2013b) reported decreases in factors involved in mitochondrial biogenesis after RSH, whereas no such data are available for SIH. Especially for this adaptation, the importance of comparing different sprinting protocols as well as hypoxia doses is evident as applying a slightly different RSH protocol and including hypoxia living led to an opposite finding (i.e., increased mRNA levels for PGC--1[alpha]) (Brocherie et al., 2018).
In regard to performance, RSH compared to normoxia training was found to improve sea-level repeated sprinting abilities (Brocherie et al., 2017; Faiss et al., 2013b; Gatterer et al., 2014; Hamlin et al., 2017; Kasai et al., 2015) and to some extent intermittent endurance exercise performance (Galvin et al., 2013). SIH, in contrast, improved the anaerobic threshold to a greater extent (Puype et al., 2013). However, other reports suggest that adding hypoxia to both training modalities does not affect performance outcomes at all (De Smet et al., 2016; Montero and Lundby, 2015; 2017; Richardson and Gibson, 2015).
As outlined before, muscle perfusion and oxygenation changes might constitute adaptations related to performance improvements after RSH whereas such changes have not yet been reported for SIH. Additionally, to the best of our knowledge, no study investigated actual mitochondrial function after RSH and SIH mostly because invasive procedures are necessary. Recent research, however suggests that the mitochondrial function of peripheral blood mononuclear cells (PBMC) may provide a measure of physical ability similar to skeletal muscle mitochondrial function (Tyrrell et al., 2015a). Moreover, high-intensity interval training in normoxia enhanced oxidative phosphorylation of lymphocyte, which again indicates the usefulness of this analysis (Tsai et al., 2016). The effect of RSH and SIH on blood cells mitochondrial function is not established yet and its relationship to muscle oxygenation and performance has not been studied so far. Thus, the present study aimed at investigating and comparing the effects of RSH and SIH on sea level running and cycling performance, and to elucidate potential common or divergent adaptations of muscle perfusion and oxygenation as well as mitochondrial respiration of blood cells, possibly related to performance improvements.
Twelve healthy subjects competing in intermittent sports (i.e. basketball, handball, soccer) at an amateur level (2-3 regular training sessions per week and one competition game) were recruited and provided their written informed consent to participate in the study. The final sample size consisted of eleven participants (age 24.0 [+ or -] 2.4 yr., height: 1.83 [+ or -] 0.05 m, weight: 84.0 [+ or -] 9.3 kg) since one subject dropped out during the study due to personal reasons. The study was approved by the Institutional Review Board of the Department of Sport Science of the University of Innsbruck.
After performing the pretests as outlined in detail below, the participants were randomly assigned to a short (RSH, n = 6, age 24.8 [+ or -] 2.5 yr., height: 1.84 [+ or -] 0.04 m, weight: 83.7 [+ or -] 10.7 kg) or long (SIH, n = 5, 23.0 [+ or -] 2.1 yr., height: 1.82 [+ or -] 0.06 m, weight: 84.5 [+ or -] 8.5 kg) repeated-sprint training regime, stratified for the pretest outcomes of the repeated-sprint ability (RSA, mean sprint time) and the Yo-Yo intermittent recovery test level 2 (YYIR2, running distance). The subsequent training intervention lasted for 3 weeks and included 3 training sessions per week (training procedures are detailed below). 3-5 days after the last training session, the posttests were performed using the same procedures described for the pretests. In a subgroup of 4 participants, venous blood samples were taken before and after the training intervention to analyze mitochondrial respiration of PBMCs.
Before and 3 days after the training intervention, participants performed a 30 s anaerobic Wingate test followed by short repeated bursts of maximal cycling (5x6 s with 20 s recovery). Tests were separated by 30 min of rest and were performed on a cycle ergometer (Cyclus II, RBM elektronik-automation GmbH, Germany). Since residual fatigue from the Wingate test was probable, the testing procedure was identical for all subjects and at all training phases, allowing for comparison of changes due to the training. The cycle ergometer was set at a fixed torque corresponding to 0.85 x body weight during the Wingate test and 0.95 x body weight during the repeated sprints. The outcomes of the Wingate and the repeated sprinting tests included the peak and the mean power output. For the Wingate test, the fatigue index was calculated as the performance loss per second. Performance decrement of the repeated cycling sprint was calculated according to Girard et al. (2011b) (i.e., [1 - (SUM of mean power output accumulated / (best mean power output x 6)] x 100 for cycling)).
On a separate day (day 4 and 5 after the last training session), participants performed the RSA and YYIR2 test in a gym. The testing session started with performing the RSA test first, followed by the YYIR2 test. Tests were separated by 1 hour where participants freely moved in the gym. Similar to the cycling tests, residual fatigue could have influenced YYIR2 test outcome, but as testing procedures before and after the training period were the same, comparison of changes due to the training should be valid. The test procedures are outline elsewhere (Gatterer et al., 2014; 2015). Briefly, the RSA test consisted of 6 x 34 m sprints (17 m back and forth) with 20 s of passive recovery between sprints. Participants started 0.5 m ahead of a photocell system (Brower-Timing-System, Utah, USA), sprinted linear 17 m, touched a cone with one hand and sprinted back through the timing system as fast as possible. Best and mean RSA times were recorded. Furthermore, the performance decrement was calculated according to Girard et al. (2011b) (i.e., [total sprint time accumulated / (fastest sprint time x 6) - 1] x 100).
The YYIR2 consisted of repeated 2 x 20 m runs back and forth between the starting, turning, and finishing line at a progressively increasing speed until exhaustion. The speed was controlled by audio bleeps. Between each shuttle, the participants had a 10 s active rest period, consisting of 2 x 2.5 m walking. The end of the test was considered when participants failed twice to reach the front line in time (objective evaluation) or if they felt unable to complete another shuttle (subjective evaluation) (Krustrup et al., 2006). The completed shuttle and the resulting distance covered were registered for analysis.
During the Wingate tests and the repeated cycling sprints muscle oxygenation was measured continuously by NIRS (Niro 200, Hamamatsu Photonics K.K., Hamamatsu City, Japan). The NIRS optical sensor was placed longitudinally over the distal part of the belly of the vastus lateralis. NIRS provides the tissue oxygenation index (TOI) and the normalised total haemoglobin index (nTHI). TOI is the ratio of oxygenated to total tissue haemoglobin and reflects changes in tissue [O.sub.2] saturation relative to rest, provides information on oxygen availability and rate of oxygen utilization and, therefore, indicates crude alterations in tissue oxygen extraction (Boushel et al., 2001; Gatterer et al., 2013; Ihsan et al., 2013). The nTHI on the other hand represents a measure of total haemoglobin and is suggested to reflect blood flow (Highton et al., 2013; Montero and Lundby, 2017). During the Wingate...