Hypoxic vs. normoxic HIIT training

All training sessions were supervised by the investigators and consisted of unilateral knee extensions on a leg-extension apparatus (GLCE365, Body-Solid, Illinois, US). The leg-extension apparatus was adapted to limit knee-extension amplitude during training sessions from 95° to 175° knee angle. During each training session the subjects performed two sets of knee extensions with a 5-min passive recovery interval in between. Each set consisted of 4–6 series of 30 contractions at 20–25% of the 1-repetition-maximum (1RM), with 30 s of rest in between the series. The number of contraction series increased from four in sessions 1–5, to five in sessions 6–10, and eventually six in sessions 11–14. The relative load increased from 20% of 1RM in sessions 1–7 to 25% of 1RM in sessions 8–14. During each series the rate of muscle contraction and relaxation was paced by both auditory and visual feedback to obtain a 3-s duty cycle with 1s extension:1s relaxation:1s rest. 1RM was re-evaluated at the start of each next week to adjust training workloads. The subjects were randomly assigned to a group training with the dominant leg in hypoxia, whilst the contralateral leg was trained in normoxia, or vice versa. In each session the subjects thus trained one leg in the normobaric hypoxic facility (b-CAT, Tiel, The Netherlands) at 12.3% F_iO₂ (~4,300 m), whilst the contralateral leg was trained in an adjacent normoxic room at 20.9% F_iO₂. The subjects performed 3 training sessions per week, and the order of left vs. right leg training was alternated between the sessions. The oxygenation status of m. vastus lateralis was assessed by near-infrared spectroscopy (NIRS) during all training sessions in week 1 and week 5, as previously described (Van Thienen and Hespel, 2016). Immediately following each training session the subjects ingested 25 g of a standard whey protein concentrate mixture (Whey Shake, Sports 2 Health, Hofstade, Belgium) to stimulate muscular repair.