Although numerous studies have reported the effect of inspiratory muscle training for improving exercise performance, the outcome of whether exercise performance is improved by inspiratory muscle training is controversial. Therefore, this study investigated the influence of inspiratory muscle-loaded exercise training (IMLET) on peak oxygen uptake (VO2peak), respiratory responses, and exercise performance under normoxic (N) and hypoxic (H) exercise conditions. We hypothesised that IMLET enhances

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... respiratory muscle strength and improves respiratory response, thereby improving VO2peak and work capacity under H condition. Methods: Sixteen university track runners (13 men and 3 women) were randomly assigned to the IMLET (n=8) or exercise training (ET) group (n=8). All subjects underwent 4 weeks of 20-min 60% VO2peak cycling exercise training, thrice per week. IMLET loaded 50% of maximal inspiratory pressure during exercise. At pre-and post-training periods, subjects performed exhaustive incremental cycling under normoxic (N; 20.9 ± 0%) and hypoxic (H; 15.0 ± 0.1%) conditions. Results: Although maximal inspiratory pressure (PImax) significantly increased after training in both groups, the extent of PImax increase was significantly higher in the IMLET group (from 102 ± 20 to 145 ± 26 cmH2O in IMLET; from 111± 23 to 127 ± 23 cmH2O in ET; P < 0.05). In both groups, VO2peak and maximal work load (Wmax) similarly increased both under N and H conditions after training (P < 0.05). Further, the extent of Wmax decrease under H condition was lower in the IMLET group at post-training test than at pre-training (from -14.7 ± 2.2% to -12.5 ± 1.7%; P < 0.05). Maximal minute ventilation in both N and H conditions increased after training than in the pre-training period. Conclusions: Our IMLET enhanced the 3 respiratory muscle strength, and the decrease in work capacity under hypoxia was reduced regardless of the increase VO2peak. background There are some difficulties in attaining success in hypoxic training for improving sea-level competitive performance among athletes; this is related to the lack of sufficient absolute training intensity caused by a substantial decrease in aerobic capacity (1). In the acute hypoxic (H) condition, peak oxygen consumption (VO 2peak ) is reduced by impaired gas exchange due to a reduction in the ambient partial O 2 pressure. The effect of hypoxia (2,400-3,000 m) on V O 2peak (2, 3) is more pronounced in subjects with higher VO 2peak with large decrease in haemoglobin O 2 saturation (SaO 2 ). Notably, individuals with a large increase in maximal minute ventilation (V Em ax ) under acute H condition, relative to those under normoxic (N) condition, show a smaller reduction in VO 2peak (4, 5), suggesting that a high V E under H condition is beneficial for minimising the reduction in VO 2peak . However, high V E under H condition leads to higher O 2 cost in the respiratory muscles than that under N condition (6). Since respiratory muscle work during heavy exercise accounts for 10%-15% of the whole body VO 2 (7) even under N condition, a compromised blood flow to the active muscle during heavy exercise was observed (8, 9). Further, diaphragm fatigue has been observed following strenuous exercise (10, 11) , which limits exercise performance (12, 13). In this regard, under H condition, an increase in V E with a minimum increase in respiratory muscle work may be essential to prevent a greater reduction in the VO 2peak and exercise performance under H condition.