A higher-than-average maximal oxygen consumption (VO_2max), is closely associated with decreased morbidity and mortality and improved quality of life and acts as a marker of cardiorespiratory fitness. Although there is no consensus about an optimal training method to enhance VO_2max, nevertheless training of small muscle groups and repeated exposure to hypoxia seem to be promising approaches. Therefore, this study was aimed at gaining innovative insights into the effects of small muscle group training in normoxia and hypoxia. Thirteen healthy participants were randomly assigned to the hypoxic (HG, n = 7) or normoxic (NG, n = 6) training group. Both groups completed nine high-intensity interval training sessions in 3 wks. The NG performed the training in normoxia (F_iO₂: 0.21; ~ 600 m) and the HG in hypoxia (F_iO₂: 0.126; ~ 4500 m). Each session consisted of 4 x 4 min one-legged cycling at 90% of maximal heart rate separated by 4 min recovery periods. Before and after the intervention period, VO_2max and peak power output (W_max) and responses to submaximal cycling (100 and 150 watts) were assessed in a laboratory cycling test. Peak power output significantly improved within both groups (9.6 ± 4.8% and 12.6 ± 8.9% for HG and NG, respectively) with no significant interaction (p = 0.277). However, VO_2max only significantly increased after training in hypoxia from 45.4 ± 10.1 to 50.0 ± 9.8 ml/min/kg (10.8 ± 6.0%; p = 0.002) with no significant interaction (p = 0.146). The maximal O₂-pulse improved within the HG and demonstrated a significant interaction (p = 0.040). One-legged cycling training significantly improved VO_2max and peak power output. Training under hypoxic conditions may generate greater effects on VO_2max than a similar training in normoxia and is considered as a promising training method for improving cardiorespiratory fitness.