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We measured oxygen consumption during walking per unit distance (Cw) values for 12 human healthy young males at six speeds from 0.667 to 1.639 m s−1 (four min per stage) on a level gradient under normobaric normoxia, moderate hypoxia (15% O2), and severe hypoxia (11% O2). Muscle deoxygenation (HHb) was measured at the vastus lateralis muscle using near-infrared spectroscopy. Economical speed which can minimize the Cw in each individual was calculated from a U-shaped relationship. We found a significantly slower economical speed (ES) under severe hypoxia [1.237 (0.056) m s−1; mean (s.d.)] compared to normoxia [1.334 (0.070) m s−1] and moderate hypoxia [1.314 (0.070) m s−1, P<0.05 respectively] with no differences between normoxia and moderate hypoxia (P>0.05). HHb gradually increased with increasing speed under severe hypoxia, while it did not increase under normoxia and moderate hypoxia. Changes in HHb between standing baseline and the final minute at faster gait speeds were significantly related to individual ES (r=0.393 at 1.250 m s−1, r=0.376 at 1.444 m s−1, and r=0.409 at 1.639 m s−1, P<0.05, respectively). These results suggested that acute severe hypoxia slowed ES by ∼8%, but moderate hypoxia left ES unchanged.