Schematic illustration of Cost of Transport (CoT) and gait speed (<i>v</i>) at normoxia and hypoxia.

<p>(a) Upward and leftward shifts of the U-shaped CoT-<i>v</i> relationship in walking and an upward shift of the linear CoT-<i>v</i> relationship in running result in a slower energetically optimal transition speed (EOTS, circles) and economical speed (ES, triangles) at hypoxia. (b) Upward shifts of both CoT-<i>v</i> relationships result in unchanged ES and EOTS. Solid and dotted lines mean normoxia and hypoxia, respectively. Both red arrows indicate a possible ‘shifting model’ for explaining slower ES and EOTS at hypoxia.