Energy cost and lower leg muscle activities during erect bipedal locomotion under hyperoxia

BACKGROUND: Energy cost of transport per unit distance (CoT) against speed shows U-shaped fashion in walking and linear fashion in running, indicating that there exists a specific walking speed minimizing the CoT, being defined as economical speed (ES). Another specific gait speed is the intersection speed between both fashions, being called energetically optimal transition speed (EOTS). We measured the ES, EOTS, and muscle activities during walking and running at the EOTS under hyperoxia (40% fraction of inspired oxygen) on the level and uphill gradients (+ 5%). METHODS: Oxygen consumption [Formula: see text] and carbon dioxide output [Formula: see text] were measured to calculate the CoT values at eight walking speeds (2.4–7.3 km h(−1)) and four running speeds (7.3–9.4 km h(− 1)) in 17 young males. Electromyography was recorded from gastrocnemius medialis, gastrocnemius lateralis (GL), and tibialis anterior (TA) to evaluate muscle activities. Mean power frequency (MPF) was obtained to compare motor unit recruitment patterns between walking and running. RESULTS: [Formula: see text] , [Formula: see text] , and CoT values were lower under hyperoxia than normoxia at faster walking speeds and any running speeds. A faster ES on the uphill gradient and slower EOTS on both gradients were observed under hyperoxia than normoxia. GL and TA activities became lower when switching from walking to running at the EOTS under both FiO(2) conditions on both gradients, so did the MPF in the TA. CONCLUSIONS: ES and EOTS were influenced by reduced metabolic demands induced by hyperoxia. GL and TA activities in association with a lower shift of motor unit recruitment patterns in the TA would be related to the gait selection when walking or running at the EOTS. TRIAL REGISTRATION: UMIN000017690 (R000020501). Registered May 26, 2015, before the first trial. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s40101-018-0177-7) contains supplementary material, which is available to authorized users.