Normalized economical speed is influenced by aging and not by exercise habituation

OBJECTIVE: A U-shaped relationship between energy cost of walking (C(w)) and walking speed indicates that there is a specific speed minimizing the C(w), called economical speed (ES). It is mostly slower in older adults than young adults; however, effects of leg length on the ES have been ignored. We investigated effects of aging and exercise habituation on the normalized ES by leg length (ES(normalized)). We quantified time delay of stride length and step frequency in sedentary young (SY), active young (AY), and active elderly (AE) adults in response to sinusoidal gait speed change at 30-s and 180-s periods with an amplitude of ± 0.56 m･s(− 1). RESULTS: The ES was significantly slower in the following sequence: AE, SY, and AY, whereas ES(normalized) was slower in the AE than in other young groups, with no difference between AY and SY. AE and SY showed greater step variabilities at the 180-s period, whereas AY showed relatively smaller step variabilities at both periods. Collectively, the ES(normalized) slowed due to aging, not due to exercise habituation. When optimizing the appropriate SL-SF combination for sinusoidal speed changes, young and elderly adults may adopt different strategies. Exercise habituation may reduce step variabilities in young adults.