Body weight support (BWS) apparatus and its functional characteristics. a A schematic illustration of the participants walking with a custom-made BWS apparatus. This setting ensured that the BWS apparatus applied vertical force without disturbing leg swing motion. VO2 and VCO2 are oxygen uptake and carbon dioxide output, respectively. CoT is the energy cost of transport per unit distance. b Fluctuations of the vertical force in four representative participants from 0.67 to 2.00 m s− 1. Five steps in each participant were time-aligned into percentage of gait cycle, and those data were overlapped at each speed. The frame position becomes the lowest at the heel contact, so the timing at the lowest vertical force was regarded as 0% gait cycle. %BW means percentage of supported body weight.