How walking and running work: insights from reduced gravity analyses

The energetic cost of transport for running decreases directly with gravity level, while walking cost is only modestly affected [1]. Speed has the opposite effect: increasing walking speed results in increased cost of transport while that of running is relatively insensitive (power increases but cost per distance remains nearly constant). An optimization model generates the same results. Probing the model provides insight into why these effects occur. For walking, the main cost derives from doing positive work to replace collision loss following heel strike, loss that depends only on speed and step length (not gravity) while costs associated with both heel-strike and leg swing increase with speed. For running, as gravity decreases the collision cost of each step remains constant but flight time increases. Likewise, as speed increases at any gravity level, running collision cost increases are balanced by longer stride length to give near constant cost of transport.