Neglected losses and key costs: tracking the energetics of walking and running.

As one of the most energetically demanding daily activities, locomotion has attracted substantial investigative attention. Although legged locomotion has been well described, it is currently not well understood. Looking at energy accounting might be a good pathway with which to solve this problem. One relatively simple way of analyzing energy management is to look directly at the flow of mechanical energy into and out of the system, in terms of costs and losses (with some attention to the mechanisms responsible for this flow). In this commentary we argue that a key source of energetic loss has largely been neglected: the redirection of body motion from downward to upward at each step. We discuss the role of this loss and the compensating energetic costs, identifying some of the general features of the trade-offs that determine gait optimization strategies. We find that even at a conceptual level, a focus on the main mechanism of loss and the strategies available to the organism to effectively compensate for losses can yield substantial insight into observations as diverse as the functional limits of a playground swing through to the strikingly different effect of reduced gravity on human walking and running. Such insight changes the interpretation of fundamental features of leg function, such as push-off timing and the role of elastic deflection during stance.