Coordination of push-off and collision determine the mechanical work of step-to-step transitions when isolated from human walking

In human walking, each transition to a new stance limb requires redirection of the center of mass (COM) velocity from one inverted pendulum arc to the next. While this can be accomplished with either negative collision work by the leading limb, positive push-off work by the trailing limb, or some combination of the two, physics-based models of step-to-step transitions predict that total positive work is minimized when the push-off and collision work are equal in magnitude. Here, we tested the importance of the coordination of push-off and collision work in determining transition work using ankle and knee joint braces to limit the ability of a leg to perform positive work on the body. To isolate transitions from other contributors to walking mechanics, participants were instructed to rock back and forth from one leg to the other, restricting motion to the sagittal plane and eliminating the need to swing the legs. We found that reduced push-off work increased the collision work required to complete the redirection of the COM velocity during each transition. A greater amount of total mechanical work was required when rocking departed from the predicted optimal coordination of step-to-step transitions, in which push-off and collision work are equal in magnitude. Our finding that transition work increases if one or both legs do not push-off with the optimal coordination may help explain the elevated metabolic cost of pathological gait irrespective of etiology.