The work the legs perform on the body center of mass (COM) is an important determinant of the metabolic cost of walking. Much COM work is performed to redirect the center of mass from a downward to an upward velocity during transitions between successive stance legs, termed step-to-step transitions. We elucidate the links between COM velocity fluctuation, COM work, and metabolic cost through several experimental manipulations of gait.

We show that foot length and foot bottom curvature affect COM work and metabolic cost in fixed-ankle walking. In dynamic walking models, longer feet lead to decreased work requirements for gait. We measured COM work and metabolic cost while subjects walked with locked ankles and artificial foot bottom shapes. COM work decreases with increasing foot length, because longer feet reduce the angular redirection of COM velocity during the step-to-step transition. Foot bottom curvature has no significant effect for humanlike foot sizes. In this range, COM work using arc shapes is less than for normal walking, though metabolic cost is higher. Metabolic cost is minimized by feet having length 28% of leg length.

We also show that COM work for mechanically unconstrained walking depends on COM speed and angular redirection of COM velocity during the step-to-step transition. We measured variations in COM velocity while subjects walked at a wide range of speeds and step lengths. COM work scales quadratically with COM speed at heel strike times angular redirection of COM velocity. We introduce a sagittal plane plot of COM velocity trajectory, called a hodograph, to visualize these variables and understand abnormal gait.

We use these hodographs to show that unilateral transtibial amputees walk asymmetrically with respect to several kinetic variables of gait. Amputees exhibit higher mid-stance COM speed and weaker push-off on the prosthetic side, and more positive and negative step-to-step transition COM work on the intact side.

Finally, we introduce the Rock’N’Lock foot, a reconfigurable foot prosthesis that implements a rigid foot bottom shape with the goal of reducing metabolic cost. In preliminary results this foot has cost equal to other prostheses’, but forthcoming design improvements may lower the cost further.