Mechanics and energetics of walking and running up and downhill: A joint-level perspective to guide design of lower-limb exoskeletons

Abstract

Lower-limb wearable robotic devices can provide effective assistance to both clinical and healthy populations; however, how assistance should be applied in different gait conditions and environments is still unclear. We suggest a biologically-inspired approach derived from knowledge of human locomotion mechanics and energetics to establish a roadmap for wearable robot design. In this study, we characterize the changes in joint mechanics during both walking and running across a range of incline/decline grades and then provide an analysis that informs the development of lower-limb exoskeletons capable of operating across a range of mechanical demands. Eight subjects (6M,2F) completed five walking (1.25 m -1) trials at -15%, -10%, 0%, 10%, and 15% grade and five running (2.25 m s-1) trials at -10%, -5%, 0%, 5%, and 10% grade on a treadmill. We calculated time-varying joint moment and power output for the ankle, knee, and hip. For each gait, we examined how individual limb-joints contributed to total limb positive, negative and net power across grades. For both walking and running, changes in grade caused a redistribution of joint mechanical power generation and absorption. From level to incline walking, the ankles contribution to limb positive power decreased from 44% on the level to 28% at 15% uphill grade (p < 0.0001) while the hips contribution increased from 27% to 52% (p < 0.0001). In running, regardless of the surface gradient, the ankle was consistently the dominant source of lower-limb positive mechanical power (47-55%). In the context of our results, we outline three distinct use-modes that could be emphasized in future lower-limb exoskeleton designs 1) Energy injection: adding positive work into the gait cycle, 2) Energy extraction: removing negative work from the gait cycle, and 3) Energy transfer: extracting energy in one gait phase and then injecting it in another phase (i.e., regenerative braking).