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

Lower-limb wearable robotic devices can improve clinical gait and reduce
energetic demand in healthy populations. To help enable real-world use, we
sought to examine how assistance should be applied in variable gait
conditions and suggest an approach derived from knowledge of human
locomotion mechanics to establish a ‘roadmap’ for wearable robot design.
We characterized the changes in joint mechanics during 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. We hypothesized that the
distribution of limb-joint positive mechanical power would shift to the
hip for incline walking and running and that the distribution of
limb-joint negative mechanical power would shift to the knee for decline
walking and running. Eight subjects (6M,2F) completed five walking (1.25 m
s -1 ) trials at -8.53º, -5.71º, 0º, 5.71º, and 8.53º grade and five
running (2.25 m s -1 ) trials at -5.71º, -2.86º, 0º, 2.86º, and 5.71º
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 ankle’s contribution to
limb positive power decreased from 44% on the level to 28% at 8.53º uphill
grade ( p < 0.0001) while the hip’s 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).