Mechanics 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 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 lowerlimb 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-8.53˚,-5.71˚, 0˚, 5.71˚, and 8.53˚grade and five running (2.25 m s-1) trials at-5.71˚,-2.86˚, 0˚, 2.86˚, and 5.71˚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 8.53˚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).

The Effects of Incline Level on Optimized Lower-Limb Exoskeleton Assistance

“…This improvement is slightly smaller than single-joint assistance on level-ground (around 18% relative to walking in no exoskeleton) [9], [12]. It is possible that exoskeletons might deliver larger improvements to incline walking than level-ground walking, because steeper inclines require larger biological joint powers [30], [31], [32] and incur higher metabolic energy costs [16], [17], [18], giving a larger opportunity for the device to effectively assist. Alternatively, it is possible that assisting incline walking may be less effective because of the related effect of walking speed: people tend to walk slower up inclines [33], [34], [35], [36], and exoskeletons have been less effective at slower speeds [7], [37].…”

“…Assisting the whole leg during incline walking could lead to greater reductions in metabolic cost. Single-joint devices have focused primarily on the ankle and hip for level-ground walking, where they do most of the biological work of the leg [30], [31]. Each of the devices that have reduced the metabolic cost of walking on inclines have also assisted only a single joint [13], [27], [28], [29], [39].…”

“…Each of the devices that have reduced the metabolic cost of walking on inclines have also assisted only a single joint [13], [27], [28], [29], [39]. However, the hips, knees, and ankles all contribute to the total positive power of the leg during incline walking [30], [31], so assisting all joints simultaneously could be best. While knee assistance has been the least effective of all joints on level ground [38], biological knee torque and power increase as grade increases [31], meaning the knee may be increasingly helpful to assist with inclines.…”

“…However, the hips, knees, and ankles all contribute to the total positive power of the leg during incline walking [30], [31], so assisting all joints simultaneously could be best. While knee assistance has been the least effective of all joints on level ground [38], biological knee torque and power increase as grade increases [31], meaning the knee may be increasingly helpful to assist with inclines. For level-ground walking, whole-leg assistance produced greater improvements to metabolic cost compared to single-joint and two-joint assistance [38], consistent with expectations from biomechanical simulations [40], [41].…”

“…By understanding how optimal whole-leg assistance changes with incline, exoskeleton designers could build mobile devices that are most effective in real-world settings. While biological torques change with incline, including large increases in hip extension torque and smaller increases in knee extension and ankle plantarflexion torques [31], it is unknown how exoskeleton torques should change with incline as well. If we understood the relationship between incline level and optimal assistance parameters, mobile devices could adapt their control strategy online to best assist current terrain.…”