Comparing optimized exoskeleton assistance of the hip, knee, and ankle in single and multi-joint configurations

Abstract

Exoskeletons that assist the hip, knee, and ankle joints have begun to improve human mobility, particularly by reducing the metabolic cost of walking. However, direct comparisons of optimal assistance of these joints, or their combinations, have not yet been possible. Assisting multiple joints may be more beneficial than the sum of individual effects, because muscles often span multiple joints, or less effective, because single-joint assistance can indirectly aid other joints. In this study, we used a hip–knee–ankle exoskeleton emulator paired with human-in-the-loop optimization to find single-joint, two-joint, and whole-leg assistance that maximally reduced the metabolic cost of walking. Hip-only and ankle-only assistance reduced the metabolic cost of walking by 26 and 30% relative to walking in the device unassisted, confirming that both joints are good targets for assistance (N = 3). Knee-only assistance reduced the metabolic cost of walking by 13%, demonstrating that effective knee assistance is possible (N = 3). Two-joint assistance reduced the metabolic cost of walking by between 33 and 42%, with the largest improvements coming from hip-ankle assistance (N = 3). Assisting all three joints reduced the metabolic cost of walking by 50%, showing that at least half of the metabolic energy expended during walking can be saved through exoskeleton assistance (N = 4). Changes in kinematics and muscle activity indicate that single-joint assistance indirectly assisted muscles at other joints, such that the improvement from whole-leg assistance was smaller than the sum of its single-joint parts. Exoskeletons can assist the entire limb for maximum effect, but a single well-chosen joint can be more efficient when considering additional factors such as weight and cost.

Reducing the energy cost of walking with low assistance levels through optimized hip flexion assistance from a soft exosuit

“…Franks et al . used HIL optimization on a hip-only exoskeleton and found that across three participants the average optimal assistance magnitude on the hip extension was 0.404 Nm kg −1 47 , ~ 47% of peak biological hip extension torque (Supplementary Table S6 ; condition). Furthermore, Zhang et al .…”

“…One recently developed technique to identify an individualized, optimal profile of assistance is human-in-the-loop (HIL) optimization, where the value of a physiological objective (e.g., metabolic cost) is estimated in real-time (with a human in the loop) and used to identify optimal device parameters for each participant 8 , 11 , 22 , 44 , 47 – 52 . The motivation of this approach is to tackle high inter-participant variability in response to any given assistance, implying that assistive profiles that help one participant may hinder another.…”

“…The motivation of this approach is to tackle high inter-participant variability in response to any given assistance, implying that assistive profiles that help one participant may hinder another. Since the HIL optimization was first introduced 48 , many research groups have tried to identify the optimal assistance of ankle plantarflexion 8 , 11 , 44 , 49 , 53 , hip extension 22 , and multi-joint systems 47 , 51 , 52 during walking 8 , 22 , 44 , 47 – 53 and running 11 . Studies have used a variety of optimization algorithms such as brute-force searches like a discrete step or continuous sweep, gradient descent, and gradient-free stochastic optimization methods like covariance matrix adaptation evolution strategy (CMA-ES) or Bayesian optimization.…”

The Effects of Incline Level on Optimized Lower-Limb Exoskeleton Assistance: A Case Series

“…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]. To see if larger improvements to incline walking are possible, the best strategy may be to optimize whole-leg assistance using a device with large torque capabilities [7], [38].…”

“…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].…”