Simulating ideal assistive devices to reduce the metabolic cost of walking with heavy loads

Abstract

Wearable robotic devices can restore and enhance mobility. There is growing interest in designing devices that reduce the metabolic cost of walking; however, designers lack guidelines for which joints to assist and when to provide the assistance. To help address this problem, we used musculoskeletal simulation to predict how hypothetical devices affect muscle activity and metabolic cost when walking with heavy loads. We explored 7 massless devices, each providing unrestricted torque at one degree of freedom in one direction (hip abduction, hip flexion, hip extension, knee flexion, knee extension, ankle plantarflexion, or ankle dorsiflexion). We used the Computed Muscle Control algorithm in OpenSim to find device torque profiles that minimized the sum of squared muscle activations while tracking measured kinematics of loaded walking without assistance. We then examined the metabolic savings provided by each device, the corresponding device torque profiles, and the resulting changes in muscle activity. We found that the hip flexion, knee flexion, and hip abduction devices provided greater metabolic savings than the ankle plantarflexion device. The hip abduction device had the greatest ratio of metabolic savings to peak instantaneous positive device power, suggesting that frontal-plane hip assistance may be an efficient way to reduce metabolic cost. Overall, the device torque profiles generally differed from the corresponding net joint moment generated by muscles without assistance, and occasionally exceeded the net joint moment to reduce muscle activity at other degrees of freedom. Many devices affected the activity of muscles elsewhere in the limb; for example, the hip flexion device affected muscles that span the ankle joint. Our results may help experimentalists decide which joint motions to target when building devices and can provide intuition for how devices may interact with the musculoskeletal system. The simulations are freely available online, allowing others to reproduce and extend our work.

From a biological template model to gait assistance with an exosuit

“…For simplicity the introduced exosuit was massless with ideally perfect 285 force tracking without power or force limitations. in [5]). Reserve actuators added small torque about each joint to compensate for 298 unmodeled passive structures (e.g., ligaments) and potential muscle weakness.…”

“…devices are also helpful for other categories of potential users, like firefighters, laborers 4 and soldiers [4,5]. They reduce the risk of injuries during work and improve the 5 ergonomics of the work conditions.…”

“…The model was based on 21 cadavers and 24 young healthy humans [70]. Our 270 simulation workflow was based on [5]. At first, the musculoskeletal model was scaled to 271 match the anthropometry of each subject.…”

“…in which w f,j denotes the constants for weighting the compensating actuator forces [5]. 302 In the assisted mode, the CMC's cost function J A includes the force f k (t) applied by 303 the exosuit actuators in addition to the previous terms.…”

“…Nevertheless, such kind of neuromuscular experiment-based models (e.g., OpenSim 562 models) are the best known software tools for proof of concept before experimental 563 investigations. Furthermore, by using such simulation models, one can examine the 564 effects of assistive devices on forces and metabolic consumption of individual muscles 565 which is extremely difficult by experiments [5]. Hence, first we used the simulation 566 model of gait assistance to evaluate our proposed approach (compared with optimal 567 solution resulting in maximum reduction in metabolic cost).…”