Restoration of bilateral motor coordination from preserved agonist-antagonist coupling in amputation musculature

Tony Shu,Shan Shan Huang,Christopher Shallal,Hugh M Herr

doi:10.1186/s12984-021-00829-z

Abstract

BackgroundNeuroprosthetic devices controlled by persons with standard limb amputation often lack the dexterity of the physiological limb due to limitations of both the user’s ability to output accurate control signals and the control system’s ability to formulate dynamic trajectories from those signals. To restore full limb functionality to persons with amputation, it is necessary to first deduce and quantify the motor performance of the missing limbs, then meet these performance requirements through direct, volitional control of neuroprosthetic devices.MethodsWe develop a neuromuscular modeling and optimization paradigm for the agonist-antagonist myoneural interface, a novel tissue architecture and neural interface for the control of myoelectric prostheses, that enables it to generate virtual joint trajectories coordinated with an intact biological joint at full physiologically-relevant movement bandwidth. In this investigation, a baseline of performance is first established in a population of non-amputee control subjects (n = 8). Then, a neuromuscular modeling and optimization technique is advanced that allows unilateral AMI amputation subjects (n = 5) and standard amputation subjects (n = 4) to generate virtual subtalar prosthetic joint kinematics using measured surface electromyography (sEMG) signals generated by musculature within the affected leg residuum.ResultsUsing their optimized neuromuscular subtalar models under blindfolded conditions with only proprioceptive feedback, AMI amputation subjects demonstrate bilateral subtalar coordination accuracy not significantly different from that of the non-amputee control group (Kolmogorov-Smirnov test, P ge 0.052) while standard amputation subjects demonstrate significantly poorer performance (Kolmogorov-Smirnov test, P < 0.001).ConclusionsThese results suggest that the absence of an intact biological joint does not necessarily remove the ability to produce neurophysical signals with sufficient information to reconstruct physiological movements. Further, the seamless manner in which virtual and intact biological joints are shown to coordinate reinforces the theory that desired movement trajectories are mentally formulated in an abstract task space which does not depend on physical limb configurations.

Highlights

Neuroprosthetic devices controlled by persons with standard limb amputation often lack the dexterity of the physiological limb due to limitations of both the user’s ability to output accurate control signals and the control system’s ability to formulate dynamic trajectories from those signals
These results agree with previous literature on bimanual coordination and rhythmic reaching, suggesting that lower-extremity movements may be formulated in dimensionallyreduced, perceptive task space variables which do not rely on vision [27]
We provided additional evidence that motor coordination tendencies are relatively invariant across the different workspaces of the body, and that movement trajectories are formulated in task space coordinates independent of underlying biomechanical structures

Summary

Introduction

Neuroprosthetic devices controlled by persons with standard limb amputation often lack the dexterity of the physiological limb due to limitations of both the user’s ability to output accurate control signals and the control system’s ability to formulate dynamic trajectories from those signals. While the mechanisms to achieve full embodiment and agency over a prosthetic limb remain elusive and difficult to define, one reasonable intermediate requirement is achieving parity between bionic and intact physiology in motor control tasks. Afferent cutaneous signaling from the external prosthesis has been shown to improve prosthetic controllability, gait mobility, upper-extremity arm functionality, confidence, and mental and physical acuity during activities of daily living [6,7,8,9,10]. This is but one feedback pathway available from the peripheral nervous system. In one ideal control paradigm for free-space prosthetic movements, these afferent neural signals provide vivid proprioception of perceived ankle and subtalar phantom joints whose kinematics are perfectly tracked by a powered prosthesis [18]

Methods

Results

Discussion

Conclusion