Muscle Stimulation for Natural Torque Feedback through the Agonist-antagonist Myoneural Interface (AMI)
Tyler R Clites, Ph.D.1; Matthew J. Carty, MD2; Matthew E Carney, M.S.1; Shriya S Srinivasan, BS1; Hugh Herr, PhD1
1Massachusetts Institute of Technology, Cambridge, MA, 2Division of Plastic Surgery, Brigham and Women's Hospital, Boston, MA
Background: In standard clinical practice, there is no robust, repeatable method of reflecting proprioceptive feedback from a prosthesis onto the nervous system. The Agonist-antagonist Myoneural Interface (AMI) is a bi-directional neural communication paradigm comprised of two muscles – an agonist and an antagonist – surgically connected in series so that contraction of one muscle stretches the other. The AMI preserves important dynamic muscle relationships that exist within native anatomy, thereby allowing proprioceptive signals from mechanoreceptors within both muscles to be communicated to the central nervous system. It is possible to directly modulate force borne on an AMI muscle via functional electrical stimulation (FES) of that muscle's mechanically-coupled antagonist. In this way, torque information from a prosthetic joint can be communicated to the nervous system by direct manipulation of mechanoreceptors within both muscles.
Materials and Methods: Afferent feedback of prosthetic joint torque was provided to a subject having two AMIs within his transtibial residuum. In response to torque measured on the prosthesis, FES was delivered through fine-wire intramuscular electrodes to the antagonist muscle within one of the subject's AMIs. Stimulation amplitude was modulated in linear proportionality to the torque measured in the associated prosthetic joint. Two psychometric evaluations were performed to quantify Subject A's perception of torque intensity in absence of the prosthesis. A functional test was then performed to evaluate the impact of the closed-loop control system on the subject's ability to modulate prosthetic torque output.
Results: The first psychometric evaluation, a magnitude estimation test, showed significant correlation between perceived dorsiflexion torque and stimulation amplitude (P < 0.0001, R2 = 0.88, n = 25). The second evaluation, a just noticeable difference (JND) test, showed a smooth psychometric curve with a JND of 0.065 mA and a Weber fraction of 0.033. Torque feedback was associated with a significant improvement in performance on a pedal task that required targeted modulation of prosthetic joint torque.
Conclusions: Stimulation of AMI muscles is perceived as natural torque about the phantom ankle, and improves performance in a torque modulation task. Reliable closed-loop control of joint torque has the potential to provide functionality to prosthetic users that was heretofore impossible, in both the lower and upper extremities.
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