Gait Adaptation in the Context of the Agonist-Antagonist Myoneural Interface (AMI)
Tyler R Clites, PhD1; Matthew J. Carty, MD2; Matthew E Carney, MS1; 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. Swing phase adjustments to joint position and impedance play a critical role in the adaptation of gait to varying terrains. Replicating these adaptations has long been a goal of lower-extremity prosthetic research, with the majority of efforts focused on intrinsic or EMG-based terrain prediction and recognition methodologies. By reproducing the dynamic muscle relationships that are fundamental to joint control and proprioceptive sensation, the Agonist-antagonist Myoneural Interface (AMI) has the potential to restore emergent reflexive behaviors in persons with lower extremity amputation.
Materials and Methods: In subjects having AMIs in their transtibial residua, reflexive activity was evaluated during stair ascent and descent tasks as the subjects walked with both a passive and a myoelectrically controlled ankle-foot prosthesis. During these tasks, subjects were instructed to walk as naturally as possible, and to avoid active volitional movement of the prosthesis. This subjects' electromyographic signals and joint angle trajectories were compared with those of a group of persons with standard transtibial amputation, using the same prosthetic limb and (where applicable) under the same myoelectric control architecture.
Results: The AMI subjects displayed normalized gait behaviors while traversing various terrains, such as ankle dorsiflexion during the swing phase of stair ascent, and plantarflexion during the swing phase of stair descent. These behaviors were described as "automatic" by the AMI subjects, and were not observed in the subjects having traditional transtibial amputation.
Conclusions: These findings highlight the AMI's potential to reinstate the central nervous system as the primary mediator of gait adaptation, by providing the afferent proprioceptive sensations that are crucial to this function. Future work will explore the impact of these adaptations on gait normalization during traversal of these and several additional terrains.
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