Extraction and selective activation of muscle synergies through spinal stimulation for SCI

Authors: *R. CHENG, J. W. BURDICK;

Lab Abstract:

Epidural spinal stimulation (ESS) has been shown to enable recovery of motor control in patients with clinically complete spinal cord injury (SCI). It is hypothesized that this results from activation of postural and locomotor circuitry in the spinal cord, but the physiological mechanisms are still unknown. In this study, we extracted muscle synergies for standing in a complete SCI patient under ESS using a novel factorization algorithm, and compared them to muscle synergies in healthy subjects in order to better understand the physiological mechanisms enabling motor control under ESS.
Muscle synergies represent the coordinated recruitment of a group of muscles co-activated by a specific neural activation signal. Standard muscle synergy extraction algorithms (e.g. NNMF, PCA) fail when applied to SCI patients under ESS, because they do not compensate for the physiological delays of an electrically stimulated neural signal to reach different muscles (e.g. a signal takes longer to reach a thigh versus calf muscle). These delays are prevalent in SCI patients under ESS, since an activating signal with fixed frequency is externally induced at a specific area of the spinal cord. Therefore, we utilize a new algorithm — regularized ShiftNMF — that accounts for these delays when extracting muscle synergies. We find that muscle synergies extracted by this algorithm are significantly better at reconstructing EMG activity, they are much more reliable when cross-validated on other sections of the EMG, and their resulting features are more physiologically meaningful.
Using this algorithm, we examine muscle synergies for standing from SCI patients under different spinal stimulation conditions, and also compare them to muscle synergies in healthy subjects. We find that (1) SCI patients exhibit fewer muscle synergies than healthy subjects, (2) when stimulated with a fixed stimulation pattern during standing, the patient’s muscle activity is composed of only a single muscle synergy, and (3) ESS with certain stimulation conditions (interleaving of multiple stimulation patterns) can activate an additional, distinct muscle synergy that greatly enhances patient standing quality. We provide evidence suggesting that muscle synergies are encoded in the human spinal cord, remain intact but possibly dormant after SCI, and are critical to quiet standing. The results allow us to hypothesize that an important physiological mechanism enabling motor control under ESS is the activation of muscle synergies in the spinal cord.

*R. CHENG, J. W. BURDICK;
Caltech, Pasadena, CA. Extraction and selective activation of muscle synergies through spinal stimulation for SCI. Program No. 296.07. 2018 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2018. Online.

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