Multi-Channel Functional Electrical Stimulation to Improve Specificity of Hand Extension

Abstract

The long-term goal of this research is to improve the functionality of non-invasive transcutaneous functional electrical stimulation (FES) by improving specificity in upper extremity paralysis. Spinal cord injuries result in an estimated 10,000 new cases of upper extremity paralysis yearly. FES has been used extensively to assist and rehabilitate individuals with lower-limb paralysis, but it has not been widely adopted for upper-limb paralysis. This is in part due to the complex setup necessary to target the right combination of small muscles densely packed in the forearm to create a desired hand gesture. To address this challenge, here we introduce a new multichannel FES system that enables complex current steering with a high compliance voltage to selectively recruit small extrinsic hand muscles in isolation or in combination. Using this system with real-time motion capture data of the hand, we stimulated unique electrode configurations each second to rapidly identify a rich repertoire of hand gestures. Additionally, as an initial demonstration of the unique current steering capabilities, we show that FES involving three active electrodes, instead of the traditional 2-electrode setup, provides greater specificity and redundancy, which in turn could support more therapeutic motions and interleaved stimulation to reduce fatigue Thus, this work constitutes an important step towards more efficient, dexterous, and robust upper-limb FES to improve neurorehabilitation outcomes for people with spinal cord injuries.Clinical Relevance- A new multichannel FES system can rapidly configure FES stimulation to evoke a variety of single-digit and multi-digit hand gestures.

Figure 1.

Experimental setup. A) Image of the custom multi-channel stimulator and a table of all the possible electrode combinations when selecting a subset of electrodes to serve as either cathodes, anodes, or an open circuit. B) A 2×4 electrode array was placed over the extensor digitorum while the hand rested on a table. An infrared motion capture camera housed in a 3D printed case was used capture hand kinematics.

Figure 2.

Data analysis. A) Example of a 2-electrode combination that evoked extension of D2, D3, and D5 at 12 mA (top) and a 3-electrode combination that produced D2–5 extension at 14 mA (bottom). Arrows represent hypothetical current flow between the cathode (red) and anode(s) (black). The amplitude (milliamps) at which the gesture was recorded, and the digit/s that crossed the threshold to be classified as an activation. B) Image of the participant’s hand as a result of the stimulation. C) Hand kinematics recorded before, during, and after stimulation. Stimulation is on during the green window. Colored traces show the kinematic position of each digit, normalized between −1 (full extension) and +1 (full flexion). Note how increasing the current and incorporating an additional anode causes D4 to extend as well.

Figure 3.

Three electrodes outperform 2 electrodes. Stimulation using 3-electrode combinations had more specificity, as indicated by a greater number of gestures for a given stimulation current (A). As expected, 3-electrode combinations also had a greater number of combinations overall (B) and redundant combinations per gesture (C), which would facilitate interleaving stimulation to reduce fatigue. Power demands were not significantly different between the 3-electrode and 2-electrode combinations, as indicated by similar minimum stimulation currents necessary to evoke gestures. Violin plots show kernel density estimation. Black horizontal lines denote the mean. Asterisk (*) indicates statistical significance (p < 0.05, paired t-test). Data show one value per participant (N = 10 participants).