Reducing Abnormal Muscle Coactivation After Stroke Using a Myoelectric-Computer Interface: A Pilot Study

Abstract

Background A significant factor in impaired movement caused by stroke is the inability to activate muscles independently. Although the pathophysiology behind this abnormal coactivation is not clear, reducing the coactivation could improve overall arm function. A myoelectric computer interface (MCI), which maps electromyographic signals to cursor movement, could be used as a treatment to help retrain muscle activation patterns. Objective To investigate the use of MCI training to reduce abnormal muscle coactivation in chronic stroke survivors. Methods A total of 5 healthy participants and 5 stroke survivors with hemiparesis participated in multiple sessions of MCI training. The level of arm impairment in stroke survivors was assessed using the upper-extremity portion of the Fugl-Meyer Motor Assessment (FMA-UE). Participants performed isometric activations of up to 5 muscles. Activation of each muscle was mapped to different directions of cursor movement. The MCI specifically targeted 1 pair of muscles in each participant for reduction of coactivation. Results Both healthy participants and stroke survivors learned to reduce abnormal coactivation of the targeted muscles with MCI training. Out of 5 stroke survivors, 3 exhibited objective reduction in arm impairment as well (improvement in FMA-UE of 3 points in each of these patients). Conclusions These results suggest that the MCI was an effective tool in directly retraining muscle activation patterns following stroke.

Keywords: EMG; arm; coactivation; muscles; rehabilitation; stroke; synergies.

Figure 1

MCI task overview. (A) Experimental setup with a stroke subject viewing a circular cursor and square target on the monitor. (B) Schematic of MCI task and muscle mapping directions. Subjects moved the cursor to one of either 2 or 8 (shown here) outer targets. Relaxing all muscles moved the cursor to the center. (C) Muscle mapping directions in both the 8-target task (left) and 2-target task (right) for both healthy (top) and stroke (bottom) subject groups. BR, brachioradialis; FFlex, flexor digitorum superficialis; FExt, extensor digitorum; ADelt, anterior deltoid; PDelt, posterior deltoid.

Figure 2

Healthy subjects' task performance and co-activation changes. (A) Time-to-target, (B) path length measures and (C) mean R between biceps and brachioradialis activity averaged across all subjects for pre- and post-training (dashed lines) and early and late training (solid lines). Error bars represent standard error (SE).

Figure 3

Mean movement trajectories to each target and representative EMG traces from a stroke subject during (A) pre-training in the first session (left) and post-training in the last session (right) and (B) early training in the first session (thin lines) and late training in the last session (thick lines). Blue and red trajectories represent cursor movement in the biceps direction and anterior deltoid direction, respectively. Representative EMG traces of biceps (blue) and anterior deltoid (red) for single trials during movement in the (C) anterior deltoid target direction and (D) biceps target direction show reduction of co-activation from early training in the first session (left) to late training in the last session (right). Black curves represent the cursor's distance to the intended target (dashed boxes in B) over time. Dashed lines in (C) and (D) represent the time at which target was reached. Vertical gray bars represent EMG scale (0.5 mV).

Figure 4

Stroke subjects' task performance and co-activation changes. Performance measures, including (A) time-to-target, (B) path length, and (C) success rate averaged across all subjects show steady improvement during both training (solid lines) and pre- and post-training (dashed lines) tasks. Mean R between biceps and anterior deltoid during the (D) training and (E) pre- and post-training tasks. Correlations steadily and significantly decreased during the training task.

Figure 5

Evolution of muscle tuning curves in stroke subjects. (A) Tuning curves of biceps and anterior deltoid averaged across subjects from post-training tasks of the first (dashed line), ninth (gray line), and last (black line) sessions. Control signals are normalized to the maximum level in both sessions and averaged across trials. (B) Tuning curves averaged across all 4 muscles and subjects over sessions. Tuning depth in all muscles gradually increased over time.