Muscle-to-action mapping for intuitive training of muscle synergies in post-stroke upper-limb rehabilitation.

BACKGROUND: Effective motor task execution relies on precise muscle coordination, which is often disrupted after a stroke, leading to impaired motor functions. Post-stroke, alterations in intermuscular coordination, including abnormal coupling of shoulder abductor muscles, are commonly observed and contribute to these impairments. Traditional rehabilitation often overlooks this complex intermuscular coordination, and there is a need for intuitive strategies to modify abnormal muscle synergies.

OBJECTIVE: This study introduced a novel "muscle-to-action mapping" approach to alter activation profiles of stroke affected muscle synergies. Muscle-to-action mapping trains complex muscle synergies by mapping them to intuitive motions or force directions. By mimicking target actions, patients can achieve desired muscle activation patterns. The feasibility of this approach for correcting abnormal intermuscular coordination and improving force control during reaching was tested in stroke survivors.

METHODS: A force tracking training system using muscle-to-action mapping was developed to modify abnormal synergy activation profiles during isokinetic reaching tasks. The system guided muscle activation by predicting the direction of endpoint force needed to activate specific muscle synergies, deviating from habitual patterns. The system's effectiveness was evaluated in eleven chronic stroke survivors, measuring changes in muscle synergies, endpoint force control, and clinical assessment scores.

RESULTS: The intervention significantly enhanced targeted muscle synergy activations and endpoint force control, demonstrating the training's ability to induce desired muscle synergy activation profiles through muscle-to-action mapping. The overall structure of muscle synergies remained mostly unchanged post-training, highlighting the potential to modify activation profiles without altering synergy vectors. Functional improvements were reflected in the Fugl-Meyer Assessment for the Upper Extremity and Wolf Motor Function Test scores, which increased by 3.36 and 6.45 points, respectively.

CONCLUSION: This study validates muscle-to-action mapping for training muscle synergy activation profiles in stroke survivors. Using a biomechanical model to generate endpoint forces, this method effectively altered synergy activation profiles and improved force control during reaching tasks, leading to clinical improvements. These findings indicate that muscle-to-action mapping could be a valuable addition to stroke rehabilitation, offering an intuitive method for enhancing intermuscular coordination and motor recovery.

TRIAL REGISTRATION: Registered in Clinical Research Information System of Korea National Institute of Health (KCT0005803).