The purposes of this study include:

1. - To identify whether features of aberrant intermuscular coordination patterns can be used to predict motor impairment after stroke.
2. - To test whether muscle synergies are malleable to a non-invasive EMG-guided exercise that induces changes in intermuscular coordination of upper extremity muscles after stroke.

Stroke is a leading cause of long-term disability in the United States. Of the more than 700,000 Americans who experience a stroke each year, two-thirds survive. 69% of patients who were admitted to a rehabilitation unit following stroke have mild to severe upper extremity dysfunction. Currently, there are more than seven million stroke survivors in the U.S., many of whom have long-term motor and sensory impairments, especially in the arm. The objective of this study involves both scientific and clinical aspects:

For the first purpose, intermuscular coordination patterns emerging under isometric reaching and dynamic conditions will be identified to predict impairment of both non-motion or motion-involved task performance and severity of motor impairment after stroke. This aim will enroll 15 age-matched (age of 40-75 yo) healthy adults and 25 adult (age of 40-75 yo) stroke survivors. Each stroke participant will have two visit sessions while age-matched will have a single measurement session.

For the second purpose, in total, 74 stroke survivors will perform an electromyographic signal-guided exercise through human-machine interaction to ameliorate motor impairment post-stroke by normalizing abnormal intermuscular coordination patterns in the arm after stroke, and improve motor impairment; also, assessment of the intermuscular coordination, UE Fugl-Meyer (FM), and Action Research Arm Test (ARAT) will be performed. Participants will have three visits per week for six weeks for training sessions. Finally, to test retention of the intervention effect, they will perform two assessment sessions one and three months after finishing the training.

Inclusion criteria for aged matched healthy group

• Male or female whose age range between 40 and 75
• no known neurological injuries

Exclusion criteria for aged matched healthy group

• have an orthopedic disorder involving upper limbs;
• have a history of any neurologic disease;
• have any history of epilepsy of the potential participants and/or their family members;
• are unable to consent;
• are pregnant.

Inclusion criteria for stroke group

• male or female hemiparetic chronic stroke survivors;
• age ranging between 40-75 year;
• with single unilateral ischemic or hemorrhagic middle cerebral artery stroke;
• neurologically stable for >6 months;
• have an expectation that current medication will be maintained without changes for at least 3 months. Stable use of anti-spasticity medication (e.g., baclofen, diazepam, tizanidine) is accepted;
• without severe spasticity (Modified Ashworth (MA) <4);
• have not received botulinum toxin on the impaired arm within 3 months.

Exclusion criteria for stroke group

• have an orthopedic disorder involving upper limbs;
• cognitive impairment sufficient to interfere with informed consent or successful completion of the protocol (Montreal Cognitive Assessment (MoCA) score < 26);
• a history of another neurologic disease;
• anesthesia of joint position sense in upper limbs;
• are pregnant or have a chance that they might be (self-reported);