The over-a-decade quantitative study of the flexion synergy has provided concrete evidence that the stereotypical movement coupling of shoulder abduction with elbow flexion and wrist/finger flexion (i.e. flexion synergy) significantly affects the ability to perform functional upper extremity movements that are essential for activities of daily living (e.g. reach-to-grasp an object). However, it is still not entirely clear what the underlying mechanisms are responsible for the flexion synergy, and furthermore, how this knowledge can advance the development of effective approaches in regaining functional upper extremity movements, especially those with moderate to severe impairment. In the current research, we will provide new insights in trying to answer these questions with three studies conducted in individuals with chronic hemiparetic stroke.

In the first study, we seek to determine the extent to which the shoulder abduction loading affects the ability to open the hand and grasp an object in individuals with chronic hemiparetic stroke. It is understood that when stroke participants are only abducting the shoulder, the torques and EMG activity of a relaxed hand increase as a function of the load applied to the shoulder. But it is not clear whether voluntary efforts of opening the hand or grasping can overcome the involuntary flexion torque generated due to the flexion synergy.

In the second study, the goal is to find out whether the expression of flexion synergy on the hand is a result from increased shared neural drive to the shoulder abductor and wrist/finger flexors. To achieve this goal, wavelet coherence of EMG signals in the shoulder abductor and wrist/finger flexors was calculated and compared with the non-flexion synergy muscle pairs and with age-matched able-bodied individuals. It is found that the alpha band (8-13 Hz) flexion synergy muscle coherence in the stroke group is significantly higher than in the ablebodied group, suggesting an increased use of the slow-conducting, polysynaptic motor pathways after stroke (e.g. cortico-reticulo-spinal tract).

In the third study, we explored the possibility of improving the classification accuracy of paretic hand opening, grasping and relaxing in individuals with chronic hemiparetic stroke. Specifically, we demonstrated that the classification accuracies of hand opening, grasping and relaxing decreased in the presence of the flexion synergy. Furthermore, the use of a subjectspecific, muscle-specific, coherence-based filter to reduce synergy-induced EMG component can improve the classification accuracy. This approach promises a better classification accuracy of hand movement intent during functional upper extremity activities.

Overall, the work in this dissertation provides valuable insights on movement control of the paretic hand during voluntary hand opening and grasping within the context of flexion synergy, advances our understanding of the underlying neural mechanisms responsible for the expression of the flexion synergy, and demonstrates a novel approach that has the potential to improve classification accuracies of hand movements during functional upper extremity activities. We hope work in this thesis will open new possibilities for clinicians, scientists and engineers to develop and deliver neural machine interfaces for individuals with chronic hemiparetic stroke to regain basic hand function.