Manipulating abnormal synergistic coupling of joint torques through force applications at the Hand: A Simulation-Based study.

Loss of independent joint control due to abnormal coupling of shoulder and elbow torques (i.e., abnormal synergies) is a common impairment after stroke and has been linked to poor upper-extremity function in stroke survivors. Previous research has shown that the flexor synergy (i.e., shoulder abduction coupled with elbow flexion) can be treated by progressively increasing shoulder abduction loading during elbow extension exercises. However, this finding has not been implemented in planar reaching exercises, as this requires a clear understanding of the relationship between external forces on the hand and elicited joint torques when reaching for different targets on a table. The objective of this study was to model this relationship and determine reach/force combinations that could be used to counteract either the flexor or extensor synergies. We used a musculoskeletal model to compute shoulder and elbow joint torques when reaching for targets on a table against different force directions and magnitudes. We found that force direction modulated the coupling of shoulder and elbow torques and force magnitude scaled each torque uniformly such that the extent of coupling remained the same. Additionally, we found that forces on the hand could be used to gradually increase the magnitude of simultaneous shoulder and elbow torques that counteract either the flexor or extensor synergy. These results provide the foundation to develop therapeutic interventions that address abnormal joint couplings following stroke using forces on the hand during planar reaching. Future studies should examine the therapeutic benefits of these findings in patient populations such as stroke.