Disturbances in movement coordination are the least well understood but often the most debilitating with respect to functional recovery following stroke. These deficits in coordination are expressed in the form of abnormal muscle synergies and result in limited and stereotypic movement patterns that are functionally disabling. The result of these constraints in muscle synergies is an abnormal coupling between shoulder abduction and elbow flexion (i.e. the flexion synergy), which significantly reduces the reaching function of an individual with stroke when they lift up the weight of the impaired arm against gravity. The investigators previous neurotherapeutic research, supported by a NIDRR Field Initiated research grant, has shown that the abnormal synergy between shoulder abduction and elbow flexion can be significantly reduced thus increasing total reaching range of motion in individuals with severe stroke. The previous work established progressive abduction loading as a key element to the rehabilitation of reaching. Although individuals with severe stroke benefited from the investigators previous work, residual flexion synergy continued to hinder normal arm function in most participants with severe stroke. This study will utilize the ACT3D robot, developed as part of the investigators previous NIDRR project, to incorporate resistance to reaching while accounting for the known benefits of progressive abduction loading. The investigators propose to randomize forty participants with severe stroke into two closely related interventions. The groups will both practice reaching under abduction loading, however, the experimental group will also move against resistance while reaching. Rigorous and quantitative investigation of therapeutic elements such as resistance to reaching and progressive abduction loading is only possible with a device such as the ACT3D. the investigators will be able to standardize the delivery of each intervention using kinematic and kinetic parameters, which will allow for a clear identification of the therapeutic effect of resistance to reaching. the investigators hypothesize that resistance to reaching in combination with progressive abduction loading will further increase dynamic multi-joint strength, increase total reaching range of motion, and increase arm function thus enhancing actual amount of use of the arm, participation in life roles, and quality of life in individuals with severe stroke.

Inclusion Criteria:

• Paresis confined to one side, with substantial motor impairment of the upper limb
• Absence of motor impairment in the unimpaired limb
• Absence of severe concurrent medical problems (e.g. cardiorespiratory impairment, changes in management of hypertension)
• Absence of any acute or chronic painful condition in the upper extremities or spine
• Absence of hemineglect
• Capacity to provide informed consent.