An exoskeleton robotic knee system for stroke rehabilitation is proposed in this study. This robotic system has been developed to facilitate functional motor recovery by practices walking with a one joint motor powered exoskeleton to reduce the muscle weakness in the shank and reduce the hyperextension in the knee joint. The robotic knee system is able to sense the gait pattern from the stroke patients and assist the walking and control the knee angle. The clinical trial will use repeated measurements to evaluate the effectiveness of the robotic knee system in gait rehabilitation post stroke.

Inclusion Criteria:

1. Ischemic or hemorrphagic stroke with drop foot problem.
2. Sufficient cognition to follow simple instructions and to understand the content and purpose of the study. (Mini-Mental State Examination > 20)
3. Capable of standing and walking independently for an extended period of time. (Functional Ambulation Category > 3, Berg Balance Scale > 40)

Exclusion Criteria:

1. Any medical or psychological dysfunctions that would affect their ability to comply with test study protocol, such as lower back pain, neuralgia, rotational vertigo, muscloskeletal disorders, injuries, and pregnancy.
2. Any severe contractures in hip, knee, or ankle joint that would preclude passive range of motion in the lower extremity.
3. Participation in any therapeutic treatment ("outside therapy") performed with the lower extremity during the planned study, including the baseline and the followup.