Stroke affects multiple joints in the arm with stereotypical patterns of arm deformity involving the shoulder, elbow, wrist, and hand and with disrupted coordination of multiple joints in active movements. However, there is a lack of systematicmethods to evaluatemulti-joints and multi-degree of freedoms (DOF) neuro-mechanical changes, especially for complex systemswith three ormore joints/ DOFs involved. This paper used a novel systematic method to characterize dynamics and control of the shoulder, elbow, and wrist of the human arm individually and simultaneously, including the couplings across themultiple joints during controlled movements. A novel method was developed to decompose the complex system into manageable single-joint level for more reliable characterizations. The method was used in clinical studies to characterize the multi-joint changes associated with spastic impaired arm of 11 patients post stroke and 12 healthy controls. It was found that stroke survivors showed not only increased stiffness at the individual joints locally but also significantly higher couplings across the joints. The relative increases in couplings are often higher than that of the local joint stiffness. The multi-joint characterization provided a tool to characterize impairment of individual patients, which would allow more focused impairment-specific treatment. In general, the decomposition method can be used for even more complex systems, making characterization of intractable system dynamics of three or more joints/DOFs manageable.