Lower-limb multi-joint (knee and ankle) stiffness may play an important role in functional activities such as walking, and may be significantly altered post stroke. Thus, determination of lower-limb multi joint stiffness matrix is important for better understanding of gait and of pathological changes post stroke. In this study, using novel dynamics decomposition, the knee and ankle joint stiffness matrix including cross-coupled stiffness terms between the two joints were determined and reported ever first. The determined stiffness matrix may be useful for gait studies, and can be served as a baseline for studying pathophysiological changes post stroke.