The anatomical foot and ankle joint provide important functions during gait, so fitting lower-limb prosthesis users with components that replicate these functions is rational and desirable. However, it is not currently known how to best combine and tune prosthetic foot and ankle stiffness to enhance a prosthesis user’s walking and standing performance simultaneously. Furthermore, altering prosthetic foot-ankle stiffness could adversely affect prosthetic knee stability in transfemoral prosthesis users. Therefore, we propose to systematically vary the stiffness of the prosthetic foot and ankle joint combination in transfemoral prosthesis users to help establish prescription guidelines for prosthetists to consider when fitting prosthetic ankle units with prosthetic feet. The Veterans Health Administration (VHA) reported that between 1989-1998, 29% of amputations were performed at the transfemoral level. This emphasizes the strong relevance and potentially high impact of this research study on current Veterans, which is directly applicable to the VHA’s Patient Care Mission.