A major challenge in lower-limb prosthesis design is achieving functional mobility while minimizing prosthesis mass. Although hydraulic actuators provide effective active control, their relatively high mass limits broader application, especially in transtibial and transfemoral prostheses. This paper presents a passive ankle joint actuation concept based on a preloaded linear extension spring as a light-weight alternative to a hydraulic actuator. Required ankle joint torques for stand-ing and walking are analyzed, spring forces are calculated, and a suitable spring is dimensioned. The results demonstrate that stable standing and walking can be achieved using a single preloaded spring, providing a viable interim solution until lighter active components become commercially available.