The purpose of this thesis project is to design a new concept of sprinting prosthesis to enhance the acceleration phase performance of bilateral transtibial amputees (BTAs) competing in sprinting races. Amputee sprinters are hampered in the acceleration phase of a sprint race by the compliance of their prostheses. Increased contact times, and small and too vertical ground reaction forces limit the acceleration performance of BTAs. A variable stiffness prosthesis, with a higher stiffness at the initial steps of the acceleration phase, would reduce contact times and increase forward propulsion. A new variable stiffness design is proposed in this report. A prototype of this new design is created and characterized by retrieving the stiffness matrices and ellipses. Ottobock's 1E90 prosthesis is also characterized, and the results are compared. For this, a new testing methodology to obtain the stiffness ellipses of running prostheses (RPs) is proposed.