I created predictive gait simulations with a custom model for optimizing the design of a transtibial prosthesis. This work was based on the idea that many prostheses focus more on emulating the form of normal anatomy than working with the wearer's altered anatomy. To unbias the design process, I created a model where the prosthesis is represented only by its resultant forces on the surrounding limb segments, then optimized those forces in a muscle-driven simulation to minimize muscle effort and peak socket loads. I found that both objectives could be improved by allowing the foot to move forward and backward relative to the tibia, and used these simulations to design a powered ankle-foot prototype.