The objective of the proposed clinical trial is to fully evaluate the biomechanical and energetic effects of using a powered knee-ankle prosthesis, identify task-specific training strategies for every-day device use, and quantify functional performance and quality of life changes at home and in the community. The hypothesis is that the powered knee-ankle prosthesis will demonstrate a significant improvement in function over existing technology. Furthermore, the hypothesis is that incorporating neural control information will provide enhanced, intuitive control of this device. Thus the approach is to directly compare the powered device with current state-of-art passive microprocessor controlled (MP) prostheses using a randomized cross-over study to control for subject-specific variation.