Transfemoral amputees suffer the loss of the knee and ankle joints, as well as partial or complete loss of many of the lower extremity muscle groups involved in ambulation. Recent advances in lower limb prostheses have involved the design of active, powered prosthetic knee and ankle-foot components capable of generating knee and ankle torques similar to that of normal gait. The associated onboard motors, conditioning/processing, and battery units of these active components result in increased mass of the respective prosthesis. While not an issue during stance, this increased mass of the prosthesis affects swing. The goal of this study is to develop and validate mathematical models of the transfemoral residual limb and prosthesis, expand these models to include an active ankle-foot, and investigate counter-mass magnitude(s) and location(s) via model optimization that might improve kinematic symmetry during swing. Single- (thigh only, shank only) and multi-segment (combined thigh and shank) optimization of counter-mass magnitudes and locations indicated that a 2.0 kg counter-mass added 8 cm distal and 10 cm posterior to the distal end of knee unit within the shank segment approximated knee kinematics of able-bodied subjects. This location, however, induced artificial hip torques that reduced hip flexion during swing. While such a counter-mass location and magnitude demonstrated theoretical potential, this location is not clinically realistic; mass can only be added within the prosthesis, distal to the residual limb. Clinically realistic counter-masses must also keep the total prosthetic mass to less than 5 kg; greater mass requires supplemental prosthetic suspension, would likely increase energy expenditure during ambulation, and contribute to increased likelihood of fatigue even with active prosthetic components. The ability to simulate the effects of active prosthetic components inclusive of varying placement of battery and signal conditioning units may advance the design of active prostheses that will minimize kinematic asymmetry and result in greater patient acceptance. Advisors/Committee Members: Silver-Thorn, Barbara, Voglewede, Philip, Harris, Gerald.