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 componentry results in increased mass of the respective prosthesis, which affects the swing phase of gait. The goal of this study was to develop a computer model of the transfemoral residual limb and prosthesis, inclusive of an active ankle-foot, and investigate counter-mass magnitude(s) and location(s) via model optimization that might improve lower limb kinematic symmetry between the residual/prosthetic limb (approximated by the computer model) and the sound limb (approximated by able-bodied motion data) during swing phase. Single- (thigh only, shank only) and multisegment (both 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 the knee unit within the shank segment approximated knee kinematics of the sound limb. This counter-mass location, however, reduced hip flexion during swing phase. While such a counter-mass location and magnitude demonstrated theoretical potential, the location is not clinically realistic; mass can only be practically 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 may require supplemental prosthetic suspension, may increase energy expenditure during ambulation and may increase the likelihood of fatigue, even with active prosthetic components. The ability to simulate the kinematic 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.