Introduction:The functional goal for unilateral transfemoral prosthesis users during ambulation is to restore the proper biomechanics and minimize the compensatory actions caused by limb asymmetries. These goals can often lead to embedded design elements that assume the prosthetic limb to look and move like the intact limb. An unconventional artificial limb design can transfer the focus from mimicking the skeletal structure to producing functional symmetry between limbs. In this pilot study, we introduce a length-actuated prosthesis design with the goal of functional biomimicry and collect qualitative feedback from two prosthesis users. Materials and Methods: The length-actuated apparatus is attached to the distal end of a transfemoral prosthetic socket and consists of an external motor and a cam and Bowden cable system for powering and dynamically adjusting the prosthesis' length in synchronization with the gait cycle. Two transfemoral prosthesis users ambulated on a treadmill with the length-actuated prosthesis and gave anecdotal feedback. Results:The prosthesis was able to support the patient's full weight and adjust to the lengths needed during the gait cycle, including toe clearance. The overall response to the prosthesis and concept was positive; prosthesis users stated that the prosthesis and the action of the knee seemed "comfortable," and physicians concluded a sufficient and confident ambulation by the prosthesis users. The next iteration will address specific concerns of the prosthesis users by incorporating a more compact electromechanical system with sensors. This will aid in matching and continuously adjusting the prosthesis' length change timing with the cadence of the user's ambulation. Conclusions: Overall, the prototype and study proved that linear motion could replace the leg shortening functions of lowerjoint rotations, with anecdotal improvements in kinematics and comfort. Future work will consist of a quantitative evaluation of center of gravity displacement, ground reaction forces, toe clearance distance, and the possibilities of knee buckling. Clinical Relevance: There are no commercially available lower-limb prostheses that control leg length with dynamic linear motion rather than user flexion and extension of a knee joint. Possible benefits that may come from a length-actuated prosthesis include improved loading symmetry and balance, as well as decreased muscle activity. (