Introduction The CYBERLEGs Beta-Prosthesis is a new transfemoral prosthesis with two active degrees of freedom in the knee and the ankle designed primarily to help those with limited ambulation ability with standard prostheses due to weakness from advanced age or complicating illness. Although the device has two powered joints, a MACCEPA based ankle mechanism with a parallel spring mechanism and a series elastic knee mechanism paired with a parallel spring mecha-nism that engages using a thread based locking mechanism, it is designed to allow a high level of passive action during the gait cycle through the use of passive components that are inserted or taken out of the gait cycle by locking mecha-nisms. Through the use of these passive energy storage components, it is possible to achieve a fluid motion while, with simple control, remaining relatively energy efficient. The prosthesis is capable of providing the full ankle and knee tor-ques during walking, as well as a large percentage of the torque required for normal sit to stand and stair climbing. Methods The CYBERLEGs Beta-Prosthesis was tested using two different control systems. The first was the CYBERLEGs sys-tem which contained a finite state machine triggered through a combination of 7 IMUs and pressure sensitive insoles. The second system was a finite state machine driven by a simple 3 DOF rate gyro, an augmented and modified phase angle system using elements of the system developed by SpringActive, and a touch screen for determining action transi-tions. Testing of the device has been performed on patients at the Don Gnocchi Foundation, Italy, as well as in Brussels and used in competition during the first Cybathlon held in Zurich, Switzerland in October 2016. The CYBERLEGs Beta-Prosthesis system was tested in Italy on four healthy, elderly (>50 years), male individuals under a variety of conditions including level ground walking in a catwalk, treadmill walking, sit to stand and stand to sit, and stair assist (step by step) and used the first control system. For the Cybathlon competition, the tests were performed with a fifth healthy, elderly and male amputee and also included slope walking and decent, stair climbing (step over step) with a hand rail, and hurdle navigation. These experiments used the simplified control system and intention detection system. Results All but one walker was able to perform the tasks as described and reported benefit from the prosthesis particularly in the sit to stand and stand to sit operations. The prosthesis was capable of climbing stairs step over step with the use of the handrail, with limited training of the pilot. This allowed the amputee to surpass the capabilities of his habitual prosthesis for this task. When the control parameters are well tuned to the needs of a specific amputee, the walking gait is stable and fluid, yet still requires a high amount of cognitive effort from the amputee compared to the habitual prosthesis. Conclusion The CYBERLEGs Beta-Prosthesis is capable of providing a significant amount of torqueto the knee and ankle while walking and actively add energy enabling low activity level amputees to perform tasks they would not be able to per-form with passive devices. Due to design decisions, the prosthesis is limited in joint velocity because the device was intended for individuals with poor locomotor ability and condition. Therefore it is not optimized to be used with high-functioning amputees, but regardless the device performed well at low velocities and enables the users to perform tasks they would otherwise not be able to perform. Although the performance during the Cybathlon did not compare to the other top of the market designs when looking at the time it takes to fulfil the tasks, the device performed all of the tasks that were deemed possible by the start of the competition, moreover tasks the amputee would not have been able to per-form with a passive device and the same limited training. Future studies will focus on decreasing weight, increasing re-liability, incorporating better control, andincreasing the velocity of the device.