Robotic knee prostheses have been improving, thereby raising the quality of life for above knee amputees. To continue this progression requires robust and versatile research tools to study the subtle biomechanical interactions between the human and the prostheses. To serve this purpose, we have created a tethered knee prosthesis with flexion and extension torque control. To reduce weight and complexity, the flexion motor, extension motor, and control hardware have been moved offbody. Power from the motors is transmitted to the prosthesis by means of bowden cables, and measurements of state are sent back through this tether. The resulting knee prosthesis weights only 0.8 kg. Benchtop tests were conducted to calibrate the strain gauges which measure torque, find the step response rise-time of the system, and find the closed loop torque bandwidths of each motor. The RMS error for flexion torque was 0.27 N·m and extension torque was 0.54 N·m . The 90% rise time was 14.3 ± 1.1 ms for flexion, and 11.6±1.0 ms for extension. The bandwidth was 28.0 Hz for flexion and 27.2 Hz for extension. Such high bandwidths and peak torques make this emulator suited for studying normal walking and running gaits as well as stumble recovery. This versatile system can also be used to design and test new control strategies for stumble recovery, walking, and running. This can lead to improved balance, metabolics, and quality of life for those with above knee amputations.