A robotic tendon is modeled and the stiffness of the spring is tuned so that the spring power reduces the peak motor power and energy required for ankle gait. When determining stiffness from gait literature, it is usually assumed that one side of the spring is fixed. We assume that the spring is translating to derive a second method to calculate stiffness. By choosing a tuned spring based on a "dynamic stiffness", the motor velocity was shown to be constant during the loading phase of ankle gait. We simulated this system and showed that energy was reduced and peak power was dramatically reduced. The constant velocity controller was implemented on a powered ankle foot orthosis and test data was correlated with the simulation.