An adjustable Robotic Tendon is a spring based linear actuator in which the properties of a spring are crucial to its successful use in a gait assistance device. Like its human analog, the adjustable Robotic Tendon uses its inherent elastic nature to both reduce peak power and energy requirements for its motor. In the ideal example, peak power required of the motor for ankle gait is reduced from 250W to just 81W. In addition, ideal energy requirements are reduced from nearly 36 Joules to just 25 Joules per step. Using this approach, an initial prototype is expected to provide 100% of the power and energy neccessary for ankle gait in a compact 0.84kg package. This weight is 8 times less than that predicted for an equivalent direct drive approach.