From Biomimetics MIT Cheetah project:

The high speed legged locomotion of the MIT Cheetah requires high accelerations and loadings of the robot’s legs.  Because of the highly dynamic environmental interactions that come with running, variable impedance of the legs is desirable; however, existing actuation strategies cannot deliver.  Typically, electric motors achieve their required torque output and package size through high gear ratios.  High ratios limit options for control strategies.  For example, closed loop control is limited to relatively slow speed dynamics.  Series elastic actuation adds additional actuators and increases system complexity and inertia.  We believed a better option existed.  In the end, we developed a novel actuator, optimal in many applications... (project homepage) (full published article)