1. An apparatus comprising: a manipulandum; a rotor in communication with the manipulandum; an actuator in communication with the rotor, the actuator configured to vary a force on the rotor; and a biasing element disposed between the manipulandum and the rotor, the biasing element configured to store at least a portion of energy input to the apparatus when the actuator increases the force on the rotor and to provide at least a portion of the energy to the manipulandum when at least one of the following events occurs: the actuator decreases the force on the rotor; and the manipulandum is released.