This paper proposes a novel mechatronic approach for on-line robust estimation of both torque and stiffness of Variable Stiffness Actuators (VSA). The proposed solution is demonstrated robust with respect to uncertainties affecting both the dynamic parameters of the motor and to the efficiency of the motor gearbox. The strategy adopted to gain robustness compared to existing techniques is based on adaptive algorithms that exploit an additional measurement, i.e, the instantaneous elastic energy stored in the transmission. A novel optoelectronic sensor to perform such measurement in a minimally invasive and low-cost manner is also proposed and experimentally tested.