This paper presents an algorithm to adjust the configuration of a variable-stiffness actuator to reduce its electrical energy requirements during the execution of repetitive tasks. The algorithm is based on the gradient descent optimization method with a modification of the adaptation step size, a forgetting term, and a projection rule to cope with the variation of the actual objective function and signal noise. The performance of the algorithm is validated with experimental results that confirm its capability to reduce the energy requirements of the actuator's driving mechanism. Simulation results illustrate the use of the algorithm in a two-degree-of-freedom system.