Soft Robotics and Variable Stiffness Actuation (VSA) are active research domains in robotics. The aim is to design human-friendly machines that can perform safely dexterous tasks close to or in contact with humans. Whereas most of current works focus on biomimetic VSA by using two antagonistic motors to simultaneously deal with motion control and stiffness adaptation, in this work, we present a control approach that takes advantage of the mechanical reversibility of series elastic actuators and the recent availability of structured H_∞ synthesis. The paper introduces a methodology for the design of a control law that allows to turn a force-sensorless actuator that includes a flexible link into a variable stiffness actuator where the stiffness can be set arbitrarily in some interval. With the proposed framework, it has been possible to accurately control the motion and the stiffness of a cable-screw actuator.