This paper develops a backstepping-based algorithm to control the position of an electropneumatic actuator while allowing the precise tuning of the closed-loop stiffness and damping. The proposed strategy offers an efficient method to choose the controller parameters based on a physical and linear analysis. The strict feedback form of the model, which is required in order to apply the backstepping methodology, is obtained through the use of a transformation of the system's inputs. The proposed multiple input multiple output control law as well as its parameters tuning method are validated experimentally. The experimental results are provided using an innovative test bench combining an electropneumatic cylinder and an electric linear motor. The two main contributions are: 1) the use of a new decoupling transformation to control the system's 2 DOF and 2) the description of a closed-loop damping and stiffness tuning strategy. Simultaneous position and stiffness control result in a more precise and adjustable variable stiffness actuator than the simultaneous pneumatic force-stiffness control laws generally encountered in the literature. Moreover, a specific study is conducted to clarify the interaction between pneumatic and closed-loop stiffnesses in order to combine the advantages of passive and active compliant actuators.