In this paper, an ongoing work for the implementation of a variable stiffness joint actuated by a couple of twisted string actuators in antagonistic configuration is reported. The twisted string actuation system is particularly suitable for very compact and light-weight robotic devices, like artificial limbs, exoskeletons and robotic hands, since it renders a very low apparent inertia at the load side, allowing the implementation of powerful tendon-based driving systems, using as actuators small-size DC motors characterized by high speed, low torque and very limited inertia. The basic properties of the twisted string actuation system are firstly presented, and the way how they are exploited for the implementation of a variable stiffness joint is discussed. A simple control algorithm for controlling the joint stiffness and position simultaneously is discussed, and a the feedback linearization of the device is taken into account and validated in simulation.