We introduce a six-actuator robotic joint mechanism with biarticular coupling inspired by the human limb which neither requires pneumatic artificial muscles nor tendon coupling. The actuator can independently change monoarticular and biarticular stiffness as well as both joint positions. We model and analyse the actuator with respect to stiffness variability in comparison with an actuator without biarticular coupling. We demonstrate that the biarticular coupling considerably extends the range of stiffness with an 70-fold improvement in versatility, in particular with respect to the end-point Cartesian stiffness shape and orientation. We suggest using Cartesian stiffness isotropy as an optimisation criterion for future under-actuated versions.