Fig. 8. General block diagram of the musculoskeletal postural coordination model. Input Xd is the prescribed head position on X axis, which is measured onto the subject. PD represent a neural controller to track the target, Uss is the supraspinal neural signal, the output is the joint position θ. The reflex loops on joint positions and velocities (provided via spindles) and on torque (GTO) change the Uss control signal, and hence modify muscle stiffness. The muscle group block is described on Fig. 7. θ0 and θ̇0 are quiet stance values, θ=(π/2, 0)T and θ̇0 = (0, 0)T .
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