This paper deals with a new approach to the trajectory tracking of a double-pendulum cart system, solved with nonlinear control. The configuration proposed is a study for the potential application in rehabilitation robotics of lower limbs. Dynamic is solved using Lagrange formulation, including viscous friction model on the joints. Simulation are performed using Simmechanics. Human gait trajectories are set to be controlled, cart movement is analyzed. PD with pre-compensation controller found to be effective to track gait trajectories.