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Powered orthoses are external mechanical devices used to stabilize human limbs, to restore or to reinforce lost or weak functions of people with reduced mobility. The embodied actuators produce the necessary joint torques to compensate gravity and passive effort as well as to generate the intended human movements. Nonlinearities due to human orthosis coupling, as well as modeling errors, parameter uncertainties, and external disturbances, necessitate the use of a robust closed-loop controller in order to guarantee precise movement generation. This paper aims to present a new prototype of an actuated knee joint orthosis using a robust controller. This orthosis is designed to restore or to assist knee-joint movements of dependent people. Dynamic modeling of the lower limb/orthosis is presented, and its parameters are estimated using different techniques. Control strategies based on second-order sliding mode are applied, which show satisfactory performance compared to classical controllers in terms of tracking errors and robustness with respect to parameter uncertainties and external disturbances. Real-time experiments are conducted on healthy subjects to illustrate the efficiency of the proposed approach.