Recently, the actuated orthosis is highly utilised to support the knee-joint movement of rehabilitees. However, uncertain disturbances arising in a nonlinear orthosis system is difficult to control, which in turn reduces the stability of the system. In this paper, an Adaptive Second-order Sliding-Mode Control (ASoSMC) is proposed to control the shank-foot orthosis system. Here, the actual position, velocity, and acceleration of the system are estimated using the final torque to determine the overall movement of the system. Moreover, these actual values are estimated by varying the sliding surface of the sliding-mode control and the positive gain of the system. Further, the performance is compared with conventional methods like Second-order Sliding-Mode Control (SoSMC) and Fuzzy SoSMC in terms of root-mean-square error, sliding surface, and positive gain. The controlling by ASoSMC is found to be precise in defining the position, velocity, and acceleration of the system against conventional methods.