This paper deals with the control of a lower limb orthosis. In which a model of the shank-orthosis system is given, we consider the human effort as external torque acting on the system. A comparative study, through a number of simulations in different operational scenarios, highlights the limits of a standard PID controller. On another hand, this work brings out the benefits of orienting the control strategies toward model reference approaches. Hence, a better exploitation of the nonlinear system dynamics; by deriving adaptive control strategies to enable the management of parameters uncertainty could be a relevant approach for such systems.