In recent years, the use of the pneumatic muscle actuator (PMA) to acquire greater power from the actuation system especially for the development of medical rehabilitation robotic for gait training system has increased. Usually, the biarticular actuators are treated as a redundancy in actuation since the number of actuators is greater than the number of joints. However, these actuators are able to generate a strong force due to wider range of motion compared to the mono-articular actuators and it is thought to generate instantaneous force. In the case of lower orthotic gait training system, the implementation of antagonistic bi-articular actuators along with mono-articular actuators plays a major role to achieve the required afferent input for the lower limb and hip joint as well as smooth and precise movements at the endpoint. One of the important characteristics of PMA is based on its muscle contraction. In this study, we modelled mathematical equations to determine the muscle contraction pattern for the antagonistic mono- and biarticular PMAs as a function of the hip and knee angles in which its magnitude is influenced by the anterior and posterior muscle activation levels. From this model, we are able to determine the input pressure for each of the antagonistic mono- and bi-articular PMAs and then control the system using a feedback controller.