Pneumatic artificial muscles belong to the group of nonconventional actuators intended for various biomedical or industrial applications. The one-DOF actuator (DOF – degree of freedom) consists of two muscles acting in opposition to each other (antagonistic connection) with simplified control scheme (stiffness control loop is excluded in order to simplify the control and to achieve as high stiffness as possible). The actuator exhibits nonlinear behaviour attributable to the compliant nature of pneumatic muscles. The muscle model is based on modified two-element muscle model consisting of a variable damper and a variable spring connected in parallel. From kinematic point of view, it is a simple mechanism with one degree of freedom (arm with added mass rotating around one revolute axis) with plane of movement being parallel to the ground. The main purpose of this model is a control design, so the stringency of accuracy criteria is lowered compared to a truth model. The model is validated using the results of dynamic experiments with real plant in laboratory conditions.