In this paper, an active modeling approach for a pneumatic artificial muscle (PAM) is being considered. The PAM's nonlinearity, hysteresis and time-varying characteristics cause difficulties in modeling its behavior and designing high-performance controllers. Here, a generalized three-element model of a PAM is proposed to obtain the reference model in the reference mode (in a given small pressure range). The equivalent state space representation of the reference model is obtained by applying the linearization approach. The model error is introduced to the equivalent state space model, and the joint estimation is adopted to estimate the system state and the model error simultaneously. Experimental studies are conducted to demonstrate the existence of the model error, and verify the necessity of introducing the model error to the reference model, and evaluate the effect of the proposed active modeling approach in changing modes (not in the given pressure range).