The potential effectiveness and adoption of robotic assisted therapy for neural rehabilitation has attracted increased attention to the development of rehabilitation robots. Our research group had developed a lightweight exoskeletal rehabilitation robot actuated by pneumatic muscle actuators. To design a robust control system with intelligent adaptation to individual patient's condition is a challenge with pneumatic muscle actuators (PMA). The dynamics of a PMA is affected by dimension (length and diameter), pressure and load. Thus, it is crucial to choose the appropriate PMA with the desired dynamic response for each of the joints to be actuated In this study, 2nd order phenomenological models have been developed to describe the dynamic behavior of nine pneumatic muscle actuators (3 different lengths and 3 different diameters). The important differences between these pneumatic muscle actuators are compared based on the model parameters. Some of the model parameters like relative muscle contraction, rise natural frequency are affected more by the PMA dimensions, while some of the other parameters are relatively unaffected by the dimensions of the PMA. In addition, analytical expressions were determined for the individual model parameters as functions of the input pressure and external load.