Muscle injuries caused by traffic accident are difficult to determine the degree of symptoms and causes. Injuries analysis for back muscles are mostly carried out using dummy, cadaver, and volunteers. However, it is difficult to consider the effeet of muscle reaction when dummy and cadaver is used. The volunteer experiment has a limitation that it should proceed only in the environment where the injury does not occur in the safety of the volunteer. The numerical method of finite elements is a powerful tool for analyzing body behavior under various impact conditions. This study developed 3-dimensional(3D) active continuum model for back muscle for analyzing the back muscle injuries under rear impact loading condition. The 3D active continuum muscle model is modeled by combining active beam element and passive non-linear, viscoelastic solid elements. Magnetic resonance imaging(MRI) and anatomical data was used for developing muscle model.