Muscle contraction mechanism is discussed by reforming the model described in an article by Mitsui (Adv. Biophys. 1999, 36, 107-158). A simple thermodynamic relationship is presented, which indicates that there is an inconsistency in the power stroke model or the swinging lever model. To avoid this difficulty, a new model is proposed. It is assumed that a myosin head forms a polaron-like complex with about three actin molecules when it attaches to an actin filament and the complex translates along the actin filament producing force. Various experimental data on the muscle contraction are well explained based upon the model.