This study aimed to perform range of motion exercises on a rat knee joint contracture model and clarify changes in the joint range of motion and number of myofibroblasts in the joint capsule. Eighteen male Wistar rats were used and randomly divided into the following three groups: control, immobilized, and exercise. The right hindlimb knee joints of rats in the immobilized and exercise groups were immobilized, and the animals in the exercise group started the range of motion exercises the day after the joint immobilization. After the two weeks experimental period, the range of knee joint extension angle was measured, and the knee joint was collected. To observe the posterior joint capsule of the rat knee joint, hematoxylin and eosin staining and immunostaining were performed. Differences in knee extension restriction angles were significant between all groups, and differences in the number of alpha smooth muscle actin-positive cells were significant between the control and immobilization groups. These results indicate that joint immobilization leads to myofibroblast proliferation, whereas gentle exercise that does not maintain sufficient range of motion in the joint may inhibit myofibroblast proliferation.