To investigate contribution of articular and muscular structures to the limitation of range of motion after joint immobility, we carried out an experimental study. Unilateral ankle joints of 36 rats were immobilized in full plantar flexion with casts from the toes to below the knee joint. The rats were divided into three sets, each set having 12 rats im-mobilized for durations of 1, 2, or 4 weeks. Each set of rats was then divided into two groups (A and B), each group containing 6 rats. At the end of each period the casts were removed from 12 of the rats consecutively, and the measurements of the range of motion of the ankle joint were performed as follows. For Group A, the measurements were performed in four stages: Stage 1: before skin resection, Stage 2: after skin resection, Stage 3: after a myotomy (resection of the flexor muscles of the ankle joint), and Stage 4: after an arthrotomy (resection of capsule and ligaments of the ankle joint). For Group B, the measurements were also per-formed in four stages: Stage 1: before skin resection, Stage 2: after skin resection, Stage 3: af-ter an arthrotomy, and Stage 4: after a myotomy. There is a general tendency to presume that the muscular factor is dominant over the articu-lar factor, but there were no differences found. As a result, both the arthrogenic and the myo-genic factors contribute to joint restriction at 1, 2, and 4 weeks of immobilization.