Joint contracture is caused by the alteration of soft tissue around joints, and muscle changes regarding immobilization is an important factor. However, the mechanism underlying muscle contracture remains unclear. Therefore, this study investigated changes in muscle extensibility and collagen expression, and examined the molecular mechanism underlying muscle contracture in the soleus muscles of immobilized rats.

Wistar rats were divided randomly into immobilization and control groups. In the immobilization group, both ankle joints were fixed in full plantar flexion with plaster casts for 1, 2, 4, 8, and 12 weeks. One soleus muscle was used for measurement of passive tension in extension (a parameter for muscle extensibility) and collagen contents (a parameter for muscle fibrosis). Other samples were examined for changes in fibrosis-related cells and molecules (macrophage, IL-1β, TGF-β1, HIF-1α, myofibroblast, type I and III collagen) using methods such as immunostaining and/or RT-PCR.

The passive tension and collagen contents were significantly higher in the immobilization group than in the control group for all experimental time points. Additionally, a significant positive correlation was observed between the passive tension and collagen contents. Macrophage, IL-1β, TGF-β1, and type III collagen increased with immobilization for 1 week, but there were no changes after a long immobility period. Myofibroblast and type I collagen levels also increased with immobilization for 1 week and further increased after 4 weeks of immobilization. On the other hand, only HIF-1α increased after 4 weeks of immobilization.

The present study indicated that the decrease in muscle extensibility depended on collagen overexpression in immobilized rat soleus muscles. Additionally, in the early stages of immobilization, upregulation of IL-1β/TGF-β1 via macrophages might promote fibroblast differentiation that affects muscle contracture. The soleus muscle became hypoxic in the later stages of immobilization, suggesting that hypoxia influences the progression of muscle contracture.