Although macrophage accumulation plays a key role in the development of immobilization-induced muscle fibrosis, the underlying mechanisms remain unclear. Therefore, we focused on the alterations of myonuclear apoptosis via cleaved caspase-3, and investigated whether these changes may be related to macrophage accumulation. In the results, myonuclear apoptosis via the upregulation of cleaved caspase-3 might induce macrophage accumulation. Moreover, these alterations are related to immobilization-induced muscle fibrosis with the differentiation of fibroblasts into myofibroblasts via IL-1β/TGF-β1 signaling.