In healthy young men undergoing 14 days of unilateral knee immobilization, the skeletal muscle proteome exhibits no significant upregulation of the ubiquitin ligases MuRF1 or MAFbx despite the presence of proteolysis, indicating that non-ubiquitin pathways contribute substantially to muscle atrophy.

When a leg is immobilized, the muscle stops working, so it needs less energy. This causes the energy-producing parts of the muscle cells to break down, leading to a buildup of harmful molecules. The cell responds by turning on cleanup systems that break down proteins, especially through lysosomes and the proteasome, which destroy muscle proteins without needing the usual ubiquitin signals. This results in muscle shrinking even though the typical ubiquitin markers stay the same.