A total of 30 ACLR patients, at least 9 months post-surgery, were recruited. Patients were divided into RTS and non-RTS groups based on their ability to return to sport, recovery to pre-injury Tegner levels, and a leg symmetry index of 85% on the single-leg hop test. Urine samples were analyzed using liquid chromatography-mass spectrometry (LC-MS) to identify differentially expressed proteins associated with RTS.