This single-centre, exploratory study at Montpellier University Hospital will investigate whether having low muscle mass and strength before hip-replacement surgery, a condition called sarcopenia, impact the return to everyday independence in people aged 75 years or older. About 20 volunteers, scheduled for surgery because of a recent femoral-neck fracture or severe arthritis that no longer responds to usual care, will give consent, answer brief health questionnaires, attempt simple walking and chair-rise tests if possible, and provide a small blood sample. While they are already under anaesthesia for their planned operation, the surgical team will take a tiny muscle sample through the same incision, so no extra cuts are needed. During the hospital stay or shortly after discharge, each participant will have a painless MRI scan of the thigh muscles and a very-low-dose bone scan (DXA) to measure muscle and bone health. The research team will then telephone participants at 3 and 6 months to ask about daily activities, walking ability, and any complications. The study lasts about six months for each person and does not alter their usual medical or rehabilitation care. Potential benefits include close follow-up, personalised feedback on muscle and bone results, and helping doctors learn whether pre-surgery muscle weakness predicts slower recovery-information that could guide future, more personalised exercise and nutrition programs. Extra study procedures carry only minimal risks: a routine blood draw, scans with none (MRI) or very little (DXA) radiation, and a muscle biopsy taken during surgery. Taking part is entirely voluntary, and participants may withdraw at any time without affecting their current or future care. The whole project will run for 18 months.