A sore knee after a long walk. Stiff fingers in the morning. A hip that aches after sitting. After 40, joint pain can start to feel like a warning light, and the first fear is often arthritis. Sometimes that fear is right. Osteoarthritis becomes more common with age, and inflammatory arthritis can appear in midlife too. But not every ache, twinge, or stiff joint is arthritis. Tendons, hormones, muscles, nerves, old injuries, medications, sleep, and even stress can all play a