Why do I get knee pain after sitting for a long time?

Knees that ache and stiffen after long sitting - then loosen after a few steps - show the classic movie-theater sign, and it points at the patellofemoral joint, where the kneecap meets the thigh bone. Sitting with bent knees keeps the kneecap compressed against the groove for an hour or more; an already-irritated joint protests when you finally load it. The two usual causes of the irritation: patellofemoral pain syndrome - runner's knee, common in younger active people, driven by weak quads and hips, tight legs, and tracking problems - and early osteoarthritis in the over-50 crowd or after old knee injuries, per NIAMS. What helps: break up long sitting - stand and straighten the knees every 30 to 45 minutes; extend the legs or adjust so knees are not jammed at 90 degrees for hours; strengthen quads and hips - straight-leg raises, mini-squats to a chair, side leg lifts - which is the actual treatment, not a supplement; and manage the flare with ice after long days. See a doctor for swelling, locking, giving way, pain that wakes you at night, or pain persisting despite weeks of strengthening - those suggest meniscus, significant arthritis, or other structural issues worth an exam and possibly an image. Pymander is a free AI doctor, 24/7 by text. Describe your sitting pattern and the pain, and it can help you plan the fix.