Why do my knees hurt when I climb stairs?

Stairs are a knee stress test: climbing loads the kneecap joint at three to four times body weight, so knees with a brewing problem complain on stairs first. The most common cause in active adults is patellofemoral pain - runner's knee - where the kneecap tracks slightly off its groove and the cartilage underneath gets irritated. The clues: aching around or behind the kneecap, worse going down than up, stiff after sitting long - the movie-theater sign. Weak quads and hips, tight hamstrings, and flat feet all contribute, which is why physical therapy is the main treatment and works well. The other big cause, especially past 50 or after old injuries: osteoarthritis - NIAMS describes it as cartilage wear bringing pain with use, stiffness after rest, and sometimes grinding. Early knee arthritis responds surprisingly well to strengthening and weight management. What helps now: strengthen quads and hips - straight-leg raises and wall sits are the classics - avoid deep kneeling and squatting during flares, ice after activity, and pick supportive shoes. See a doctor for locking, giving way, swelling, or pain that persists off the stairs - those suggest meniscus, ligament, or arthritis issues worth an exam. Pymander is a free AI doctor, 24/7 by text. Describe where the knee hurts and when, and it can help you plan next steps.