I have pain in tendon below knee cap when i walk. it quickly goes away when i sit down. then i can walk again and it starts to come back if i walk too long, but if i sit down it goes away again.

Chondromalasia pat: Sounds like degeneration in the underside of your kneecap (patella), but may involve the joint as well. The knee cap rides in a groove in the femur but sometimes it can slip to the side, or degenerative changes can cause a grinding, leading to inflammatory changes including pain, When you stand and walk. See an orthopedist for accurate diagnosis and treatment options . A ‘cho’ strap can help too.