Runner's knee - patellofemoral pain syndrome - is very common, felt as a pain in the front of the knee. Forces on the patella (kneecap) can be half a person's weight when walking but up to three times their weight during stair climbing and seven times their weight during squating!
A review in American Family Physician highlights how common this condition is, causing16-25% of all injuries in runners. However, it commonly affects people who don't run at all, causing stiffness and pain around or behind the kneecap, with prolonged sitting (the 'theatre sign'), going up and down stairs and bending down.
A study published in British Journal of Sports Medicine found no benefit from keyhole surgery.
Until recently it was thought the muscles around the knee were the cause. Recently, the importance of weakness of hip muscles - specifically the gluteal muscles - has been appreciated. There is emerging evidence (Barton CJ et al Br J Sports Med 2013) of the importance of gluteal muscle strength and programmes which included hip strengthening have been more successful than programmes that focused on the knees alone.