The patella (knee cap) is exposed to incredibly high loads during sporting activities.
Even activities of daily living, squatting and jogging can put loads equivalent to seven times body weight through the patella - even stair climbing puts loads of three times body weight through it.
So it is hardly surprising things go wrong with this joint. But what we should really be asking is why it doesn't hurt all the time!
So what goes wrong?
Very simply, this fantastic load distribution system stops working optimally and tissues are exposed to higher loads than they can tolerate. Load distribution can fail because the limb alignment is not correct, this could be from the bottom up (foot posture and movement) or top down (pelvic and thigh alignment). In this case the tracking of the patella is affected so loads are no longer spread evenly across the surface; this creates stress on the overloaded tissues and results in pain.