They call it "jumper's disease". But the complaint of patellar tendonitis, with which Owen Hargreaves has become so painfully acquainted, has ruled out a considerably wider range of physical activities than just that.
The midfielder's four-year career at United was seriously hampered by injuries to both knees as a result of patellar tendonitis, which led to surgery on both joints and long periods of rehabilitation.
The patellar tendon connects the kneecap (the patella) to the shin bone. This is part of the "extensor mechanism" of the knee, enabling the knee to bend and straighten out. Like other tendons it is made of tough string-like bands which are surrounded by a vascular tissue lining, which provides food and oxygen to the tendon.
Patellar tendonitis is a painful condition that arises when the tendon and the tissues that surround it become inflamed and irritated. This is usually due to overuse, especially from jumping activities such as basketball, volleyball and football.
It causes pain in the knee area, made worse by jumping or kneeling, and can be accompanied by swelling around the tendon.
Standard treatments include: rest, avoiding all activities that cause discomfort, anti-inflammatory pain medications, ice treatments, more rest and special support straps that work for some people, but no one knows how and why.
Surgery is actually quite rare, generally reserved for those patients who have persistent symptoms that cannot be resolved by more conservative treatments.
In November last year, after several operations on his knees, Hargreaves made his first start for United in over two years against Wolves, but limped off after six minutes. His contract was not renewed at the end of the season.
The player says that he was given "prolotherapy", or regenerative injection therapy, by United's medical team. It has known to have been used for his former team-mate, Rio Ferdinand, to try and heal his persistent back injury. Prolotherapy is thought to work by tricking the inflamed, painful tissue into generating new healthy tissue by injecting irritants or "harmless" substances such as sugar, cod liver oil or glycerine into the affected tendons or joints.
The strengthened tissues should no longer send pain signals to the brain. This experimental treatment is used by a minority of doctors as the research evidence is mixed. However, several clinical trials testing its effectiveness in conditions such as osteoarthritis and tennis elbow are underway.