The cause behind "runner's knee" or patellofemoral pain syndrome among athletes has been identified by researchers.
The debilitating injury apparently affects one in four physically active people.
Darin Padua, Ph.D., associate professor of exercise and sport science in the UNC College of Arts and Sciences, said: "Earlier studies have usually looked at people after the problem sets in.
"That means that while previous research has identified possible risk factors related to strength and biomechanics, it's been unclear whether those caused the injury, or whether people's muscles and the way they moved changed in response to their injury."
A total number of 1,600 midshipmen from the United States Naval Academy were observed as part of the study.
After many years, it was found that some of the observed people had developed patellofemoral pain syndrome.
In fact a sum of 40 participants (24 women and 16 men) had developed the syndrome.
The researcher noted participants with weaker hamstring muscles were 2.9 times more likely to develop the syndrome that those with the strongest hamstrings, while those with weaker quadriceps muscles were 5.5 times more likely.
Also, those with a larger navicular drop (a measure of arch flattening when bearing weight) were 3.4 times more likely.
And participants with smaller knee flexion angle (those whose knees bent less on landing during a jump test) were 3.1 times more likely.
Padua explained: "Overall, these people generally have weaker quads and hamstrings. As a result, they don't bend their knees as much when doing task, such as running or jumping.
"That means the contact area between the kneecap and the femur is smaller, so pressure is focused and pinpointed on a smaller area.
"Also, the more a person's arch falls when bearing weight, the more their whole leg may rotate inwards. That will mean their kneecap won't track properly, leading to yet more pressure and more potential pain."
The syudy was published in the November issue of the American journal of Sports Medicine.