Under extreem stresses such as those involved in jumping a partial rupture can occur. This can then lead to inflammation.
Inflammation can also result from overuse.
Injury to this often affects athletes involved in jumping or throwing sports. Weight lifters, Tennis and Badminton players can also be effected. Pain at the bottom of the kneecap especially when pressing in. Aching and stiffness after exertion. Pain when you contract the quadriceps muscles.