Patellar tendonitis is usually caused by an overuse of the patellar tendon. People who play sports involving regular squatting and jumping movements are most at risk.
Another cause is the abnormal alignment of the lower limbs, such as with knock knees or flat feet. These altered postures change the angle between the quadriceps muscle (a group of four muscles in front of the thigh) and the patellar tendon.
Increased tension in the patellar tendon may occur during growth spurts and the tendon may not be able to keep up with the growth of the lower leg. This is known as the Sinding-Larsen-Johansson disorder, where the tendon becomes too short, thus causing it to pull on the bottom tip of the knee cap.