The patellar tendon attaches the lower pole of the kneecap (patella) to the lower leg bone (tibia). The quadriceps muscle attaches to the upper pole of the kneecap and pulls through the kneecap and patellar tendon to allow one to straighten out ones leg or to support ones weight while squatting. When the patellar tendon is torn, one cannot support their weight when the knee is bent, such as getting up from a chair or going down the stairs.
The patellar tendon is usually injured with a sudden high force across the tendon, such as landing from a jump. The patellar tendon can be injured by a direct blow or a sharp laceration across the tendon.