Anatomy
The quadriceps tendon attaches the main group of muscles in the front of the thigh (quadriceps) to the upper pole of the kneecap (patella) The quadriceps muscle attaches to the upper pole of the kneecap through the quadriceps tendon to allow one to straighten out their leg or to support ones weight while squatting. When the quadriceps 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.
Injury Mechanism
The quadriceps tendon is usually injured with a sudden high force across the tendon, such as landing from a jump. The quadriceps tendon can be injured by a direct blow or a sharp laceration across the tendon. Often, the quadriceps tendon is weak prior to the injury due to chronic illness or a period of inactivity.