The quadriceps tendon connects the muscles at the front of your thigh (the quadriceps muscles) to the knee. A fall or a sports injury can put too much pressure on the tendon or the muscles, causing the tendon to tear.
When you want to straighten your leg, you contract (shorten) your quadriceps muscle. The force of that contraction is transmitted through the quadriceps tendon to the patella (the kneecap), then to the patellar tendon, and finally to the tibia (the calf bone). The tendons themselves do not contract, but they transmit the force of the muscle to the knee. When you bend your knee, the quadriceps muscle lengthens.
Tendon ruptures occur most often in middle aged people who play sports or engage in athletic activity, but they can happen to anyone. Falling on a partially bent knee is the most common cause.