Located just below the kneecap, tendonitis of the patellar tendon is usually caused by having a seat that is too low or from riding too long using big gears. Because of this, the gluteal muscles aren't being utilized as well as they should be. As a result the quadriceps muscles become overworked and fatigued, leading to tendonitis in the tendon.
Tip: Try variation in your pedal cadence. While every cyclist has their preference, breaking up your training by pedaling in a high cadence (90-120 revolutions per minute) can help prevent injury if you do most of your riding in big gears. Increasing pedal cadence will develop your cardiovascular system too, which is another added benefit.
Raising the seat if it's too low will also help to utilize more of the hamstring and gluteal muscles, taking some of the strain off of your quadriceps and the patella tendon. Pedal mashers use more of the quadriceps muscles rather than incorporating other of the body's largest muscles groups used when pedaling in circles. Incorporate the quadriceps, the hamstrings and the gluteal muscles to avoid any one muscle group from becoming fatigued.