Patellar tendinopathy (or as it is commonly known patellar tendonitis or tendinitis) is an overuse injury affecting your knee. It is the result of your patella tendon being overstressed. A common name for it is Jumper's Knee.
More info about: Tendinopathy
Anatomy of the Knee
The patella tendon is located just below the patella (knee cap). It has attachments on the patella and the tibial tuberosity on the tibia (shin bone). The role of the patella tendon is to transfer the force of the quadriceps muscles, much like a rope around a pulley, as your knee straightens.
Your quadriceps is even more important when controlling your knee as you bend from a straight position eg walking downstairs, landings. Your quadriceps muscles are heavily involved in most sports, especially those which involve jumping, running and kicking.