Inflammation of the patellar tendon (ligament). The patellar tendon is actually a ligament that connects the patella (kneecap) to the tibia (shinbone).
Most commonly Overuse. This tendon absorbs much force, and repeated stress often times due to extensive jumping or running can cause tiny micro tears in the patellar tendon. Tight Hamstrings and Quadriceps also play a role in knee flexion and extension.
Localized tenderness at the point of insertion where the tendon meets the bone (below the kneecap). Pain may be more noticeable when going downhill.
R.I.C.E. Rest, Ice, Compression, Elevation. Reduce activity to promote healing. Strengthen surrounding muscles with a professionally recommended program. Apply Ice for 10-15 minutes after each workout to reduce inflammation and pain. A proper stretching routine is highly recommended pre and post workouts. As with any orthopedic injury, a consultation with a sports medicine professional is recommended. Stretches and strengthening exercises can be found here.