Patellar tendinopathy (also known as patellar tendonitis, patellar tendinitis, patellar tendinosis, and jumper's knee) is not inflammatory and is instead a degenerative condition of the tendon with associated micro-tears. It presents as gradually increasing pain over the distal pole of the patella / patellar tendon. Pain can be of insidious onset after increasing exercise intensity or frequency; alternatively, pain can gradually increase in the course of days to weeks.

Repetitive knee motion that is typically forceful with eccentric load being placed upon the knee extensor, most often during jumping, can result in patellar tendonitis. In addition to micro-tears in the tendon, there is occasionally angiofibroblastic hyperplasia of the tendon or mucoid degeneration of the tendon in advanced cases.

The condition is thought to occur secondary to poor flexibility of the thigh musculature, specifically the hamstrings and the quadriceps. This condition occurs in about 20%-30% of jumping athletes (such as in volleyball and basketball), with a predilection for adolescents and young adults. Men have a higher incidence than women.

Symptoms of pain at the distal aspect of the patella and pain at the patellar tendon typically occur with an insidious onset after higher risk (eg, jumping) activities, but in later phases of the disease, pain can occur during activity or with prolonged knee flexion (sitting for long periods of time).