Tendonitis—also known as tendinitis—refers to acute (short-term) inflammation of a tendon. Inflammation is characterized by pain, swelling, or irritation. These symptoms develop in response to an injury to initiate the natural healing process.

Alignment or overuse problems of the knee structures can lead to strain, irritation, and/or injury. This produces pain, weakness, and swelling of the knee joint. Patellar tendonitis (also known as jumper's knee) is a common overuse condition of one of the knee tendons that is associated with running, kicking, and repetitive jumping and landing.