Patellar tendonitis (also called “jumper’s knee”) occurs when the quadriceps muscles are overused or inflexible. When quadriceps are overused, the patellar tendon has to absorb more of the shock created by movement. If the quadriceps are inflexible, the patellar tendon has to withstand more friction during movement. In either case, the tendon may become irritated or damaged. This can result in pain during activity–and pain may continue even when you’re resting.