Patellar tendonitis, or jumper’s knee, is when there is pain in the knee right at the attachment of the patellar tendon to the kneecap (patella). These stretches & exercises should help with patellar tendonitis.
The first thing is to get the patella loosened up and moving by doing some patellar mobs. Make sure not to push downward when moving the patella.
Then you want to stretch all the muscles around the knee including the quads, hamstrings, and calf muscles. The all can cause a lot of pain in the knee if they are tight.
Finally, you want to strengthen the muscles around the knee and hip. Straight leg raises, sidelying hip abduction, prone hip extension, prone leg lifts, bridges, and clamshells work muscles around the knee and hip including the hip flexors, hip abductors, gluteus medius and maximus, hamstrings, and core muscles.