The way some muscles attaches into the growth plates of your bones can cause problems if you are doing a lot of sport involving jumping and running, which is why a lot of girls and boys who dance or do athletics get issue with their knees. This happens especially around periods of growth when the growth plates are more active.
If you have pain below the knee cap, and you think it may be the growth plates, it may be a syndrome called Osgood Schlatters Disease. This is not really a disease but is just named after the guy who discovered it.
The tendon that attaches the patella (knee cap) to the tibia (shin bone) is called the patella tendon. The tendon attaches to your tibia right on the growth plate. The muscles of the front of the thigh (quadriceps) attach into the top of the patella which acts as a pulley to allow you to straighten your leg without the muscle rubbing over the edges of the bone. If you bend and straighten the knee many times, the constant pulling on the growth plate can aggravate it and you can get quite a bit of pain.
Things you can do to reduce the amount of pain and pulling include
Gentle stretching of the Quadriceps