I remember a few years ago being really irritated that my son's soccer coach didn't have them stretch before practices and games. I didn't know if I should say something and be one of "those" parents. So I told my son that he should stretch anyway, and then sort of forgot all about it. Then, I noticed he did not sit quite right on the floor. It literally hurt him to sit up. I also noticed that when his entire Tae Kwon Do class was "touching their toes" his was the only back that was curved, with his shoulders pulled forward. Soon after trying to correct this, he was running in the yard playing tag when a pain hit his back and he could not move. Also, for a while, we noticed his ribs were not matching on both sides, but I kept thinking it would correct itself.
I got him under chiropractic care, where we learned that his spine was not straight (hence the ribs being "off" and the pain in his spine). Soon after, the knee pain began. My son (12) plays sports year round. The knee problem came up during soccer, so we got online to see what might be going on.
That's where we found out about Patellar Tendonitis, "Jumper's Knee", and Osgood Schlatter's disease (or syndrome). I immediately made an appointment at the chiropractor, and the doctor spent 30 minutes explaining how to take care of the problem.
The tell-tale symptom that narrowed down the diagnosis was that I knew my son's hamstrings were incredibly tight. You can read why this causes knee problems online. There are even stretches you can have your child do daily at home. I am not here to tell you what to do, as I'm not a doctor. But I'm telling you to have your child athlete stretch before performing sports! The problem is also common during adolescent growth spurts, especially in athletes.
Ironically, the next day at his soccer tournament, I noticed that every player (and referee) with a knee brace on had it on his/her right knee. The chiropractor had said that the prominent knee is the one most affected. How funny I'd never noticed that before!
Anyway, years of not stretching properly, a growth spurt, and his crooked spine all led to tendonitis in his knee. I'd also like to say that as an athlete, and a coach who stretches with my players, it just feels good to stretch, and everything feels better afterwards.