About 1 & 1/2 years ago, I had a severe knee problem. Every step I took was painful, it felt like someone took a hammer to the inside of my knee. At first, I ignored this, thinking it to be a sprain, or a minor injury of some sort (I used to always get sprained ankles and what not, so I didn't think this would be much different). However, after a couple of weeks it did not heal. Concerned, I started to wear a knee brace and went to visit my doctor. I went for some tests, and one of the first possibilities that popped up for me was Arthritis (a juvenile form?). It didn't fit perfect, but they suspected that was what I had.
While all of this was happening, my knees were getting worse. Not only could I not walk without pain, I couldn't jog or run - not because of the pain, but when I tried to, my body couldn't support the impact. After about 6 steps, my knees would simply give out. The same thing went for stairs; I had to take them slowly, and change how I walked.
We did more searching, and after more tests, they came up with what they thought I really had - a cracked miniscus (the miniscus is like a pad that sits within your knee to prevent scraping and what not). The miniscus almost never regrows. With a cracked miniscus, my legs would be like this for life. And for someone who likes being outdoors, that's a pretty awful sentence.
Suspecting that something wasn't right, our next stop was a sports medicine clinic. There, I was examined, and I soon after went to a physiotherapist for another examination. However, after I was examined and my legs massaged, I found that I could walk easier!
After over a year of intensive physiotherapy, and 2 hours of stretching and strengthening each day, I was back on my feet.. It has been 2 years since then, and I can run, jog, take steps two at a time (which before, I would have fallen flat on my face if I tried to do) and even do squats (which before, when I sometimes could, it was painful beyond belief - as was crouching down and getting up).
What was it? Growth. I had grown 6 inches in a year and a half (or so). At the time, I wasn't doing a lot of physical activitiy, so the bone grew and the muscle couldn't keep up. It was stretched a lot, and I had massive muscle knots in my legs.
I hope this helps someone else out there who had, or is having, the same problem as I did.