Answer:
The other person was right on both guesses.. Growing pains or sleeping/resting on it incorrectly.
It could also be caused by overworking it.
All three of those options can lead to cramping or what feels like an ache near the joint (unless it`s not at a joint, then it`s probably just a little muscle cramp).
I’m pretty sure it’s the second period. I think that’s when most of the development happens
Since someone would be performing wind sprint activities, it is guaranteed when performing any activity is to get better overtime so I would say to "work above your target heart rate"