does strength training and weighted balls help increase your velocity? and also is it true that strength training stops you from growing?
Strength training stopping growth is a myth. You could start strength training at 5 if you wanted to.
Weighted balls have mixed reviews on this board. They do increase velocity, but there is a risk of injury associated with their use. I would suggest that you find a good pitching instructor before beginning any type of weighted ball program.
That being said, you can gain anywhere from the 2-3 mph range all the way up to 10 mph from weighted balls.
Injuries associated with improper weight training have the potential to damage growth plates. Excessive training (not just weight training) to the point where it causes or weight loss or prevents normal weight gains can temporarily stunt growth. This is what happens with female gymnasts.
When my son started the HS baseball conditioning program his growth slowed or stopped despite being in the middle of a growth spurt. When he had to shut down due to a growth related injury he grew an inch over the next month.
In the long run almost everyone grows to their potential despite temporary slowing during over training.