As young people go through their growth spurts their bones become longer. In the short term this can be detrimental to skill and strength as they become accustomed to their longer levers. They have become long, but not strong. Imagine rolling modeling clay out on a table. You start off with a solid ball and watch as it gradually gets longer and thinner. You pick it up and it flops around, useful for shaping, but more likely to fall apart.