content: I can't explain what you want, but I can explain the opposite.  During developing years, children's muscles and bones grow at different rates and in different ways than adults.  Working a child's body like this during these developing years can negatively impact his growth later on down the line. It's the same concept with little league pitchers: their arm muscles don't grow/develop the same way an adult's would, and they don't grow/develop naturally, so they're useless as pitchers when they get older.