Honestly i have seen so many kids try to throw hard.
Whenever you see someone reach back for a little bit of extra speed, the first thing they do is try to use more arm.
In my own experience i can tell you, that your legs and your core make up 95 percent of your velocity.
Here is why i believe this.
I haven’t thrown a single baseball in over a year time. I have been bodybuilding, hence i haven’t thrown at all. i kind of gave up baseball, but i am back into for the summer time.
Anyways i have been bodybuilding, so hence i have been working my legs out, with squats as well as working my back out with deadlifts and heavy weighted pullups.
Anyways: I haven’t thrown a single ball in a year right.
2 days ago i got radared. Last year i was throwing 85mph average and hit 89mph for a few pitches.
2 days ago i was clocked at 83mph. Thats after 1 year of not throwing a single baseball. I was also not in cleats, not on a mound and was throwing uphill as well. (i was in my backyard). I know for a fact, that with a month or two of getting back into things as well as being on a mound with a slight downward slope i have no doubts i will be able to hit 90mph consistently.