hey, im 13 too and i feel your pain. I read that if you condition your legs then, in transition it makes you overall body stronger. do push ups and a couple pull ups (i can only do like 2). something that a lot of people dont realize is that your pecs are running muscles along with your biceps (push ups make you faster too) and to get your shoulders a little bigger you could jog a few laps around the hood. hope this helps
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