I think it is not so much about playing multiple sports than it is about not throwing for 2-3 months (which means no pitching for 4 months since you have to get back to strength). Multi sports means a larger chance kids won't touch a baseball for some months (Really not just not pitching but also no throwing).
But you can also have a break without other sports. Cressey pitchers will stop throwing and instead focus on lifting and specific exercises to get stronger).
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