There are several reasons to delay single sport specialization if possible, but these are often sport and child dependent.
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Jayanthi N, Pinkham C, Dugas L et al. Sports specialization in young athletes: evidence-based recommendations. Sports Health. 2013;5(3):251-257.
Myer GD, Jayanthi N, DiFiori JP, et. al. Sports Specialization, Part II: Alternative Solutions to Early Sport Specialization in Youth Athletes. Sports Health. 2016;8(1):65-73.
Brenner JS and AAP Council on Sports Medicine and Fitness. American Academy of Pediatrics Clinical Report: Sports Specialization and Intensive Training in Young Athletes. Pediatrics. 2016;138(3):e20162148. https://pediatrics.aappublications.org/content/138/3/e20162148