Vänttinen, T, Blomqvist, M, Nyman, K, and Häkkinen, K. Changes in body composition, hormonal status, and physical fitness in 11-, 13-, and 15-year-old Finnish regional youth soccer players during a two-year follow-up. J Strength Cond Res 25(12): 3342–3351, 2011—The purpose of this study was to examine the changes in body composition, hormonal status, and physical fitness in 10.8 ± 0.3 ...
https://journals.lww.com/nsca-jscr/Fulltext/2011/12000/Changes_in_Body_Composition...