Development and tracking in fitness components: Leuven Longitudinal Study on Lifestyle, Fitness and Health. - Beunen,G.; Ostyn,M.; Simons,J.; Renson,R.; Claessens,A.L.; Vanden Eynde,B.; Lefevre,J.; Vanreusel,B.; Malina,R.M.; van't Hof,M.A. (Int J Sports Med, 1997)