In this study, we examined the relationship between birth-month distribution, skeletal age, height, and weight in adolescent soccer players. The participants were 271 well-trained male adolescent soccer players aged between 9.1 to 15.9 years (N=101 for U12, N=132 for U15, and N=38 for U18). They were divided into four groups according to their month of birth (Q1: Apr-Jun, Q2: Jul-Sep, Q3: Oct-Dec, and Q4: Jan-Mar). Then, the number of players in each group was compared with the standard population of children in Tokyo. Our result showed significant asymmetries in the birth-month distribution for participants of all age groups (p<0.001); it was less than 10% for participants belonging to group Q4. However, no significant difference in maturation ratio (skeletal age-chronological age) was seen among the groups. Moreover, skeletal age did not differ significantly among birth-month for all groups except for Q3. These tendencies were also seen in height and weight distributions. These findings imply that players who biologically mature at an early stage may be selected systematically as soccer players at competitive levels; however, biases based on maturation and physique may cause asymmetric birth-month distribution, as seen in this study.
