We have studied the physical characteristics of strength and the electrocardiographs of 291 boys, between 9 and 15 years of age, and 284 girls from 8 to 14 years old, divided into groups according to their sexual maturation stage (SMS). During this age period, children pass from SMS 1 to SMS 5. We have used the Ruffier-Dickson test as a test of strength and have also obtained an electrocardiographic index. Analysis of variance (ANOVA) showed that in girls, the number of hours of sport activity/week, systolic blood pressure (SBP) and diastolic blood pressure (DBP) both during rest and post-effort (SBPe and DBPe), Ruffier-Dickson index (RDI), pressure frequency product (PFP), increase of diastolic blood pressure (Inc DB) and the indices of Lewis and Sokolow right show a statistically significant dependence on the SMS. In boys, the number of hours of sport activity/week, SBP, DBP, heart frequency post-exercise, SBPe, RDI, PFP, increase in systolic blood pressure and the index of Sokolow right and left are dependent on the SMS. The objective of this paper is to draw the pediatrician's attention to the need to use the SMS for evaluating biological functions, not the chronological age.