The aim of this study was to examine the relationship between morphological
characteristics and motor abilities of boys and girls aged from 11 to 14 so that the process of
physical education could be in line with the dynamics of growth and development as well as
the differences between the sexes. The total sample consisted of 730 subjects, divided into
four age groups of 11, 12, 13 and 14. Eight motor tests were conducted and 9 anthropometric
measures were taken. The boys of all ages achieved better results in tests of coordination, repetitive and explosive power, while girls of all age groups scored better in flexibility tests. The largest differences between the sexes at the ages of 11 and 12 years were the results of
variables evaluating motor abilities. At the age of 13, a significant difference in skinfold
thickness was found in favor of girls and at the age of 14 in the voluminousness in favor of
boys. In boys aged from 13 and 14, the increase in body volume was negatively correlated
with the increase in skinfold thickness, indicating that the increase of voluminousness is
caused by the increase of muscle mass. It is therefore concluded that the linear increase of the
differences in motor performance tests under 14 years of age is caused by increased muscle
mass in boys, especially in tests where achievement depends on the strength and production
of force.