The aim of the paper was to determine the biological age (growth age proportional age) of school-aged boys and girls (8-9.99 and 12-13.99 years old) with low level of physical fitness in the Czech test battery UNIFITTEST. The growth age results of 4 selected groups (17-25 individuals in one group) showed that classification of the individuals into normative height bands was in no relation to their age, sex or level of physical fitness. Considering the height-weight proportionality, we found out that the pubescent individuals had the relatively higher values of body weight than prepubescent ones of the same sex and physical fitness. The selected individuals have been growing up proportionately to their hereditary predisposition. We did not find any significant differences among KEI values in the relations to physical fitness. The range of differences between the biological and chronological age in the prepubescent and pubescent low-fit samples was more than 3 years. The values of the interquartile range of differences indicate higher variability in the low-fit samples than in samples from the Czech population. The results of differences between the biological and chronological age in the low-fit children approximately correspond to the findings in an average child population.
