Muscle strength of the hands (carpal dynamometry)--is an integral indicator of the functional state of the nervous and muscular systems of the child, as well as the index of fitness and stamina. The drop in functional indices of children has been noted by many researchers from different regions of Russia. In this study, an analysis of endogenous and exogenous factors in children and adolescents with different muscle strength has been performed. The level of the biological development, especially in adolescence, was found to have a significant impact on the muscle strength. The reduction in muscle strength in pupils is due to exogenous factors, such as low physical activity, high study loads.