Purpose: To determine whether the observed phenotypic stability in static strength during adolescence, as measured by inter-age correlations in arm pull (ARP), is mainly caused by genetic and/or environmental factors. Methods: Subjects are from the Leuven Longitudinal Twin Study (LLTS) (n=105 pairs, equally divided over five zygosity groups). ARP data were aligned on age at peak height velocity (APHV) to attenuate the temporal fluctuations in inter-age correlations caused by differences in timing of the adolescent growth spurt. Developmental genetic models were fitted using structural equation modeling. Results: After aligning the data on APHV, the annual inter-age correlations conformed to a quasi-simplex structure over a 4-year interval. The best fitting models included additive genetic and unique environmental sources of variation. Additive genetic factors that already explained a significant amount of variation at previous measurement occasions explained 44.3% and 22.5% of the total variation at the last measurement occasion in boys and girls respectively. Corresponding values for unique environmental sources of variance are 31.2% and 44.5% respectively. Conclusions: The observed stability of static strength during adolescence is caused by both stable genetic influences and stable unique environmental influences in boys and girls. Additive genetic factors seem to be the most important source of stability in boys, while unique environmental factors appear to be more predominant in girls.