This study was designed to evaluate the effects of long-term (2 years) strength training on reactive strength. To control the effect of the strength training intervention, reference data in trained and untrained adolescents were determined. Therefore, part 1 of this investigation evaluated a cross-sectional analysis of untrained adolescents (students, n=430) performing drop jumps from 24 and 32 cm height (DJ24, DJ32). The participants were divided into age groups (under 19 years old (U19) to U14). The performance differences between groups were expected to indicate age-related developmental changes. Part 2 investigated the same physical tests performed by trained adolescents (soccer players, n=114, U19 (A) to U15 (C)). The tests were conducted before (pretest) and 1 year and 2 years after the training period (posttest). One group performed periodised strength training (twice per week) in addition to their regular soccer training (strength training group (STG)), while the other group served as control group (CG). The cross-sectional data of untrained adolescents showed significant differences among age groups in drop jump performance (e.g. mean Δ(under 17 years old (U17) to U19)=1,3% to  Δ(U16 to U17)=14,8%). Trained participants reached significantly higher performance levels than did untrained participants (e.g. mean performance index of DJ24 of U15: e.g. 146 vs. 111, respectively). A comparison between the CG and STG showed significantly higher performance gains in the STG for most age groups over time (e.g. mean performance index of DJ24 of C Δ10% vs. 0%, respectively). First, it can be concluded that performance gains in the DJ occur with age in both trained and untrained youth participants. Second, playing soccer positively influences DJ performance. Third, long-term strength training has a moderate effect on DJ performance.