Optimal Drop Height in Prepubertal Boys Is Revealed by the Performance in Squat Jump

Drop jump (DJ) performance gain with increasing drop height is well documented in adults, but there is still no clear evidence of such gain in children. This study aimed to examine the differences in DJ performance gain in male adults and prepubescent boys by comparing drop heights tailored to each individual's performance and expressed as a percentage of their squat jump (SJ) performance. Fifteen boys (9-11 y) and 15 men (19-27 y) executed DJs from drop heights that were set at 75%, 100%, 125%, and 150% of their best performance in SJ (DJ₇₅, DJ₁₀₀, DJ₁₂₅, and DJ₁₅₀, respectively). Vertical ground reaction force (vGRF), contact time and kinematics of the lower extremities were captured. The results showed that boys jumped significantly lower than adults in DJs, and both age groups presented jumping gain with increasing drop height, up to DJ₁₂₅. Boys demonstrated longer total contact time, lower angular velocity and vGRF during the propulsive phase, as well as smaller knee flexion at touchdown and lower reactive strength index. vGRF in DJ₇₅ and DJ₁₀₀ was lower than in DJ₁₂₅ and DJ₁₅₀. The highest value for maximum knee flexion was also presented at DJ₁₅₀. It is concluded that in prepubescent boys, the appropriate drop height for an effective DJ is linked to their performance in SJ and might be between 75% and 125% of their maximum SJ performance.