The effect of box height on joint stiffness performance in drop jumping: a cross sectional study

This study aimed to examine the impact of performing drop jumps from different heights on joint stiffness, particularly focusing on the ankle and knee joints. In sports and training environments, joint stiffness plays a vital role in force absorption and performance output during jumping activities. Understanding how stiffness responds to varying jump heights can contribute to optimizing training strategies and minimizing injury risk. A total of 32 athletes (16 males and 16 females; mean age: 21.6 ± 2.6 years; height: 178.1 ± 10.2 cm; weight: 68.7 ± 16.1 kg) voluntarily participated in this study. Each participant performed two drop jumps from three different box heights (26 cm, 35 cm, and 42 cm), executed on both the dominant and non-dominant lower limbs. Reflective markers were positioned on anatomical landmarks, and motion data were captured using the Qualisys Track Manager system. The collected kinematic data were processed with Visual 3D software to calculate gravity displacement and vertical stiffness values. The results demonstrated that knee joint stiffness was significantly greater than ankle joint stiffness across all jump heights. Moreover, peak knee joint moments increased with higher drop heights, with the highest values observed at the 42 cm box (p < 0.05). Vertical stiffness was higher at Box 3 than Box 1 (p < 0.05), but no other significant differences were observed. The findings indicate that the knee joint serves as the primary contributor to force generation during drop jumps, while the ankle joint contributes mainly to stabilization. Although joint moments increased with drop height, vertical stiffness only differed between Box 1 and Box 3, suggesting adaptation occurs mainly through joint-level adjustments.