The relationship between the knee flexion-extension strength ratio and lower limb stiffness at different movement speeds involves the interaction of biomechanics, neuromuscular control, and muscle physiology. The knee flexion-extension strength ratio reflects the balance of strength between the quadriceps and hamstrings, which typically varies across different movement speeds. Optimal lower extremity stiffness plays a crucial role in enhancing movement efficiency and preventing injuries. This study aims to investigate the influence of the knee flexion-extension strength ratio, assessed under different isokinetic testing conditions, on lower limb stiffness during jumping. By examining the relationship between strength characteristics and lower limb dynamic parameters, this study seeks to provide scientific evidence for optimizing athletic performance, injury prevention, and rehabilitation training. Fifty-five healthy male college students were selected as subjects. The knee flexion-extension muscle groups were tested at slow, medium, and fast speeds (60°/s, 150°/s, and 240°/s) using a Biodex isokinetic muscle strength tester to calculate the flexion-extension muscle strength ratios.High-speed cameras and a Kistler three-dimensional force platform were used to collect mechanical parameters related to the push-off and buffering phases of vertical jumps in place, and to calculate the vertical stiffness of the lower limbs.Variance analysis was employed to compare the differences in knee flexion-extension muscle strength ratios and lower extremity stiffness across different testing speed modes. The influence of different knee flexion-extension strength ratios on the peak vertical ground reaction force and the maximum vertical displacement of the center of mass showed significant differences. Specifically, comparisons between the high F/E ratio group and the low H1 group (P = 0.016), the high F/E ratio group and the low-to-moderate groups (P = 0.037), and the high F/E ratio group and the low group (P = 0.009) all demonstrated statistical significance (P < 0.05). Additionally, under different speed test conditions, the calculated lower limb vertical stiffness among different F/E ratio groups exhibited significant differences. The P-values for the significant differences among the slow (60°/s), moderate (150°/s), and fast (240°/s) speed groups were 0.018, 0.006, and 0.024, respectively. Under the same velocity mode, lower limb vertical stiffness during both the propulsion and absorption phases increases with a higher knee flexion-extension strength ratio (F/E). Across different velocity modes, the knee F/E ratio exhibits a general increasing trend with the acceleration of test speed among the low, medium, and high groups. This study is based on a specific isokinetic testing mode and jump task, without accounting for the effects of different movement contexts and individual variability. The causal relationship between muscle strength and jumping ability remains unclear. Therefore, future studies should adopt broader testing methodologies, such as integrating electromyography (EMG), biomechanical modeling, or longitudinal experiments, to further investigate the long-term effects of the flexion-extension strength ratio on lower limb stiffness and athletic performance.
