The purpose was to evaluate the effect of the arms action during vertical jump landing on ground reaction forces, joint kinematic and vertical displacement of the center of gravity (CG). 29 athletes where the vertical jump constitutes a basic skill in their sport have participated in this study. All participants had to drop from a height of 0.5 m and cushion the fall in two experimental situations: a) without arm action and b) with free participation of the arms. A force platform, operating at 500 Hz, was synchronized temporarily to a video camera at 210 Hz which recorded the sagittal plane of the jumps. The jumps have been considered as a symmetrical movement that develops in a plane, composed by a simplified mechanical model of eight segments. The results have shown that the free participation of the arms contributes to reduce the second peak of force, which is due to the greater braking distance of the CG and a smaller reduction of the vertical velocity of the CG, which could contribute to reducing the risk of injury. The segmental contribution to the vertical displacement of the CG indicates that the restriction of the arms during landings produces a significant change in the mechanism of absorption of the forces that could increase the tension in the anterior cruciate ligament of the knee.
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