THE VARIABILITY OF VERTICAL GROUND REACTION FORCES DURING UNLOADED AND LOADED DROP JUMPING

Abstract

The increase of the resistance of the movement - on the basis of the dynamic fundamental law F.t = m.v (Force Impulse = Change of the Kinematic Impulse) - can be achieved either by increasing the drop velocity v (initial height of the drop jump) or by increasing the body mass m of the athlete (adding a weighted vest). The purpose of this study was to investigate the behavior of the forcetime parameters of the force impulse by varying the kinematic impulse, using unloaded and loaded drop jumping. METHOD Ten top track & field and diving athletes (6 male and 4 female) participated in this study, 17-24 years old. The subjects were instructed to perform three unloaded drop jumps from heights (drop velocities) of 0.30 m (2.43 d s ) , 0.45m (2.97 d s ) , 0.55m (3.28 d s ) and two loaded drop jumps from a standard height of 0.30m (2.434s) where the additional load was defined based on the Kinematic impulses that arise from the unloaded drop jumps the height of 0.45m and 0.55111 respectively. A Kistler fore platform was used to record the force-time curves of the jumps. RESULTS AND DISCUSSION The calculated values of the average vertical jumped distances for the unloaded conditions (drop height 0.45m,0.55m), as it was expected, were found to be 20% and 31% higher in comparison to the loaded drop jumps respectively. The results of the analysis of the variability in the force-time parameters have shown no significant differences (p>.05, student's t-test at .05 level) for the total average force (acceleration force and loaded force) between unloaded and loaded jumps while for the acceleration force significant differences (p<.05) were found. On the otherhand the average time values were significantly (p<.05) increased for the loaded drop jumps. The above results indicate that the athletes can perform during the unloaded drop jumps significantly higher peak power values due to the higher velocities of the moving body mass. Also the average time from the point of the peak power to the take off point for both the unloaded and load drop jumps, was found to be approximately the same, ranging from.0.52-.53 s. This data indicate that, with a given kinematic impulse, for unloaded as well as for loaded drop jumps the actual performance level (in the means of muscular strength) that the athletes can reach, is dependent on their present training level.