Different solutions in generation of angular momentum during imitation jump take-offs among diverse level ski jumpers
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Introduction: During the push-off motion in ski jumping, the athlete strives to generate both a high vertical velocity and a correct angular momentum. The latter is important as it brings the athlete into the flight position. The ground reaction force (GRF) acts with a moment arm (d) upon the ski jumpers centre of mass (CoM). The interplay between the d and the GRF is responsible for creating the angular moment from which the necessary angular momentum of the ski jump transpires. This study aimed to describe how ski jumpers generate the angular momentum in indoor imitation ski jumps, and how different kinetic parameters of the generation are related to skill. Methods: Eleven male ski jumpers performed 13-15 consecutive jumps from an indoor take-off ramp. Kinematics and the GRF-vector was recorded. CoM was calculated with base in kinematic data, using the individual body-segments length and mass. The athletes were ranked according to expected level of performance at the actual test day. To relate the analysed variables to performance level, Spearman’s rank correlation analysis was used. Rho were plotted as a function of time in order to look at trends throughout the push-off phase. Results: The angular momentum was generated in the later part of push-off motion. The late development is linked to the ski jumper’s late alteration of d. Significant correlations to performance were found in both vertical force production, the length of d, as well as in the rate of change in d. Total angular momentum showed no correlation to performance either during the push-off nor at the actual take-off (first time sample with GRF≤0). The correlations between performance and vertical force were found during a brief interval of the early push-off, when the better jumper tended to exert the greater force. Correlation trends in d showed wider intervals of significant correlations: The better jumper tended to enter the transition phase between the in-run and the push-off with a d closer to zero in comparison to the lower ranked athletes, who typically entered this phase with small negative values of d. Also, the better jumpers tended to both delay the onset of change in d, and have the lesser rate of change in d during a large part of the push-off. Eventually, this means the better jumper both entered and left the push-off with values of d closer fixated around zero. Discussion: It seems as the athlete’s different solutions in generation of angular momentum might be a result of the interplay between force production and orientation of the GRF-vector; trends in data suggests the poorer athletes to make up for a slight lack of force by having the longer d throughout the push-off. During the in-run athletes seem to control d in order to enter the push-off in a specific state.