Biomechanical Analysis of the Takeoff, the Early and the Mid-flight Phases in Ski Jumping During the Salt Lake City Winter Games

Abstract

Introduction: The early flight phase in ski-jumping is characterized as the transition from take-off into a stable flight. This phase covers the first 15 to 25 meters or 0.7 to 1.0 seconds, respectively, depending on the size of the jumping hill. The data on the early flight kinematics analyzed during the Lillehammer Games (Arndt et al. 1995) reported that less than 15% of the variance of total performance (distance jumped) can be explained by the CM parameters (position, velocity) of the take-off. The explained variance at 15m after the take-off through the CM ballistic variables was shown to be less than 20%. When adding five parameters describing the body configuration and indicating aerodynamic variables the explained variance of total performance increased up to about 85%. These results demonstrate that the early flight or the transition take-off to stable flight should be the most sensitive phase to determine the total performance in ski jumping. The purpose of the biomechanical analysis in ski jumping during the Salt Lake City Winter Games was to evaluate the results of the biomechanical studies performed at the 1994 and the 1998 Olympic Games in Lillehammer and Nagano, respectively, to identify changes in the techniques used in the period of performing the V-style (firstly performed in 1992 in Albertville), and to increase the general knowledge of the identification of performance limiting factors in the transmission from take-off to flight. Methods: The jumps of all participants of the 90m and the 120m hills were recorded by three synchronized cameras. A set of three cameras operating at 50 fps were genlocked and recorded the take-off, the early flight, and the mid-flight of the jumpers. The cameras were panned and tilted in order to allow a small object field. The panning and the tilting were controlled via well calibrated landmarks in the background. A specific DLT routine was used to transform the digitized data into the metric space coordinate system. Fifty trials of each competition were chosen for further analysis. From the first round of each competition the best ten jumps and the shortest ten as well as all thirty jumps of the final round were analyzed. Joint centers, reference points on the ski and calibration points were digitized manually. Data of segments' relative mass and moment of inertia were taken from Hanavan's model. Results and Discussion: The crucial point for the jumper during the take-off is to adjust to the external forces (aerodynamic forces, friction between ski and track, ground reaction forces) and to control the posture during the take-off to achieve the best initial conditions for the early flight. Different groups of athletes can be identified using different and individual strategies to solve the problem. The direction of the take-off and the release velocity vector has a higher effect on the rotational component than on the linear component. The rotational component determines the angular momentum of the body's forward rotation during the take-off substantially. From dial one can conclude that the angular momentum of the body rotation during the take-off play the major important role concerning the preparation of the early flight. A high angular momentum at take-off is more important than a high release velocity in order to reach an early compact flight position. From the results we can conclude that there exists an optimum of the angular momentum for the early flight which means that there is an optimal relationship between ballistic and aerodynamic take-off parameters. The optimum angular (somersault) momentum will ensure the early compact flight position and thus the necessary lift and the drag forces for an optimum flight. The take-off and the transition to early flight are the dominant prerequisites for total performance in ski-jumping. An optimum angular momentum at take-off brings the athlete in an advantageous position in the early flight for an optimum use of lift and drag forces. It is substantial for the jumping performance to optimize angular momentum during the take-off in accordance with the torque due to the air steam after take-off, which has to stop the forward rotation at the right moment. Acknowledgement: This project was made through the support of the Medical Commission of the IOC and Pfizer.