THREE TECHNIQUES OF SKI JUMP TAKE-OFF MODELED BY CHANGES OF JOINT ANGLE

Abstract

INTRODUCTION: In ski jumping, take-off action is the most important factor for ascent force. Jumpers should aim for optimum movements of the joints, because reaction force is the result of the integrated kinetic parameters of each joint or segment. In our recent studies, three techniques of take-off action were classified by manner of joint power generation. However, such kinetic parameters are difficult to explain in coaching situations. It would be more useful for coaching to represent the three techniques of take-off action by joint angle rather than joint power. The purpose of this study is to establish visual models of ski jump take-off action of world class jumpers based on changes of joint angle which would be especially useful in coaching. METHOD: The take-off actions were analyzed from videos taken at Hakuba Intercontinental Cup summer competition at 1997. Camera speed was 240 frames per second. The data from the videos were collected by computer. An inverse kinematics solution was applied to analysis. Jump performance of four jumpers, who received first prize in the team competition of Nagano Olympic games, was analyzed. RESULTS: The maximum value of angular velocity in the thigh was observed at close to the take-off platform edge in all jumpers. Peak angular velocity in the thigh was larger than in the trunk segment. The action at the hip joint represented the characteristics of jumping techniques rather than the action at the knee joint. The technique was classified by three manners of angular velocity. The three types of jump action were represented simply as visual models. In the technique of Type-A, motion was observed with regular order in each joint. The angular velocity of the trunk stayed at 2 rad/sec until the thigh’s angular velocity’s appearance. Type-A can be defined simply as an action moving from hip to knee joint. In technique Type-B, the angular velocity in the thigh was 4 rad/sec, higher than in the trunk at the initial jump action, and after some delay, the value in the trunk raised up from a negative value. Action Type-B can be defined simply as moving from knee to hip joint. In the last technique, Type-C, both the trunk and thigh angular velocities increased synchronously. Both hip and knee joints were extended at the same time. Type-C can be defined simply as the technique of synchronous movement at the knee and hip joint. These motions classified by changes of joint angle were shown by three visual models from Type-A to C. CONCLUSION: Three types of jump action could be represented simply by joint angle as visual models. There are advantages and risks involving jump hieght and the amount of body area subjected to aerodynamic drag force associated with each technique.