INTRODUCTION: There are several differences between imitation jumps and actual jumps in ski jumping, such as equipment, speed, air resistance, and ground friction. No or little Information is available on the effect of having full friction between jumper and surface in imitation jumps on the coordination pattern. This is important for the evaluation of imitations jumps with regard to technique training for actual ski jumping. In actual ski jumping, the athlete has no option to use horizontal ground forces for propulsion because the friction always acts against the movement direction. In imitation jumps on a full friction surface, jumpers have that possibility, regardless the instruction on jumping technique. We examined this issue in imitation jumps that ski jumpers regularly perform during technique training. METHOD: Six elite male ski jumpers performed imitation ski jumps under 2 different conditions. Five imitation jumps were performed with full ground friction, allowing the option to apply horizontal propulsion force (FO), and 5 jumps from a moving trolley, rolling down on a ramp with low velocity (1.3 ms-1) minimizing this Option (MO). In all jumps, the subjects were caught by their coach directly after take off, minimizing the need to use horizontal propulsion forces. The subjects were asked to perform these jumps as they would do during training, i.e., mimic as closely as possible the technique as is used during an actual hill jump. The differences in kinematics and kinetics were measured during the jumps by means of a Kistler force plate and ProReflex system. Joint moments and power were calculated using inverse dynamics. RESULTS: No differences were found in the kinematic variables. However, in FO a substantial backwards horizontal force was applied (Fig. 1), which was low in MO. This was reflected in the moment (Fig.1) and power Output differences between the two conditions. These were opposite for the hip and knee joint. DISCUSSION: This study showed that the athletes have difficulty to suppress the application of horizontal propulsion forces if this option is present. Even though kinematically the performance is unaffected by this, different coordination is applied between the two conditions. The weight of the trolley plus force plate (17kg), allowed the subject to generate some horizontal forces in MO. This was option was hardly used, as indicated by the minimal deceleration of the trolley during push-off. In the current setup a small velocity difference was present. This may have been of influence by giving athletes the feeling not having to jump forward (in terms of propulsion). CONCLUSION: The dynamic conditions under which imitation jumps are performed need to be considered carefully if this exercise is used to practice proper jumping technique.
© Copyright 2007 4th International Congress on Science and Skiing. Veröffentlicht von University of Salzburg. Alle Rechte vorbehalten.