Introduction: In ski jumping, take off ground is a only place at which the jumper can change their moving direction. Therefore, it is important to know how the take off motion affects the jump length, If jumpers have same power, the timing of power production could mostly affect the flight length. But, it is impossible to make same power production come true in human performance. If the jumping hill could be reduced to one hundred, take-off speed is approximately 1 or 2 m/s. Then the aerodynamic force can be ignored. The flight length depends on the take off velocity, which changes both the speed and moving direction with jump action. It is important to know the behavior of jumping performance. Especially, it is important to know when the jump action Starts. Taking this into serious consideration, we tried to clarify the Optimum timing of the jump action by using a compact jumping doll which four springs is incorporated. Methodology: The doll, one tenth scale model of a jumper, and an one hundred scale of jumping hill were designed in our laboratory. The doll has four spring devices to produce same jumping force, since this model was designed to take the same squat position. Although this model was made by wood, the resistance of the knee and hip joints during the jump action was minimized by four bearings. The trigger System for jumping action was mounted on the model and jumping hill. This System enables the doll to keep the squat position and storage the force until the release point on the take off area. When the doll passes the release point, the doll jumps. Therefore, the flight length can be controlled by the release point, which can be changed by every 1cm on the take off area. Results: Figure 1 shows the relationships between timing and flight length. When the jumping action started at 8 cm before frorn the edge, the doll reached P point, but the doll reached K point when the jump action started at the 3 cm before from the edge. The difference in the release point by 5 cm caused the difference in the flight length by 30 cm.. CONCLUSION: The jump action starts earlier, the flight length reaches shorter.
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