This chapter contains sections titled: Introduction, Kinematic characteristics of the current take-off technique, Optimal aerodynamic position after the takeoff, Kinetics of the take-off performance, Conclusions, References. Ski jumping is an exciting and primarily a competitive sport involving both ballistic and aerodynamic factors. The ballistic factors include release velocity and release position from the take-off table, whereas aerodynamic factors influence the gliding properties of the jumper/skis system (velocity, surface area, posture of the jumper/skis system, suit design, turbulence, and resisting and lifting forces) during the in-run, take-off, flight, and landing phases of the jump. The role of both ballistic and aerodynamic factors has undergone some changes during past years mainly due to changes in jumping hill profiles and other regulations concerning equipment and anthropometric characteristics of jumpers' body type. These changes will be discussed in more detail later in this chapter. Table 6.1 shows the importance of some selected factors to the length of the jump in ski jumping and it can be clearly seen that the in-run speed is the strongest factor influencing the jumping distance (see also Virmavirta et al., 2009). The table also shows that ski jumping performance is highly dependent on the aerodynamic forces acting on the jumper during the flight phase. Since the aerodynamic forces are proportional to the square of jumper's speed, the effect of these two factors is strongly linked together. However, the takeoff is still considered to be the most important phase in ski jumping as it determines the initial conditions for the subsequent flight. The overall development of ski jumping performance during past years has made FIS (Fédération Internationale de Ski) to modify the equipment regulations in order to control the progressive increase in jumping distances. After the dramatic effect of size of the modern jumping suit on the jump length was fully realized (Figure 6.1, Virmavirta & Kivekäs, 2009), many changes have been done to reduce its aerodynamic effect. Hovvever, the current regulations on suit design reduce utilization of this particular aerodynamic effect. Probablv, the most important change was done in 2004 when the maximum ski length that a jumper is allowed to use in competition was related to the body mass index (BMI). A current rule states that the maximum ski length, 145% of the jumper's body height, is only allowed if the jumper's BMI is 20.5 or more. The rule was introduced to make the low body weight less important for successful ski jumping performance. The effect of body mass on jumping distance is well known (see Table 6.1 and Figure 6.2a) and the BMI rule was assumed to stop the tendency among ski jumpers toward extremely low body weight (Schmölzer & Müller, 2002; Müller et al., 2006; Müller, 2009). The possible effect of BMI rule on the body type of ski jumpers and furthermore to the final jump distance is well demonstrated in Figure 6.2b where the relationship between body mass and jumping distance of best jumpers in the Olympic ski jumping competitions is presented. The above-mentioned changes added by the changes in jumping hill profiles certainly places special demands on the competition organizers when attempting to guarantee equal conditions for all jumpers with different performance levels even in the Olympic level (Virma\'irta et al., 2005, 2009). Reducing aerodynamic effects is often compensated by increased in-run speed which inevitably results in some problems that ski jumping has faced in previous years when the best jumpers accidentally fly too far. Despite apparent easiness, the modern ski jumping is consequently a very complex sport whose comprehensive understanding requires knowledge of several factors related to jumping distance, including application of computer simulation. This chapter updates the Information presented by Komi and Virmavirta (2000) on the same topic and a special effort will be made to characterize kinematics and kinetics of the take-off perform-ance. The presentation is based primarily on the published and unpublished material obtained in the various ski jumping research projects of the Neuromuscular Research Center, University of Jyväskylä. For a comprehensive review of bio-mechanical research in ski jumping, the reader is referred to the recent paper of Schwameder (2008) including more than 100 references.
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