Determinants of ski-jump performance and implications for health, safety and fairness.
ABSTRACT Ski jumping puts high demands on the athlete's ability to control posture and movement. The athlete has to solve extremely difficult optimization problems. These implicit decisions and the resulting control manoeuvres can be understood by means of computer simulations. Computer simulations based on wind tunnel input data can identify the determinants for high performance and answer many questions of training methods, safety and health, role of weight, fairness, optimized hill design, sport development, and changes to the regulations. Each of the performance determinants has to be seen in the context of all others in order to understand its importance; the predominant factors are: high in-run velocity, high momentum perpendicular to the ramp at take-off due to the jump and the lift force, accurate timing of the take-off with respect to the ramp edge, appropriate angular momentum at take-off in order to obtain an aerodynamically advantageous and stable flight position as soon as possible, choice of advantageous body and equipment configurations during the entire flight in order to obtain optimum lift and drag values, and the ability to control the flight stability. Wind blowing up the hill increases the jump length dramatically and decreases the landing velocity, which eases the landing, and vice versa for wind from behind. Improvements to reduce unfairness due to changing wind are urgently needed. The current practice of the judges to reduce the score when the athlete has to perform body movements in order to counteract dangerous gusts is irrational. The athletes should rather be rewarded and not punished for their ability to handle such dangerous situations. For the quantification of underweight it is suggested to use the mass index: MI=0.28 m/s2 (where m is the jumper mass and s is the sitting height), which indirectly considers the individual leg length. The MI formula is similar to the body mass index (BMI) formula: the height is replaced by the sitting height s and a factor of 0.28 effects that the MI is equal to the BMI for persons with average leg length. The classification of underweight is not only a question of the cut-off point, as much it is a question of the measure for relative bodyweight used. Low weight is one of the performance determinants; however, it should be considered that very low weight can cause severe performance setbacks due to decreased jumping force, general weakness, reduced ability to cope with pressure, and increased susceptibility for diseases. In the past, several cases of anorexia nervosa among ski jumpers had come to light. The development toward extremely low weight was stopped in 2004 by new Fédération Internationale de Ski ski-jumping regulations, which relate relative body mass to maximum ski length. The 2006/7 and 2008/9 seasons showed that light athletes who had to use skis with just 142% of their height could still win competitions. A further increase of the borderline weight is being discussed. The current regulations are based on the well known BMI; the use of the MI instead of the BMI should be explored in future studies.