A kinematic and kinetic study of alpine skiing technique in slalom
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Despite a large body of lay and professional literature covering numerous aspects of alpine skiing technique, only a limited number of published scientific investigations have examined the relationship between skier technical and tactical characteristics and racing performance. As a consequence, our scientific understanding of how the underlying mechanics of alpine ski racing technique relate to performance is surprisingly weak. The purpose of this research project was therefore to identify, describe, and study aspects of alpine ski racing technique which play an important role in determining skier performance. Since this is the first project of its kind for our research group, an additional purpose was to establish the theoretical and methodological foundation for a line of future research on this topic. Initially, a conceptual model of turning technique was developed to provide the theoretical framework for defining research questions. This model was built on the basis of a literature review and 17 in-depth interviews with highly-experienced coaches from 6 nations. Research questions were then defined on the basis of this model for a quantitative, motion analysis study of turning technique, the purpose of which was to quantitatively describe the various components of the aforementioned model and to explore their interrelationships with the ultimate aim of further developing our understanding of how turning mechanics relate to racing performance. Towards this end, a 3-dimensional, video-based photogrammetric method utilizing multiple panning cameras was implemented to capture the performances of six highly skilled athletes during slalom race simulations on courses with 10 and 13 m linear gate distances. The resulting data were then used to examine kinematic, kinetic, and energetic questions related to ski motion, skier technique, and performance. A comparison between courses was made in the hope that understanding how skiers adapted to the differing circumstances would shed further light on the relationship between turning mechanics and performance. This dissertation describes in detail the methods and results of this investigation. In terms of ski motion, the results of this investigation provide observational evidence in support of current theoretical models of ski snow interaction mechanics and may provide insight into both ski design and injury prevention issues. Many of the differences observed between courses in terms of ski motion, skier kinematics and skier kinetics seem to be reminiscent of differences between carving and skidding mechanics, which is perhaps not surprising considering that there was a greater degree of carving on the 13 m course. Perhaps a finding of particular significance was the difference in turn cycle structure observed between courses. While the skiers’ trajectories were symmetrically distributed about the gate on the 10 m course, there was a prolonged Initiation Phase on the 13 m course, resulting in an asymmetrical trajectory about the gate. This difference in turn structure and trajectory shape may to a large degree be the result of the different gate distances and the skis’ physical properties. Further work to understand why this difference occurred may help to improve our understanding of how ski characteristics influence turn mechanics. In terms of kinetics, maximal snow reaction forces were high, reaching over 3000 N and 3.5 times bodyweight on both courses. There was a clear difference in the timing of the snow reaction force relative to the turn cycle between courses. On the 10 m course, large reaction forces were generated at about 50 % of the turn cycle, or approximately gate passage. In contrast, peak forces were delayed on the 13 m course, occurring just after gate passage, or about 65 % of the turn cycle. In addition, there was a greater degree of unloading in the vertical component of the snow reaction force during the Initiation Phase on the 13 m course. The air drag force was larger on the 13 m course due to increased skier speeds. Mechanical energy dissipation, introduced by Supej, Kugovnic, and Nemec (2005a), was calculated based on skier center of mass motion. A cyclical pattern of energy dissipation was observed on both courses with high dissipations occurring during the turn and low dissipations during the transition between turns. Negative dissipations— situations where skier kinetic energy increased by more than what can be attributed to changes in skier potential energy—were observed at the transition between turns on the 10 m course and early in the turn cycle on the 13 m course. This may provide evidence of skiers increasing their kinetic energy through muscular work, although in this study these increases were small, representing less than 3 % of skier kinetic energy gains. To help understand what factors acted to slow the skier, the negative work of each of the external forces acting on the skier was calculated. About 20 % of skier total mechanical energy loss in the investigated situation was due to air drag. The remaining majority of mechanical energy losses were attributed to a drag component of the snow reaction force. However, this study does provide evidence suggesting that the energy cost of gate clearance in slalom may play a significant role and needs to be accounted for in future work. In terms of racing performance, both skier vertical and fore/aft actions correlated well with elapsed time through the investigated sequence. The relatively strong correlation observed between skier fore/aft position and mechanical energy loss further indicates that this is a parameter whose role in skiing mechanics and performance should be investigated further. In summary, this project has provided a conceptual model of turning technique in alpine ski racing that is based on both scientific and practitioner knowledge and that can be used to guide future research efforts. The results of this investigation are limited in that generalization from a single situation, and a single group of athletes, to other circumstances needs to be considered very carefully, particularly in light of the infinitely variable conditions possible in alpine ski racing. Moreover, the observational design of this study does not provide evidence of cause and effect relationships. In light of these limitations, perhaps the most important scientific contribution of this study lies in the identification of turning technique parameters which need to be better understood to further develop our knowledge of how the mechanics of turning technique relates to performance. Perhaps of particular importance in future work will be the development of our understanding of how skier actions influence the ski snow interaction and, ultimately, ski and skier motion.
Avhandling (doktorgrad) - Norges idrettshøgskole, 2010.