Introduction: In competitive skiing on of the most important factors influencing the performance is to keep a high level of average velocity while maintaining an optimal trajectory. In each turn around the gates, the skiers have to change his instantaneous trajectory and this produces a very high centripetal force acting on the CoG. A great amount of eccentric and concentric muscular activity is needed to counteract this kind of load (Berg et al., 1995). Provided the limited amount of studies reporting movement kinematics during ski competitions (Pozzo et al., 2001), we focused our work on collecting three-dimensional kinematics data on athletes during a world cup giant slalom race. Methods: The data reported in this work are related to the World Cup men`s slalom, which took place in December 2002 in Val Badia (Italy). Four digital camcorders (Canon XM2- 50 Hz) were located along the slope and acquired the skier`s motion throughout three gates in the middle part of the race. A dedicated software for video analysis, allowing the operators to freely pan, tilt and zoom the TV cameras, was used in order to ensure the largest possible working volume (Baroni et al., 1998). The system calibration was performed by means of the DLT method. The 3-D localization of the control points (gate poles, nets supports, etc.) was obtained by means of a geodetic theodolite immediately before the start of the race. The biomechanical model of the skier consisted of 17 body landmarks in correspondence of joint centres, foot edges, head centre, as well as skis and poles extremities. The total body centre of mass (CG) was calculated according to Gubitz. The joints ranges of motion were calculated on average slope plane (xz horizontal plane), uphill 25°, and on the orthogonal direction to this plane, namely vertical direction (y). The instantaneous radius of rotation was calculated for the path of the CoG in the critical phase of turning the gates. This allows evaluating the centripetal acceleration which, in turn, is an indirect indicator of the centripetal force acting on the CoG of the skiers. Results: Data acquired and elaborated for 8 top skiers are presented in this work. Fig. 1 shows the corresponding trajectories of the CoG of three skiers onto the snow plane and the time course of vertical CoG displacement and velocity for one exemplifying skier . Mean skiing velocity was 21,5±2 m/s. At the phase of "change of the edges" of each turn, the CoG average vertical displacement was 50±30 cm, with an average vertical velocity of 1,3±0,5 m/s. According with previous works (Raschner et al., 1997) the knee joint angle reached a minimum value of 50° and a maximum of 150°. The instantaneous radius of rotation of the CoG path showed values between 11 m and 23 m, whereby the centripetal acceleration values were between 16 m/s2 and 32 m/s2 . Fig. 2. Upper panel: trajectories of CoG on the snow plane (xz). Lower panel: time course of CG vertical motion We also found some correlations between the radius of rotation and joint angles. Calculated centripetal forces acting on the CoG revealed analogous values compared with those collected directly via force plates mounted on the ski. The kinematics of additional biomechanical parameters is discussed with reference to the potential biomechanical significance for the optimisation of the movement performance. Discussion/Conclusions: The main findings of this work support the possibility to obtain relevant data to be used for determine the actual loading conditions in competitive situation and for evaluate specific constrains which are to be matched by the biomechanical system. Nonetheless, it is possible to make some distinctions in the individual patterns of joint motion, which could be of great relevance for training purposes
© Copyright 2005 International Congress Mountain & Sport. Updating study and research from laboratory to field. 11th-12th November 2005. Rovereto (TN) - Italy. Programme and book of abstracts. Published by Centro Interuniversitario di Ricerca in Bioingegneria e Scienze Motorie. All rights reserved.