25 Supej, M., Kugovnik, O., & Nemec, B. (2002). New advances in racing slalom technique KinSI 8(1), 25–29 NEW ADVANCES IN RACING SLALOM TECHNIQUE NOVA NAPREDOVANJA V TEKMOVALNI SLALOMSKI TEHNIKI Matej Supej 1 Otmar Kugovnik 2 Bojan Nemec 3 Abstract In the past years, the radical changes in the shape and materials of the slalom skis affected the skiing technique. This area is the subject of intensive inve- stigations. Our previous work in measurement of ki- nematic and dynamic parameters in the alpine ski- ing initiated new advances in slalom racing technique. The changes are so significant that we propose the new racing technique. In this study we present several biomechanical advantages of the new technique according to the current one. Sub- stantial dynamic analyses have been performed and have been proven with the Kolmogorov-Smir- nov statistical tests. Up to now, all performed mea- surements, even with the elite ski racers, show the superiority of the proposed technique. Keywords: alpine skiing, slalom, technique, dyna- mics, measurement 2 University of Ljubljana, Sports Faculty, Ljubljana, Slovenia 3 Institute Josef Stefan, Ljubljana, Slovenia 1 Ski Instructors Association of Slovenia Contact address 1 Matej Supej University of Ljubljana – Faculty of Sport Department of Biomechanics Gortanova 22 SI-1000 Ljubljana Slovenia Tel.: +386 1 520-77-00 Fax : +386 1 520-77-50 E-mail: Matej.Supej@sp.uni-lj.si Izvleček V zadnjih letih se je zelo močno spremenila geome- trija slalomskih tekmovalnih smuči. Naglemu razvo- ju mora slediti tudi tehnika, ki mora čimbolj izkori- stiti novo opremo. Predhodne meritve kinematičnih in dinamičnih parametrov pri alpskem smučanju ter študij tekmovalne tehnike so nas privedli do novih izboljšav tekmovalne slalom tehnike. Spremembe so tako značilne, da predlagamo novo tekmovalno tehniko. Opravljena je bila izdatna analiza dinami- ke, ki so jo potrdili tudi Kolmogorov-Smirnov stati- stični testi. Vse meritve do sedaj, tudi tiste z vrhun- skimi tekmovalci, so se izšle v prid nove tehnike. Ključne besede: alpsko smučanje, slalom, tehnika, dinamika, meritve (Received: 25. 10. 2001 – Accepted: 03. 07. 2002) 26 Supej, M., Kugovnik, O., & Nemec, B. (2002). New advances in racing slalom technique KinSI 8(1), 25–29 INTRODUCTION Many changes in the shape and materials of the al- pine skis have happened in the last few years. That is why skiing technique is also under radical chan- ges. Especially in racing new improvements in equipment and technique contribute to better re- sults. These are the reasons for several biomecha- nical studies that have been introduced in skiing. For example mathematical modeling have been used for better understandings of the skiing tech- niques and proposing new, more efficient techni- ques (Renshaw, & Mote, 1989; Hirano, & Tada, 1994; Hirano, 1996). On the other hand optimum controlling procedures were used for minimization of the total time needed (Maronski, 1990) and for maximization of the speed (Hertzen, Hollund, & Ranta, 1997) in the downhill racing. Also, variation calculus was used to obtain optimum trajectories in giant slalom (Supej, & Kugovnik, 2000). Several re- search groups measured and analysed different ki- nematical parameters that also help to improve the skiing technique (Kugovnik, Nemec, Pogačar, & Čoh, 2000; Pozzo, Canclini, Cotelli, Martinelli, & Roeckmann, 2001; Supej, Kugovnik, Nemec, & Šmitek, 2001; Seifriz, & Mester, 2001). According to new carving skis and consecutively different turn trajectories many dynamic analyses of forces and vibrations have been performed (Kugovnik, Ne- mec, Pogačar, & Čoh, 2000; Kugovnik, Nemec, & Supej, 2000; Nemec, Kugovnik, & Supej, 2001; Tscharner, & Schwameder, 2001). However, the most radical changes in the men’s world cup races have happened in slalom skis. Ra- cers started to use skis from 155 to 170 cm long in- stead of over 190 cm in the last two years. New skis are significantly different also in the side cut and radius, which was decreased from approximately 35m to 11m. Such change results in the evolution of the racing technique (Supej, Kugovnik, Nemec, & Šmitek, 2001). The main goal of our work is to propose new advances in slalom technique. Due to the significant changes compared to the current technique we decided to call the proposed techni- que »new technique«. METHODS Sample We took under the investigation the current and the new proposed slalom racing technique. In the current race technique, there is evident double up and down body motion during the ski turn. Conse- quently, this results in very high ground reaction forces just under the ski gate. Because of the doub- le body motion, the ground reaction forces are the sum of the radial, gravitational and on-weighting forces. Additionally, double motion results also in non-optimal motion of the skier mass centre. Detai- led analyses of the current technique can be found in (Supej, Kugovnik, Nemec, & Šmitek, 2001). In order to overcome mentioned problems, we pro- pose a new slalom technique. The key points of the new technique are: Fig. 1: The pictures on the »film« represents one turn in the new technique (top) and one turn in the current technique (bottom). 27 Supej, M., Kugovnik, O., & Nemec, B. (2002). New advances in racing slalom technique KinSI 8(1), 25–29 • there is no double up-down body motion during the ski turn. The skier is in the most bent position at the time of the weight change. This is the be- ginning of the new turn. The upright position is around the ski gate. • The resulting skier mass center trajectory is shor- ter with the new technique. • The radial, gravitational and on-weighting forces do not reach the maximum values at the same time in the same direction. This results in decrea- sed ground reaction forces. The comparison between new and current techni- que is shown in the Fig 1. A high-skilled ski instructor, member of the natio- nal demonstration ski team (Ski Demo Team Slo- venia) and former ski racer, performed 18 slalom runs. In the first 10 runs the gates were placed with the absolute distance d s =13m between each other and rapt by d r =4m (Fig. 2). For the other 8 runs the gates were placed with the absolute distance d s =11m between each other and rapt by d r =2.5m. The skier alternately used once the current and once the new technique. Both setting-ups were pla- ced on the well-prepared ski slope (18.5° at the be- ginning and ending with 13°). There were 14 gates for the each setup. The first and the last ski turn were excluded from the analyses. The snow was in the first runs hard, but due to the high temperatu- re conditions (+6°C) it was slowly getting wet. The skier was using race extreme carving skies (Head Cyber XTi, 160cm) with 11m radius that suits FIS (International Ski Federation) requirements. The weather was sunny and the visibility was excellent. Measuring device The forces during the runs were measured with four strain-gauge sensors with 50 measurements per se- cond on each ski build in ski boots and synchroni- zed with a mini DV digital camcorder. Detailed des- cription of the device is presented in the article (Kugovnik, Nemec, Pogačar, & Čoh, 2000). Data analysis For analysing the performance of the skier runs we used the AviComp 3.0 program and for analysing the recorded data combined with the video signal XSki 1.0 (Fig. 3). The forces measured during each run were analy- zed with histograms, where the measuring area from 0N to 4000N was divided into 100 intervals. The norm of histograms was set to 100 and the da- ta was smoothed by Butterworth digital filter su- broutine in Matlab 6.0. The effect of the filtering is shown in Fig. 4. The Kolmogorov-Smirnov statistical test is used for zero hypotheses whether certain two histograms belong to the same statistical population. All com- binations of each population and between the po- pulations were made. Two histogram regions were especially considered. The first region from 0N to 120N determines the low contact forces between the skis and the snow. According to this, we intro- duce a new parameter p c , which describes the por- tion of the ski turn in percentage that was perfor- med with the low contact forces. The second region with forces over 2000N determines undesi- red forces that contribute to the dissipation of the kinetic energy. Additionally, the mean histogram values in each population were smoothed with But- terworth digital filter for easier analyses. RESULTS From 18 runs we made four different populations of results. Two runs from the first population were excluded due to bad skier performance. That is why we have two populations of four runs for the cur- rent technique and two populations of four runs for the new technique. The associated histograms and the filtered mean values to these four populations are presented in Fig. 5. Statistical significance (5%) has been proven with the Kolmogorov-Smirnov test. When we tested whether the samples in one population are the same (combination of tests), the tests were positi- Fig. 2: The schematic shows the absolute distance between neighboring gates ds and the rapture dr between the gates. 28 Supej, M., Kugovnik, O., & Nemec, B. (2002). New advances in racing slalom technique KinSI 8(1), 25–29 ve in 5 of 6 in both first populations and 6 of 6 in both second populations. When we tested the ine- quality between the first two populations, the tests were positive in 10 of 16 and between the second two populations in 11 of 16. The determination of the poor snow contact brought the following results. On the first gate com- bination the factor pc was 8.8% in average with the current technique and only 0.5% with the new technique. On the second gate combination fac- tor was 14.5% and 2% respectively. From Fig. 5 it can be seen that the force distribution is different in the new technique compared to the current one. Detailed analysis of undesirable for- ces over 2000N shows that the density of these for- ces is greater with the current technique. The ave- rage value with the current technique in the first and second gate combination exceeds the new technique for 20% and 35% respectively. DISCUSSION From the measurement of the ground reaction for- ces we can notice that the average force does not change with the new technique. However, we have statistically proven that the force distribution is sig- nificantly different. From Fig. 3 different force distribution in one turn can be noticed. With the new technique, the skier compensates the radial and gravitational forces with the down body un-weighting motion (see ground reaction force graph between 14.7 and 15 sec of the ski run performed with the new techni- que). The force distribution is even more evident from the force histograms (Fig. 5). The ratio p c is 17 and 7 times greater with the current technique for the first and the second gate combination respectively. Please note that large p c factor denotes poor con- tact between the skis and the snow. This reduces steering ability of the skier. However, very high for- ces in the ski turn usually mean increased dissipa- tion of the kinetic energy and increased friction. Therefore, the portion of the very high forces du- ring the ski turn should be as small as possible. Also Fig. 3: Analyzing the current (top) and the new (bottom) slalom ski technique with XSki 1.0. The diagrams shows the forces during the runs for the left ski, right ski and the sum. Both pictures on the right side pre- sent the skier du- ring the weight change (marked with the black line in the middle of the diagrams) once with the current (top) and once with the new technique. Fig. 4: Histogram of the skier first run using the current slalom technique. The curves show non-filtered data and smoothed da- ta with Butterworth digital filter. 29 Supej, M., Kugovnik, O., & Nemec, B. (2002). New advances in racing slalom technique KinSI 8(1), 25–29 in this parameter we can see the benefit of the new technique. The average value of the high force with the current technique in the first and second gate combination exceeds the new technique for 20% and 35% respectively. Although the total time of the ski runs were not un- der the investigations, we obtained a 0.2 to 0.4 sec shorter time in average on ski runs of 12 sec. Encou- raged with these results we performed tests with elite ski racers, where we obtained significantly shorter total times with the new technique. 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Ljubljana: Published by the author. 12. Supej, M., Kugovnik, O., Nemec, B., & Šmitek, J. (2001), Novi po- gledi na slalomsko tekmovalno tehniko [New views to the slalom racing technique]. Šport, 49(4), 49-55. 13. Tscharner, V., & Schwameder, H. (2001). Filtering of force variab- les in skiing by specified wavelet analysis. In E. Mueller, H. Scwa- meder, C. Raschner, S. Lindinger, & E. Kornexl (Eds.), Science and Skiing II (pp. 55-68). Hamburg : Verlag Dr. Kovač. Fig. 5: Each of the diagrams presents four of the non-filtered histograms and the filtered mean value drawn with bold line. Both left diagrams show the populations of the current technique and the right two show the new one. One of the diagrams in each pair be- longs to the first gate combination and the other one to the second gate combination.