(2005), where the authors talk, on carving turn, about the initiation phase being longer than steering phase when compared with a parallel turn. A rough estimate ...
GROUND REACTION FORCES MEASUREMENT BASED ON STRAIN GAUGES IN ALPINE SKIING SOŇA VODIČKOVÁ & FRANTIŠEK VAVERKA* Technical University of Liberec, Faculty of Education, Czech Republic * Palacky University, Faculty of Physical Culture, Olomouc, Czech Republic Ski experts in technique of turning on skis are dealing with problems concerning load of skis during particular kinds of joined curves. There are many theoretical works, most of them based on kinematographic or dynamographic record or on a combination of both methods. Observation of reaction forces between the ski and the plate has been performed in the past. Many studies have been performed to eliminate the number of injuries of knee-joints. However, significant changes occurred in the construction of skis and ski boots in recent years. Also a shift to the use of carving skis appeared. With such a way of skiing, bigger side-cut radius is exploited than it was with the traditional skis. This is the reason, why recently the number of studies dealing with biomechanics of alpine skiing with emphasis on measurement of trajectory and ground reaction forces. We have developed a special measuring device, which enables to measure forces in three directions and also their torsional moments around these three axis. This system works on the principle of strain gauges. During the measurement all the data are collected on Compact Flash disc with the frequency of 100 Hz. We were interested whether it is possible with help of the system, to detect interindividual deviation during a carve and thus to participate on improving of skiing technique. Our measurement refered to measurement of ground reaction forces arising during a carving turn. The investigation was carried out on a group of 6 skiers (3 racers and 3 ski teachers) with the weight of M = 78,8 ± 5,46 kg. Their height was M = 1,80 ± 0,04 m and age was M = 26,5 ± 1,61 years. We realised that during a carving turn the strongest ground reaction forces occur during the steering phase after the fall line and during the initiation phase skis are alleviated and edging is changed. It was interesting that from the point of view of ground reaction forces there is no identical pass of right and left turn from all measured skiers. Comparison of measured left carving turn and comparison of right carving turn showed very similar cycle. Ground reaction forces measurement has already become a part of preparation of advanced national teams and it can help to improve quality of skiing not only top-class skiers, but also coaches of young gifted individuals can contribute to quick progress in technical performance of some motoric activities owing to measured data.
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Introduction
Ski experts in technique of turning on skis are dealing with problems concerning load of skis during particular kinds of joined curves. There are many theoretical works, most of them based on kinematographic or dynamographic record or on a combination of both methods(Fukuoka, 1971; Nigg, Neukomm & Lüthi, 1977; Müller, 1986, 1991, 1994 and others). Observation of reaction forces between the ski and the plate has been performed in the past. Many studies have been performed to eliminate the number of injuries of knee-joints (Fetz, 1977; 1991; Nachbauer, 1986; Nachbauer & Kaps, 1995; Niessen & Müller, 1999; Senner, Lehner, Wallrapp & Schaff, 2000). However, significant changes occurred in the construction of skis and ski boots in recent years. Also a shift to the use of carving skis appeared. With such a way of skiing, bigger sidecut radius is exploited than it was with the traditional skis. This is the reason, why
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2 recently the number of studies dealing with biomechanics of alpine skiing with emphasis on measurement of trajectory and ground reaction forces. At present, there are in existence a number of objective methods enabling the determination of kinematic and dynamic parameters of a skier’s drive in the turn, which have been applied in many studies (Kugovnik, Supej & Nemec, 2003; Pozzo, Canclini, Cotelli & Baroni, 2005; Supej, Kugovnik & Nemec, 2005 and others). The basic goal of our work was construction and test of the recording device of forces between the ski and the plate during carving turn and measurement of the reaction forces. 2
Method
Based on literature published in recent years concerning ski load during course we performed an approximate calculation of dimensions, shape and stiffness of measuring elements for strain gauges application. Taking advantage of these calculations, we constructed a measuring device usable on the slope. For our study we used carving ski Blizzard SLK Kompressor of the length of 167 cm, with radius 16 m. At the original version there was a carving plate mounted under the bindings. This plate raises position of the body’s center of gravity and thus facilitates bringing the ski into turning. At the ends it is supplemented with rubber blocks which limit deflection and also oscillation of the ski at its bending. Recording appliance was verified in February 2003 in Liberec on a 200 meters long sector of a slope. The original carving plate was substituted by specially adapted element for the experiment. External shape and dimensions are similar to the original including the damping rubber elements on its ends. The new plate consists of two parts, between which the measuring elements were fixed. The bottom part creates a “tank”, which is firmly fixed to the ski together with the measuring elements in the same way as on the original system but is slightly raised. A complete connection of miniature strain gauges system on the elements is made here. The strain gauges system enables measuring of all required moments of force; it is effects of force in three axes of the Cartesian system of coordinates and relevant flexural (torsional) moments around these axes (Fig.1). The upper part creating “cover” carries the bindings and is fixed to the lower part so that the system “tank” – “cover” can do mutual relative motion and thus enable measurement of force proportions.
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Figure 1 Cartesian system of coordinates The complete measurement system for the ride data record required to be developed in the smallest size and weight possible, shock resistant and easily placed on skier’s body without limiting his/her motion. A single-chip microcontroller was chosen for this purpose and adapted specially for this case using custom-made set of amplifiers and Compact Flash Disc for data record. The system operates using battery and thanks to its compact dimensions can be placed on skier’s back in a small bag. To determine the time courses of single components of the six mentioned moments of force for each ski from the measured signals subsequent analysis and mathematical processing is necessary. The mathematical computation consists of the following steps: 1. Offset correction to get normalized scale in relation to state when the skier stands still on a flat horizontal surface 2. The strain gauges system outputs transformation to axial forces and appropriate moments including correction of cross-affection of force components in the strain gauges system and correction of vertical force with respect to weight of the skier. 3. Filtering the obtained forces data by a low-pass filter with cutoff frequency set to 10 Hz. By linking the forces data to a picture record, information about force proportions in given moments of the skier’s movement can be obtained. This picture – data synchronization is ensured by a LED diode placed on skier’s forehead blinking in defined intervals. This approach gives us possibility to determine the real forces affecting the measured system, which is later subject to further analysis in connection to the picture record of skier’s movement on the slope. Forasmuch as our video recording is possible to synchronize with recording of running forces in individual turns with the help of an optical signal, we could determine the ratio of loading between an outer and inner ski in the phases of a turn.
4 Our measurement refered to measurement of ground reaction forces arising during a carving turn. The investigation was carried out on a group of 6 skiers (3 racers and 3 ski teachers) with the weight of M = 78,8 ± 5,46 kg. Their height was M = 1,80 ± 0,04 m and age was M = 26,5 ± 1,61 years. 3
Results and Discussion
We realised that during a carving turn the strongest ground reaction forces occur during the steering phase after the fall line and during the initiation phase skis are alleviated and edging is changed. The first annurately detected results of analysing of proportional shape of two phases of the turn indicated that the initiation phase represents approximately 40% and the steering phase 60% of the total duration of a carving turn. Interindividual variability of the ascertained data, expressed by standard deviation, is relatively very small and standard deviation fluctuates between 6 – 7% of duration of individual phases. In literature, we have only encountered qualitative consideration of the two phases in studies of Raschner et al. (2001), Müller & Schwameder (2003) and Müller et al. (2005), where the authors talk, on carving turn, about the initiation phase being longer than steering phase when compared with a parallel turn. A rough estimate in the published graphs of Raschner et al. (2001) study is approximately 37 % initiation duration phase It was interesting that from the point of view of ground reaction forces there is no identical pass of right and left turn from all measured skiers (Fig. 2).
Figure 2 Ground reaction forces during left and right turns
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Conclusions
Ground reaction forces measurement has already become a part of preparation of advanced national teams and it can help to improve quality of skiing not only top-class skiers, but also coaches of young gifted individuals can contribute to quick progress in technical performance of some motoric activities owing to measured data.
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