Biomechanical Analysis in Sports: Application to Alpine Skiing

A NEW BIOMECHANICAL MEASUREMENT AND TESTING METHOD FOR TURNS IN ALPINE SKIING

O. Rađenović*, B. Nemec**, and V. Medved***

*Zagreb Sport Association, Zagreb, Croatia

**Department of Automation, Biocybernetics and Robotics, Jožef Stefan Institute, Ljubljana, Slovenia

*** Faculty of Physical Education, University of Zagreb, Zagreb, Croatia

E-mail:

Abstract: We have measured electromyographic (EMG) signals from four left leg muscles: m.rectus femoris (m.r.f.), m.vastus lateralis (m.v.l.), m.vastus medialis (m.v.m.) and m.biceps femoris (m.b.f.) and ground reaction force below ski boots, during performance of wedge turns. Standard and carving skis were used and two types of wedge turns were performed, the Croatian and the Austrian variant. Video recordings were used to monitor kinematics. EMG signals and ground reaction forces were compaired for both types of skis and both types of turns. The least strain develops while performing the Austrian variant of the wedge turn using carving skis. These results may have implications for the use of this type of skis in teaching basic skiing techniques.

Introduction

In teaching basic and advanced skiing techniques, performance of the elements of the ski-school depends mostly on the skiing knowledge and physical preparation of the skier 1.

The wedge turn is one of the most essential and basic elements of the ski-school. Therefore, the way this element is taught will influence all subsequent steps and elements of basic and advanced ski technique.

One of the first researches involving EMG in the biomechanics of skiing was done by Asang et al. 2, where the authors have monitored EMG from four leg muscles in conjunction with force measurements using a dynamometer between the binding and the boot during six elements ranging from a herringbone step to a parallel turn. They reported substantional activity from the anterior tibialis and adductors relative to the quadriceps femoris, and low levels of activity from the triceps surae.

Exstensive research has been done on 21 Austrian ski instructors that have performed a progression of turns based on ski teaching curricula 3. The instructors skied in a variety of snow conditions and over different terrains. Measurements included EMG from eight muscles, force from strain gauges mounted on the skis, knee angls from goniometers, and qualitative kinematics from film. Turns were partitioned into initiation and steering phases and distinguished by unique characteristics in the initiation phase. Involvement of the anterior tibialis and peroneus longus in edging, and gluteus maximus and quadriceps muscles in extension of the knee and the hips were among the general patterns of muscle activity reported.

Clarys et al. 4 examined the effects of three types of skis (racing, soft, and compact) on muscular activity during three types of teaching turns and a straight run. They monitored six leg muscles on eight ski instructors and reported mean EMG activity of all muscles combined. They concluded that since the soft ski elicited the least muscular activity, it was the best ski for recreational and competitive use.

Clarys and Cabri 5 have summarized the research that has been done using EMG in different sports: 13 different studies using EMG have been done in alpine skiing, during slalom, giantslalom, supergiantslalom and different competition conditions.

In this report, we present biomechanical analysis of the wedge turn using video information, EMG and a system for ground reaction forces determination 6,7. This approach has permitted an assesment of strains that arise during performance of this element using either carving or standard skis, and the comparison of the Croatian and Austrian variants of this turn.

Materials and methods

The measurements were performed on a member of the Croatian Ski Demo Team (age 30, 165 cm height, 68 kg mass). Surface EMG signals were measured using Muscle Tester ME3000 Professional 8. Measurements were performed on Rogla ski resort in Slovenia. The skier had the electrodes of the Muscle Tester ME3000P put directly on the skin, above the muscles tested 9, and the device was used in a terrain mode. After A/D conversion, EMG signals were smoothed (full-wave rectified and low-pass filtered-100 ms time constant). Portable parts of the both devices: for ground reaction force measurements and EMG were put on a belt that was attached to the skier's waist. Kinematic information was simultaneously acquired by a VHS-video camera. The measurement begins when a performer pulls a trigger: the EMG, the unit for ground force reaction measurement, as well as the flash, are connected to the trigger. The time is recorded on the VHS-video camera when the flash goes on, and that moment is considered a zero point of the measurement.The procedure is exactly the same and the end of the measurement. The terrain task consisted in the following: at the ski slope of 50 m in length and an incline of 15-17, and the slope was groomed daily to maintain consistent snow conditions. The skier performed eight wedge turns, four using the Croatian variant of the turn, and another four using the Austrian one. The procedure was repeated three times. This paradigm was performed twice, using standard and carving skis.

Ground reaction force was determined by a device described in 6. The force transducers are composed of two measuring plates, each plate containing two sensors. The measuring plates were mounted onto the ski boot sole (Fig.1.). The scheme of the tensiometric plate is shown in Fig.2.

Fig.1. Ski boot instrumented with four force sensors.

Fig.2. Tensiometric plate, sensors, and corresponding acting forces.

From the sensor data, fij (see Fig. 2), it is possible to estimate the total vertical force, Fr, in the z axis and the torques around the x and y axes, or in a more convenient form, the point of the application of the vertical forces, tpx and tpy, using the following equations derived from the balance of angular momentum:

Fr = f11 + f12 + f21 + f22 - F01 - F02

, (1)

where F01 and F02 are the forces of the binding, and fij are the forces from the corresponding sensors according to Figure 2. The binding forces can be calculated by measuring the force foij on sensors loaded only with ski binding force, using the equations:

(2)

.

If we insert eq. 2 into eq. 1, we get a simplified relation in the form of:

. (3)

The above equation shows that it is not necessary to calculate the binding forces F01 and F02 as we can instead subtract the signal of each force sensor loaded only with ski binding force 6.

During analysis of measurement records, in each video record three time instants were identified (Figs. 3. and 4.): 1. the entrance, 2. the steering and 3. the exit. They represent corresponding phases. The second phase of the turn is the most important during performance; as shown in Figs. 3 and 4, and the most marked difference in technique between the Croatian and Austrian variants is precisely in this phase. Video recordings were used to determine precisely the beginning and the end of each phase.

Results and discussion

Differences between Croatian and Austrian variants of the wedge turn can be seen from kinematic records in Figs. 3 and 4. When performing the Croatian variant, in the second phase of the turn the outside (dominant) leg is mostly loaded, in contrast to the Austrian turn, in which during the same phase both legs are loaded almost equally.

Tables 1, 1a, 2 and 2a show the average results of six independent measurements during performance of both variants of the wedge turn, on standard and carving skis, in three phases of the right turn.

As shown in Tables 1, 1a, 2 and 2a, the developed muscle force estimated via EMG signals is almost the same for the m.b.f. for both variants of the turn and both types of skis. The wedge turn, however, is a relatively static element, with minimal vertical movements of the body, and the activity of m.b.f. is very low.

As shown in Tables 1, 1a, 2 and 2a the more intense strain in m.v.m. and m.r.f. develops during performance of the Croatian variant on standard skis. The m.v.l. is maximally loaded in the Croatian variant on carving skis, which is influenced by performing technique.

instant 1 instant 2 instant 3

Fig. 3. Croatian variant of the turn with EMG signals for each phase.

instant 1 instant 2 instant 3

Fig. 4.Austrian variant of the turn with EMG

signals for each phase.

Slightly higher activities of this muscle are seen in the Austrian variant of the turn on both types of skis, because of different technique used, namely the centre of gravity is closer to the centre of the turn (see Fig. 4., instant 2). More important are the activities of m.v.m., which drags the knee to the centre of the turn, and of m.v.l. and m.r.f., which are required to strengthen the knee during conduction of the wedge turn. As shown in Figs. 6. and 7., the large scattering implies that the difference across selected instants is large for standard skis; also the maximum strain is large when performing the turn on standard skis. Importantly, on standard skis the muscles are constantly loaded and the difference between phases is minimal. The ground reaction force (Fl.l. - left leg force, and Fr.l. - right leg force), is higher during performance of the wedge turn on standard skis (Figs. 6. and 7.). For carving skis (Figs. 6. and 7., the right side of the graph.), the reaction force is smaller and equally distributed on both legs which results in less strain in contrast to standard skis for which just one leg is loaded.

Conclusions and perspectives

We have measured and studied the performance of the wedge turn, which is one of the basic elements of the ski school. The skier who performed the measurements is a member of the Croatian Ski Demo Team and uses model technique, which makes our results a plausible model for the performance of this element Since carving skis are being widely used even by begginers, we have chosen to analyze the wedge turn which is often the first element taught in the ski school and compaired its performance on standard and carving skis, using the Croatian and the Austrain variant of the turn. Our results show that the least strain develops when performing the Austrian variant of the wedge turn using carving skis. Similar measurements can be done for other elements of the ski school and changes in teaching ski technique can be made accordingly to, hapefully, reduce time and effort required to learn new sking skills.

Table 1. Average values of smoothed EMG signals and ground reaction force measurements in selected time instants during performance of the Croatian variant on carving skis.

Table 1a. Average values of smoothed EMG signals and ground reaction forces measurements in selected time instants during performance of the Croatian variant on standard skis.

Table 2. Average values of smoothed EMG signals and ground reaction forces measurements in selected time instants during performance of the Austrian variant on carving skis.

Table 2a. Average values of smoothed EMG signals and ground reaction forces measurements in selected time instants during performance of the Austrian variant on standard skis.

Acknowledgment

This study was supported by The Ministry of Science and Technology of the Republic of Croatia (Project No. 034-004).

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Fig.6. Graphical representation for the measurements of the Austrian variant on carving (the right side of the column) and standard skis.

Fig.7. Graphical representation for the measurements of the Croatian variant on carving (the right side of the column), and standard skis.

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