$186 Journal o f Biomechanics 2006, Vol. 39 (Suppl 1) Oral Presentations

Material and Methods: 23 healthy subjects participated in this study. NW was carried out at self-paced speed. Speed of W was adapted to that of NW (±0.2 m/s). Knee kinematics and ground reaction forces (GRF) were measured using a Vicon system (250 Hz) and a Kistler force-plate (1250 Hz). Resultant joint moments were calculated. Data were normalised to 100% stance phase and body-mass, smoothed and averaged. Paired t-test was used to verify the differences between NW and W (p < 0.05). Results: NW speed was 2.0±0.2 m/s. Paired differences showed significant lower knee flexion (1.8 °) and knee adduction moment (0.03Nm/kg) in NW compared to W. In absolute values average maximal knee flexion was 128.90 and average maximal adduction moment 0.52 Nm/kg in NW respective 127.10 and 0.54Nm/kg in W. No differences were detected in the maxima of knee endorotation, exorotation moment and flexion moment. Discussion: Results showed the highest joint loading in the sagittal plane. As these moments did not differ significantly, the propagated effect of using walking sticks on load reduction at the knee joint could not be supported. Sagittal plane moments were three-times higher than frontal plane moments. Moreover only in 24% of the subjects, variances between trials of one condition appeared to be minor than the averaged difference between NW and W within this subject. Further studies should investigate if the detected differences in some persons were evoked by technical skill level.

References [1] Willson J.W., et al. Med Sci Sports Exerc. 2001; 33(1): 142-7.

additionally had to put the left pole on a second force plate. The selected step was recorded digitally by two video cameras (3D-Analysis). Study 1 shows that different movement techniques lead to varying times of contact and curves of force. The electromyographical analysis indicates that the different performance styles strain the relevant muscles differently. Study 2 particularly shows that the lengths of the poles do not lead to significant technique changes. In conclusion it seems to be legitimate to speak of a sound movement pattern. Merely the difference in the vertical COG-amplitude revealed a significant difference (p < 0.05). To sum it up, the analysis shows that Nordic Walking is an independent kind of sport concerning biomechanical parameters. The technique does not vary with the use of slightly different pole lenghts.

References Burger R. (2005). Biomechanische Betrachtung der Nordic Walking Technik. In:

Kongressband zum 3. Internationalen Nordic Walking Kongress, Bad Tatz- mannsdorf, pp. 40-52.

Burger R. (2005a). Nicht Entlastung macht uns fit, sondern Belastung. Nordic Fitness 4: pp. 57-58.

Schwameder H., Roithner R., M(iller E., Niesen W., Raschner C. (1999). Knee joint forces during downhill walking with hiking poles. Journal of Sports Sciences 17: pp. 969-978.

Willson J., Torry M.R., Decker M.J., Kernozek T., Steadman J.R. (2000). Effects of walking poles on lower extremity gait mechanics. Medicine & Science In Sports & Exercise, 1: pp. 142-147.

7852 Th, 09:15-09:30 (P38) Field test ing to determine biomechanical loading o f the lower l imb dur ing nordic walk ing versus walk ing-Compar ison between nordic walk ing instructors and exper ienced nordic walkers

C. GriJneberg 1 , T. J611enbeck 2, D. Leyser 1 , M. Mull 2, C. Classen 2. 1Faculty ef Health, University of Applied Sciences - Europa Fachhochschule Fresenius, Idstein, Germany, 2Institute of Biemechanics, Klinik Lindenplatz, Bad Sassendorf, Germany

A common opinion is that walking with nordic walking poles provides 30-50% load reduction to the lower limb (e.g. Geyer 2005). However, some studies already focused on biomechanical loading in experimental set-ups and found only little evidence of load reduction (Willson et al., 2001). The aim of this study was to analyse the loading of the lower limb between nordic walking instructors (NWI) and experienced nordic walkers (ENW) during nordic walking compared to walking in real field circumstances with different walking tracks. For that purpose 14 experienced, middle aged ENW and 6 NWI have been asked to walk a 1575m track. The track consisted of different walking tracks (asphalt, cast, moderate downhill and uphill). All subjects wore a backpack with mobile data acquisition equipment. Vertical ground reaction force, forces in and positions of the walking poles have been recorded. First, the results show no lower vertical reaction forces between ENW and NWI during nordic walking compared to walking. Neither a reduction of the loading response at heel contact and push off, nor due to one of the different walking tracks, have shown less vertical reaction forces between groups. In contrast, for both groups in most of the tracks higher vertical reaction forces during heel contact were recorded for the nordic walking condition. Though, these differences were not significant. Second, the total vertical reaction forces measured in the walking poles are not significant different between groups and condition. The common opinion of a load reduction of the lower limb by 30-50% during nordic walking has to be rejected for ENW and NWI.

References Geyer (2005). Mit Stockeinsatz zum Ziel. physiopraxis 4: S.36-38. Willson et al. (2001). Effects of walking poles on lower extremity gait mechanics.

Medicine & Science in Sports & Exercise 1 : 142-147.

7863 Th, 09:30-09:45 (P38) Biomechanical analys is o f the Nordic walk ing- technique R. Burger, K. Schwaben, J. Fischer. Institute ef Sports Science, Johannes Gutenberg-University, Mainz, Germany

This research contains a crictical examination of statements published by in- dustrial companies. Furthermore, it analyses the morphological consequences of the poles as a sporting aid. The claim of a biomechanical examination of different technique-parameters is based on these two qualitative-theoretical aspects. The first pilot scheme (n =2) had the aim to describe the kinematic, dynamometric and electromyographic structures of different Nordic Walking techniques. A second study examined the variability of the technique depen- dent on the use of different pole lengths. The sample consisted of trained Nordic Walking instructors (n =8) who all possessed the highest qualification of their association. The subject had to place one step on a force plate within a testing track with the length of 12 metres. In the course of this the person

6. 4.3. Skiing

7811 Th, 11:00-11:15 (P41) Freestyle aerial ski ing mot ion analys is and s imulat ion A. LLithi, G. B6ttinger, T. Theile, H. Rhyner, W. Ammann. WSL Swiss Federal Institute for Snow and Avalanche Research SLF, Davos, Switzerland

In freestyle aerial skiing, up to three twisting somersaults are performed consecutively. In the present study both motion analysis and numerical sim- ulations were used to support the Swiss National freestyle skiing team from a biomechanical point of view. In total, 37 freestyle skiing jumps performed by 5 different athletes were captured using 20 VICON 4-Megapixel infrared cameras operating at 120 frames per second. A scaffold was specially built up around the ski jump to position the cameras adequately for filming. 48 sphere markers were attached to the athletes' jumping suit, helmet and ski equipment. From the 3-dimensional space evolution of these markers, somersault, twist and tilt angles were derived. The experimental uncertainty on these orientation angles was found to be smaller than 50 and allowed to compare different jump techniques as regards performance. In parallel to the motion analysis, a rigid body model of the human body in airborne movement was implemented. Given the segment parameters, the relative motion of adjacent segments, the initial body orientation angles and the total angular momentum, which remains constant during the flight phase, the simulation solves the conservation of angular momentum equation to get the evolution of the somersault, twist and tilt angles. Filmed and computed evolutions were found to agree well, with an average absolute difference smaller than 40 for the somersault angle, 130 for the twist angle and 20 for the tilt angle. The validated rigid body simulation was finally used to investigate alternative jump techniques with the aim to increase the number of twists per somersault.

4293 Th, 11:15-11:30 (P41) From red cells to ski ing: a real ist ic model for lift mechanics o f downhi l l ski ing and snowboard ing Q. Wu 1 , '~ Andreopoulos 2, S. Weinbaum 2. 1Department efMechanical Engineering, Villanova University, USA, 2Departments of Biomedical and Mechanical Engineering & New York Center for Biomedical Engineering, The City College of New York, USA.

The great enhancement in lift generated by a planing surface as it glides over a soft porous media is a new concept deduced from a red blood cell moving in a tightly fitting capillary. This concept is applied in the present study to the development of a realistic model to describe the lift mechanics of downhill skiing and snowboarding, where for the first time, the lift contributions due to both the transiently trapped air and the compressed solid phase (snow crystals) are determined. The model uses Shimizu's empirical relation to predict the local variation in Darcy permeability due to the compression of the solid phase. The forces and moments on the skier or snowboarder are used to predict the angle of attack of the planing surface, the penetration depth at the leading edge and the shift in the center of pressure for two typical snow types, fresh and wind-packed snow. We present numerical solutions for snowboarding and asymptotic analytic solutions for skiing for the case where there are no edging or turning maneuvers. The force and moment balance are then used to develop a theory for control and stability in response to changes in the center of mass as the individual shifts his/her weight. Our model predicts that for fine-grained, wind-packed snow when the velocity of the snowboarder or skier, U =20 m/s,

Track 6. Spor t Biomechanics - Jo int ISB Track

approximately 50% of the total lift force is generated by the trapped air for snowboarding and 40% for skiing. For highly permeable fresh powder snow the lift contribution from the pore air pressure drops substantially. This paper develops a new theoretical framework for analyzing the lift mechanics and stability of skis and snowboards that could have important application in future ski and snowboard design.

7864 Th, 11:30-11:45 (P41) Jo int loading on the lower extremit ies in ski ing and snowboard ing: methodologica l procedure

M. Klous, H. Schwameder, E. MLiller. Department of Sport Science and Kinesiology, University of Salzburg, Austria, Christian Doppler Laboratory 'Biomechanics in Skiing', University of Salzburg, Austria

To determine the loading on the lower extremities in skiing and snowboarding accurately, representative 3D kinematic and kinetic data has to be collected which serves as input data to calculate the loading parameters. The goal of this presentation is to give an overview and validation of the methodological procedures to collect and analyse these data. 3D kinetic data was collected with a mobile force plate (KISTLER). This device was placed between the binding plate and the binding for skiing and between the board and the binding in snowboarding. Kinematic data of both legs and pelvis were collected with five analogue cameras (50Hz). The 3D marker positions were calculated with Simi Motion. Several tests validated the methods used to collect the kinetic and kinematic data. (Stricker et al., 2005; Klous et al., 2004). Further analyses of kinetic and kinematic data as well as the development of the inverse dynamic model were programmed in Matlab. Validation of the mobile force plate shows accuracy comparable with laboratory systems. An error of 1-2% was found for 3D measured forces and an error of 1-5% was found for the determined torques (Stricker et al., 2005). In the 3D video analysis also a high accuracy was found. Error margins are in the range of 1 -2cm on a measuring range of 20m. In the kinematic data analysis, in which the position and orientation of each of the segments was calculated, the error margins increased only slightly. It can be concluded that the methods used in this project to collect and analyse the kinetic and kinematic data are sufficient and acute enough to use the collected data as an input for the inverse dynamics calculation.

References Klous M., Schwameder H., MUller E. (2004). Beitrag der 10. Tagung der (~SG,

14-15. Stricker G., Scheiber P., MUller E. (2005). In: Book of Abstracts, 10th Annual

Congress - ECSS, Belgrade.

7871 Th, 11:45-12:00 (P41) Rel iabi l i ty o f EMG median power f requency analysis in recreat ional alpine ski ing

J. Kr6111,2, J. Seifert 3, E. MLiller 1,2. 1Department of Sport Science and Kinesiology University of Salzburg, Salzburg, Austria, 2CD-Laboratory "Biomechanics in Skiing" Salzburg, Austria, 3St. Cloud State University, St. Cloud, United States

The mean frequency of the power spectrum of an EMG signal is an accepted index for monitoring fatigue in static contractions. There is indication that it may also be a useful index for dynamic contractions. In a previous study it was demonstrated that the median power frequency (MPF) of vastus lateralis (VL) and vastus medialis (VM) show tendencies to change in recreational skiing. After a fatiguing skiing session the MPF of the VL shifted downwards and the MPF of the VM shifted upwards during a 100s run. The purpose of this study was to determine whether this signal behaviour is reproducible in two measurement runs, and whether the individual results of the two measurement runs correlate. Muscle fatigue was induced in 10 recreational skiers by skiing 20 runs on a slope with 300m elevation ( -3 hours). EMG of VL and VM was measured after the fatigue process during skiing twice a 100 s run (Biovision, 2000Hz, right leg). To compare the MPF, five turns at the start (InMPF) and five turns at the end (EnMPF) of both runs were analysed by setting a time window of 500 ms over the steering phase of each turn. The same signal behaviour was observed in both runs whereas MPF of VL decreased (Run1: 64.23Hz/61.2Hz, p<0.134; Run2: 59.8Hz/55.7Hz, p < 0.061) and MPF of VM increased (Run 1 : 47.1 Hz/52.6 Hz; p < 0.111 ; Run2: 45.6Hz/51.1Hz; p<0.091). No significant difference for InMPF and EnMPF was found between run1 and run2. The correlation between run1 and run2 was significant for both muscles in InMPF and EnMPF (VL: InMPF r=0.784", EnMPF r = 0.890**; VM: r = 0.798", EnMPF r = 0.956**). Due to the fact that the general signal behaviour was observed in two different measurement runs and a high correlation between the two runs is observed, the present investigation demonstrates the usefulness of EMG frequency analysis as a reliable method for fatigue determination in alpine skiing.

6.4. Sports Activity - Skiing $187

7911 Th, 12:00-12:15 (P41) Safety in ski ing

V. Senner 1 , S. Lehner 2. 1Technische Universit#t MEtnchen, Department Sport Equipment and Materials, Munich, Germany, 2BASIS, Applied Biomechanics, Munich, Germany

Injury rate for skiing is relatively low at approximately 2 -3 per 1000 skier days, and have remained relatively steady over the last few seasons. It is not difficult to define the two major problems regarding safety in skiing, reduce the rate of knee injuries and keep lower leg fractures low. Achievements and commonly agreed knowledge regarding the other three steps of van Mechelen's Sequence of Prevention (van Mechelen, 1992) however are difficult to obtain. In the first part the presentation will give an overview of the efforts and results of various research groups working on the identification of risk factors and accident mechanisms, the development of counter measures and - most difficult - the validation of efficiency of these measures. Norwegian scientists use an anthropometric model based motion analysis to more precisely describe knee injury mechanisms from observed accidents. The US research group around Shealy and Johnson empirically determine the speeds of skiers (and snowboarders) on alpine slopes as a function of gender, visibility and the use or not use of helmets. Using nation wide data on fatal injuries in skiing over fourteen years, Shealy was also able to validate the effect of helmet utilization. An interesting and still ongoing controlled study in French ski resorts intends to validate the effect of reduced binding setting for specific target groups, especially women. Under the leadership of an Italian institute a grant research of the European Commission, called Best Practises in the Prevention of Skiing Accidents, tries to establish and promote prevention measures on a European level. The second part of this talk will present some of our work in the field of skiing safety. In total seven different studies mainly dealing with equipment related injury prevention will briefly be explained, focusing on biomechanical and methodological aspects.

References van Mechelen W, Hlobil H, Kemper H. (1992 Aug). Incidence, severity, aetiology and

prevention of sports injuries. A review of concepts. Sports Med. 14(2): 82-99.

7870 Th, 12:15-12:30 (P41) Biomechanical character ist ics o f fur ther deve loped classical and skat ing techniques in c ross-county ski ing spr int compet i t ions

S. Lindinger 1,2, T. Stoeggl 1,2, E. Mueller 1,2. 1Department of Sport Science and Kinesiology, University of Salzburg, Austria, 2Christian Doppler-Lab "Biomechanic in Skiing", Salzburg, Austria

Since cross-country (XC) skiing sprint was introduced in World Cup, techniques have developed but have not been biomechanically analysed yet (older studies Smith et al. 1996; Millet et al., 1998; etc.). Purposes of two studies were 1) to perform a complex biomechanical analysis of the new double poling (DP) and double-push skating (DPS) technique compared to traditional ones and 2) to advance hypotheses, which biomechanical aspects contribute to DP and DPS performance. Eleven elite XC skiers performed DP at a submaximal velocity roller skiing on a treadmill (1°). Three national elite skaters performed DPS and double dance skating (DDS) at maximal speeds on a flat snow track. Pole and plantar ground reaction forces, joint angles, cycle characteristics and EMG of several muscles were analyzed (standard statistical methods). DP velocity was correlated to peak pole forces (PPF) (r=0.70), angular velocity in elbow flexion (r=0.80) and minimal knee and elbow angle (EAmin) ( r=-0.72; [P<0.05]; r=-0.88, [P<0.01]). PPF in DP correlated to EAmin (r=-0.71), relative poling/recovery time (r= -0.72/r= 0.73) and hip angle at pole plant ( r=-0.89) (all P <0.05). The best skiers' technique showed higher PPF, impulses of pole force (P < 0.05), angular elbow and hip flexion velocities and smaller minimum elbow and hip angles (all P<0.01) at noticeable high preactivation and activation of trunk, hip, shoulder and elbow flexors and/or extensors. DPS showed additional push-off impulses at the outside ski edge, higher push-off impulses of force, higher flexion and extension velocities in leg joints, higher maximum sprint velocities and different EMG patterns (lower leg and adductor muscles) compared to DDS. Both new techniques were found to be complex movements with changed muscular and technical demands, which have to be further investigated, supplemented also by physiological studies.

References Smith G. A., Fewster J. B., and Braudt S. M. (1996). Double poling kinematics

and performance in cross-country skiing. J. Appl. Biomech. 12: 88-103. Rest of literature at the authors.