Biomechanical analysis in freestyle snowboarding: application of a full-body inertial measurement system and a bilateral insole measurement system

DOI: 10.1002/jst.89 Research Article Biomechanical analysis in freestyle snowboarding: application of a full-body inertial measurement system and a bilateral insole measurement system Andreas Kru ger and Ju rgen Edelmann-Nusser Otto-von-Guericke University, Department of Sport Science, Germany Several investigations show that 5 28% of all snowboarding injuries relate to the ankle joint complex. To reduce the risk of ankle injuries, the development of enhanced snowboard equipment is considered. Therefore, it is essential to understand the biomechanics in snowboarding. Scientific studies investigating the ankle joint complex in freestyle snowboarding, including inrun, flight phase, and landing, are so far not available. An auspicious method to determine relevant kinematical and kinetic parameters is based on the utilization of an inertial measurement suit in combination with a bilateral insole measurement system. This pilot study aims at the application of these two systems in freestyle snowboarding for data collection in a real snowboarding environment. The accuracy of the used measurement systems is assessed. The insole measurement system shows a root mean square error of 28% (76.6%) in reference to a force plate. A maximum mean deviation of 4.81 (70.31) is found in the inertial measurement system compared to an optical video-based system. The on-snow data collection reveals valuable information and provides a better understanding of the biomechanics in freestyle snowboarding. Ranges of movement of the ankle joint complex and forces essential to perform snowboarding maneuvers are measured. In addition, combinations of joint angles and landing forces occurring during the landing phase of an aerial maneuver are found that have the potential to cause injuries. Critical values of 251 dorsiflexion and 81 external rotation in combination with a normal force of 3020 N are measured at the back leg. r 2009 John Wiley and Sons Asia Pte Ltd Keywords:. snowboarding. motion analysis. ankle joint. injury. safety 1. INTRODUCTION Snowboarding is one of the premier alpine winter sports. With respect to the anatomic location of injuries, several investigations show that 5 28% of all injuries relate to the ankle joint complex [1 4]. Furthermore, correlations between *Otto-von-Guericke University, Department of Sport Science, Brandenburger Str. 9, 39104 Magdeburg, Germany. E-mail: andreas.krueger@ovgu.de the type of equipment used and the injury were found [2,3,5,6]. Thus, the development of enhanced snowboard equipment, to reduce the risk of equipment-related injuries, is considered in literature [2,5,6,7]. In a current research project at the University of Magdeburg in Germany, a new safety snowboard binding concept is in development, which in particular protects the snowboarder s ankle joint. The usage of soft boots increases the risk of ankle injuries compared to hard boots [2,3,5,6]. Additionally, most snowboarders prefer the use of soft boots [3,6,8]. Following this, the aim of the research and development project is to develop of a soft boot-binding Sports Technol. 2009, 2, No. 1 2, 17 23 & 2009 John Wiley and Sons Asia Pte Ltd 17

Research Article concept. The understanding of the biomechanics in snowboarding is crucial for this research project. It is essential to gain knowledge concerning specific biomechanical parameters, especially with regard to the ankle joint complex, to prevent equipment-related injuries. The literature provides several studies investigating kinetics and kinematics in snowboarding. Corresponding to the determination of kinematical parameters of the ankle joint complex, Delorme et al. [8] analyzed this complex while onslope snowboarding. McAlpine and Kersting [9] ascertained kinetic and kinematical values for the landing phase in freestyle snowboarding. However, scientific studies investigating kinematical parameters of the ankle joint complex in freestyle snowboarding, including turns, flight phases, jumps, and landings, are so far not available. A potential reason can be assigned to limitations of traditionally-used kinematical measurement systems, in particular, to the capture volume. Today, as a result of the ongoing miniaturization of micro electromechanical systems, inertial measurement systems are used in a number of sports disciplines [10 13]. There are several studies showing the usage of such systems in the research of alpine winter sports [14 21]. Harding et al. [20,21] utilized inertial sensors in freestyle snowboarding for the determination of objective key performance variables; for example, air time and degree of rotation. The research of Brodie et al. [16,17] and Supej [18] demonstrate the application of an inertial measurement suit (IMS) for the determination of kinematical parameters in alpine skiing. It specifies advantages over optical methods in terms of capture volume, measurement preparation, and analysis time [18]. Thus, the application of an IMS is a potential method for the determination of kinematical parameters of the ankle joint complex in freestyle snowboarding. Aside from kinematic parameters, kinetic parameters also need to be known. The knowledge of joint angles, as well as forces, is vital for the development of equipment. McAlpine and Kersting [9] measured ground reaction forces of jump landings in freestyle snowboarding by means of a snowboard-mounted force platform. The acquired loads reveal a potential injury risk. However, the mechanical characteristics (weight, standing height) of such a system can affect a snowboarder adversely. Aside from snowboard-mounted force platforms, the application of pressure-sensitive insoles is another possibility to measure ground reaction forces [14]. These systems are limited to the measurement of the normal force, which acts perpendicular to thebootsole.duetothelowweightandsmallsizeofthese systems, the application of a bilateral insole measurement system is a potential method for the determination of kinetic parameters in freestyle snowboarding. The accuracies of the IMS and the insole measurement system have to be known in order to draw conclusion from the measured parameters for the current research and development project. Thus, the aim of this study is divided into three parts: (i) to assess the accuracy of a commercially-available IMS system in a field-based setting; (ii) to assess the accuracy of a bilateral insole measurement system against a laboratorybased force plate; and (iii) to collect and analyze data with these systems during the on-snow performance of one freestyle snowboarding maneuver in order to provide important biomechanical information necessary for the development of enhanced snowboarding equipment. 2. METHOD A. Krüger and J. Edelmann-Nusser 2.1 Evaluation of the IMS in a Field-Based Setting The accuracy of an IMS depends on the specific application and the sensor fusion algorithm used by the system [22]. A quantitative validation of a full-body IMS with respect to the achievable measurement accuracy for application in a real snowboarding environment is not yet available. Due to the severe conditions in snowboarding, an evaluation in a laboratory is not sufficient. On that account, the determination of the measurement accuracy was conducted in a real snowboarding environment. Given the common application of optical video-based systems for kinematic analyses in snowboarding and alpine skiing, it was chosen as reference system for evaluation. For that purpose, a snowboarder equipped with a full body IMS (Moven, Xsens Technologies, Enschede, The Netherlands [23]) performed a test run on a well-prepared slope (Figure 1a). The system consists of 16 sensor units and two transmission units. The sensor units include gyroscopes, accelerometers, as well as magnet field sensors [24], which are placed in a suit worn by the snowboarder. The sensor units Figure 1. Setup for the determination of the accuracy of the IMS using a traditionally used optical camera based system and a differential GPS system (a) and the computed 3-D model (b). 18 www.sportstechjournal.com & 2009 John Wiley and Sons Asia Pte Ltd Sports Technol. 2009, 2, No. 1 2, 17 23

were fixed to the boots, lower legs, upper legs, pelvis, shoulders, head, upper arms, forearms, and hands of the test person. The sample rate was 120 Hz. Based on the measured sensor data, the software (Moven Studio 2.1, Xsens Technologies, the Netherlands) computed a 3-D model of the snowboarder (Figure 1b). A second-order, low-pass filter was used with cutoff frequencies of 10 Hz (pelvis) and 20 Hz (other segments). According to the disadvantages of the IMS for the application in skiing [18], a differential GPS (DGPS) system (GPS1200, Leica Geosystems, Heerbrugg, Switzerland) was additionally employed to determine the absolute position of the snowboarder in space (Figure 1a). The system measured the trajectory of the snowboarder with a sample rate of 20 Hz. The calculated accuracy of the measured data is 1 4 cm (LGO software, Leica Geosystems, Germany). Due to the weight and size of the DGPS system, the evaluation was conducted on a prepared slope to ensure the safety of the snowboarder. A video-based kinematical analysis was carried out in order to determine the accuracy of the IMS. For this purpose, one turn of the test run was filmed by three synchronized cameras (50 Hz). The capture volume of the video-based system was limited to 1.5 m of the turn. Passive markers were placed at anatomical landmarks of the snowboarder (Figure 1a). The markers were manually digitized in the lab using Simi Motion software (Simi Reality Motion Systems, UnterscleiXheim, Germany). Subsequently, the software computed 3-D coordinates of the markers and a 3-D model of the snowboarder. The data were filtered using a moving average filter (n 5 3). The analysis of the ankle joint complex was limited owing to poor visibility of the snowboard boot markers. For this reason, the knee angle was chosen as the parameter and used for the evaluation. To determine the accuracy of this setting, the distance between two markers placed at the knee and hip was measured and compared to the computed data. A maximum error of 3 cm was found. The DGPS data and the data of the optical system were interpolated to a frequency of 120 Hz. The GPS data were linked to the data of the IMS. The manufacturer s sensor fusion algorithm (Moven Studio 2.1, Xsens Technologies, the Netherlands) calculated the sensor kinematics of each sensor unit and translated the kinematics to segment kinematics of a 3-D model (Figure 1b). The detection of contact points with the surroundings and supplementary information from the DGPS system were used to correct the computed model [25]. Knee angles of the computed 3-D model of the snowboarder were subsequently calculated. To quantify the results, the mean deviations between the data of the optical video-based measurement system, the IMS, as well as the combined usage of the IMS and a DGPS system were computed. Winterthur, Switzerland) as the reference system. For that purpose, 10 trials with 10 consecutive squats of a test person equipped with snowboard boots, bindings, and a board similar to the equipment used in the field test were analyzed using both systems simultaneously. Furthermore, the clamping force of the binding system was measured by the insole measurement system. The weight of the snowboard equipment was metered by the force plate. All data were captured at a sampling frequency of 120 Hz. The mean force values for each trial were calculated. Subsequently, the root mean square error (RMSE) of the bilateral insole measurement system was computed. 2.3 Data Collection in Freestyle Snowboarding The investigation was carried out during the winter of 2007/2008 in Kappl (Austria). An experienced recreational snowboarder (n 5 1, male, 28 years, 73 kg, snowboarding for 16 years) performed a single test run (3601 indie grab) in a prepared snowboard park (Figure 2). The size of the park jump was approximately 8 m. The IMS (Moven, Xsens Technologies, the Netherlands) was used to measure kinematical parameters at a sample rate of 120 Hz. The set-up time, including the calibration procedure, was approx. 15 min. Additionally, the pressure distribution on the plantar surface was measured applying the bilateral insole measurement system (T&T Medilogic, Germany) with 120 Hz. The total weight of the complete measurement equipment was approximately 2.2 kg. The measured data were sent wirelessly to data loggers placed beside the track. Data synchronization was established by a distinctive movement of the snowboarder. The test run was additionally filmed. The ankle joint axes were simplified as a set of three orthogonal axes. To determine the angle of the ankle joint, the joint rotation of the foot with respect to the shank was calculated using the orientations of the segments given by the IMS. The normal force (forefoot and heel) was calculated from the pressure distribution data. The IMS and the insole measurement system provided the necessary data right after the test run. The calculation of the joint orientation and the normal force using these data took approx. 10 min. 2.2 Evaluation of the Bilateral Insole Measurement System in a Laboratory The bilateral insole measurement system (T&T Medilogic, Scho nefeld, Germany [26]) consists of two insoles, two amplifiers, a wireless data transmission unit, and a data logger. Each insole includes 64 pressure sensors and was inserted in the athlete s snowboard boot. The accuracy of the system was tested in a laboratory environment using a force plate (Kistler, Figure 2. Snowboarder performing the aerial maneuver (3601 with an indie grab). Sports Technol. 2009, 2, No. 1 2, 17 23 & 2009 John Wiley and Sons Asia Pty Ltd www.sportstechjournal.com 19

Research Article 3. RESULTS 3.1 Evaluation of the Full-Body IMS The accuracy of the full-body IMS is moderate with regard to the optical video-based system. The results of the inertial measurement exhibit slightly different knee angles, especially for the left leg (Figure 3). The mean deviation between both systems for the left leg and the right leg are 4.81 (70.31) and 3.11 (71.71), respectively. At the same time, high correlation coefficients (left knee angle r 5 0.96; right knee angle r 5 0.77) are observed. No differences in the measured angles between the IMS and the combined usage with a DGPS system are found. Thus, the IMS can be used solely to measure joint angles. With respect to the calculation of the absolute position in space of the snowboarder, the IMS shows a drawback. By solely using the IMS, the determination of the snowboarder s position is not possible. Therefore, additional systems, such as DGPS, must be utilized [18]. 3.2 Evaluation of the Bilateral Insole Measurement System The accuracy of the bilateral insole measurement system is limited. The result of the measurements represents a mean RMSE of 28% (76.6%) for 10 trials (Figure 4). The test person reported no adverse effects. 3.3 Data Collection in Freestyle Snowboarding The measurement results reveal ranges of movement and normal forces for all phases of the performed freestyle snowboarding test run (Figure 5). Internal rotations of 2 111 for the front leg and up to 151 external rotation for the back leg are measured in the inrun phase. In the initiation phase of the jump, the snowboarder pushes off actively. Thus, an increased normal force of 930 N (1.2 body weight, front leg) and 1080 N (1.35 body weight, back leg) is detected. Furthermore, an increased plantar flexion of 121 of both legs, an internal rotation of 411 (front leg) and an external rotation of 321 (back leg) are measured, which is caused by the rotation of the upper body around the longitudinal axis. Due to the performed aerial maneuver, comparatively high peak values, in particular for the front leg, are identified (Table 1). At the end of the flight phase, the snowboarder is in a neutral position to ensure a safe landing (Table 1). Throughout the landing phase, the snowboarder leans towards the tail of the snowboard. Thus, an increased inversion of 251 is measured for the front leg. In contrast, the back leg shows an eversion of 151. In addition, an increased normal force of 3020 (3.8 body eight) is detected at the back leg compared to the 920 (1.2 body weight) measured at the front leg (Figure 5). 4. DISCUSSION 4.1 Evaluation of the Full-Body IMS A. Krüger and J. Edelmann-Nusser It is important to consider that the presented accuracy of the IMS is determined in reference to an optical video-based system. Given the common application of such systems for kinematic analyses, it is chosen as reference system. A reason for the found deviations can be a systematic error, as both systems resulted in high correlations of the knee angle data. This might be caused by the misalignment of the optical system s marker and the calibration procedure of the IMS. As a consequence of the poor visibility of the optical boot markers in the capture volume of the cameras, the knee angles instead of the ankle joint angles are validated. However, the determination of both joint angles is based on equal sensors and the same algorithms of the IMS. Due to soft snow conditions, no strong vibrations potentially influencing the measurements are observed. In addition, the validity of the ankle joint angle data is confirmed by agreement between the calculated 3-D model and the video data, as performed by Figure 3. Calculated knee angles of the left and the right leg measured by means of the IMS (inertial) and the optical system (optical). Figure 4. Calculated mean forces of 10 trials measured by means of a force plate and the insole measurement system. 20 www.sportstechjournal.com & 2009 John Wiley and Sons Asia Pte Ltd Sports Technol. 2009, 2, No. 1 2, 17 23

Figure 5. Ankle joint angles and normal force of the front (left) leg of the snowboarder for a complete test run. Gray and white background indicates the phases of the test run (start, inrun, flight phase, ground contact); dashed lines indicate start and end of heel-side (H) or toe-side (T) turns. EV, eversion; IV, inversion; PF, plantar flexion; DV, dorsiflexion; IR, internal rotation; ER, external rotation; F, normal force. Source of illustration describing the rotations of the ankle joint [30]; with permission. Table 1. Peak values of the rotation of the ankle joint complex for each phase of the test run. Rotation Leg Inrun (1) Take off (1) Flight phase (1) End-of-flight phase (1) Ground contact (1) PF/DF Front 5 PF 30 DF 12 PF 25 PF 7 PF 4 DF Back 3 34 DF 12 PF 14 PF 14 PF 25 DF EV/IV Front 7 IV 2 EV 11 IV 33 IV 8 IV 25 IV Back 13 26 IV 30 IV 7 IV 9 IV 15 EV IR/ER Front 2 11 IR 41 IR 26 IR 6 IR 11 ER Back 3 IR 15 ER 32 ER 13 ER 2 IR 8 ER DF, dorsiflexion ER, external rotation; EV, eversion; IR, internal rotation; IV, inversion, PF, plantar flexion. Brodie et al. [17]. Therefore, it is assumed that the rotations of the ankle joint have the same measurement accuracy as the knee angle data. On account of the recommended combination of an IMS, and for example, a GPS system [18], a DGPS system is used additionally. Unfortunately, the DGPS system affects the snowboarder adversely. Therefore, the evaluation study is not conducted in the snowboarding park, but on-slope to ensure the safety of the subject and the measurement equipment. However, the results show that additional data have no influence on the posture of the calculated 3-D model of the IMS. Joint angles can be determined by using the IMS solely. However, it is important to keep in mind that the accuracy of the IMS depends on the conditions while snowboarding [18,22]. Thus, the accuracy of the data might vary in the different phases of a run, particularly in the landing phase. Considering these facts, further studies have to be conducted to evaluate the IMS, in particular for the landing phase and the ankle joint complex. If only joint angles are of interest, an evaluation can be conducted in a snowboard park without any additional systems (e.g. DGPS). 4.2 Evaluation of the Bilateral Insole Measurement System The arrangement of the sensor elements in the insole, as well as the mechanical characteristics of the snowboard boot, can be considered as potential influencing factors. Due to the comparable high RMSE, the data of the insole measurement system are only used to determine the phases of the test run and to estimate acting loads. Nevertheless, the application of the system in the current research and development project for data collection in freestyle snowboarding is considered to be useful. In particular, the lack of any adverse effects is beneficial. 4.3 Data Collection in Freestyle Snowboarding The measurement results reveal a potential injury risk in the landing phase. Boon et al. [27] found a 201 dorsiflexion, 101 inversion, and 01 201 external rotation with an applied axial load of 2200 8900 N, which was required to produce fractures in nine out of 10 specimens. In addition, Sports Technol. 2009, 2, No. 1 2, 17 23 & 2009 John Wiley and Sons Asia Pty Ltd www.sportstechjournal.com 21

Research Article Funk et al. [28] subjected 10 cadaveric leg specimens to dynamic inversion and eversion of 48 621 with an axial load of 2500 N in a dorsiflexed position, whereby producing injuries in nine out of 10 specimens. Considering these results, the measured values in the landing phase (in particular, 251 dorsiflexion; 81 external rotation; 3020 N load) may be sufficient to cause an ankle injury, in particular, at the snowboarder s back leg (Table 1). It is important to consider that the loading time was shorter than 0.1 s. Cadaver studies show that the injury threshold of human structures increases up to 17% and 25% applying rapid loading times of 0.1 s and 0.01 s, respectively [29]. Considering the increased threshold and assuming a similar behavior of the ankle joint complex, the measured loads still have the potential to cause injuries. Similar results were found by McAlpine and Kersting [9] (for the investigated front leg). They measured higher peak values of 3.74 to 4.79 body weight during the impact. A reason for the lower normal peak force of the front leg compared to the data presented by McAlpine and Kersting [9] is deemed to the backward leaning of the snowboarder and the limitations of the insole measurement system. With respect to the internal and external rotation of the ankle joint in the inrun, the measured values differ to the values found in literature. Specifically, the internal rotation of the front leg is observed to be 3 231 higher than that measured by Delorme et al. [8]. A potential reason for that is the varied binding stance. In the current study, a binding stance of 121 (front leg) and 61 (back leg) is chosen by the snowboarder. In contrast, the binding stance in the investigation of Delorme et al. [8] was set to 211 (front leg) and 61 (back leg). Due to less rotated bindings, as well as the turning of the snowboarder s trunk towards the nose of the snowboard, the joint ankle complex might show an internal rotation for the front leg and a higher external rotation at the back leg ankle joint. 5. CONCLUSION This pilot study illustrates the application of an IMS in combination with a bilateral insole measurement system for biomechanical analyses in freestyle snowboarding. The main advantage of this set up is the measurement of kinetic and kinematical data without limitations with regard to a restricted capture volume. However, it is important to realize that the accuracy, in particular of the kinetic data measured by the bilateral insole measurement system, is limited. Referring to the athlete the adverse effects caused by the weight and the mechanical characteristics of the IMS, as well as the insole measurement system, are almost negligible. Thus, the application of an IMS in combination with an insole measurement system in freestyle snowboarding is considered to be beneficial. The systems can provide useful information to improve the understanding of the biomechanics in freestyle snowboarding. The measured data reveal ankle joint angles and loads, which have the potential to cause injuries. In the context of the current research and development project, the measured data provide valuable information. The knowledge of the estimated acting loads and the range of movement of joints will be implemented in the current design process. This is a vital source of information for the definition of the requirement list. The measurement results are going to be integrated in the requirement list for the development of the new safety snowboard binding concept. Further studies are planned to collect data with a higher sample of test runs and snowboarders. Acknowledgements This research was funded by the Arbeitsgemeinschaft industrieller Forschungsvereinigung (AiF) Otto von Guericke e.v. The authors would like to thank Kappl ski resort and Fabian Wolfsperger. REFERENCES A. Krüger and J. Edelmann-Nusser 1. Abu-Laban RB. Snowboarding injuries: An analysis and comparison with alpine skiing injuries. Canadian Medical Association Journal 1991; 145: 1097 1103. 2. Davidson TM, Laliotis AT. Snowboarding injuries: A four-year study with comparison with alpine ski injuries. The Western Journal of Medicine 1996; 164: 231 237. 3. Kirkpatrick D, Hunter R, Janes P, Mastrangelo J, Nicholas R. The snowboarder s foot and ankle. The American Journal of Sports Medicine 1998; 26: 271 277. 4. Made C, Elmqvist LG. A 10-year study of snowboard injuries in Lapland Sweden. Scandinavian Journal of Medicine and Science in Sports 2004; 14: 128 133. 5. Pino EC, Colville MR. Snowboard injuries. The American Journal of Sports Medicine 1989; 17: 778 781. 6. Bladin C, Giddings P, Robinson M. Australian snowboard injury data base study a four year prospective study. The American Journal of Sports Medicine 1993; 21: 701 704. 7. Sutherland AG, Holmes JD, Myers S. Differing injury patterns in snowboarding and alpine skiing. Injury 1996; 27: 423 425. 8. Delorme S, Tavoularis S, Lamontagne M. Kinematics of the ankle joint complex in snowboarding. Journal of Applied Biomechanics 2005; 21: 394 403. 9. McAlpine P, Kersting U. Development of a field testing protocol for the biomechanical analysis of snowboard jump landings a pilot study. In: Schwameder H, Strutzenberger G, Fastenbauer V, Lindinger S, Mu ller E, (eds). Proceedings of the XXIV International Symposium on Biomechanics in Sports, pp. 79 82, University of Salzburg; Salzburg, 2006. 10. Mayagoitia RE, Nene AV, Veltink PH. Accelerometer and rate gyroscope measurement of kinematics: an inexpensive alternative to optical motion analysis systems. Journal of Biomechanics 2002; 35: 537 542. 11. King K, Yoon SW, Perkins NC, Najafi K. The dynamics of the golf swing as measured by strapdown inertial sensors. In: Hubbard M, Mehta RD, Pallis JM, (eds). The Engineering of Sport 5, Vol. 2, pp. 276 282, ISEA; Sheffield, 2004. 12. James DA. The application of inertial sensors in elite sports monitoring. In: Moritz EF, Haake S, (eds). The Engineering of Sport 6, Vol. 3, pp. 289 294, Springer; New York, 2006. 13. Tsunoda K, Sasaki T, Hoshino H, Ichikawa K. Development of data acquisition system for dynamic analysis in ski jumping. In: Mu ller E, Lindinger S, Sto ggl T, Fastenbauer V, (eds). Abstract Book of the 4th International Congress on Science and Skiing, pp. 178, University of Salzburg; Salzburg, 2007. 14. Kru ger A, Edelmann-Nusser J, Spitzenpfeil P, Huber A, Waibel KH, Witte K. Zum Einsatz eines Fussdruck- und Inertialmesssystems im alpinen Skirennlauf [Pilot study about the use of a foot pressure and inertial measuring system in ski racing]. Leistungssport 2007; 37(5): 44 47. 15. Brodie M, Walmsley A, Page W. Force vector analysis of ski racing technique using fusion motion capture. In: Menzel HJ, Chagas MH, (eds). 22 www.sportstechjournal.com & 2009 John Wiley and Sons Asia Pte Ltd Sports Technol. 2009, 2, No. 1 2, 17 23

Proceedings of the XXV International Symposium on Biomechanics in Sports, pp. 71 74, Ouro Preto, Brazil, 2007. 16. Brodie M, Walmsley A, Page W. How to ski faster: Art or science? In: Mu ller E, Lindinger S, Sto ggl T, (eds). Science and Skiing IV, pp. 162 174, Meyer & Meyer Sport; UK, Maidenhead, 2009. 17. Brodie, M, Walmsley A, Page W. Fusion motion capture: a prototype system using inertial measurement units and GPS for the biomechanical analysis of ski racing. Sports Technology 2008; 1: 17 28. 18. Supej M. A step forward in 3D measurements in alpine skiing: A combination of an inertial suit and DGPS technology. In: Mu ller E, Lindinger S, Sto ggl T (eds). Science and Skiing IV, pp. 497 504, Meyer & Meyer Sport; UK, Maidenhead, 2009. 19. Wägli A, Skaloud J. Turning point trajectory analysis for skiers. InsideGNSS 2007; 3: 24 34. 20. Harding JW, Toohey K, Martin DT, Mackintosh CG, Lindh AM, James DA. Automated inertial feedback for half-pipe snowboard competition and the community perception. In: Fuss FK, Subic A, Ujihashi S, (eds). The Impact of Technology on Sport II, pp 845 850, Taylor & Francis; London, 2007. 21. Harding JW, Mackintosh CG, Martin DT, Hahn AG, James DA. Automated scoring for elite half-pipe snowboard competition: important sporting development or techno distraction? Sports Technology 2008; 1, No. 6, in press. 22. Brodie M, Walmsley A, Page W. Fusion motion capture: accuracy of motion capture without cameras for alpine skiing. In: Mu ller E, Lindinger S, Sto gglt,fastenbauerv,(eds).abstract Book of the 4th International Congress on Science and Skiing, pp. 128, University of Salzburg; Salzburg, 2007. 23. Moven. Wireless Inertial motion Capture. 2009. Available: http://www. moven.com/en/home_moven.php [accessed March 2009]. 24. Xsens. MTx. 2008. Available: http://www.xsens.com/en/products/human_ motion/mtx.php [accessed March 2009]. 25. Roetenberg D, Luinge H, Slycke P. Moven: Full 6DOF human motion tracking using miniature inertial sensors (technical whitepaper). 2008. Available: Moven Studio 2.1 software documentation. 26. Medilogic. Foot pressure measurement system. 2009. Available: http:// www.medilogic.de/index.php?id 5 48&L 5 1 [accessed March 2009]. 27. Boon A, Smith J, Zobitz M, Amrami K. Snowboarder s Talus Fracture. The American Journal of Sports Medicine 2001; 29: 333 338. 28. Funk JR, Srinivasan SC, Crandall JR. Snowboarder s talus fractures experimentally produced by eversion and dorsiflexion. The American Journal of Sports Medicine 2003; 31: 921 927. 29. Hauser W, Asang E. StoX- und Schlagbelastung des menschlichen Schienbeins im Drehversuch. Archives of Orthopaedic and Trauma Surgery 1979; 93: 125 131. 30. Lamontagne M, Delorme S, Tavoularis S. Biomechanics of the ankle joint complex in snowboarding. Oral presentation at the 4th International Congress on Science and Skiing, St. Christoph am Arlberg, Austria, Dec. 2007. Received 8 December 2008 Revised 6 April 2009 Accepted 6 April 2009 Sports Technol. 2009, 2, No. 1 2, 17 23 & 2009 John Wiley and Sons Asia Pty Ltd www.sportstechjournal.com 23