INTRODUCTION: The dynamics of an athlete's center of mass (COM) is central for calculating external work and energy fluctuations (Kehler et al. 2014). However, accurate measurements of COM dynamics are usually restricted to a laboratory environment. To investigate the possibility for simple COM measurements in field, a Sacrum model (SM) has been proposed for ski skating (Myklebust et al. 2013). However, results were satisfactory only in the medio-lateral direction. Here we augment SM with measurements of pelvis orientation in addition to translation. Our hypothesis is that such a model can be used to predict COM displacement also in the sagittal plane. METHOD: Six elite skiers volunteered for roller-skiing on a treadmill using the V2 technique. Their movement patterns were recorded using (1) a motion capture (MOCAP) system that recorded the trajectories of 41 markers distributed over the athletes' skin and equipment, and (2) an IMU consisting of a 3-axis accelerometer and gyroscope positioned at the Sacrum. The MOCAP-system enabled calculation of the COM using a 14-segment model (Myklebust et al. 2013). The IMU allowed computation of Sacrum translation r and orientation "Theta". Considerations from a simple 3-segment model (Fig. 1a), along with a 1. order Taylor expansion suggested that the COM displacement vector could be estimated by eq. (1) (axis convention shown in Fig. 1a). The parameters C1 to C5 were calculated by minimizing the RMS-difference between the COM displacement of the 14-segment model and the IMU-model using the Nelder-Mead algorithm. Finally, the model was validated on the same six athletes using a jackknife-approach. RESULTS: Displacement curves for MOCAP model, SM, and IMU model is shown in Figure 1b. RMS deviation between MOCAP and IMU model averaged over all athletes was < 11 mm in the sagittal plane. DISCUSSION & CONCLUSION: The presented model substantially improves estimated COM displacement, and suggests that in-field measurements of COM displacement with minimal equipment are possible. The validity of the model for higher order derivatives (e.g. COM acceleration), and for other level of skiers, needs investigation. In summary, combining measurements of sacrum translation and orientation substantially improves estimates of COM displacement compared to a sacrum translation model only. This suggests that very simple measurement setups can estimate highly important kinematics of skiers. --- The hosting University of Jyväskylä is planning to publish conference proceedings "Science and Nordic Skiing III". In case you are interested in this publication please contact the editors (Anni Hakkarainen anni.s.j.hakkarainen@jyu.fi) to become registered for the book.
© Copyright 2015 3rd International Congress on Science and Nordic Skiing - ICSNS 2015. 5-8 June 2015, Vuokatti, Finland. Veröffentlicht von University of Jyväskylä; University of Salzburg. Alle Rechte vorbehalten.