INTRODUCTION: Recent studies have investigated the mechanical energy (ME) fluctuations of the center of mass (CoM) in cross-country (XC) skiing techniques (1,2). The most used method of CoM ME calculation in human movement is by use of force platforms to record ground reaction forces (GRF). This method, however, fails to include the rotational component of the kinetic energy (Ekin-r), which arguably is important in XC skiing, especially in double poling (DP). The present study compared the use of force platforms to that of the sum of segmental energy calculations, both with and without Ekin-r. METHOD: Nine XC skiers double poled at four increasing exercise intensities on a DP ski ergometer equipped with a force cell while standing on a Kistler force plate. Total dynamic ME of CoM (DME) was calculated as the sum of translational kinetic (DEkin-t) and gravitational potential (DEpot) energy, using total force acting on the participants calculated as the sum of GRF and poling force in the horizontal and vertical directions. Simultaneously, Qualisys motion capture analysis was used to capture characteristics of six segments (foot, shank, thigh, trunk+head, upper arm, forearm). Mass, moment of inertia, and position of each segments' CoM was calculated according to de Leva (3). Total segmental ME was calculated as the sum of all segments' Ekin-t, Ekin-r and Epot, both with (SMER) and without (SME) Ekin-r. RESULTS: External work increased from 157 J per cycle at low to 214, 257 and 316 J across intensities. Total ME fluctuations within the DP cycle yielded similar values when using DME and SME calculation and ranged from 127 J at low to 149, 174 and 220 J at maximal intensity (Fig. 1). Rotational kinetic energy contributed strongly to kinetic energy fluctuations (Fig. 1 top). However, SMER maximal total energy fluctuations were only about 6 J higher than DME and SME (all p<.05). This is because fluctuations in Ekin were mostly out-of-phase with, and smaller than Epot. Still, rate of change of ME during propulsion, and thus rate of exchange to external work are affected by accounting for Ekin-r (p<0.05). DISCUSSION & CONCLUSION: The present study shows that when examining ME fluctuations in the DP technique or other movement forms with a significant rotational component, care should be taken with regard to what method of calculating the ME is used (4). Ignoring rotational energy may lead to erroneous conclusions. --- 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.