INTRODUCTION: Human locomotion can be analyzed by computing the mechanical work necessary to sustain it. Starting from the knowledge of the center of mass (COM) displacement it is possible to calculate work associated to the potential and kinematic energy changes. For walking and running this approach has allowed to identify the models associated to these forms of locomotion as well as to understand the transfer between these energy forms. The aim of this study was to analyze the mechanical work associated to COM motion in cross-country skiing for the diagonal stride (DS) and the double poling (DP= techniques. METHOD: Six cross-country skiers skied with roller-skis on a treadmill at 2° and at 5 different speeds, from 6 km/h to 14 km/h. They were asked to use DP and DS. The 3D movement of COM was calculated from the coordinates of 12 body Segments acquired by means of an optoeletronic motion capture system (Qualisys). The COM curve was low pass filtered and used to calculate the potential (PE), vertical (KEv) and forward (KEf) kinetic energy curves. The mechanical work for vertical (WPE+KEv) and forward (WKEf) motion was computed over 15 cycles as the sum of positive increments of the respective energy curves and was expressed per unit of body mass and distance. The "percent recovery" that account for the interchange between these energies was also calculated. Significant differences were checked by a two way (technique x speed) ANOVA. RESULTS: No significant differences in WKEf were found between techniques; however, WPE+KEv for vertical motion was found to be significantly higher for DP (WKEf= 0.28+0.08 J/kg*m) compared to DS (0.46 ±0.07 J/kg*m). Increments of speed caused a significant and linear increment of (WKEf) while no effect was shown for (WPE+KEv) in both techniques. Percent recovery was found to be of about 55% for DP and between 15 and 30% for DS. While the fluctuations of the curves for PE+KEv are roughly in phase with the KEf curves for DS, they are substantially in phase opposition for DP. Percent recovery showed no variation across speed for DP while for DS a significant and linear decrease with the speed was found (%rec = 27.2±2.2% at 6 km/h and for %rec = 14.9±0.5% at 14 km/h). DISCUSSION: Even if the work done for moving the COM in the forward direction is not different between techniques, DP required a higher mechanical work to rise the center of mass in the vertical direction with respect to DS. The most evident difference between techniques concerned the transfer between PE+KEv and KEf that was found to be higher for DP, as accounted by the value of %rec, and to reach a value comparable to that commonly found in walking. The decrease of energy transfer observed with increasing speed for DS could be considered as a possible determinant for the abandoning of DS at high speed. CONCLUSION: In this study we applied a methodology traditionally being used to analyze human locomotion to cross-country skiing. The results showed that this approach is useful in the analysis of the peculiarity of the different techniques in regard to the mechanical work done.
© Copyright 2010 Book of Abstracts. 5th International Congress on Science and Skiing, Dec. 14 - 19, 2010, St. Christoph am Arlberg. Published by University of Salzburg, Interfakultärer Fachbereich Sport- und Bewegungswissenschaft/USI. All rights reserved.