INTRODUCTION: The external forces acting on a skier in alpine skiing include gravitational, air drag, and snow reaction forces. A skier controls trajectory and speed primarily through regulating the ski's interaction with the snow and, thereby, controlling the size and orientation of the snow reaction force. The purpose of this investigation was to gain further insight into skier technique through examining the magnitude and timing of the external forces acting under competitive conditions and to describe the relative contributions of each external force towards skier mechanical energy changes. METHOD: Skier kinematic data from a previous investigation (Reid et al., 2009) were used to estimate the external forces acting on six highly-skilled skiers during slalom race simulations on courses with 10 and 13 m linear gate distances. Skier 3-D positions were reconstructed using 4 panning cameras and control points distributed near the course. Center of mass resultant acceleration (acom) was calculated using finite central differences. Air drag force (FD) was modeled according to Gilgien (2008) and gravitational force (FG) was calculated knowing ski and equipment masses (m) as well as the line of action of gravity. Finally, the equation of motion was rearranged for the snow reaction force, FR = maCOM - FD - FG. Mechanical work of each external force was calculated as the line integral of the dot product between the force and center of mass velocity vectors. Skier mechanical energy dissipation (EDiss) was calculated as described by Supej (2008). RESULTS & DISCUSSION: Maximal snow reaction forces were greater than 3000 N and 3.5 bodyweights on both courses. Timing of the snow reaction forces was different between courses. On the 10 m course, large reaction forces were generated at about 50% of the turn cycle, or approximately gate passage. With 13 m gates, peak forces were delayed, occurring after gate passage at about 65% of the turn cycle. Air drag force was larger on the 13m course due to increased skier speeds. High energy dissipations occurred near mid-turn and low dissipations during the transition between turns. Negative dissipations—where skier kinetic energy increased by more than what can be attributed to changes in skier potential energy—were observed near the transitions between turns. This may be evidence of skiers increasing kinetic energy through muscular work, although this represented less than 3% of skier kinetic energy gains. About 20% of skier total mechanical energy loss was due to air drag. The remaining, majority of mechanical energy loss was due to a drag component of the snow reaction force. Peaks in energy dissipation occurred frequently at gate passage and may be associated with the work required to accelerate the gate shaft out of the skier's path CONCLUSION: Large snow reaction forces were observed of 3 to 4 times body weight. Energy losses were primarily through snow reaction force. However, the energy cost of gate clearance may also play a significant role and needs to be accounted for in future work.
© Copyright 2012 Science and Skiing V. 5th International Congress on Science and Skiing, Dec. 14 - 19, 2010, St. Christoph am Arlberg. Published by Meyer & Meyer Sport (UK) Ltd.. All rights reserved.