Mechanical Energy Dissipation and Performance in Alpine Ski Racing

Abstract

Supej et al. (2005) introduced a method of computing an alpine skier's instantaneous mechanical energy losses to his surroundings as a measure of turn quality. This energy dissipation represents the difference between the change in skier potential energy (EPOT) and the corresponding change in kinetic energy (EKIN). The turning phase of a turn cycle is associated with high levels of energy dissipation. It is thought that one factor differentiating fast and slow skiers may be the pattern of energy loss over a turn cycle. PURPOSE: To determine if fast and slow skiers differ with respect to the distribution of energy dissipation over the turn cycle. METHODS: Six members of the Norwegian men's Europa Cup team were analyzed while skiing through 2 complete turns during a slalom race simulation. Skier 3-D positions were determined using a DLT-based method with 4 panning cameras and calibration control points distributed near the course (Nachbauer et al., 1996). Skier EKIN and EPOT were determined knowing the center of mass velocity and altitude. Energy dissipation was then calculated for each 10% increment in the turn cycle knowing the changes in EKIN and EPOT (Energy dissipation = −(ΔEPOT + ΔEKIN)/m), where m is the skier's body mass. Based on performance time, skiers were divided into two groups, the 3 fastest (FAST) and 3 slowest (SLOW). The average energy dissipation for each 10% turn increment for each group was calculated. RESULTS: The group differences in energy dissipation for each increment varied systematically throughout the turn cycle. The greatest differences were found early in the turn, where FAST skiers had 0.84 J/kg greater dissipation (p=.507), and the later part of the turn phase, where SLOW skiers had 1.24 J/kg greater dissipation (p=.053). The shift of distributions of energy dissipation from early for fast skiers to later for slow skiers was apparent across multiple increments in time, however none were statistically different due to low power associated with the small sample size of elite skiers. CONCLUSIONS: Faster skiers tend to dissipate energy earlier, and slower skiers later in the slalom turn cycle. Although these differences are not statistically significant, they may be practically significant.