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. 2013 Aug 2;13(8):9821-35.
doi: 10.3390/s130809821.

Determination of external forces in alpine skiing using a differential global navigation satellite system

Matthias Gilgien  1 , Jörg Spörri, Julien Chardonnens, Josef Kröll, Erich Müller
Affiliations

Determination of external forces in alpine skiing using a differential global navigation satellite system

Matthias Gilgien et al. Sensors (Basel). .

Abstract

In alpine ski racing the relationships between skier kinetics and kinematics and their effect on performance and injury-related aspects are not well understood. There is currently no validated system to determine all external forces simultaneously acting on skiers, particularly under race conditions and throughout entire races. To address the problem, this study proposes and assesses a method for determining skier kinetics with a single lightweight differential global navigation satellite system (dGNSS). The dGNSS kinetic method was compared to a reference system for six skiers and two turns each. The pattern differences obtained between the measurement systems (offset ± SD) were -26 ± 152 N for the ground reaction force, 1 ± 96 N for ski friction and -6 ± 6 N for the air drag force. The differences between turn means were small. The error pattern within the dGNSS kinetic method was highly repeatable and precision was therefore good (SD within system: 63 N ground reaction force, 42 N friction force and 7 N air drag force) allowing instantaneous relative comparisons and identification of discriminative meaningful changes. The method is therefore highly valid in assessing relative differences between skiers in the same turn, as well as turn means between different turns. The system is suitable to measure large capture volumes under race conditions.

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Figures

Figure 1.
Figure 1.
Illustration of the experimental setup at turn eight. The gates are plotted in blue, the calibration points for the video-based 3D kinematic system in black, skier CoM trajectory in red, the analyzed area in yellow, the analyzed turn cycle in green and the analyzed turning phase in pink.
Figure 2.
Figure 2.
Left side: The pendulum model approximating the CoM in the dGNSS model. CoM position (⊗), antenna position (□). Right side: Skier with GNSS antenna mounted on the helmet.
Figure 3.
Figure 3.
(a) Illustration of the reduced skier amplitude (D) applied for the drag area calculation. PGNSS (□), intersection point of the pendulum and the snow surface (PSKI); (b) Illustration of the ski friction force (FF) calculation. The direction of FF is defined by the vertical projection of the velocity vector (vdGNSS) onto the snow surface (vdGNSS'). FF is finally calculated by projection of the ground reaction (FGRF,dGNSS) onto vdGNSS'.
Figure 4.
Figure 4.
Comparison of the ground reaction force computed from the dGNSS method (FGRF,dGNSS, solid line) and the reference system (FGRF,REF, thick dashed line) with their standard deviations (FGRF,dGNSS, gray area; FGRF,REF, thin dashed lines) in the upper part of the graph. The bottom part of the graph shows the instantaneous average vectorial difference (solid line) and its instantaneous AVD-Within-SD (dashed lines) across the turn cycle. Gate passage and the points where the turn radius is less than 30 m are marked as turn start and turn end.
Figure 5.
Figure 5.
Comparison of the ski friction force computed from the dGNSS method (FF,dGNSS, solid line) and the reference system (FF,REF, thick dashed line) with their standard deviation (FF,dGNSS, gray area; FF,REF, thin dashed lines) is provided in the upper part of the graph. The bottom part of the graph shows the instantaneous average vectorial difference (solid line) and instantaneous AVD-Within-SD (dashed lines) across the turn cycle. Gate passage and the points where the turn radius is less than 30 m are marked as turn start and turn end.
Figure 6.
Figure 6.
The comparison of the air drag force calculated from the dGNSS method (FD,dGNSS, solid line) and the reference system (FF,REF, thick dashed line) with their standard deviations (FD,dGNSS, gray area; FD,REF, thin dashed lines) is provided in the upper part of the graph. The bottom part of the graph shows the instantaneous average vectorial difference (solid line) and instantaneous AVD-Within-SD (dashed lines) across the turn cycle. Gate passage and the points where the turn radius is less than 30 m are marked as turn start and turn end.
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