A MEASURING METHOD FOR 3D-ANALYSIS OF FOOTPOSITIONS IN FIGURE SKATING

Abstract

INTRODUCTION-The courses of angles in the area of the foot are of essential importance for jumping technique in figure skating (Knoll & Witt, 1994). It is yet not possible to deduce from video-recordings of the whole body - because of limited resolution - an exact statement on footpositions. This results is the inevitable necessity to restrict the picture field to the interesting part of the body - lower leg and foot. The space of movement is, as a rule, the whole icy surface. To track the figure skater the camera must be permanently panned, tilted as well as zoomed. The object-backround of recorded details has almost no additional information for conclusions concerning the actual optical axis. Obviously usual evaluation strategies are not enough and new approaches are required. Our aim was to develop such new approaches and to apply them in the analysis of detail-recording during the European Championships in figure skating in Dortmund in 1995. Consequently the determination of the current orientation of the detail-camera is only possible if the measuring object itself provides calibration informations. This can be done using two pairs of cameras, synchronized with and to each other and a transfer of measurement cascadely. The first camera-pair records, as usual, the figure skater with large format. The analysis of the whole body provides the typical biomechanical parameters as well as objectpoints in the area of the lower legslfeet. Now clear-cut object points in this area serve as reference points for the photogrammetric evaluation of the camera-pair which is concentrated on recording feet. Both pairs of cameras are fixed on a tripod, panned, tilted and zoomed. For calculation we use the photogrammetric evaluation procedures presented by Drenk (1 994). RESULTS- In the training sessions during the European Championship 1995 in Dortmund difficult jumps were recorded with this double-pair camera-configuration. Our defailed analysis of angular courses in the areas of feet was performed successfully. CONCLUSION- In figure skating the assessment of footpositions is of great interest, but when measuring them based on video recordings we were faced with considerable problems. One of our two camera-pairs recorded the area of feet. This pair was connected with a second pair of cameras that was synchronized with the first one and which recorded the usual large format. Using our methods which allow free rotation and zooming of the cameras we were able to develop a special measuring method. The application of this procedure to record and process complicated jumps during the 1995 European Championship in Dortmund was successful. REFERENCES Drenk,V. (1 994). Photogrammetric evaluations procedures for pannable and tiltable cameras of variable focal length. In Proceedings of the 12th International Symposium on Biomechanics in Sports. Budapest-Siofok, July 26,1994, 27-30. Knol1,K. & Witt,M. (1 994). Entwicklung eines Schlittschuhs fur Eiskunstlaufer mit verbesserten Absprungmoglichkeiten im Fungelenk bei gleichzeitiger Sicherung der erforderlichen Stabilitat. lnstitut fur Angewandte Trainingswissenschaft Leipzig .