A marker has to be seen by at least two cameras for its three-dimensional (3-D) reconstruction, and the accuracy can be improved with more cameras. However, a change in the set of cameras used in the reconstruction can alter the kinematics. The purpose of this study was to quantify the harmful effect of occlusions on two-dimensional (2-D) images and to make recommendations about the signal processing. A reference kinematics data set was collected for a three degree-of-freedom linkage with three cameras of a commercial motion analysis system without any occlusion on the 2-D images. In the 2-D images, some occlusions were artificially created based on trials of real cyclic motions. An interpolation of 2-D trajectories before the 3-D reconstruction and two filters (Savitsky–Golay and Butterworth filters) after reconstruction were successively applied to minimize the effect of the 2-D occlusions. The filter parameters were optimized by minimizing the root mean square error between the reference and the filtered data. The optimal parameters of the filters were marker dependent, whereas no filter was necessary after a 2-D interpolation. As the occlusions cause systematic error in the 3-D reconstruction, the interpolation of the 2-D trajectories is more appropriate than filtering the 3-D trajectories.
Mickaël Begon and Patrick Lacouture
Patrice Holvoet, Patrick Lacouture and Jacques Duboy
The aim of this study was to objectively predict individual improvements in a release-regrasp tkatchev skill. The prediction was based on a kinematic analysis of failed and successful trials. The modification of release conditions, and the correction of hip and shoulder joint motions during the aerial phase of failed trials, were determined by considering the successful trials as target executions. Computer simulations were used to confirm the effect of the corrected parameters on the flight trajectory and angular motion of the body over the bar. The results indicated that when time of release is initiated earlier, this presents a major problem the gymnast must overcome in order to grasp the bar. Moreover, the moment when the body’s center of gravity is vertically above the bar represents a critical instant for the gymnast in initiating the hip and shoulder movements. The rotation motion analysis of the segments indicated that the stabilization motion of the upper limbs could be a good strategy for improving the failed tkatchev. This study showed that simple computer simulation using hypothetical data based upon real data could be an effective tool for improving acrobatic skills.