Kinematic analysis of figure exercises in artistic roller-skating

Obligatory exercises (OE) are key exercises in artistic roller-skating, and thus they are introductory to all other more complex exercises. Therefore, carrying out movement analysis studies on OE is very important. The only available data on OE come from a federal handout (Merlo 2008). In that handout, the movement is explained using a qualitative approach, and the wheel displacements are analysed through signs left by the athlete on the chalk. Amongst the many OE described in the handout, we decided to consider in this study the: -Forward outside bracket (FOB), consisting in a turn performed on one foot with a change of pressure and a clockwise or counter-clockwise body rotation in a sense opposite to that of the movement. -Backward inside bracket (BIB), following the same principles of the FOB, but with a backwards execution. -Forward outside counter (FOC), consisting in one-foot turn from one circle to another, without a change of pressure and with a body rotation that is opposite to the initial direction of the movement. The exercise is executed on the longitudinal axis of the circles, in their tangency point. It must have the initial length of one skate and a depth between the length of one truck and one-half the length of one skate. -Backward outside counter (BOC), following the principles of the FOC, but with a backwards execution. Eight word-class skaters, representing the best European and world athletes, were involved in the study. A motion analysis system constituted by ten synchronized cameras was used. (BTS SMART-D Motion Capture System). The cameras have a maximum resolution of 0.48 Megapixel with an acquisition rate of 250 photograms per second. The ten cameras were placed around the tangency point of the athlete’s trajectories at various heights. The longer duration of pivot phases of the wheels in the BIB can be related to the wider spaces in the direction of the movement observed in this exercise. The counters, being more precise on the linear spaces, show a shorter duration of the pivot phases. The brackets are the exercises with the highest error trend in the direction of the movement. It seems that the athletes, to maintain the trace, delay the movement on the x-axis. In the BIB the athletes perform the exercise most delayed. A possible explanation is that they have a worst visual feed-back compared to the other exercises, due to the body and the swinging limb position, blocking a good vision of the trace. This leads to conclude that the BIB is the most difficult exercise. The counters are more precise on the x-axis, but they show a wide trend to make mistakes on the z-axis. In this case, the subjects search for the following trace, as they must perform a change of circle. Concerning the support leg, we can state that the initial, more marked in the BOC, loading and maximum extension can be explained as an attempt of the athlete to lighten the skate during the rotation. This can be related to the shorter times of the pivot phases of the wheels. Furthermore, this explains a more fluid action of the skate in the counters. The loading at the end of the rotation is more marked in the FOB. This is due to a wider seeking of the trajectory and the dynamic equilibrium. Different strategies in the sequence of movements are observed also about the free leg adduction. This is justified by the pelvis rotation, being counterclockwise in forwards exercises, and clockwise in backwards exercises. The different behaviour in BIB about the trunk flexion can be explained considering that the BIB is the only, amongst the three examined exercises, that is performed backwards. The initial flexion is more marked due to a bigger difficulty to see the trace. At the end of the rotation the athlete, extends more the trunk compared to the other exercises. The shoulder rotation with respect to the x-axis and the pelvis is due to the position of the athlete that is required in the exercise.