*2.1. Participants*

In this experiment, the participants were two world class hurdlers: Colin Jackson (body mass 75 kg, and height 182 cm) from Great Britain and Dayron Robles from Cuba (body mass 79 kg and height 191 cm). Both competitors specialized in 110 m hurdle, and were or are world record holders in 110 m hurdles. Some more personalized and anthropometric data of both athletes are shown in Table 1. The participants provided informed consent and were informed of the protocol and procedures for the study prior to the o fficial video recording. The selection of athletes to conduct the experiment was specific and dependent on the possibility of making a video recording with its entire comprehensive procedure during an international meeting, and above all dependent on the level of participants in these competitions. Due to the fact that the experiment concerns the analysis of the hurdle technique for only two competitors, it can be qualified as a case study—the work reports scientifically sound experiments and provides a substantial amount of new information. The study was approved by the Human Ethics Committee of the University of Ljubljana.



BMI (Body Mass Index), \* World Record in 1993, \*\* World record in 2008.

#### *2.2. Experimental Design*

The experiment design used was a comparison of dynamic and kinematic variables between two 110 m hurdles races at the segmen<sup>t</sup> between hurdles 4 and 5 and hurdle clearance of two world record holder. Both recordings of hurdles took place during regular international athletics competitions, although in two di fferent places and two di fferent years. These two conditions forced the experiment to match two di fferent race recording methodologies. The hurdle races of Jackson and Robles were both recorded using two cameras each, although of di fferent resolutions of 50 Hz frames per second and 100 Hz per second, respectively. From a methodological point of view, this may be a significant di fference, but the conditions of variability were respected when processing data. In order to avoid the errors involved in analysis, real measurements were recalculated, taking into account the measurement error, which actually means that they corresponded (e.g., 50 Hz means 0.04 s between frames, so a hurdle clearance time of 0.5 s vs. 0.54 s represents a single frame). In both analyses the model of Dempster [30] was used for the calculation of the body's COM and the kinematic program ARIEL (Ariel Dynamics Inc., Trabuco Canyon, CA, USA) for the digitization was applied.

#### *2.3. Procedure of Measurements—Colin Jackson*

Colin Jackson's biomechanical analysis was carried out on 28 June 2002, at the International Meet in Velenje (EA Classic). His finish time was 13.47 s. The weather conditions were optimal; the outside temperature was 27 ◦C with a wind speed of + 0.2 m/s. Authorization to perform the experiment was approved by the Slovenian Track and Field Association. Biomechanical measurements were performed by a team of experts from the Laboratory for Movement Control of the Faculty of Sport in Ljubljana. Two synchronized cameras, namely Sony DSR-300-PK DVCAM Camcorders with Fujinon 17x lenses, were located at the main stands (the zone of hurdle 5) and operating at 50 Hz (shutter speed: 1/1000) were used to film the races. To record all kinematic parameters, the cameras were set at an angle of 120 to the direction of the moving hurdler in the segmen<sup>t</sup> between hurdles 4 and 5 (Figure 1). The zone of the 5th hurdle was calibrated with a calibration cube, one at the beginning of hurdle 4 and one at the end of hurdle 5. A 15-segment Dempster's model [30] and the ARIEL kinematic program (Ariel Dynamics Inc., Trabuco Canyon, CA, USA) were used to calculate the center of mass.

**Figure 1.** Comparison of biomechanical parameters of hurdle clearance.

#### *2.4. Procedure of Measurements—Dayron Robles*

Biomechanical analyses of Dayron Robles's 5th hurdle clearing technique was performed at the 2011 IAAF World Challenge—Zagreb International Race. Weather conditions were optimal; the outside temperature was 23 ◦C, and the wind speed was −0.2 m/s. Authorization to perform biomechanical measurements was obtained from the Technical Delegate of the European Athletics Federation and the Organizing Committee of the competition. The running track lane in the zone of the 5th hurdle was covered by two Casio high-frequency digital Casio EX-F1 512 × 384 (300 fps) sampled down to 100 fps cameras (Casio Computer Co., Ltd., Tokyo, Japan), which were interconnected and synchronized. The shutter speed of the Casio cameras was 1/300 s. The cameras were set perpendicular to the zone of the 5th hurdle (running hurdler) at an angle of 90◦. The zone of the 5th hurdle was calibrated with a 2 m × 2 m × 2 m reference frame, within which eight points were measured. Data processing utilized an APAS computer system for 3D kinematic analysis (Ariel Performance Analysis System). Digitization of a 15-segment athlete body model was carried out, defined by 15 reference points [30] The point coordinates were smoothed with a 14 Hz digital filter. The center of mass (COM) was calculated

from the digitized points based on Dempster's (1955) model of determination of COM via the ARIEL kinematic program (Ariel Dynamics Inc., Trabuco Canyon, CA, USA).
