**4. Discussion**

The entire process of hurdle clearance took 0.50 s for Robles; it took 0.10 s for take-o ff, 0.33 s for the flight phase, and 0.07 s for the landing phase. Meanwhile, Colin Jackson completed the hurdle clearance a little more slowly, as it took him 0.54 s. Jackson also spent 0.10 s for take-o ff, 0.36 s for the flight phase, and 0.08 s for the landing phase. For comparison, the measurement of Amara [23]—a medium level athlete (13.90 s at 110 m hurdles) showed di fferences in the abovementioned parameters of 0.60 s, 0.36 s, 0.21 s, and 0.12 s (respectively for each variable). Jackson's slower clearance of the hurdle is associated with a higher rise in his COM above the hurdle and a longer landing distance over the hurdle, extending both the flight phase and the shock absorption phase. A slower hurdler [30] had a similar problem; his excessive height of the vertical COM displacement together with a high take-o ff angle had a negative impact on the time to clear the hurdle. The di fference in the flight parabola between the two athletes can be attributed mainly to the di fference in their height and the di fference in their functional abilities. Based on the kinematic parameters of the parabola, we can, therefore, conclude that Dayron Robles has a more rational hurdle clearance technique (Figure 1).

The take-o ff distance for Robles was 2.43 m, which was 66.4% of the total clearance length over the hurdle. For Jackson, the take-o ff distance was 2.09 m, which was 57.0% of the total length of clearance. Jackson's landing distance was 1.58 m (43.0% of his total stride length), while Robles's was 1.23 m (33.6% of his total stride length). It can be compared with some other studies [10,30], which indicate that the optimal ratio between take-o ff spot and landing place should be 40–60%, which is comparable with Amara's [17] findings (i.e., 58:42). This ratio was confirmed by previous researchers [8,18,24,28,31,32], which indicated that take-o ff distance should range from 2.04 cm to 2.31 cm. In turn, the landing distance was shorter. We can identify two di fferent hurdle clearance strategies. Robles has a faster hurdle clearance; his take-o ff is elongated, and his landing is closer to the hurdle. The duration of Robles's flight phase is 0.33 s, and that of Jackson is 0.36 s. A technical model of When [33] indicated that the optimal over the hurdle time should range between 0.30 and 0.33 s for a world class hurdler. This confirms the importance of the take-o ff (the angle between the top of the foot and the hip) and landing distances in high hurdler races, as was previously mentioned by Coh and Iskra [31] and Lopez at el. [24].

In the concentric phase, Robles had a take-o ff angle of 78.7 ◦, and Jackson's was 72.9 ◦. The COM velocity resultant during the braking phase of the take-o ff was 8.73 m/s for Robles and 8.82 m/s for Jackson. This velocity resultant of COM is defined as the vector sum of the vertical COM velocity (0.70 m/s for Robles, –0.43 m/s for Jackson) and horizontal COM velocity (8.70 m/s for Robles and 8.81 m/s for Jackson). It changes until the last contact of the take-o ff when it measured 9.18 m/s for Robles and 9.41 m/s for Jackson. Robles's vertical COM velocity at that time was 1.80 m/s, and Jackson's was 2.35 m/s; their horizontal COM velocities were 9.00 m/s and 9.11 m/s, respectively. The COM horizontal velocity during take-o ff thus increased by 0.30 m/s for both Robles and Jackson. The relative increase in the horizontal velocity of COM for Robles was 3.30% and 3.29% for Jackson (Figure 2). For both athletes, the duration of their take-o ff was the same. Robles's COM height during take-o ff increased by 0.13 m, equal to Jackson's (Figure 1). It is comparable with data of Amara [17], Li and Fu [34], and Lopez at el. [24], who claimed that during take-o ff (propulsion phase), the average height of the COM should be around 1.12 m.

**Figure 2.** Comparison of the biomechanical parameters of take-off before the hurdle.

The transition between hurdle clearance and the sprint between hurdles is dependent on the landing phase. For Robles, the horizontal velocity at landing was 8.80 m/s, which means that the horizontal velocity decreased by 0.20 m/s (2.2%). For Jackson, the horizontal velocity decreased by 0.34 m/s (3.7%). During the landing phase, Robles's height of COM decreased by 0.07 m (5.4%) and 0.09 m (7.8%) for Jackson. The short duration of the landing phase (0.07 s for Robles and 0.08 s for Jackson) indicated a high level of reactive power [35] for both athletes (Figure 3), and an efficient transition to sprinting between hurdles [4,36].

**Figure 3.** Comparison of the biomechanical parameters of the landing.

For Jackson, the reduction in the horizontal velocity of COM was greater than that of Robles, and the height of his center of mass (COM) was lower at landing, so it can be concluded that Robles has a slightly more biomechanically rational hurdle clearance technique. In addition, our results do not contradict the research of Amara [23], who claimed that the vertical component of COM velocity and the lead-leg/trail-leg at take-off and at flight phase constituted key factors of optimum hurdle clearance. According to Amara [17,23] and Shibayama et al. [37], in addition to the take-off angle, the knee and the hip angles are very important in high hurdles clearance, as also found in previous studies done by Coh [18,38], Xi et al. [22], Bubaj [21] and Sidhu [39]. Liu [40] just confirmed this statement and additionally indicated that the flight-phase duration is also defined by the takeoff angle, which should be lower.
