# Comparative Study of the Sprint Start Biomechanics of Men's 100 m Athletes of Different Levels
###### Abstract  
Background: This study sought to investigate the kinematic and kinetic differences in sprint start between high-level and medium-level sprinters. Methods: Twenty male sprinters were dichotomized according to their personal 100 m performance. Each sprinter performed three block starts. Six high-speed cameras were used for 2D kinematic analysis, and kinematic and dynamic forces were determined by Kisprint. Results: There was no significant differences between high-level and medium-level athletes in the antero-posterior distance of two blocks, block inclination, reaction time, push time, and the joint angle of set position (\(p>0.05\)). The ankle angle of the front leg and swinging leg at the time of front leg exit were significantly greater in high-level athletes than in medium-level athletes (\(p<0.05\)). The vertical RFD (rate of force development) and RFD of high-level athletes were significantly lower than that of medium-level athletes (\(p<0.05\)). The relative maximum horizontal force generated by the front leg of high-level athletes was significantly larger than that of medium-level athletes (\(p<0.05\)), and the maximum vertical force of the rear leg was significantly lower than that of medium-level athletes (\(p<0.05\)). Conclusions: Our finding shows that the start kinematics of high-level sprinters is characterized by a greater ankle angle of the swinging leg and front support leg at the front block exit. High-level sprinters generate greater maximum horizontal force in the front block and smaller maximum vertical force in the rear block. t +
Footnote †: journal: Medical  
in high-level athletes than in medium-level athletes [15]. Nevertheless, a decrease in hip angle may proportionally increase the load on the extensors and thus decrease their contractive force, and therefore a smaller hip angle is not recommended for sprint start [17]. However, there has been little agreement on the correlation between reaction time and 100 m performance. One of the studies showed that there are no significant differences in reaction time among the elite groups [18; 19]. It has been observed from another study that the athletes' 100 m performance improved from 9.94 to 9.87 s, and their reaction time also improved accordingly from 0.163 to 0.133 s, indicating that improvement in reaction time is significant for overall performance improvement [20]. Related to sprinter joint angles configuration in the set position, a previous experimental study has shown that a rear knee angle of 90\({}^{\circ}\) can generate higher block velocity than rear knee angles of 115\({}^{\circ}\) and 135\({}^{\circ}\)[21]. In contrast, [PERSON] believes that elite athletes' rear knee angle of 135\({}^{\circ}\) is an optimal angle in the set position, while 130\({}^{\circ}\) and 140\({}^{\circ}\) both fall into the poorer category [22]. Consequently, it is unclear whether the knee angle of the rear support leg in the set position affects sprint start velocity. In addition to external kinetics, a consensus has been reached according to which the horizontal block impulses are the determining factor of the horizontal velocity at block exit [23; 24; 25]. However, the evident important force generated against the blocks could help gain a deeper understanding of block velocity. Fewer comparative studies based on the force-velocity curves during the pushing phase have been carried out in sprinters of different levels, and this will further constrain the targeted training practices. To reconcile these contradictory findings and to answer the question of the key factors of sprint start, this paper has three objectives. The initial objective is to clarify whether the start block setting varies among different levels of sprinters. The second objective is to demonstrate whether the joint angular kinematics affects sprint start performance. Third, to analyze the forces generated during the pushing phase to identify which specific force affects the start performance. Such detailed comparisons are much better to understand sprint start biomechanics.