# Effects of Age and Sex on the Kinematics of the Sprinting Technique in the Maximum Velocity Phase
## 1 Introduction
Research revealed a difference regarding the secondary acceleration phase [45], which is very short in prepubescents and is completed between 20 and 30 m [46; 47]. Furthermore, children achieve their maximum velocity around 30 m, they present a very short steady period [15; 46], and a decrease afterward [46]. Although it is widely acknowledged that RV is determined by the combination of stride frequency and stride length [48], the leg joint angles are essential factors for the identification of the effectiveness of running gait [49; 50]. Nevertheless, comprehensive research specifically addressing the developmental aspects of these characteristics during childhood is sparse. Studies in adults have indicated that faster runners achieve longer strides by increasing the application of ground reaction forces while reducing the duration of ground contact time [51; 52], and enhancements in strength and power have been linked to improvements in both stride length and running speed [53]. Additionally, SF and SL values are not constant but vary across different phases of the sprint. In children, there is some controversy regarding stride frequency at maximum velocity. An earlier study reported that it remains consistent regardless of age [54] while another study indicated lower stride frequency for the less mature (pre-PHV) children [55]. Furthermore, boys' speed developed at a low rate from 8.8 to 12.1 years of age and there was a decreased stride frequency in the older boys within this age span [40], whilst in age-matched girls, speed presented an accelerated improvement that was accompanied with a stride length enhancement at the phase of maximal speed--attributed to their probable ability to apply higher propulsive forces [56]. However, it seems that younger and less mature children rely more on stride frequency during maximum sprinting due to their lower musculotendinous stiffness and their lower ability to apply high rates of force development during stretch-shortening cycle (SSC) movements [57]. The comprehension of biomechanical factors in maximum sprint running is valuable due to their essential role in determining performance outcomes. Previous biomechanical analyses on young athletes' sprint kinematics focused mainly on basic parameters such as SF, SL, duration of support, and flight time [26; 31; 42; 43]. [PERSON] et al. [9] provided some information about the knee joint angles at touchdown and take-off in 12-year-old boys and girls. Sex differences were not observed, and the same result was revealed in the comparison of the lower limb angular kinematics in a long (100 m) compared to a short (50 m) sprinting distance [9]. In general, the existence of this limited information in terms of understanding the mechanical determinants of youth's sprint ability has been noted elsewhere [30]. Taking into consideration the previous conflicting results regarding sex differences in biomechanical parameters in untrained prepubescent children during sprinting, it would be of great interest to examine the possible age- and sex-related differences in the kinematic parameters in prepubescent and adult trained sprinters during the maximum sprinting phase. The purpose of this study was to compare the step parameters, the Body Center of Mass (BCM) kinematics, as well as the linear and angular kinematics of the lower extremities' joints of prepubescent and adult sprinters of both sexes. The hypotheses are as follows: (a) the spatiotemporal step parameters, i.e., SF, SL, duration of support, etc., will be influenced by age as they are strength-related; (b) the angular kinematics (angle and angular velocity of the lower limb joints) will not be altered between the age groups, since sprinting is a congenital ability; and (c) sex differences in RV and the biomechanical parameters will be present only in adults.