# Effects of Age and Sex on the Kinematics of the Sprinting Technique in the Maximum Velocity Phase
## 1 Introduction  
Sprint speed refers to the capacity of the human musculoskeletal system to facilitate forward body movement till reaching the point of maximum velocity [1]. Velocity assessments are frequently incorporated in talent identification tests [2; 3] as they are recognized as a crucial factor for success in various adult [4; 5] and youth sports [6; 7]. The 100-m dash comprises the most representative and popular sprinting event in athletics. Data from both elite-level competitions and research studies have identified a typical running velocity (RV) development pattern during the 100-m sprint. This pattern consists of a rapid acceleration after the start (acceleration phase, 0-40 m), the RV regulation that leads to its maximization (maximum velocity phase, 40-80 m), and the decrease in RV as the finish line is approached (deceleration phase, 80-100 m) [8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. Each phase of sprinting exhibits variations in the neuro-muscular, kinetic, and kinematic demands of the movement [18]. In the maximum velocity phase, RV is a function of step length (SL) and step frequency (SF). The deterministic model of sprinting [19] suggests that SL comprises the stancedistance and the flight distance. SF is defined by stride time, which is the sum of the duration of the support and flight phases within the stride. The body segment positioning at the instances of touchdown and take-off affects the duration of support and the stance distance, while the air resistance and the ground reaction forces at the stance phase affect the flight time and distance. It is suggested that the level of sprinting ability is associated with differences in key step kinematical parameters [20; 21; 22; 23; 24]. During development, RV is constantly increased [25; 26; 27; 28; 29; 30; 31]. However, the development of sprint speed follows a non-linear trajectory during childhood and adolescence [29; 32] as a performance \"spurt\" in both preadolescence (between the ages of 5-9 years) and adolescence (between the ages of 12-14 years) has been reported [33]. In more detail, this cross-sectional study led to the conclusion that sprint RV was primarily increased in boys aged 7-11 years old and in girls between 6 and 9 years old [33]. Additionally, the most intensive improvement in RV, as noted in sprinting tests (30-50 m), has been reported to exist in children aged 9-11 years old [34; 35; 36]. However, comprehensive information regarding the biomechanical characteristics of running at speed in track-and-field athletes of this age is lacking. During preadolescence, there is a development of sprinting ability without any apparent sex differences [32]. Sex differences in 30-m-sprint performance appear negligible until after 10 years of age [34] but after the age of 11, the rate of progression of speed development is significantly increased in boys compared to girls [37; 38] and between early- and late-maturing girls [39]. In more detail, when examining sex differences in sprinting ability during puberty, there is a rapid development of sprinting ability in boys [33], whereas girls present a \"plateau\" in sprinting ability around 13 years of age [40]. These differences are suggested to be attributed to the larger increment in strength production by males [41]. The established relationship between the parameters composing the deterministic model of sprinting is decomposed at the age of 11-15 years old [26; 31; 42; 43]. In general, the period of peak height velocity affects RV since SL increases and SF decreases [44]. The relative information for younger athletes--in order for developmental changes of running characteristics to be better understood--is not present in the respective literature. The main difference between adult and young individuals in terms of sprint speed, apart from the lower maximum speed achieved by prepubescents, lies in the duration and total distance of the acceleration phase.