# Age-Related Changes in Landing Mechanics in Elite Male Youth Soccer Players: A Longitudinal Study
## 1 Introduction  
Reaching the top level of performance in sports, including soccer, requires long-term age-appropriate training. In this process, successive development of motor skill competencies and physical fitness through appropriate training and competition workload, which consider physical and psycho-social developmental changes, must be accentuated [1; 2; 3]. The optimization of this process requires minimizing the occurrence of injuries due to an acute injury risk as well as overtraining and overreaching. Soccer is a team sport that has a high occurrence of injuries, with the most common injuries in both adult and youth soccer players reported in the lower limbs, specifically the hamstring, knee joint and ankle [4; 5; 6; 7]. In the case of knee joint injuries in soccer, the highest risk is of anterior cruciate ligament (\(\Lambda\)CL) injury [4]. Although not so frequent, ACL injuries represent the most serious injury burden in terms of long-term consequences, including high re-injury risk, decreased knee function and degenerative changes [8]. Epidemiological studies show that ACL injuries in adolescent soccer players are mostly of a non-contact nature, and during soccer match-play and training, they usually occur during dynamic high-intensity movements [9; 10; 11; 3]. Numerous modifiable and non-modifiable risk factors have been associated with non-contact ACL injuries [13; 14], and it has been suggested that poor lower extremity mechanics of jump landing is one of the movement patterns for ACL injury, as it stressesthe passive ligament structures beyond their capacity [15; 16; 17; 18]. Specifically, the magnitude of the mechanical load on passive structures and the risk of injury are affected by movements in the frontal, sagittal and transverse planes [19; 20]. Especially, increased medial knee displacement in the frontal plane and valgus moments of the knee joint during the impact phase of jump-landing tasks are considered important potential predictors of ACL injury [21; 22; 23; 24]. In the sagittal plane, injury risk decreases with greater angular joint displacement, which can be expressed by the magnitude of the change in the flexion angle in the hip, knee and ankle joints from the time preceding the initial contact of the foot with the surface to the time of maximum flexion, the position of the torso at the initial contact and changes in torso flexion [17; 25]. Importantly, limited sagittal plane joint displacement (SPJD) during landing is associated with less mechanical energy absorption and greater ACL loading through quadriceps muscle actions [26; 27] and increased frontal plane knee motion and moments [28]. Given that jumps and subsequent landings are common in soccer, and also given the trainability of this aspect in landing technique, consideration of this knowledge may help in LCA injury risk management in players. Other biomechanical influential risk factors are restricted ankle dorsiflexion range of motion, hip external rotations, asymmetries between the lower extremities and peak vertical ground reaction forces (pVGRF) [13; 22; 26; 29; 30]. Current research also indicates that restricted ankle dorsiflexion range of motion is associated with decreased SPJD in the ankle, knee and hip joints when landing [23; 30]. Contrarily, increased ankle mobility may improve landing mechanics by increasing sagittal plane joint displacement due to a softer landing reducing pVGRF [24; 26; 31]. The incidence of ACL injury in youth players increases with age [3; 4; 32; 33]. The increase in risk of injury is also related to periods of accelerated growth during adolescence [6; 34; 35]. In this context, adolescence is an important period in the development of specific motor abilities (e.g., running speed, jump height) that might be influenced by \"adolescent awkwardness\" [36].