# Age-Related Changes in Landing Mechanics in Elite Male Youth Soccer Players: A Longitudinal Study
## 1 Introduction
From this perspective, monitoring jump-landing mechanics during adolescence, especially during peak height velocity (PHV) and following the age of PHV, could provide useful information, given that injury incidence rates are greatest during this period [35]. To date, only a few cross-sectional studies have examined the effect of growth and maturation on the landing mechanics in youth athletes, and their results are conflicting. In a recent study [37], in the sagittal plane, no significant differences were demonstrated between pre-PHV, circa-PHV or post-PHV players in joint kinematics from a Drop Vertical Jump test. However, in the Tuck Jump Assessment, the group of pre-PHV players achieved significantly higher values of flexion in the hip and knee joint at initial foot contact compared with older players. Significantly higher values were also observed for maximal flexion in the hip joint and for plantar flexion in the ankle at the initial foot contact in the pre-PHV group compared with post-PHV. In a cross-sectional study on youth soccer players aged 10-18 years [34], vertical jump height and absolute pVGRF from single leg jumping increased during maturation; however, relative to body weight, a significant difference was observed only on the left leg in circa versus post-PHV players. A trend of reduction in knee valgus with maturation on both legs was observed, but the between-group differences were only between post-PHV and pre-PHV players on the left leg. Current knowledge indicates that appropriate landing mechanics may reduce ACL load during jump landing and decrease the risk of knee injury [26; 28; 38], and that movement patterns can be modified during growth and maturation in youth [34; 38; 39; 40; 41]. In this context, video-based movement screening can be used to identify potential ACL risk factors related to movement characteristics [13; 16; 30]. The \"Landing Error Scoring System\" (LESS) is a comprehensive, valid and reliable approach to clinical assessment of jump-landing biomechanics, and was developed by [PERSON] et al. [17]. By means of this tool, jump-landing quality is assessed by analyzing the records of landing in the sagittal and frontal planes during the jump-landing screening task [16; 42]. A study on elite youth soccer players [16] showed that the LESS could be used as a potential screening tool to determine ACL injury risk. The authors also suggested that whereas a high LESS score may lack precision in identifying which athlete will sustain an ACL injury, the LESS score may be effective in separating athletes into high-risk and low-risk subgroups [16]. The key cut-off value suggested for high injury risk is five points [17]. In conclusion, although many studies have focused on the quality of neuromuscular control during jump landing, only a few studies have focused on the assessment of kinetic and kinematic injury risk factors during landing. Moreover, these studies were of a cross-sectional design; longitudinal studies on changes in landing mechanics in youth soccer players with high validity are missing. Such studies with longitudinal design can provide new information in terms of the changes in movement mechanics during growth and maturation in soccer players, which could be useful for injury prevention management and performance enhancement. Therefore, the aim of this longitudinal observational study was to examine the age differences in landing kinematics in 13-17-year-old elite male soccer players. The hypothesis was that the landing kinematics would be improved with the chronological age in regularly trained 13-17-year-old elite soccer players.