## 4 Discussion
Dynamic valgus and increased valgus moments during landing are considered to be contributing factors to anterior cruciate ligament (ACL) injuries, based on research carried out by [PERSON] et al. [47]. In this study, professional male tennis players show significantly increased knee abduction ROM, moments, and peak moments compared with amateur players, indicating that the professional group might face an elevated risk of knee injury. [PERSON] et al. [48] also observes that a larger knee abduction moment raises the likelihood of ACL injury. Excessive external moments are recognized to increase knee-joint laxity, which may result in damage to medial and lateral collateral ligaments, the anterior cruciate ligament, and the posterolateral region [49]. This study's findings reveal that knee-joint moment in the horizontal plane is significantly greater in the professional group. This implies that professional male tennis players may face a greater risk of knee-joint injuries compared with amateur players. Inversion is the most common mechanism of injury for ankle joints [50]. However, this study finds no notable variation in ROM in the frontal plane when comparing professional and amateur male tennis players. Excessive peak forces and training loads have been recognized as contributing factors that elevate the likelihood of muscle injuries and bone tissue damage in professional athletes [51; 52]. The findings of this study indicate that \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Joint** & **Plane** & **ROM** & **Joint Moment** & **Peak Joint Moment** & **Joint Stiffness** \\ \hline \multirow{4}{*}{Hip} & X & - & - & - & - & \textless{}0.001 \\  & Y & - & - & - & - \\  & Z & 0.026 (45–100\%) & 0.036 (59–91\%) & 0.029 & - \\  & X & 0.003 (0–82\%) & - & - & - \\ Knee & Y & 0.042 (0–29\%) & \textless{}0.001 (12–100\%) & \textless{}0.001 & - \\  & Z & - & \textless{}0.001 (19–100\%) & - & - \\  & X & \textless{}0.001 & 0.004 (6–74\%) & - & - \\ Ankle & Y & - & - & - & - \\  & Z & 0.041 (27–67\%) & - & \textless{}0.001 & - \\ \hline \hline \end{tabular} The values are presented as \(p\), with only significant \(p\)-values reported.  
\end{table} 
Table 4: Variations in lower-limb joint mechanics between male tennis players of two skill levels. professional male tennis players exhibit considerably higher ROM and moments in the sagittal plane, along with higher ROM and peak moments at the ankle joint in the transverse plane compared with amateur male players. It is speculated that the landing phase of the serve may pose a higher risk of ankle injuries to professional male tennis players. Athletes must control and optimize the efficiency and stability of their movements during the serve to sustain optimal performance levels [37]. The lower-limb movement and pelvic thrust initiate the kinetic chain during the serve, playing a vital role in generating powerful trunk rotational moments [53; 54]. Athletes' leg strength positively correlates with serve speed, as found by [PERSON] et al. [55]. Furthermore, it is important to note that upper-limb and trunk movements also influence lower-limb biomechanics [56; 57]. Improvements in upper-limb function help to enhance overall stability [58; 59], which affects the joint angles, forces, and overall biomechanics during landing. Therefore, additional studies are required to gain a comprehensive understanding of the complex relationships among upper-limb, trunk, and lower-limb biomechanics, especially in the context of injury prevention. This study focuses on lower-limb injury risks during the serve landing phase. Aligning with the results of this study, [PERSON] et al. [60] found that lower limbs are more prone to injury during athletic activities, and [PERSON] et al. [61] observed that young high-level athletes are more susceptible to lower-extremity injuries. This might be because athletes with higher skill levels are more likely to display greater intensity or to engage in risk-taking behaviors compared with their amateur counterparts, thereby increasing their injury risk.