## 4 Discussion  
In tennis matches, the number of serves in each service game exceeds that of any other stroke type [35]. Serves directly influence match outcomes [36], as players strive to score points through serves to secure victory. Tennis serving requires the coordination of lower-limb joints [37]. The serve motion is categorized into three stages: the pre-serve jump, lift-off, and the landing phase [38]. The landing phase primarily relies on the action of a single lower limb [39], imposing significant demands on the muscular control of the athlete's lower limb, potentially leading to an elevated injury risk. Differences in the serve performance among tennis players with varying skill levels [40] may result in varying risks of lower-limb joint injuries. Thus, the study investigated differences in ROM, joint moments (including peak moments), and lower-limb joint stiffness in the landing phase \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Joint** & **Plane** & **Amateur** & **Professional** & \(p\) \\ \hline \multirow{4}{*}{Hip} & X & \(-0.866\pm 0.589\) & \(-1.151\pm 0.438\) & 0.212 \\  & Y & \(-0.069\pm 0.506\) & \(0.110\pm 0.661\) & 0.482 \\  & Z & \(1.094\pm 0.417\) & \(1.511\pm 0.417\) & 0.029 \\  & X & \(0.235\pm 0.955\) & \(0.337\pm 0.608\) & 0.764 \\ Knee & Y & \(0.521\pm 0.259\) & \(1.354\pm 0.244\) & \(<\)0.001 \\  & Z & \(-0.034\pm 0.257\) & \(-0.063\pm 0.468\) & 0.087 \\  & X & \(-0.569\pm 0.296\) & \(-0.755\pm 0.211\) & 0.104 \\ Ankle & Y & \(0.217\pm 0.215\) & \(0.173\pm 0.287\) & 0.690 \\  & Z & \(0.360\pm 0.175\) & \(0.678\pm 0.137\) & \(<\)0.001 \\ \hline \hline \end{tabular} The values are presented as the mean \(\pm\) SD; \(n\) = 30 (\(n\) = 15 per group).  
\end{table} 
Table 2: Peak joint moments of the left lower limb. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Joint** & **Plane** & **Amateur** & **Professional** & \(p\) \\ \hline \multirow{4}{*}{Hip} & X & 0.166 \(\pm\) 0.049 & 0.278 \(\pm\) 0.079 & \(<\)0.001 \\  & Y & 0.088 \(\pm\) 0.054 & 0.075 \(\pm\) 0.022 & 0.478 \\  & Z & 0.161 \(\pm\) 0.112 & 0.162 \(\pm\) 0.065 & 0.987 \\  & X & 0.036 \  
of the serve between professional and amateur male tennis players (Table 4), highlighting differences in lower-limb movements and force dynamics during this phase for players with varying skill levels. Consistent with the hypothesis, professional male tennis players adopt a more aggressive approach during the serve phase. At the hip joint, professionals exhibit significantly higher internal-external rotation moments and peak moments in the horizontal plane, whereas amateurs demonstrate a significantly greater ROM. This suggests that professionals can generate greater force with a smaller joint ROM, reflecting more efficient force transmission and control, as confirmed by the findings of [PERSON] et al. [41]. However, prolonged exposure of the hip joint to high-load forces may result in chronic injuries, while atypical force trajectories in the hip joint could potentially impact the knee joint as well, consequently increasing the likelihood of knee injuries [42]. The results indicate that during the landing phase of the serve, professional male tennis players demonstrate greater hip flexion-extension stiffness. [PERSON] et al. [43] found a positive relationship between lower-limb stiffness and explosive strength. However, excessive joint stiffness, which limits ROM, may increase the risk of joint injuries under impact [44; 45]. During the landing phase, athletes experience substantial ground reaction forces and internal muscle forces, which could pose a higher risk of hip injuries for professional male tennis players. The results indicate that during the landing phase of the serve, professional male tennis players exhibit significantly greater knee-joint ROM in the sagittal plane compared with amateur male tennis players. An excessive ROM may increase the load on the lower-limb extensor muscles [46], thereby elevating the risk of injury.