## 4 Discussion
### The Role of Support Leg in Ball Velocity  
Ball velocity increased with greater peak extension angular velocity of the support hip for preferred leg kicks. Following SLTG, the hip continued to extend, reaching its peak angular velocity prior to ball impact. Simultaneously, the kicking leg executed a forward-downward motion, characterized by hip flexion and knee extension [13, 23, 28]. These findings align with [PERSON] et al., who conducted a technical refinement intervention [23]. In their study, participants were instructed to increase both knee and hip extension in the support leg, with the objective of enhancing vertical displacement on the support side. This modification elevated pelvic positioning, allowing the kicking leg to initiate its downswing from a higher point [23]. Consequently, the prolonged acceleration pathway of the kicking leg facilitated greater knee extension velocity prior to ball impact. Building upon this theoretical framework, this study extends these findings to professional-level players, across different competitive levels, in that an increased hip extension velocity in the support leg prior to ball impact contributes to enhanced ball velocity. Ball velocity also increased with a higher peak extension angular velocity of the support knee for preferred leg kicks. Following toe-off, the extension angular velocity of the support knee rapidly reached its peak, then gradually decreased, maintaining a lower extension angular velocity and a weaker flexion moment prior to ground contact [19, 22, 28]. At this stage, the primary function of the knee is to prepare for the absorption of impact forces upon landing. Currently, no studies have explicitly examined the relationship between peak knee extension angular velocity before ground contact and ball velocity. However, based on the observed kinematic patterns of the support knee, it is reasonable to hypothesize that for the preferred leg kicks, a higher knee extension angular velocity may facilitate a faster transition into an optimal support posture, ensuring greater stability and impact absorption upon ground contact. However, the findings also indicate that peak knee extension angular velocity has a positive effect on ball spin for both the preferred and non-preferred leg kicks, with this effect being more pronounced in the non-preferred leg kicks. This suggests that the peak knee extension angular velocity has an optimal range for both ball velocity and ball spin; thus, it can play a crucial role in kicking accuracy. Although no prior research has directly examined the relationship between peak knee extension angular velocity before ground contact and ball spin, existing studies have demonstrated that both the vastus medialis (VM) and vastus lateralis (VL) of the quadriceps, as well as the hamstrings, are actively engaged before landing [29]. These muscle groups work in coordination to regulate knee extension, ensuring controlled ground contact. Consequently,excessively high knee extension velocity prior to landing may lead to overactivation of the knee extensors, disrupting the flexor-extensor coordination pattern, thereby increasing ball spin. This effect is particularly pronounced in less technically proficient players in the non-preferred leg kicks, indicating a possible area of improvement for their kicks. A third finding of this study was that the ball velocity decreased with greater peak external rotation angular velocity of the support hip for non-preferred leg kicks. Given that players typically approach the ball at an angle, hip external rotation occurs in the support leg following kicking leg toe-off to adjust the foot placement angle prior to landing. [PERSON] et al. (2014) reported that the hip joint reaction force in the support leg induces a counter-clockwise rotation of the pelvis in the horizontal plane, a mechanism that plays a crucial role in pelvic stability [22].