## 3 Results and Discussion
### Shoe Heel Effect  
Heel design is a crucial factor in footwear biomechanics, influencing shock absorption, movement coordination, and force distribution during high-impact movements such as badminton lunges [46]. Variations in heel curvature, including Rounded Heel Shoe (RHS), Flattened Heel Shoe (FHS), and Standard Heel Shoe (SHS), have been studied for their effects on ground reaction forces (GRFs), joint coordination, and overall performance efficiency [16; 17; 18]. Understanding how different heel structures modify lower-limb mechanics can inform shoe optimization strategies tailored to athlete skill levels and movement demands.  
[PERSON] et al. [17] examined the impact of different heel designs on contact time and shoe-ground angles in elite badminton players, reporting no significant effects of heel curvature modifications on these variables. However, [PERSON] et al. [16] found that shoe heel curvature altered vertical loading rates during lunges, with RHS demonstrating lower maximum vertical loading rates compared with FHS and SHS. Notably, elite players exhibited significantly lower left-side peak and mean vertical loading rates in RHS, whereas intermediate players showed no measurable shoe effect on loading rate, suggesting that experience level may influence an athlete's ability to adapt to footwear modifications.  
[PERSON] et al. [18] further investigated the influence of heel structure on knee-ankle coordination during lunges, showing that RHS promoted better movement coordination based on Phase Angle (PA) and Continuous Relative Phase (CRP) metrics. These findings suggest that rounded heel designs may facilitate smoother joint coordination during dynamic footwork, potentially improving movement efficiency. The reduced vertical loading rate observed in RHS users may be attributed to more controlled heel-to-tee transitions, which optimize force distribution and minimize abrupt ground impact forces [16].  
Overall, these results highlight the importance of heel curvature in optimizing badminton lunge performance and reducing impact-related injury risks. The Rounded Heel Shoe design offers advantages in shock absorption and movement coordination, particularly for elite athletes who require precise biomechanical control during lunges. These findings support the notion that shoe optimization should be personalized based on the technical level of athletes, as biomechanical responses to footwear modifications appear to differ between highly trained and intermediate-level players.  
While these studies provide valuable insights, certain limitations should be noted. The experiments focused on single-movement analyses, limiting their applicability to real-game conditions, where fatigue and variability in footwork influence biomechanics. Additionally, muscle activation patterns were not quantified, meaning that the role of neuromuscular control in footwear adaptation remains unclear. Importantly, female athletes were not included, despite research showing that women have a larger Q-angle and greater knee valgus tendencies [15]. These factors suggest that female athletes may rely more on external stability from footwear, including RHS designs, but existing studies [16; 18] have not tested female samples. Future research should investigate how RHS influences knee-ankle coordination in female athletes, as well as integrate electromyographic (EMG) analysis to reveal the impact of heel design on muscle activation strategies.