## 1 Introduction  
The instep kick is one of the most fundamental and extensively studied movements in soccer, as it plays a decisive role in goal scoring. Biomechanically, it has been defined as an open-chain movement of the lower limb from the proximal to the distal part, involving continuous braking and acceleration [1, 2]. This movement requires precise coordination among multiple joints and muscle groups across the pelvis, hip, knee, and ankle, spanning multiple planes of motion [3]. Investigating its biomechanical characteristics is essential for understanding the mechanisms underlying kicking performance and optimizing training strategies. A high ball velocity is often used as a parameter of kicking performance [1, 2]. However, the success of a kick is determined not only by ball velocity but also by precision, and often, kicks have an inherent characteristic of ball spin included which may or may not increase precision. However, studies have shown that the ball spin is inversely related to the ball velocity [4]. Most studies have focused on the joint angles, angular velocities, and force generation of the kicking leg of male players [5, 6], showing that the coordination and efficient energy transfer in the kicking leg are important factors in achieving higher ball speeds [7; 8]. However, the role of the support leg remains largely underexplored. Existing research on the lower limb joints of the support leg has primarily focused on cross-sectional comparisons related to gender [9; 10; 11], leg preference [12; 13; 14; 15], sports injuries [16; 17; 18], and approach strategies [19; 20]. These studies have highlighted the role of the support leg in stabilizing the body and optimizing force transmission [21], and a further investigation into its kinematic characteristics and their relationship with kicking performance is essential. A study reporting on the kinetics has shown that that the joint moments at the ankle, knee, and hip of the support leg primarily counteract ground reaction forces (GRF) thus facilitating impact absorption rather than generating positive energy [22]. Another key mechanism through which the support leg contributes to kicking performance is its role in pelvic rotation. In the leg acceleration phase, when the knee of the kicking leg extends as the hip of the kicking leg flexes, the reaction force at the support leg's hip joint facilitates counterclockwise pelvic rotation, which is essential for the proximal-to-distal energy transfer in the kicking motion. While some studies have analyzed this mechanism, they have not specifically examined how different kinematic parameters of the support leg contribute to kicking performance. Previous research on the direct role of the support leg in determining ball velocity has been mixed: a transversal study found a low predictive effect of the support leg kinematics on ball velocity [21]. However, this study incorporated both the kicking leg and the support leg into a single model, which may have masked the support leg's true contribution. On the contrary, an intervention study demonstrated that technique refinement training significantly increased the knee extension velocity and vertical hip displacement of the support leg, directly enhancing the swing speed of the kicking leg and, as a consequence, ball velocity [23]. This discrepancy suggests that the relationship between support leg kinematics and kicking performance is more complex than previously assumed and requires further investigation using more comprehensive analytical approaches. Although research on the biomechanical properties of the support leg has made progress, there are still research gaps. First, existing studies mostly focus on cross-sectional comparisons of specific variables and do not include a complete kinematic analysis of the support leg during kicking. Second, almost all studies have dealt with amateur and/or university level players, and none have studied the technique in professional players.