## 3 Results  
Figure 4 shows that two events--take-off and ball contact--divide the jumping side volley into three phases: (1) the jumping phase, (2) the airborne phase, and (3) the landing phase. Our data reveal the following characteristics of the 1 st phase: Before the volley, the athlete's trunk and pelvis rotate away from the goal. During the takeoff, the non-kick-side (NKS) leg is raised, at the same time, the trunk reverses rotational directions and twists toward the goal. In order to increase the range of motion (ROM) of trunk rotation, both arms abduct to near horizontal (over \(80^{\circ}\), Table 1). Until the take-off, the ROM of the trunk twist (commonly known as the X-factor [27; 28; 29]) is about \(40^{\circ}\) (Table 1). At the end of the phase, the trunk-orientation approaches a more horizontal position (Figure 4). Correlation analyses confirm that the X-factor (\(\alpha\)), angle between thighs (\(\beta\)) at take-off, and shoulder abduction during jumping are key/dominant factors, influencing the kick quality, i.e., the ball release speed (Figure 5a,b).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**Selected Parameters**} & \multicolumn{2}{c}{**Results**} & \multicolumn{1}{c}{**CI**} & \multicolumn{1}{c}{**CV**} & \multicolumn{1}{c}{**r**} \\ \hline \multirow{6}{*}{ROM (\({}^{\circ}\))} & Trunk Twist (X-factor \(\alpha\)) & \(41.5\pm 5.1\) & 40.5–42.6 & 0.12 & 0.86 \\  & KS Hip (Flex/Ext) & \(51.6\pm 5.2\) & 50.5–52.7 & 0.10 & 0.71 \\  & KS Knee (Flex/Ext) & \(56.9\pm 6.1\) & 55.6–58.1 & 0.11 & 0.76 \\  & KS Ankle (Flex/Ext) & \(11.6\pm 2.2\) & 11.1–12.1 & 0.19 & 0.52 \\  & Max Shoulder Abduction * & \(83.7\pm 4.2\) & 82.8–84.5 & 0.05 & 0.69 \\ \multirow{2}{*}{Angle (\({}^{\circ}\))} & Take-off Angle between & & & & \\  & Thights (\(\beta\)) & \(75.2\pm 13.0\) & 72.5–77.9 & 0.17 & 0.82 \\  & Min KS Knee Angle & \(84.1\pm 9.6\) & 82.1–86.1 & 0.11 & \(-0.75\) \\ Coordination & Trunk \(\rightarrow\) KS Hip & \(83.2\pm 4.1\) & 82.3–84.1 & 0.05 & 0.57 \\ Timing (\%) & KS Hip \(\rightarrow\) KS Knee & \(74.4\pm 15.4\) & 71.2–77.6 & 0.21 & 0.89 \\ \hline \hline \end{tabular}  
* X-factor: the angle between shoulder line (upper trunk) and hip line (lower trunk). Flex/Ext: Flexion/Extension, KS: Kick Side. *: the max value is the average of both shoulders.  
\end{table}
Table 1: Average and standard deviation of selected parameters, their confidence intervals (CI), coefficient of variation (CV), and their correlation with the ball release speed (r) (\(p<0.05\)).  
Figure 4: Phase identification based on 3D motion analysis data.  
Two notable characteristics of the 2 nd phase (i.e., the airborne phase) are the flexed kick-side (KS) leg and the extended NKS leg (Figure 5c). Both legs form a scissor-movement, i.e., they move in opposite directions. The correlation analyses show that both the minimum angle and ROM of the KS knee are key factors affecting the kick quality (Table 1). From the timing perspective, the KS hip flexion starts when about 80% of the trunk twist toward the goal has finished. Similarly, the KS knee begins its extension after the KS hip finishes about 75% of its flexion (Table 1). Further, our data show that the explosive KS knee extension happens shortly before the ball contact, and is followed by an ankle flexion. The correlation analyses unveil that the sequential segment-coordination is also a key contributor to the kick quality.  
The main issue in the 3 rd phase (i.e., the landing phase) is how to dissipate the impact load produced by falling in order to avoid potential injuries. Our data reveal that the well-trained athletes apply multiple landings to share the impact loads for reducing the risk of injury. The 1 st landing is the flexed arm-hand chain (like a spring) for the 1 st damping. The 2 nd landing is the hip landing, followed by body rolling, sharing the rest load among multiple contact points.