## 4 Discussion
### Whip-like Control Smash  
The basic element enabling human movement is the angular motion of joints, wherein the angular velocity starts at zero and returns to zero at the end of each joint movement. After the commencement of angular motion, the joint angle undergoes rapid changes over time, with the fastest angular velocity (i.e., the most rapid change in joint angle over time) occurring somewhere in between. In motion analysis, the human body is mechanically described as a chain system with multiple joints. To maximize the velocity of distal segments, such as throwing, smashing, kicking, or striking in various sports, sequential joint control/coordination (i.e., whip-like control) is required. This whip-like control involves a sequential progression of motion from the core of their body outward to the distal end of their extremities. Ideal timing in whip-like control involves starting the angular motion of a joint when the previous joint reaches its maximum angular velocity. In a diagram depicting angle changes over time, the degree of whip-like control can be assessed by analyzing the timely overlaps of the joint angle changes involved in a movement [5; 14; 49]. Perfect whip-like control demonstrates that at the end of the period of the most rapid angle change in a larger joint, the adjoining smaller joint begins its period of the most rapid angle change [69]. When impeccable timing is achieved from proximal to distal control, it results in the highest possible cumulative distal-end velocity. Extensive reports and studies [61; 70; 71; 72; 73; 74] have consistently documented that, in various throwing and striking endeavors, skilled athletes adhere to the distinctive whip-like control pattern. Illustrative examples of this phenomenon can be observed in activities such as the golf swing [14; 60], tennis serve [75], baseball pitch [62; 63], javelin throw [47; 48], soccer kicking [49; 50; 76], or aekwondo kicking [24; 77]. Research evidence has substantiated that this sequential control motion represents the most effective motor control pattern of generating explosive power and transferring the resultant kinetic energy to a ball, equipment, or opponent [71]. Based on the description above, several points are worth discussing. Firstly, concerning the rapid change periods identified, the whip-like control observed in the skilled group primarily consists of trunk internal rotation (area A1 in Figure 4), shoulder internal rotation (area A2 in Figure 4), elbow extension (area A3 in Figure 4), and wrist flexion (area A4 in Figure 4). This indicates that skilled players have developed a four-segment whip-like control system. Secondly, there are no notable rapid changes in shoulder flexion and adduction during the smash (shoulder control: flexion/extension and abduction/adduction in Figure 4). This phenomenon suggests that the function of shoulder over-extension and abduction is to create a tension arc, establishing the necessary conditions/posture for enabling the whip-like smash. Therefore, their primary role is to support the whip-like smash. It is interesting to note that a previous study has revealed that the rapid shortening of the pectoralis major contributes significantly to the internal rotation of the upper arm during the badminton smash [5]. This confirms that shoulder over-extension and abduction during the tension arc formation (pre-lengthening of the pectoralis major) would enable rapid shoulder internal rotation during the whip-like control. Thirdly, due to the inertia effect, the rapid angular movement of the trunk and shoulders in the first half of the smash results in additional elbow flexion and wrist over-extension (Figure 4), constituting passive movements [51; 52]. These passive movements strengthen the SSC effect, leading to a more powerful whip-like smash [65; 66; 67]. Since these controls are passive, learners should focus their attention on trunk and shoulder controls in the initial stages of the smash.