Anatoly Antipin 2. The Tennis Serve The serve should be one of the easiest shot. It can be done from standard position. There are no hurry, no running involved etc. On the other hand, the serve is one of the most difficult shot in tennis to learn, because there is still no clear explanation on how to build the proper serving routine. Further, I’ll try to explain the most important and difficult to understand elements of the tennis serve. Andy Roddick possesses one of the best serves in the world. There are some data of his performance. According to Figure 2.1 the Roddick’s right arm generates 80% of the ball speed. All others limbs actually are not very important and contribute merely 20%. Very famous American coach Vic Braden stated, "I still hear some television announcers telling the viewing audience that the server is getting such great power on the serve because he/she is jumping up to the ball and getting full extension. But, one of our coaches, John Tichy, served a 124 mph serve while on his knees. The ball went into the service box as it had enough topspin to force the proper trajectory". That’s why I mostly pay attention on the right arm and its parts actions. Figure2.1. Body parts contributions to the Andy Roddick serve To go any further, we need to know a little bit about biomechanical terminology (Figure 2.2) Figure 2.2. Biomechanical terms of the arm movement In the tennis slang, the pronation means the counterclockwise rotation of the arm (not just forearm) and the supination is the arm clockwise rotation. The forearm pronation has restricted range around 180°. If your palm is facing the floor you basically cannot pronate at all. To make the pronation possible you should supinate first. If the palm is facing the ceiling, you can produce the most efficient pronation with range around 180°. The upper arm also can pronate/supinate around 180°. This motion also is called: the internal/external rotation of the shoulder. The forearm and upper arm together can provide pronation/supination around 360°. In case of the any tennis serve, pros usually supinate first to provide appropriate pronation (around 90°). It is obvious; to maximize the pronation angular speed we should use both: forearm and upper arm counterclockwise rotation. Definition: The Target Plane is the plane, which includes the tennis ball during impact and the imaginary target inside of the deuce or ad tennis court. This plane should be parallel to the perpendicular to the racquet string bed during the impact. The Target Plane basically determines the boll velocity direction. We shouldn’t change amount of the pronation, because it is almost impossible to control, in order to change boll’s direction, much easier to alter direction of the Target Plane. Always keep the range of the arm pronation around 90°. 2.1 The Basic Kick Serve Routine and Pronation Almost all of the modern instructions advise the tennis player to drop the racket in a backscratch position, provide appropriate supination, and swing up on edge like you are trying to use the side of the tennis racket to cut the ball in half. At the last second before impact, the player has to pronate the arm around 90° very quickly. In most cases the Continental grip is recommended. There are also a lot of words about legs, shoulders, trunk positioning and motion, which I’m not going to scrutinize here in detail. The pictures (Figure. 2.3) show the set of the video’s frames (last second before impact) taken during the pro Florent Serra’s typical kick serve and practically confirm instructions above. Let’s analyze these pictures and try to figure out what is really vital for the typical kick serve. Figure 2.3. Set of the pictures around impact Florent Serra’s kick serve What can I state about the body rotation? It looks like the body is more or less frozen (because it is very slow) and hence, it cannot contribute anything significant to the racquet’s velocity. But the arm itself and its parts are rotating in the different planes with the visible angular speeds. The arm is rotating in the vertical plane (Figure 2.3) by using mostly shoulder joint (also very slow joint). This vertical plane should be parallel to the Target Plane to provide appropriate direction of the ball’s velocity. There is also the wrist ulnar deviation, which directs the racquet upward, but this movement is not very important for the ball speed because it creates just brushing (spin) motion and I describe it later (see step 2.2). On the picture 2.3.1 the vertical ray with arrow indicates starting point of the arm vertical rotation. All others pictures include this starting point ray and its own ray for measuring angular movement of the arm between starting point and current position of the arm (the angle ϴ). The numbers next to these rays show degree of the angle ϴ as result of the vertical arm rotation. On pictures from 2.3.3 to 2.3.7 the symbol ΩV represents angular speed of the arm for particular frame, ft is time elapsed between any two consecutive frames, ft=3.33 msec. During this vertical rotation from Figure 2.3.1 to Figure 2.3.7 the arm travels 11° (Figure 2.3.7, ϴ =11°). The arm vertical rotation angular speed ΩV practically is constant on all pictures. It varies from 1.5°/ft to 2°/ft. The shoulder joint muscles do not produce any arm acceleration, it moves like a car coasts in neutral. For the reason that the arm is moving with constant speed, the acceleration was achieved on previous steps of the serve, mostly, thanks to the fast elbow extension. The previous forearm movement forced the arm to move parallel to the Target Plane with angular speed approximately ΩV=2°/ft. At the same time, the arm pronation moves the racquet in the horizontal plane around 90°. Usually pros pronate something from 80° to 110°. Suppose the pronation provides 110° path of the racquet. It means the racket rotates in horizontal plane 10 times as many as the arm and racquet moves in vertical plane (ϴ=11°). The average horizontal angular speed will be around ΩH=20°/ft, or 10 times as many as the vertical angular speed ΩV =2°/ft. Wow, this result is astonishing! Question: Have legs, shoulders, and trunk motions contributed anything to the racquet horizontal rotation (pronation)? Answer: These parts of the body produce something to the arm and the racquet vertical rotation, but they are arguably even counterproductive for the horizontal rotation since the trunk rotates (clockwise), in opposite direction to the arm pronation (counterclockwise). OK, it looks like I found the winner! The pronation can provide much bigger angular speed than others body limbs (except the wrist) altogether. Not so fast. In reality, we are interested in the linear velocity (the speed and direction) of the racquet, not just in the angular speed. Definition: Linear speed = radius × angular speed. The direction of this velocity is perpendicular to the radius of the rotation in the plane where the point of contact rotates. The angular speed already discussed above. But, what is the radius? The figures 2.4; 2.7 give an idea about calculation of these radiuses. Figure 2.4. Federer serve To be continued, see please post #7.