The most detailed, physics/math oriented article on the kick serve!
This is a very lengthy article, so I decided to post the pictures first, before the text explanations. A word of warning. This is an article that focuses on pronation in the serve. I agree with the pronation point of view in many respects. But I am still free to disagree with Anatoly Antipin as far as the statement that turning the body parts other than the forearm and upper arm do not contribute much to the EFFECTIVENESS of the serve.
Sampras' serve is said to have the weight of a bowling ball. I believe the weight comes from the degree he throws his weight behind a very wide coil. He is also said to have the RPM of 4600+ revolutions!
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Permalink Reply by Tim Prapong on
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There are alot of good tips at tt.tennis-warehouse.com, a forum otherwise known as TW forum. There are also alot of bad ones, but this one, in particular, stands out.
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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)
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, 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
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 ΩVpractically 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.
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