In tennis, the serve is a critical stroke, of key importance for the course of any match. However, the serve is only a real weapon when it is powerful, regular and fully controlled. None of these features can be achieved if a tennis player does not master a precise and repeatable ball toss.
It is widely accepted that the main reason for the irregular toss is the lack of skill. Meanwhile, the real cause usually lies elsewhere: in the complexity of the kinematic chain involved in the toss. If the chain is complicated and the brain has to control dozens of joints being moved by many muscles and tendons, it's hard to expect satisfying repeatability of the ball toss even after thousands of repetitions. However, if the number of degrees of freedom of the kinematic chain is reduced considerably, the increase in the toss regularity could be visible literally within minutes.
During the serve toss, the players nowadays use different ball grips. Beginners usually squeeze the ball with all the fingers spread apart and bent to follow the curvature of the ball. This is certainly not the most sensible solution: at the moment of release, the fingertips can disrupt the ball motion. To avoid such situations, more advanced players hold the ball only with fingertips. In another version of this grip, all four fingers are close together and the ball is pressed against them with the thumb. Joined, slightly bent fingers can also form a shallow cup, in which the ball rests on the base of the fingers, where it is stabilized with the thumb. In a more advanced type of this grip, the cup is formed of only three fingers, slightly apart. Other players hold the ball in a yet another way: along its circumference and with only two fingers, the index and the thumb.
Five popular grips for the ball toss. The grip described in the article is in the right bottom corner.
The major disadvantage of all these grips is their active character: the ball is actually held by the player, who has to release it in a strictly defined phase of the toss. A closer analysis of the kinematic chain reveals more sources of trouble. For example, if the ball is held by the fingertips, only within the palm up to fifteen joints are involved in the toss (three in each finger). Furthermore, additional degrees of freedom appear as a result of motions between metacarpals or changes in the wrist position. During the toss, this complex mechanical structure is controlled by several dozen muscles. In this situation, keeping all five fingertips in exactly the same positions in relation to each other becomes difficult not even due to the lack of experience of the player, but for more fundamental reasons of a physical and biomechanical nature. Meanwhile, with only a few points of support, even small inaccuracies will result in slightly different forces acting on the ball at the moment of its release.
Among the grips presented above, the last two stand out. In the first one - the ball is placed here in a cavity formed of four or three fingers - everything the player needs to do to release the ball is to carry the thumb away. If four fingers are used, mutual contacts between them help to suppress their accidental motions. With three fingers apart, some fluctuations may appear, but the complexity of the kinematic chain is reduced. In the second grip - the ball is held here along the circumference with the index finger and the thumb - the number of joints and muscles involved in the motion is significantly reduced1. However, the ball is still actively held by the player, which may lead to minor motions of the index finger in relation to the thumb. They can change the orientation of the plane of the ring holding the ball up to several degrees. In both grips, in order to achieve useful regularity of the toss, the player should work on a repetitive mechanism of releasing the ball in the appropriate phase of the motion sequence.
The chalice inertial grip (ChInG) presented below is one of the techniques of inertial tennis, the first internally coherent set of tennis techniques built from scratch on solid scientific foundations. Each element of inertial techniques has been designed for the most optimal use of the fundamental properties of physical reality within the biomechanical framework of the human body. All inertial techniques were constructed and falsified by the author of the article.
The inertial chalice grip. Two rings formed by the index finger and the middle finger shape the hand into a shallow chalice. Inside, the ball is held not by the muscles of the fingers, but by the Earth's gravity field. If the palm rotates and the rings become parallel to the direction of the local gravity force, the ball simply falls out.
A characteristic feature of inertial tennis is its high level of integrity: main physical ideas and biomechanical solutions are the same in any stroke, and the hitting techniques are inextricably linked to the techniques of body movement on the court. Integrity is a huge advantage in the late stages of learning. Because of it, progress in one technique drives progress in others, and the internal coherence and simplicity of all techniques become more and more visible. At the same time, integrity is a big obstacle at the introduction to inertial tennis, when it is necessary to explain a wide range of issues, including the inertial techniques and their roots in the natural sciences.
Against this background, the inertial chalice grip is particularly easy to describe. It is technically simple, easy to use and can be separated from the actual hitting sequence. Furthermore, it occurs only in one, very specific tennis technique. At the same time, the great advantage of this grip is that it can be used to illustrate some of the important ideas behind the theory and techniques of inertial tennis, like striving to reduce the probability of errors2.
The human kinematic chain is a far from trivial structure. The individual joints, surrounded by many groups of muscles and tendons of different levels of tension, work with different degrees of freedom and in different ranges of mobility. When optimizing the work of a kinematic chain, the hinged joints are of particular interest. The hinge works in one plane. If the external force acts perpendicular to this plane, the joint practically does not move. This means that skillful (and careful!) changes of the orientation of the working planes of hinge joints can functionally modify the number of joints in the kinematic chain - even in different phases of a single motion.
Comparison of the size of the areas where the ball is supported by the palm, in different grips. The more red and the more uniformely distributed around the ball's center of the mass (marked in black), the better.
In order to ensure that the toss will be as regular as possible, the number of joints working in the grip should be minimized. It could also be advisable if the contact area between the stabilized links of the kinematic chain and the ball was as large as possible and, preferably, evenly distributed around the center of the mass of the ball. This requirement means that at least part of the hand should be formed in the shape of a cup or ring. Of particular relevance is the requirement to remove the need for conscious release of the ball.
Comparison of the stability of the areas where the ball is supported by the palm, in different grips. The red color means that joints can bend under the emerging forces and therefore actively participate in motion, making it harder to control. The more red, the worse.
The above requirements are surprisingly easy to implement. The ring can be formed by two bent fingers: index and thumb. If its diameter is slightly smaller than the diameter of the ball, the fingers will not squeeze the ball and it will rest in the ring only under the influence of an external force (generated by Earth's gravity field or, if the palm is accelerating, by the inertia3 of the ball). Thus, the trivial reduction of the ring diameter eliminates the need of the conscious initiation of the ball release. As soon as the hand begins to slow down in the final phase of the toss, the ball will separate from the ring naturally, due to its inertia. After just a few attempts, it is not difficult to notice that the stability of the ball in the palm can be increased by means of a second, smaller ring, formed by the bent middle finger. The hand with two rings, the outer larger and inner smaller, takes the shape of a chalice, which explains the name of the grip.
Both rings of the chalice inertial grip should be slightly open to guarantee the proper depth of the chalice (if the chalice is shallow, the ball can wobble and the control over its flight is reduced). The opening is optimal if the ball touches the ring finger.
Errors in the inertial chalice grip: the chalice is too shallow (left) or too tight and the ball can't fall out (right).
In such a grip, the palm does not touch the ball at individual points or along a short line located on one or another side of the center of mass of the ball, but with almost the entire circumference of the outer ring (larger, formed by the index finger and the thumb) and about half of the circumference of the inner ring (smaller, formed by the middle finger). Both rings remain rigid because the working planes of the engaged finger joints are oriented perpendicular to the forces applied to them due to the gravitational or inertial mass of the ball. This means that the number of joints working in the fingers and metacarpus is functionally reduced to zero4. The only joint of the hand that is active in this grip is the wrist. During the toss, it performs a small, relatively slow, smooth and easy-to-control motion, resembling a toast. The ball is not held by the player but is pressed against the fingers by gravity (when the grip rings are horizontal), inertia (at the beginning of the toss) or by... the racquet (why, we will see in a moment).
The inertial chalice grip may cause problems in the ritual phase of the serve. Most players bounce the ball before the serve and at the end of bouncing they grab the ball. Unfortunately, the rings of inertial chalice grip will be oriented vertically at this moment and the ball will fall out of the palm. In order to prevent this, the ball should be pressed against the palm by the shaft of the racquet. This moment of additional support should be used to form the tossing hand in a chalice. If the racquet was moved away and the hand remained stationary, the ball would fall to the ground. Thankfully, the player can initiate the toss at the same time. Inertial forces will then be enough to keep the ball inside the chalice.
The racquet protects the ball from falling out during the ritual phase, when after the ball bouncing the grip changes to the inertial chalice grip.
Importantly, the inertial chalice grip changes the working regime of the kinematic segment in the upper limb. Almost all traditional grips force a vertical orientation of the elbow joint plane. Practice proves that the so oriented elbow is often bent and, furthermore, its angle can vary during the tossing sequence. The inertial chalice grip forces the pronation of the forearm, what naturally induces the rotation of the working plane of the elbow joint. Although the plane is not oriented perfectly perpendicular to the direction of motion, its rotation is sufficient to significantly reduce the probability of accidental elbow bends during the tosses. In this way, the inertial chalice grip increases the consistency of the toss.
The classic ball toss can be performed with the elbow joint bent to a greater or lesser extent. The inertial chalice grip turns the elbow working plane and naturally favors tosses with a more rigid hand.
Let's summarize. Instead of a complex kinematic chain involving dozens of joints, muscles, and tendons, and requiring a training of the moment of ball release, in the inertial chalice grip the number of joints working in the hand is reduced to just the wrist, and it is easy to control. The main contact surface between the hand and the ball is distributed around almost the entire circumference of the ball, practically uniformly around its center of the mass. The working range of the elbow joint is significantly limited. Also, the need for the conscious release of the ball is eliminated: it is not the player, but the physics itself that decides when the ball leaves the palm.
A bit of fun with the chalice inertial grip.
Does the use of such a simplified kinematic chain really translate into the accuracy of the tosses? During the development of the inertial serve, the author - then with more than 20 years of amateur tennis practice and using traditional grips - was able to perform four to six precise tosses out of ten attempts. With the inertial chalice grip, the number of good tosses increased to six-eight in just a few dozen minutes. After a few days of training, the problem of the precise ball toss ceased to be important.
The inertial chalice grip beautifully presents the potential of careful design of the kinematic chains involved in the tennis sequences. It proves that, although tennis has been practiced for a century and a half by millions of people all over the world, there is still room for many improvements.
|Thanks to the chalice inertial grip, physics itself - not the player! - decides when the ball leaves the palm.|
ABOUT THE AUTHOR:
Jarek Chrostowski, Polish physicist, popularizer of natural and technical sciences (over 30 years of experience), author of several hundred popular science articles in national media, science journalist and editor promoting in Poland and around the world achievements of the leading Polish scientific institutions, such as the Faculty of Physics of the University of Warsaw, the Institute of Nuclear Physics of the Polish Academy of Sciences, the Institute of Physical Chemistry of the PAS, the Institute of Experimental Biology of the PAS and others. He has been playing tennis since he was a child, always as a self-taught amateur who has never participated in any tennis lessons. Inspired by creative conversations with other physicists, in 2011-12 he created the inertial theory of tennis and within the next five years transformed it into the first in history of the game internally coherent set of tennis techniques, based on the phenomenon of inertia and conservation laws.
1 We will see in a moment that this grip can be treated as a non-inertial version of the impoverished chalice grip.
2 This does not mean that a tennis player using inertial techniques makes fewer mistakes than players using classic (non-inertial) techniques. The extreme dynamics of inertial strokes, combined with human perceptual and biomechanical limitations and the fundamental, chaotic properties of the physical phenomena at the core of these techniques, make errors an inevitable part of inertial tennis. Even the most careful construction of inertial tennis techniques will never completely eliminate them. However, it is possible to reduce their number to the level acceptable for the sport competition.
3 Inertia is the resistance of physical objects to accelerations.
4 Almost, because the rings can move slightly relative to each other. These small movements, however, do not have practical significance.
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