Biomechanical Principles of Tennis Technique: Using Science to Improve Your Strokes
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Biomechanical Principles of Tennis Technique: Using Science to Improve Your Strokes
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Product Details
| ISBN-13: | 9780983511182 |
|---|---|
| Publisher: | Usrsa |
| Publication date: | 04/01/2006 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 128 |
| File size: | 2 MB |
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Biomechanical Principles of Tennis Technique
Using Science to Improve Your Strokes
By Duane Knudson
USRSA
Copyright © 2006 Duane KnudsonAll rights reserved.
ISBN: 978-0-9835111-8-2
CHAPTER 1
BIOMECHANICAL PRINCIPLES OF TENNIS TECHNIQUE
"A common trait of recreational players is that they try to do things with their hands to make up for their lack of quickness and positioning with their feet as they hit the ball." - Arthur Ashe
Any meaningful discussion of variations of tennis stroke technique requires knowledge of sport biomechanics. Biomechanics is the field of study that focuses on understanding the motion and causes of motion of living things. Sport biomechanics, naturally, focuses on how humans create a wide variety of movements in sports. Fortunately for the tennis player and coach, there is a large body of research on the biomechanics of tennis movements. From the footwork to move on the court, to the adjustments in the stroke to create topspin, biomechanics is an essential tool for understanding movement in tennis.
This text will not revel in the details of this research and its limitations, but will be concerned with painting a picture of the consensus of this body of knowledge that can be applied to tennis. It is easy for biomechanical analyses to churn out hundreds of thousands of numbers representing the time varying values of a myriad of force and motion variables. What is more important, and more difficult, is the identification of key variables that are most influential and interpreting how they affect performance or injury risk. (See Advantage Box 1.1 for a brief discussion of the differences between the levels of scientific evidence and coaching opinion.) Often the biomechanical research supports the experiential wisdom of tennis coaches, but at times the research points to interesting and counterintuitive ideas. This is not surprising given the complex mechanical properties of biological tissues, the complexity of the musculoskeletal system, and the high-speeds of the game that make most aspects of the movements truly invisible to the naked eye.
Fortunately, much of the fascinating and complex nature of movement in tennis can be easily understood using general principles of biomechanics. Biomechanics scholars have proposed nine or ten generic principles of biomechanics in human movement (Knudson, 2003a). This book is based on six of these principles that are most relevant to tennis (Figure 1.1). These principles of tennis mechanics focus attention on key mechanisms of body movement (biomechanics) and ball trajectory in tennis. The trajectory principles may initially appear to be strictly mechanics (physics) with limited interaction with the biological properties of the tennis player. However, we will see that the biological factors (skill, strength, anatomical motion) do affect the range of speed, spin, and initial trajectories that tennis players can create.
Knowing what body motions were used and how they were created and may be modified are powerful tools for improving performance and reducing the risk of injury in tennis. This chapter will provide a brief introduction to these principles. These principles will underlie much of the discussion on the biomechanics of tennis strokes and movements that are explored in the rest of this book.
* * *
Knowing what body motions were used and how they were created and may be modified are powerful tools for improving performance and reducing the risk of injury in tennis.
Force and Time
The Force and Time Principle says that motion of any body can be modified by the application of force(s) over a period of time. Most tennis movements are characterized by large forces applied for a short time as opposed to smaller forces applied over a longer time. This principle may be the most important because it deals with the creation or modification of motion. For example, a tennis player rushing the net usually performs a split-step to create reaction and friction forces from the ground to redirect his body to intercept a passing shot. We will see later that the split step employs a coordination and transfer of energy strategy as well as the mechanical properties of muscles to redirect the body in the very short time available to react to the ball.
* * *
Most tennis movements are characterized by large forces applied for a short time as opposed to smaller forces applied over a longer time.
To fully understand and apply this principle, the tennis player needs to understand several key concepts related to force and motion. Many readers will notice that this principle is the direct application of one of the most important laws of physics — Newton's Second Law of Motion. This law is important because it shows the relationship between the forces that cause motion and the resulting motion. Newton originally defined this relationship using both force and time variables (impulse-momentum), but this equation can be rearranged to give the more famous formula (ΣF = ma) that shows the relationship for any instant in time. The formula — (ΣF=ma — says that the acceleration a body experiences is equal to the sum of the forces in that direction and is inversely proportional to the mass of that body. Two ideas are necessary to fully understand this relationship. The first is that force (F) and acceleration (a) are vector quantities, meaning that they are described by both a magnitude and a direction. Second, the mass of an object is the measure of resistance to change in state of linear motion (speed or direction). This fact is embodied in Newton's First Law, called the Law of Inertia, which states that bodies tend to maintain their state of motion, and the linear measure of this property is simply mass. Because a tennis player cannot decrease his body mass during a point, if he wants to move quickly to the right in our split step example, he must create large ground reaction forces in that direction (Figure 1.2). There is very little time to apply forces, so the forces created must be large. As we will see, the split step is essential to helping the leg muscles create larger forces than they could from a static start.
The example in Figure 1.2 also illustrates another subtle concept about forces. Forces represent the push/pull interaction between two bodies. This is the essence of Newton's Third Law of Motion — for every force, there is an equal force acting in the opposite direction from the other body. Notice that the player in Figure 1.2 pushes with his legs to the left, to create reaction forces to the right. Since the mass of the player is much less that the mass of the court/earth, the reaction force from the ground creates a visible change in velocity (acceleration is the rate of change of velocity) of the player, but not of the earth.
Newton's Second Law and the Force and Time Principle can also be applied in rotations. Forces applied off-center on a body create a tendency to rotate called a moment of force or torque. Torque is dependent on both the force (size and direction) and the right angle distance between the force and the axis of rotation (Figure 1.3).
The resistance to rotary motion or angular inertia is called the moment of inertia. The moment of inertia of objects depends primarily on the location of the mass relative to the axis of rotation. The sum of the mass of each piece of the object times its distance (moment arm) from the axis of rotation is the resistance of that object to rotation about that axis and is known as the moment of inertia. This is commonly called the "swingweight" of a racquet, but it is important to remember two things: objects (such as racquets) typically have different moments of inertia in different planes of motion because shape and mass are not uniform, and objects really have an infinite number of swingweights because you can grip and rotate them at different points (see the book by Cross and Lindsey (2005) for more on the many "weights" of a racquet).
If you want to create greater rotation in a stroke, you must either create greater off-center forces in a short time in the direction you want to rotate, or you must apply smaller forces over a longer amount of time. The tactical situation will dictate the optimal biomechanical strategy to accomplish your aim. You can also decrease the extension in a series of joints to decrease the moment of inertia of your body, so the lower inertia will be accelerated more by the torque you create.
* * *
If you want to create greater rotation in a stroke, you must either create greater off-center forces in a short time in the direction you want to rotate, or you must apply smaller forces over a longer amount of time.
Coordination and Transfer of Energy
The Coordination and Transfer of Energy Principle is concerned with the origins of the forces that modify motion. External forces represent the interaction of the body with the outside world, while internal force represent the interactions of the segments of the body. A good way to express this principle is that the body creates an external force through a complex coordination and transfer of energy between the various linked segments of the body. In other words, we can apply large forces to a tennis racquet because we can transfer mechanical energy from the ground through the legs, through the trunk, and up through the arm. This transfer is a very complex phenomenon because muscles have effects beyond the joints they cross and because force acting across joints can transfer energy. Mechanical energy is the ability to perform work on an object by virtue of position in space, motion, or recovery from deformation. Advantage Box 1.2 provides several examples of the transfer of energy between the segments of the body in tennis.
* * *
We can apply large forces to a tennis racquet because we can transfer mechanical energy from the ground through the legs, through the trunk, and up through the arm.
In some tennis instruction circles it has become common to talk about the linked system of body segments as a "kinetic chain." This is an adaptation of engineering mechanics terminology (used to simplify modeling and calculations) that begins to create confusion when used to classify movements as either "open" or "closed." A kinetic chain is said to be "closed" if there is significant restraint at both ends. Some would say the arm action in a push-up exercise is a closed kinetic chain because the ground and the body are significant restraints at each end of the chain formed by the hand, forearm, and upper arm. An open chain would be the segments of the opposite arm in a forehand because there is no significant restraint (force or heavy object) at the hand, with only significant restraint at the other end (shoulder joint). It is not clear if the mass of a tennis racquet is small enough to say that weight force of the racquet can be neglected to say the stroking arm in a forehand is essentially an "open" chain motion.
More important than vague classifications of linked segment systems is the concept that the causes of observed motion are often distant from what appear to be obvious sources. The large forces a player can apply to a racquet near the "backscratch" position in the serve primarily originate in the lower leg drive, hip, and trunk rotation (Figure 1.4). Because of the body's linked segment system, forces at one segment can have effects at all the other segments. Biomechanics has only started to develop two-dimensional models of the body to begin to understand how to track the distant effects of a force through a complex linked-segment system (Zajac and Gordon, 1989; Zajac et al. 2002), and the physical meaningfulness of these transfers is controversial (Chen, 2006). The complexity of the human body makes it exceedingly difficult to track the flow of mechanical energy throughout the body in three-dimensions, so documenting exactly how mechanical energy is transferred from the lower extremity to the racquet may only be possible in the future. Coaches need to understand that current use of the kinetic chain concept might communicate the important issue of transfer of energy and source of stroke power distant from high-speed motion, but it certainly is not a scientific theory with any explanatory power.
* * *
Because of the body's linked segment system, forces at one segment can have effects at all the other segments.
Many tennis coaches may base their technique suggestions on a theory of how certain body segments individually contribute to the force used to accelerate the racquet. It is good for a player to understand the theory or philosophy his coach is using. We will see in the rest of this book that biomechanics research has only provided a few tentative answers to many of these coaching/teaching issues. The primary research problems are the great complexity of the body, three-dimensional nature of most tennis movements, variability, individual differences, and a lack of funding for applied tennis research. Only a consensus of several research studies using a variety of biomechanical evidence (motion, forces, theoretical modeling, muscle activation, etc.) can provide answers to important teaching and coaching issues.
What we do know about coordination of segments and the transfer of energy between them are some general trends. First, novices or people with weakness or injury tend to use fewer body segments and poorly coordinate their sequencing. As skill increases more segments can be used and with greater consistency. Second, there is a coordination continuum ranging from nearly simultaneous motion of body segments to more sequential segment motions. The larger the external resistance in a movement, the more simultaneous the motion of the body segments, and the more speed in the movement, the more sequential will be the segment motions. For example, moving a large resistance like the whole body in a split step tends to use simultaneous flexion and extension motion in the lower extremities. A tennis racquet provides a small resistance, so trunk and arm coordination in most strokes is sequential action from the core of the body outward. Third, even though there seem to be common patterns in the movements of players, there is also variation across and within players.
* * *
The larger the external resistance in a movement, the more simultaneous the motion of the body segments, and the more speed in the movement, the more sequential will be the segment motions.
The terminology used in kinesiology (the whole academic study of human movement, not just biomechanics) to describe this commonality and diversity in movements ranges from the general to the specific. Classes of movements with a similar pattern (primarily due to biomechanics) are called fundamental movement patterns. An overarm throw or walking are fundamental movement patterns. Specific adaptations of a fundamental movement pattern for a particular objective is a skill. A tennis serve is a sport skill that is a refinement of an overarm throwing fundamental movement pattern. Skills also have variations to suit particular, often tactical, objectives that are called techniques. The slice and twist serve are techniques of a tennis serve. Variation in movement due to the individual is typically related to style. A unique style a particular player has in a technique (e.g., rhythm, kind of backswing) should not be considered an error unless it violates the principles of biomechanics. A longer, lower backswing would be a stylistic feature of a serve which typically would be consistent with the principles of biomechanics and the demands of the shot.
Of course there is much more to the study of human movement variation than trying to determine the difference between a true common tennis technique and a stylistic variation. In fact, recent kinesiology research suggests that variation in many movements, body loading, or practice stimuli may be important to promote health and learning. Tennis players certainly know that a skilled, tough opponent can adjust his game to a variety of shots. Coaches need to realize that the technique adjustments they explore with their players need to be selected within the context of the consistency or variability of the strengths or weaknesses they observe. It is tempting for some to use biomechanics to label some technique points as incorrect. This rather rigid interpretation of the Coordination and Transfer of Energy principles should be avoided. The more you learn about the biomechanics of strokes, the more you realize how much the benefits or limitations of certain techniques are variable and contextual.
Balance and Inertia
Balance is the ability to control body motion and it is critical in a sport with high body movement and accuracy requirements such as tennis. The Balance and Inertia Principle states that tennis players must seek the best compromise between motion and stability that suits the situation. This is critical because stability and mobility in a structure are inversely related. A very wide base of support tends to increase stability, but it also decreases the ability of a person to move his center of gravity beyond the limits of the base in order to run.
(Continues...)
Excerpted from Biomechanical Principles of Tennis Technique by Duane Knudson. Copyright © 2006 Duane Knudson. Excerpted by permission of USRSA.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
Contents
PREFACE,1. BIOMECHANICAL PRINCIPLES OF TENNIS TECHNIQUE,
2. INJURY PREVENTION IN TENNIS,
3. BIOMECHANICS OF THE SERVE,
4. BIOMECHANICS OF THE FOREHAND,
5. BIOMECHANICS OF THE BACKHAND,
6. BIOMECHANICS OF THE VOLLEY,
REFERENCES,
INDEX,