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.

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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.

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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...