Technique: Basic Terms and Principles in Biomechanics

(The information in this article was taken or adapted from the High Performance Coaching Program Study Guide.)

Biomechanics is the study of the causes of human motion, so tennis biomechanics is essentially the science/ mechanics tennis technique.  When tennis coaches combine their practical tennis teaching experience with knowledge of tennis biomechanics, they can accurately analyze strokes, prescribe training and exercises, and maximize skill development while minimizing the risk of injury to their players. 

Knowing the how and why of tennis motions is essential to prescribing the best possible tennis instruction. It is important to know several biomechanical terms and principles to better understand tennis technique:

  • Forces and torques,
  • Newton’s Laws of Motion,
  • Linear and angular momentum,
  • Center of gravity, and
  • Kinetic link/ Kinetic chain

Forces and Torques

A force is simply a push or pull and it changes the motion of a body segment or the racket.  Motion is created and modified by the actions of forces (mostly muscle forces, but also by external forces from the environment).

When Force rotates a body segment or the racket, this effect is called a torque or moment of force.

Example— Muscles create a torque to rotate the body segments in all tennis strokes. In the service action internal roation of the upper arm, so important to the power of the serve, is the result of an internal rotation torque at the shoulder joint caused by muscle actions (latissimus dorsi and parts of the pectoralis major and deltoid).  To rotate a segment with more power a player would generally apply more muscle force.

Newton’s Laws of Motion

Newton’s three laws of motion explain how forces create motion in tennis.  These laws are usually referred to as the laws of inertia, acceleration, and reaction.

  • Law of Inertia.  Newton’s first law of inertia states that objects tend to resist changes in their state of motion.  An object in motion will tend to stay in motion and an object at rest will tend to stay at rest unless acted upon by a force.
    Example—The body of a player quickly moving across the court will tend to want to retain that motion unless muscular forces can overcome this inertia.  Players should therefore build movement into their basic stroke production that takes them back toward the center of the “hitting-arc”.
  • Law of Acceleration. Newton’s second law precisely explains how much motion a force creates. The acceleration (tendency of an object to change speed or direction) an object experiences is proportional to the size of the force and inversely proportional to the object’s mass (F = ma).
    Example—If a player improves leg strength through training while maintaining the same body mass, she or he will have an increased ability to accelerate the body using the legs, resulting in better agility and court speed.  This also relates to the ability to rotate segments, as mentioned above.
  • Law of Reaction. The third law states that for every action (force) there is an equal and opposite reaction force. This means that forces do not act alone, but occur in equal and opposite pairs between interacting bodies.
    Example—The force created by the legs “pushing” against the ground results in ground reaction forces in which the ground “pushes back” and allows the player to move across the court (As the Earth is much more massive than the player, the player accelerates and moves rapidly, while the Earth does not really accelerate or move at all).  This action-reaction also occurs at impact with the ball as the force applied to the ball is matched with an equal and opposite force applied to the racket/body.

Momentum

Newton’ second law is also related to the variable momentum, which is the product of an object’s velocity and mass.  Momentum is essentially the quantity of motion an object possesses. Momentum can be transferred from one object to another. For tennis coaching, you need to know two types of momentum:

  • Linear momentum, which is momentum in a straight line.
    Example— Linear momentum is created by the forward step in a square stance forehand. 
  • Angular momentum, which is rotational momentum and is created by the rotations of the various body segments.
    Example—The open stance forehand uses significant angular momentum. The tremendous increase in the use of angular momentum in groundstrokes and serves has had a significant impact on the game of tennis. One of the main reasons for the increase in power of the game today is the incorporation of angular momentum into groundstroke and serve techniques.

In tennis, the angular momentum developed by the coordinated action of body segments transfers to the linear momentum of the racket at impact

Center of Gravity and Balance

Understanding the concepts of the center of gravity and balance is enormously important if you are to coach tennis successfully. These concepts are interrelated and have a profound effect on the success or failure of stroke production.  Loss of balance or poor balance is one of the single biggest causes of errors in tennis.

Center of Gravity

The center of gravity is an imaginary point around which body weight is evenly distributed The center of gravity of the human body can change considerably because the segments of the body can move their masses with joint rotations. This concept is critical to understanding balance and stability and how gravity affects tennis techniques. 

The center of gravity of a tennis racket is a far simpler process and can usually be found by identifying the point where the racket balances on your finger or another narrow object.

Balance

Balance is the ability of a player to control her or his equilibrium or stability. As a coach, you need to know and understand two types of balance:

  • Static balance: The ability to control the body while the body is stationary. 
    Example—A player uses static balance when she or he prepares for a serve.
  • Dynamic balance:  The ability to control the body during motion.
    Example—A player uses dynamic balance when she or he changes direction after hitting a shot.
 
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