INTRODUCTION

Professional tennis player, Sam Groth, holds the world record for the fastest serve that reached 263.4km/h at the Busan Open Challenger in 2012 (Wikipedia, 2013). In other words, he was able to manipulate his body in various ways to generate a ball speed more than four times faster than the average driving limit. How did he manage this? What techniques did he use and how did he execute them in a way that produced such a high velocity of the ball?

The answer lies in the area of biomechanics, which is typically concerned with the science that analyses the mechanics of the human body (Kohl & Murray, 2012, p. 21). A significant aspect of biomechanics is its role in determining optimal movement patterns during skill acquisition. This blog outlines the components of a tennis serve and applies biomechanical principles to determine the optimal technique behind it.

BIOMECHANICAL PRINCIPLES DISCUSSED

-          Kinetic chain (push-like and throw-like movements)

-          Newton’s third law of motion

-          Impulse-momentum relationship

-          Velocity

-          Conservation of vertical momentum

-          Newton’s second law of motion

-          Inertia

-          Angular momentum

-          Projectile Motion

COMPONENT 1 - THE LEGS

Positioning of the feet prior and during the serve

As a tennis player approaches the service line, they position their feet in a particular way. Girard, Micallef and Millet (2005, p. 1025) analysed the significance of foot positioning in the lead up to the serve and determined two types that generate different movements; the foot-up platform thrust style (FU) and the foot-back style (FB) (refer to Images 1 and 2).

 

Image 1: Roger Federer uses an FU style of serve, where one leg starts further back and moves forward as he strikes the ball to push off.



Image 2: Maria Sharapova demonstrates more of a FB style during serving, i.e. both legs remain relatively close together throughout the serve.

 

 

Depending on the foot stance the athlete chooses, braking and propulsive impulses can also be observed. Blazvich (I2012, p. 55) identified that when the foot lands at a smaller angle the propulsive impulse is greater, allowing the individual greater ability to accelerate forward. This is identified in the FB style where players can generate greater forward propulsive force and therefore more rapid displacement to the net (Girard, Micallef and Millet, 2005, p. 1025). Dissimilarly, the FU style allows players to develop large vertical forces, however provokes larger horizontal breaking forces, which may hinder performance by provoking slow acceleration to the net. However, a study (Reid, Elliot & Alderson, 2008, p. 312) has shown that the type of stance does not impede serving velocity.

Leg movement during the serve

Newton’s third law of motion states that ‘for every action, there is an equal and opposite reaction’ (Blazvich, 2012, p. 45). This is a fundamental concept observed in the performer’s legs during the serve. With this law in mind, it can be ascertained that applying a large force into the earth will initiate an equal and opposite reaction and propel the player in a particular direction.

Demonstrated in Image 3, is a player applying a force into the ground, followed by his projection upwards and forwards. This specific motion is a reflection of Newton’s third law.  

 

Image 3 - Demonstration of Newton's third law of motion

                                           

But why is a large force important and how do we generate this force?

A larger force that is applied for a longer time (force x time) initiates a greater change in momentum (Blazvich, 2012, p. 53). This is known as the Impulse-momentum relationship, which is observed in the tennis serve in the flexion of the knees for an extended period of time to build momentum. This allows the body to prepare for higher speed movements through a vigorous leg drive (Elliot, 2006).

The leg movements in the tennis serve can be further analysed in terms of a push-like movement pattern, where one end of the kinetic chain is completely free to move (open kinetic movement). A push-like movement extends all joints in our kinetic chain in a single movement and provides a straight line movement that creates greater accuracy (Blazvich, 2012, p. 196).

Dynamic lower limb motion in the tennis serve is vital to a higher speed of the upper torso (Reid, Elliot & Alderson, 2008, p. 311), therefore the way this movement is executed is very significant. Elliot (2006) furthermore suggested that a more forceful leg drive will account for a higher serve velocity.

 

Full leg extension as demonstrated in Image 3, can ultimately assist with accuracy and placement of the ball over the net, by lowering the player’s centre of mass, allowing them to “hang” in the air and therefore reach a greater vertical height (conservation of vertical momentum) (HLPE3531 Workshop handout).

 

COMPONENT 2: THE SERVING ARM

As stated previously, the legs provide a push-like motion in the kinetic chain and carries energy through the body until it reaches the wrist of the serving arm. Novice tennis players initially learn to use a push-like movement in the serving arm to gain accuracy, however as they progress to the elite level, players learn to use a throw-like pattern to create ball speed (Blazvich, 2012, p.198). (refer to Images 4 and 5 for differences in beginner and advanced serving techniques).

The acceleration of an object is proportional to the net force acting on it and proportional to the mass of the object (Blazvich, 2012, p. 45). Newton’s second law of motion understands that we need to apply a force to something to change its state of motion. In the case of the tennis serve, the tennis ball needs to be measured in terms of its inertia (the propensity for an object to remain in its present state) and a force needs to be applied to the tennis ball to overcome its inertia and be accelerated.

Angular momentum must be increased to increase force production. Blazvich (2012, p. 79) demonstrated that to increase the momentum the moment of inertia needs to be decreased and this can be achieved by either reducing the objects mass or keeping the mass close to the centre of rotation. This is why younger tennis players (with smaller muscle mass) tend to hold the racquet further down the handle and junior tennis racquets have shorter leavers.

Image 4: Novice server

 

Image 4: Elite serve (Novak Djokovic)


 

COMPONENT 3 - THE BALL TOSS

The ball toss in the tennis serve can be directly influenced by the concept of projectile motion that reflects the motion of an object that projects at an angle into the air; in this case, the tennis ball (Blazvich, 2012, p. 25). Projectiles are influenced by one force; gravity. Ideally, an object projected at 45 degrees will have an equal vertical and horizontal velocity. However, this is affected by the height of projection at racquet/ball contact. The optimal angle of projection decreases when the relative height is more positive. Conversely, when the landing level is more negative, the optimal angle of projection increases. In the tennis serve, the height the player reaches at their full leg extension is very significant. With a higher racquet head, the optimal angle decreases, therefore the ball reaches the target at a faster velocity (refer to Image 8).

 

 

Image 8: Roger Federer generating height through leg extension and creating a smaller angle at which to deliver the ball.

THE ANSWER

The answer to the optimal serving technique in tennis lies in the areas of the athlete’s legs, serving arm, and their ball toss. There is no optimal foot positioning, but generally players either use the foot-up thrust style, or the foot-back style. The FU style generates a high vertical movement, while the FB style creates more forward propulsive impulses, both which are beneficial to the game. Newton’s third law of motion indicated that a forceful leg drive for a longer period of time is beneficial as it builds momentum and generates a higher velocity that transfers to other areas of the body (i.e. forearm and racquet). In turn, this creates more force on the ball. While a push-like movement pattern is beneficial for beginner tennis players, as they become more advanced players should practice a throw-like motion during the serve to build velocity. Finally, the angle and height of projection during the serve is optimal when the player reaches a greater height, as the angle is decreased and the ball is projected more rapidly to the desired target.

HOW ELSE CAN WE USE THIS INFORMATION?

The information provided in this blog outlines not only the optimal technique for the tennis serve, but can provoke further understandings of techniques in various sports such as cricket, badminton and table tennis. In cricket, bowlers use a throw-like action, as opposed to a push-like motion, to generate speed. In table tennis, a push-like motion is valuable to generate accuracy on a smaller surface area. Biomechanics is also influential in the area of sports injuries and rehabilitation. For example, Brett Lee’s back injury as a result of leg positioning was recognised through understanding biomechanical principles.