What biomechanical principles can be applied to help an athlete effectively execute the tennis serve?

The tennis serve is used to start every point in a match and is therefore considered to be one of the most important shots in the game of tennis (Bahamonde, 2000). The aim of the tennis serve is to hit the ball into the opponent’s service area while making it is hard as possible for them to return. Therefore players are continually being challenged to increase their serve velocity and use a variety of different serves in order to gain an edge over their opponent (Fleisig, Nicholls, Elliot & Escamilla, 2003). The serve is the only stroke in tennis in which the player has full control over its outcome and so therefore what biomechanical principles can be applied to help an athlete effectively execute the tennis serve? In order to answer this question we will focus on the sub questions: 

How does an understanding of the biomechanical principle of levers help to increase the velocity of a serve?

How does an understanding of the Magnus effect help the server execute the slice and kick serve?

Preparation Phase

During this phase the athlete determines the location and type of serve they are going to hit. This will be based on the positioning of the opposition player, their knowledge of the player’s ability and their confidence to effectively complete the serve. The centre of gravity has vital importance in the preparation phase of the tennis serve. Blazevich (2012) describes the centre of gravity as being “the point in which all particles of the body are evenly distributed” (Blazevich, 2012). This is known to affect balance and momentum. An athlete must make sure that their centre of gravity is over the base of support in order to maintain balance.

Wind-up Phase

The purpose of the wind-up phase, explained through the use of biomechanical principles, is to produce force summation in the lead up to the connection of the ball. This force summation results in momentum and force being applied to the ball causing it to accelerate. This wind up phase can be explained through a number of different key biomechanical principles. In this phase the athlete will have already decided what shot they are planning to play and will therefore wind-up appropriately. For a kick and slice serve the athlete focus less on trying to generate speed and more on trying to compute spin on the ball, whereas if they are planning on hitting the flat serve then their sole purpose will be trying to generate as much velocity as possible.

The kinetic chain is the complex coordination of individual movements about several joints at the same time. In biomechanics there are two main categories of kinetic chain patterns: push-like and throw-like (Blazevich, 2012). In the wind-up phase of the tennis serve the athlete is using both kinetic chain patterns. The push-like pattern is where all the muscles extend in a simultaneous motion to create a pushing force (Blazevich, 2012). When completing the serve the lower half of the body must push off the ground to create an extensional force. This push-like pattern allows the athlete to exert maximal power when connecting with the ball. In this push like pattern there are other biomechanical principles at work. This push like pattern can be described through Newton’s Third Law. Newton’s Third Law states that every action has an equal and opposite reaction. This is applied in a tennis serve when the athlete bends their knees and pushes off the ground in order to elevate to hit the ball. When the athlete exerts a force into the ground they are creating an equal and opposite reaction force in the other direction, in this case upwards. This force is also known as the Ground Reaction Force [GRF] (Blazevich, 2012; Durovic, Lozovina & Mrduljaš, 2008). Force platform studies have indicated that the most significant component of the GRF is the vertical component (Bahamonde, 2000) The GRF is the beginning of the summation of forces. The upper part of the body follows a throw-like kinetic chain pattern in the wind-up phase.

                                           Ground Reaction Force (Newton's 3rd Law)

The angular momentum during the tennis serve is a three-lever system comprising the trunk, arm and racket (Bahamonde, 2000). This three lever system allows the server to produce angular momentum about the X-axis of rotation, which is shifted from the trunk to the arm and then the racket.

The tennis serve is a third class lever system. In this system the angular velocity is greater when the further the resistance is from the force. Therefore when serving a tennis ball an athlete will maximise the speed of the serve if they connect at the highest point. By increasing this distance it also increases the torque, By increasing the moment arm (lever) more power will be put on the ball which will cause a greater acceleration (Blazevich, 2012).

In the video below we can see Andy Roddick, who is considered to be one of the best servers on the ATP circuit, hitting the ball at its highest point, and hence optimizing the speed on the ball.

Centre of gravity is also important in the wind up phase of the tennis serve. By shifting our centre of gravity backwards at the beginning it allows the athlete to fully extend the racquet arm behind the back. By doing this it increases the lever and also allows more distance for the racquet to swing and hence allowing more time to accelerate and generate a greater force. The shift of centre of gravity is evident in the image below. Centre of gravity will also be important in the connection and follow through phases. 

                                               

Connection Phase

The connection phase is the most important phase as this will determine how the shot is played. As with the wind-up phase there are a number of different biomechanical principles that can help to explain and therefore improve the tennis serve.

In the connection phase the kinetic chain is continued from the wind up phase. The player will need to continue with the throw-like pattern in order to maximise the power output. Also, as with the wind-up phase, the centre of gravity of the athlete is extremely important. In the connection phase the player begins to shift their weight forward. Since the weight was shifted back in the wind-up phase, by shifting their weight forward the server is able to use centre of gravity to create power and momentum. It is important that the server does not over balance in this phase. This will be discussed in more detail in the follow through phase.

Newton’s Laws are also evident in this stage of the tennis serve. Newton’s Second Law states “the acceleration of an object is proportional to the net force acting on it and inversely proportional to the mass of the object” (Blazevich, p.45, 2012). In the connection phase this is important since the mass of the ball remains constant throughout the whole process, therefore the acceleration of the object will be as a result of the force applied on it. Therefore if the ball is hit with a greater force then the ball will travel faster. Once again Newton’s Third Law will be at play here as the server is still propelling upward from the GRF generated in the wind up phase.

When deciding to hit the slice or kick serve, decision making will be made easier with an understanding of The Magnus effect. The biomechanical principle of the ‘Magnus effect’ refers to the spinning of the ball as it moves through the air. The Magnus effect specifically refers to the boundary layer of air that ‘clings’ to the surface of the ball as the ball spins through the air (Blazevich 2012), causing the ball to deviate from its typical trajectory or flight path. In the tennis serve, an athlete is able to hit the ball in such a way that spin is applied. By putting sidespin on the ball, the Magnus effect will cause the ball to deviate sideways and hence is called the slice serve. By placing top spin on the ball the ball will kick once it hits the ground and is known as the kick serve. See below how the Magnus effect works.

Magnus efffect on tennis ball

The Coefficient of Restitution is another biomechanical principle that can help to increase the speed of tennis serve. The coefficient of restitution the proportion of total energy that reminds with the colliding object after collision (Blazevich, 2000). To increase the coefficient of restitution there is not much the athlete can do during the serve. However if they wish to increase the coefficient of restitution then they should choose an appropriate tennis rack that allows maximum energy to be retained after the collision of the ball and the racquet.

We can increase the speed of the ball after the collision by increasing the speed of the racquet. While this seems obvious it is described biomechanically since this increases the total momentum of the system. Also, by increasing the mass of the athlete’s tennis racquet this will increase the total momentum and can increase the speed at which the ball comes off. This is only effective if the weight of the racquet does not compromise the athlete’s ability to swing the racquet quickly. See figure below for a comparison in size and power.

An understanding of levers and velocity can also help the server make the ball harder for the opposition player to return. It is apparent that the ball will obtain a greater velocity if the athlete swing their arms outstretched, as long as reaching out does not slow down movement. Therefore a server can use this to their advantage and aim a serve at the opposition’s body. This will make the opposition player swing for the ball without maximising their swing and therefore reducing the speed of the return (Blazevich, 2012).

Follow Through

The follow through phase is just as important in the serve as any of the other phases. It is important that the motion of the connection face is carried out throughout the follow through. Without a follow through momentum will be lost and therefore so will velocity (Blazevich, 2012). By continuing through with the service motion the athlete is able to prepare for the well for their next shot. Centre of gravity is again important in this phase. If the centre of gravity goes too far forward then the player will then become over balanced and will struggle to prepare for their next shot (Pollock, 2000). The following video explains the follow through. 

Ground Strokes Tennis Tip: Freeze The Follow Through For A Split Second!

The Answer

There are several biomechanical principles that can be applied to improve the effectiveness of the tennis serve. In terms of increasing the speed of the serve, ultimately the faster the racquet head accelerates the greater the force and impulse will be on the ball. Since force is directly proportional to acceleration then the ball will therefore have a greater acceleration and therefore velocity. To optimise racquet head acceleration an understanding of the kinetic chain is needed. An effective kinetic chain and fluid motion will help to conduct elastic energy and hence greater velocity. Optimizing the length of the leaver (distance between shoulder and middle of racquet) will also increase the momentum and speed of the ball. This is done by hitting the ball at its highest point.

When hitting the slice or kick serve, while the velocity of the ball is still important, the focus is trying to apply spin to the ball. By applying this spin to the ball this will result in a Magnus effect and hence the ball will swing in the direction on spin. For the slice serve the server should endeavour to put side spin on the serve so that it swings away from their opponent. For the kick serve they should strive to apply top spin so that the ball ‘kicks’ up off the surface unexpectedly. In order to do this the ball is contacted though the centre of the racquet and the athlete can do one of the following: 1) apply top spin by flexing the wrist over the ball, 2) cut the ball to the side by contacting the ball through the its centre of gravity and then flexing and rotating the wrist to the side of the ball inducing sideways rotation on the ball (Hopper, 2001).

How else can we use this information?

This information can be used for a number of different sports. The biomechanical principles of tennis, in particular the principle of levers can be very easily applied to sports which require the use of a bat or some sort of hitting implement such as badminton, where the overarm shot is used very often, lacrosse, baseball and cricket. The throw-like pattern of the kinetic chain can be applied to any type of throwing sport such as shot put, cricket and baseball. Sports that require jumping such as high jump, volleyball and basketball would also benefit from the use of the push-like pattern in the kinetic chain.

An understanding of the Magnus effect is extremely important for soccer players who are trying to kick the ball to get around a defensive wall. The Magnus effect will help them to curve the ball and deceive the goal keeper. These are just a number of ways in which these principles could be used in other sports, however an understanding of these common biomechanical principles will help coaches and educators in every sport. 

References

Babolat. (2013). Babolat Tennis: Products. Retrieved June 15, 2014 from: http://www.babolat.com/product/tennis

Bahamonde, R. E. (2000). Changes in angular momentum during the tennis serve. Journal of sports sciences, 18(8), 579-592.

Blazevich, A.J. (2012). Sports Biomechanics The Basics: Optimising Human Performance (2nd Ed.). London: Bloomsbury.

Fleisig, G., Nicholls, R., Elliott, B., & Escamilla, R. (2003). Tennis: Kinematics used by world class tennis players to produce high‐velocity serves. Sports Biomechanics, 2(1), 51-64.

Hopper, T. (2001). Biomechanical Analysis of the Tennis Serve Greg Emery 9707553 PE 117.

Pollock, A. S., Durward, B. R., Rowe, P. J., & Paul, J. P. (2000). What is balance?. Clinical rehabilitation, 14(4), 402-406.

Youtube:.Ground Strokes Tennis Tip: Freeze The Follow Through For A Split Second! https://www.youtube.com/watch?v=Bh3qvR1HRJU

Youtube: Andy Roddick Serves in Slow Motion: https://www.youtube.com/watch?v=SZbxKuLEP_o