What are the ideal biomechanics for the tennis serve & which professional player has the best serve biomechanics?

The tennis serve has often been described as the most important and pivotal shot in the sport. The serve plays a larges determinant in the result of a tennis match both at the amateur and professional level. This blog will analyse and compare the service actions of Roger Federer, Andy Roddick and Ivo Karlovic. Experts refer to Roger’s service action as ‘classical’ and like ‘poetry in motion.’ Andy’s service technique is not classical but it is arguably the most effective serve in history (Radicchi & Perc, 2011) . Ivo recently recorded a record of 45 aces in a three set match when he defeated Thomas Berdych in Halle, Germany (ATP, 2015).  Each of these players have biomechanical differences when comparing their serves against one another. This blog will look at the biomechanics of the tennis serve and will determine why these players are the best servers on the ATP tour.  

                    Rodger Federer         Andy Roddick         Ivo Karlovic    

The tennis serve has been described as a throwing action, however there is a racket involved. Essentially the Racket works as an extension of the arm working to reach the ball at the highest vertical point possible. Force is also generated by shoulder turn, forward momentum, hip rotation, vertical momentum, wrist pronation and knee bend.  There are three main stage to the serve these are; the starting point to trophy position, the contact point and the follow through post contact. Each of these factors will have a large determinant in the velocity and momentum of a serve (Chiang, Nien, Chiang & Shiang, 2007). 

The trophy position of a serve is a commonly referred to reference point of a players service action.

Essentially the take back or back swing for the serve aims to get the server  in to the trophy position (pictured by each of the subjects above). Good preparation leading into the trophy position is very important for delivering a strong serve. As shown above, each of the three servers becomes quite coiled through this stage of the serve. The body weight of the server is essentially on the balls of the feet, the knees are bent and ready to push hard off the ground to create linear momentum towards the desired contact point. Each of the servers left hips are elevated higher than the right. The right shoulder has turned parallel to the baseline (this is more evident in the Federer and Roddick figure) this is to create angular momentum (torque) to rotate into the ball. The left arm is elevated higher than the right, this is for two reasons. Having the left arm up means the server is able to focus on the elevated ball before contact. Secondly, as the tossed ball is dropping, the left arm of the right handed server is dropped. This means the left shoulder is also drops, meaning the right is being enabled to elevate itself higher at the ball. The trophy position is about creating the largest potential for summation of force through the kinetic chain to launch into the ball (Fleisig, Nicholls, Elliott & Escamilla, 2003). Newton’s third law of motion is at play during this phase of the serve. For every action there is an equal and opposite reaction. The toes and knees are being pushed downward into the ground in order to propel the server both forward and upward toward the ball. The forced placed into the ground by the servers lower body dds further torque and linear momentum prior to the moment of inertia (Blazevich, 2013).  

If a server is to deliver a fast and accurate serve it is essential that the player makes contact with the ball at the correct height and angle.

The torque produced through the trophy position stage of the serve comes into play during the contact point phase of the serve. The uncoiling of the knees, hips, core muscles and shoulders makes it possible for the server to connect with the ball at the highest possible point. The height of the contact point a server is able to reach, is ultimately dependent on the amount of torque the athlete is able to produce prior to contact. As a result of the torque produced and the uncoiling of the kinetic chain, the server’s feet leave the ground. Once the servers racket has made contact with the ball it becomes property of projectile motion. Projectile motion is defined as the motion of an objects path (usually curved) which has been projected at an angle into the air through to when it lands (Blazevich, 2010). Newton’s second law comes into play during this phase of the serve. The second law of motion states that force equals mass times acceleration (Blazevich, 2013). The speed of a serve is determined by the force created by the uncoiling of the kinetic chain making contact with the ball, which weighs about 0.056 kg (ATP, 2015).  The angular velocity and acceleration of Federer’s right shoulder joint when working its way from the trophy position to contact point is said to be 2,300 degrees per second. The force produced by the uncoiling of the body’s kinetic chain meeting the ball at the moment of inertia will ultimately determine the  force of a serve (Barnett, Meyer & Pollard, 2008). Federer’s fastest serve left his racket at 135 miles per hour (ATP, 2015). 

Rodger Federer’s body is tilted significantly forward when he hits the ball, his torso is tilted 38 degrees forward on average when he hits the ball. This tilt creates a good base to launch forward into the ball. A pitcher in Major league Baseball is generally tilted at 40 degrees forward from the torso when releasing the ball for a pitch (Blazevich, 2013).  Federer’s left hip moves forward 8-10 inches on preparation to make contact with the ball. Roger's left arm and left elbow begin to move down, to make the right shoulder tilt forward towards the ball, this enables his to fully extend his right arm at the ball. The angular velocity of Federer’s right shoulder joint when working its way from the ‘trophy position’ to contact point is said to be 2,300 degrees per second. Federer’s maximum external arm rotation before contact is 79 degrees. The rotations and uncoiling created from twisting and bending the knees, hips, core muscles and shoulders gives Roger the maximum amount of torque on the moment of inertia when his racket meets the ball (Fleisig et al., 2003).

Andy Roddick is the same height as Federer at 188 cm's. He weighs in at 88 kg’s compared to Federer’s 85. Roddick has a much faster serve on average than Roger. Andy's fastest ever serve recorded was timed at 155 mph (ATP, 2015).

Andy’s front foot is placed 90 degrees to the baseline prior to beginning his action. Prior to contact Roddick's hips turn 123 degrees from the take back to the trophy position, this brings his hips  almost parallel to the baseline. His shoulders run in line with his hips and also turn 123 degrees away from the ball. During the transition from trophy position to contact point Andy's elbow is bought up and points upward toward the sky. His right arm also turns 127 degrees away from the ball toss.It is said that 1 mph is added to a serve for every extra degree of external arm rotation. Basically, the further a server is able to turn away from the ball more linear momentum and torque can be produced within the action. Andy’s hips rotate further forward than any players on tour, his hips are rotated square to the baseline on contact point (Barnett et al., 2008). 

Newton’s first law of motion is also at play throughout the projectile motion of a serve. Newton’s first law states that an object will stay at rest or in uniform motion unless it is acted on by an external force. If a tennis ball were to be served in outer space it would carry on forever if nothing interrupted its path. When a tennis player serves there are three elements which will decelerate the momentum of the  ball these are; wind resistance, gravity and the ground (Blazevich, 2013). 

Each of these three players use a continental grip when serving, this is very similar to the grip you would use when hammering a nail. If a racket is held at the mid line of the body at naval height a ‘V’ shape will appear between the index finger and the thumb, the edge of the racket will be visible, the strings should not be visible from this angle. The three subjects in this blog each grip the racket at the base of the grip. Basically, the lowest point possible before the hand begins to slip off. This is to increase the radius of rotation before contacting the ball. Essentially, holding the racket further down the handle towards the butt cap of the racket is putting more mass into the head of the racket. Some players stick lead tape to the head of the racket to increase their rackets mass. Increased mass and radius of rotation means there is far greater potential for a more explosive moment of inertia, making for a more powerful serve. The handle should be grasped firmly to ensure the racket doesn’t stray from the players hand; however it must be gripped too tightly. The grip must be loose enough to allow for sufficient pronation of the wrist joint. An overly tight grip will lock the wrist and therefore make for a less explosive moment of inertia, which will make the serve significantly slower  (Bahamonde, 2000).

Ivo Karlovic is the tallest player on tour at 211cm's, he weighs 104kgs. Karlovic is 36 years old and still playing great tennis. His current ranking as of the 8^th June 2015 is 27 on the ATP tour. Karlovics fastest serve was timed at 156 mph (ATP, 2015). 

Due to Karlovics height he does not need as much external arm rotation in comparison to the likes of Roddick and Federer to produce powerful serves. The strengths of the Karlovic action lie within the parametric acceleration of his racket arm prior to the moment of inertia and the pronation of the wrist ‘wrist snap’ upon the moment of inertia. Federer and Roddick both keep their feet close to each other during their service action. Karlovic brings his right foot closer to his left prior to contact point. This is what is referred to as a ‘foot up’ serve in tennis. Karlovic at 211 cm’s doesn’t need to create the same torque and vertical momentum to reach up to the ball as most players, he has a predisposed height advantage. This saves Karlovic’s body a lot of strain. Instead, Karlovic’s main concern is creating forward momentum into the ball. By sliding his right foot (back foot) towards his left foot (front foot) he is transferring his weight forward through the ball during the moment of inertia. Though Karlovic may not produce the same torque are linear momentum during his action prior to the moment of inertia, his height of release is still significantly higher than his other two subjects (Chiang et al., 2007). 

The follow through and landing stage of the serve occurs after the contact point. Following the moment of inertia, the pronation of the right arm continues beginning from the wrist joint. Much like the follow through of a cricket bowling action, the left arm continues to pull down and rotate around the left side of the body (Blazevich, 2013). As the left arm continues its path the left foot of the right handed server comes into land. The left arm begins to move behind the torso of the server bringing the right arm and shoulder through. Essentially the shoulders are replacing each other (the body rotates 180 degrees). If the server has successfully transferred their weight through the ball their left foot should land inside the baseline. From here the right foot should swing through after the left has landed. From here the right foot either acts as a break to halt the momentum of the servers body or continues forwards to maintain a servers momentum towards the net (McGinnis, 2013).

Though this chart is very basic, it shows how the height of a servers contact point will be a large in factor of where the ball lands in the court. Due to Federer and Roddick being the same height Michael Chang is being used as a third example. Chang is 175cms tall, one of the smaller players during his era. Chang's chances of making a flat (limited spin) first serve are relatively low, due to a low margin for error. His serve has a higher likelihood of dropping into the net or going long of the service box in comparison to the other subjects. Roddick and Federer are of reasonable height, the torque and linear momentum created through the uncoiling of their kinetic chain will ensure they have a reasonable chance of landing flat first serves. Karlovic has a very high probability of landing a high percentage of flat first serves. Due to his height and long arm span he can basically pronate his wrist straight over the top of the ball, essentially hitting the ball downwards into the court. It also gives Karlovic a wider range of options as to where he can serve the ball in the services boxes,  as the net becomes less of a hurdle. He will be able to hit flat serves into shorter angles of the service box due to the downward projectile motion of his serves trajectory (Chiang et al., 2007).

Career serving statistics (ATP, 2005)

	Ivo Karlovic	Roger Federer	Andy Roddick
Career Matches Played (ATP) win/loss	524 275-249	1,263 1030-233	825 612-213
Aces	9,613	9,198	9,074
Double Faults	1,545	2,306	1,585
Percentage of first serves made (%)	66%	62%	65%
Percentage of points won when first serve lands in (%)	83%	77%	79%
Percentage of points won on the second serve (%)	53%	57%	56%
Break points faced	1,960	5,468	2,996
Break points saved (%)	70%	67%	68%
Service games played	6,854	15,435	9,804
Service games won (%)	92%	88%	90%
Service points won (%)	72%	69%	71%

The Answer Part 1. 

A biomechanically correct service action will be different for each individual depending on height, age, body weight and gender. However, there will be some commonalities in all technically sufficient service actions. It is essential that a server strives to uses as much summation of force through the kinetic chain as possible to achieve the most explosive moment of inertia and highest possible contact point. To achieve this, a tennis player must use their knees and feet to apply force against the ground to create forward and vertical momentum (Blazevich, 2010). To do this a server must turn their hips and shoulders in the opposite direction to the ball, turning parallel to the baseline to create maximum torque through the shoulders, hips and core muscles as possible when driving up to the ball. External arm rotation from the trophy position through to the contact point will vary for each individual; at least 65 degrees of flexion from the elbow joint would provide the serving arm with sufficient angular velocity to extent vertically towards the ball. Grip will also be important, a continental must be used so individual have adequate range and flexibility upon contact. This grip will ensure the server can use both a wide and tee serve. Holding the racket at the very base of the handle will ensure the server has a greater radius of rotation and leaves more mass in the head of the racket. This makes for greater impact on contact point. If the kinetic chain uncoils correctly the server’s front foot should land inside the baseline with the back leg coming through after impact. The servers back shoulder and back foot should come through replacing the previous front shoulder and foot (Blazevich, 2013). 

The Answer Part 2.

Mathematically Karlovic has the best serving percentages, he has played 525 games on the ATP tour for 9,613 aces and leads all other statistics apart from percentage of points won on the second serve. From a sports science perspective Roddick's serve is pure genius, the amount of external arm, hip and shoulder rotation he produced is unheard of. Being able to serve the way he did at 188 cm's is astonishing. However, the strain required for this action was possibly to the detriment of the longevity of Roddick's career. Roddick suffered continual shoulder, hamstring and oblique injuries throughout the later stages of his career and eventually retired at age 30. Federer is currently 33 years old and has played 1,263 games on tour, this more than doubles Karlovic's total of 524. Federer's service statistics aren't quite as good as Karlovic's, this is to be expected given the height difference. Federer's action doesn't put too much strain on any particular part of his body, he has never suffered many injuries. Federer entered the professional ranks in 1998 whereas Karlovic turned pro in 2000 (ATP, 2015). Federer's career win/loss ratio is far better than Karlovic's, this is the main reason why he has played more games. Karlovic has not had many injury concerns throughout his career which suggests he hasn't many technical floors. It is fair to say, given his physical stature, serving percentages and equal longevity of  career |Karlovic has a biomechanically more effective serve than Federer's. Karlovic's game evolves around his serve, if it weren't for his serve he probably wouldn't be in the ATP top 100. Federer has a more holistic game style, he isn't overly reliant on one particular shot, this is why he has had the more successful career.

References

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

Barnett, T., Meyer, D., & Pollard, G. (2008). Applying match statistics to increase serving performance. J Med Science Tennis, 13(2), 24-27.

Blazevich, A. (2013). Sports biomechanics. London: A. & C. Black.

Blazevich, A. (2010). Sports biomechanics : the basics : optimising human performance (2nd ed.). London: A. & C. Black.

Chiang, C., Nien, Y., Chiang, J., & Shiang, T. (2007). KINEMATIC ANALYSIS OF UPPER EXTREMITY IN TENNIS FLAT AND TOPSPIN SERVE. Journal Of Biomechanics, 40, S196. doi:10.1016/s0021-9290(07)70192-6

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.

McGinnis, P. M. (2013). Biomechanics of sport and exercise. Human Kinetics.

Radicchi, F., & Perc, M. (2011). Who is the best player ever? A complex network analysis of the history of professional tennis. PloS one, 6(2), e17249.

Websites

ATP World Tour. 2015. Players career statistics. [ONLINE] Available at: http://www.atpworldtour.com/en/players. [Accessed 25 June 15].