WHAT ARE THE OPTIMAL BIO-MECHANICAL PRINCIPLES OF THE TENNIS SERVE?

The tennis serve is arguably the most important shot in a tennis match and one that the player can have full control of the outcome (Bahamonde, 2000). However, the serve is a complex movement sequence which requires good coordination of the upper and lower limbs to be able to be performed correctly and successfully. The optimal aim of the tennis serve is for the player to hit the ball over the net in the opponent’s service box trying to make it difficult for the opponent to return successfully or efficiently. The faster the serve, the more difficult it will be to return. How does a player serve at a high pace while still being able to control movement, coordination and prepare for the next shot? Understanding the biomechanics of a sports skill allows for player success, performance increase and reduced risk of injury (Elliott, 2006). Developing optimal stroke production in tennis requires an analysis of the biomechanical elements behind the movement (Elliott, 2006). Therefore, this blog will serve to answer the question of ‘what are the optimal biomechanics of the tennis serve?’ 

BIOMECHANICAL PRINCIPLES 

The biomechanical principles that are within the tennis serve include:

• Impulse momentum
• Kinetic chain
• Torque and center of mass
• Inertia (moment of inertia)
• Angular kinetics and the conservation of angular momentum 
• Newton's three laws of motion
• Push-like and throw-like patterns of movement
• Coefficient of Restitution 
• Velocity
• Acceleration 
• Projectile motion (angle of trajectory)

PREPARATION PHASE

The preparation phase involves the player preparing stance and a mental mind set in order to perform the tennis serve. The player’s centre of gravity plays an important role in the stance. The player must ensure their centre of gravity is over the base of support in order to maintain an optimal balance and improving momentum. The player's feet should be shoulder width apart. For a right handed player, the left foot must point diagonally across the court at roughly a 45-degree angle (it should be point to the right net post) with the right foot parallel to the base line (reverse for left handed player). To prepare for the force and momentum production of the next phase, the player's shoulders and chest should be facing the right hand tram-line. One of the principles of bio-mechanics relevant to the stance is potential energy. Potential energy is energy associated with positioning which will be further explained in the latter phases. Range of motion is a principle which is increased by standing with shoulders perpendicular to the net allowing for their rotation later in the movement and thus benefiting both the amount of force able to be generated and accuracy of the serve. Both arms extended in front of the body allows for greater rocking backwards and greater mass generation to increase forward momentum in the next phase. By having the ball toss arm placed in front of player,  the ball is then made easier to be tossed in front of the player which will allow for bigger swing and forward hitting action. These positional changes maximize the potential energy going into the tennis serve (Blazevich, 2012).

BALL TOSS

Kinetic energy can be measured to ensure ball toss is thrown slightly forward of the body for optimal principles that will be explained in this paragraph. The potential energy the ball toss gets positioned in front of the player to allow for forward motion, a high ball toss allows for a bigger swing as there is more time to position and extend the arm and complete the sequence before the ball is at the optimum height. During the ball toss phase of the tennis serve, the bio mechanical principle is a push like movement. To achieve this, the shoulder, elbow and wrist joints of the tossing arm are simultaneously rotated creating a straight line movement with the benefit being an increase in accuracy in the ball toss. It is important to have the ball toss arm straight before the ball toss to create the straight line in the simultaneous rotation of joints. Projectile motion is the trajectory of the ball toss is affected by three factors. These are the projection speed, the projection angle and the vertical distance between the landing and the release point, referred to as relative height projection. In the ball toss phase, the ball is accelerated by gravity at the same rate throughout the downwards motion, it is important to understand and recognize this in order to make contact with the ball at the optimum height. The principle of projection speed is also related in this component of the ball toss. This is because of the effect of horizontal velocity and flight time which together effect the range (the distance it travels before hitting the ground). Because in this phase the ball is thrown vertically, projection speed is what regulates how high the ball tennis ball reaches before it is affected by gravity and accelerated downwards towards the ground. The outcome of being aware of this principle, is a consistent and regulated ball toss which is a vital aspect in determining the effectiveness of the serve. The standard rate at which an object accelerates (without taking into consideration of wind resistance) is at 9.8 m/s/s. Newton’s law states that the tennis ball will continue to travel at the same velocity until acted upon by an external force. The external force in the tennis serve, is at the racquet. Gravity pulls the ball in the direction of the earth and wind resistance decreases the ball’s speed. Both these forces effect the path of the ball, in tennis these are a common challenge that need to be acknowledged. The focus of this first law is on velocity not speed, this refers to the object both continues at the same speed as well as in the same direction. The principle of Newton’s third law refers to ‘every action has an equal and opposite reaction’, therefore, the ball going in an upward motion then downward, both being affected by gravity is how this law relates to the serve (Blazevich, A, (2012).

WEIGHT TRANSFER

Kinetic energy is something that can be measured at different areas of the body with the aim of measuring and ensuring maximal weight is exerted during the connection phase to allow for maximum force. During this phase, potential energy is achieved when the optimal position is to begin with both arms extended in front of the body to allow for ‘rocking’ backwards with more ease and creating greater mass generation in the following forward rocking. It is important to clarify that the ‘rocking phase if performed when the hips are rotated forwards the backwards as far as possible causing the players trunk to ‘rock’ back and forth Kovacs, M. S., & Ellenbecker, T. S. (2011) & (Blazevich, Pp. 45(2012). In the weight transfer phase, to increase range of motion, weight is moved over the midline to increase forward moment. AS EVIDENT IN THE VIDEO, this is done by transferring the player’s weight from the back foot forwards as soon as the ball toss arm reaches its peak. To increase forward momentum and force, the player must push off their back foot, followed by an extension of the legs, proceeded by the torso and finally the hitting arm. This is a throwing like movement pattern because of the sequential extension of the joints. Newtons First, Second and Third law applies to the phase of weight transfer with its component of ‘every reaction has an equal or opposite reaction’ (Blazevich, Pp. 45(2012). Force summation is generated and increased with the principle of applying a vertical force through both feet. Ground reaction force is applied as the ground exerts and equal and opposite reaction to force. This principle is used to overcome inertia as the ground exerts equal and opposite ground reaction force which is used to accelerate the player in a forward motion overcoming the body’s inertia (Blazevich, (2012). Applying this law put simply, means applying force against something that doesn’t move (the ground) and consequently, an equal and opposite force will be exerted against the player. The benefits of applying this principle is greater force is applied to the player from the object when greatest force is applied to the object. Secondly, by applying specific force in an opposite direction causes the force summation to accelerate the player in the desired direction. The earth having more mass than a human, means propulsion created effects the player only, allowing the player to jump as high as possible making contact with the ball at the highest point possible. The two laws work hand in hand in this phase. The way in which the three laws are distinguished are; Newton’s first law, is about overcoming inertia, which is achieved through using the second law where the player applies force against themselves (F=ma). The third law comes into practice when a deliberate force is applied against the earth (ground) for an equal and opposite reaction with the outcome of a summation of force generation for the jump and leading onto the next phase of the swing (Baurès, Benguigui, Amorim, & Siegler, (2007). Force summation combination of forces from different parts of the body. Angular momentum is applied before the rocking action takes flight, the pelvis and the shoulder must be tilted. The benefit associated in doing so, is the facilitation of developing angular momentum though utilizing lateral trunk flexion as during the forward motion the body takes. Simply put, in doing this allows for greater velocity in the serve. This principle in evident in the video (Reid, Elliott & Alderson, (2008). In a stretch-shorten cycle, elastic energy that is stored during the stretch phase of the tennis serve is partially recovered and during the shortening phase it is enhanced. The performance benefit of these two actions are critical to success in tennis (Elliott, 2006). It is essential that only a short pause occur at maximum knee flexion during the serve in order for stored energy to have full benefit (Elliott, 2006).

CONNECTION PHASE

Bio-mechanics plays an integral role in successful stroke production. The connection phase determines the overall end outcome with how the shot will be played. For the connection phase of the tennis serve, there are numerous body segments (see table 1) that must be coordinated together to enforce a high racquet speed to be generated upon impact, in skill acquisition terms, this is referred to as information movement coupling as students are required to couple the relevant information in the environment (the ball toss) with the student’s movement (the action of throwing the ball). Activation of all these body segments is required in the kinetic chain to produce power. The more powerful the swing of the serve, the more momentum and force the ball will have and the more difficult it will be for the opponent to return. During this phase the player shifts their weight forward so their centre of gravity can create force and forward momentum to increase the effect of inertia.

(Elliott, 2006)

The Magnus effect is another principle involved in this phase. It can determine where the ball lands or the direction it takes when unequal pressures are forced upon the ball with the players choosing between the serve having a back, side or front spin. Players can use this effect to their advantage. To perform a slice serve, the player can place a sidespin on the ball causing it to diverge sideways. To perform a kick serve, the player can place a topspin on the ball causing it to bounce high once it hits the ground. The more force applied on the ball, the greater the speed and rotation the Magnus effect has. Elite players should learn to manipulate this effect to their advantage to confuse opponents. (Blazevich, 2012). Another biomechanical principle that can be utilised in this phase to hit a ball further and faster is the Coefficient of Restitution. This is defined as ‘the proportion of total energy that remains with the colliding objects after the collision’ (Blazevich, 2012). The colliding objects in this instance are the racquet and the ball. By increasing the speed or mass of the racquet this will increase the momentum of the system (Blazevich, 2012). This will in turn increase the coefficient of restitution, which decreases the energy lost in the collision of the racquet and the ball. While the player uses a push-like movement in the wind up phase of the serve, in the connection phase the player uses a throw-like movement pattern similar to an overarm throw movement (see figure 2) (Blazevich, 2012). 

When performing the swing movement with the racquet in the serve, the body’s joints in the kinetic chain extend sequentially and it creates angular momentum. Angular momentum is the product of moment of inertia and angular velocity and correct performance of the tennis serve requires transfer of linear and angular momentum (Bahamonde, 2000). This angular momentum during the serve is a three-lever system; trunk, arm and racket. This system allows for the players to produce angular momentum on the x-axis that is shifted from the trunk to the arm and then to the racquet (Bahamonde, 2000). Elite players use an extreme throw-like movement pattern in order to increase ball speed while still executing excellent accuracy (Blazevich, 2012). Kinetic energy is associated with how the player can manipulate the product of mass and velocity to create a more successful tennis serve. Determining kinetic energy is measured as 0.5(mass) x velocity(squared).  Holding the racquet further down to extend to leaver creates greater inertia. The ball will acquire greater velocity if the player stretches their arms while swinging to extend the leaver and as long as this does not slow the movement down this will make it more difficult for the opposition to return (Blazevich, 2012). Increasing the mass of the racquet will also increase the momentum of the service but also as long as the mass doesn’t slow the movement down. Newton’s second law is also apparent in this stage. The mass of the tennis ball remains constant unless acted on by force of an object. Therefore, the greater acceleration and force of the swing, the faster the ball will travel.

Figure 1 Pictures of service action showing shoulder‐over‐shoulder trunk rotation and internal rotation of upper arm at the shoulder joint (Elliott, 2006)

The follow through is movement following the swing phase. A player who develops a strong follow through motion will be able to successfully prepare for the next hit. The follow through helps the player keep momentum and velocity and without it they will be lost (Blazevich, 2012). In order to prepare for the next shot the player must keep their balance under control during the follow through. The players centre of gravity must be over their forward foot. Once their body moves in a forward motion they regather by doing a split step where the centre of gravity to the centre of their body where optimal stability is achieved. Figure 1 demonstrates how the internal rotation of the shoulder which begins before connection, follows through after the connection. 

ANSWER

To answer the question of "what are the optimal bio-mechanical principles of the tennis serve?", it can be summarized by breaking the action down into four different phases and applying a variety of principles specific to each phase. the first phase of the serve is called the stance, followed by the ball toss, and finally the wing. The bio-mechanical principles relevant to each phase have been identified. As well as identified, the way in which they effect the serving action has been clearly explained. For an optimal serve action, both the coaches/teachers and students must understand the concepts involved and they in which they relate to the action and specifically how they benefit the overall outcome of the skill.

REFERENCES

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

Blazevich, A, (2012), “Sports biomechanics, the basics: Optimising human performance”, A&C Black, Pp 44-202.

Elliott, B., Fleisig, G., Nicholls, R., & Escamilia, R. (2003). Technique effects on upper limb loading in the tennis serve. Journal of Science and Medicine in Sport, 6(1), 76-87.

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.

Reid, M., Elliott, B., & Alderson, J. (2008). Lower-limb coordination and shoulder joint mechanics in the tennis serve. Medicine+ Science in Sports+ Exercise, 40(2), 308.

Bosco, C., Luhtanen, P., & Komi, P. V. (1983). A simple method for measurement of mechanical power in jumping. European journal of applied physiology and occupational physiology, 50(2), 273-282.

Kovacs, M. S., & Ellenbecker, T. S. (2011). A performance evaluation of the tennis serve: implications for strength, speed, power, and flexibility training.Strength & Conditioning Journal, 33(4), 22-30.

Baurès, R., Benguigui, N., Amorim, M. A., & Siegler, I. A. (2007). Intercepting free falling objects: better use Occam’s razor than internalize Newton’s law. Vision research, 47(23), 2982-2991.