How can a tennis player increase force and speed on the forehand stroke?

The Forehand ground stroke is the most common and most used stroke in a game of tennis. Using the principles of biomechanics, we can analyse how a tennis player can increase the force and speed of their forehand stroke.

 The Kinetic Chain

The kinetic chain can be described as the linked segments of the body that move together.  (Blazevich, 2012) There are two main categories of the Kinetic chain the push like movement and the throw like movement.

Push-Like Movement:  Is exactly as expected, we move as if we are pushing something. This tends to extend all the joints in our kinetic chain simultaneously in a single movement. (Blazevich, 2012) The push like movement is used to move things that are heavy. The push like movement has two important benefits, the first being that it acts simultaneously and the cumulative forces generated by each joint result in a high overall force. The second benefit is the simultaneous joint rotations often result in a straight-line movement. (Blazevich, 2010) A push like pattern can be used to increase force production and accuracy.

Throw Like Movement: differs from push like, in the joints of the kinetic chain extend sequentially, one after another. (Blazevich, 2010) In the movement of a throw like pattern the shoulder extends before the elbow and wrist; the shoulder actually begins to extend while the elbow is still flexing during wind up, or cocking phase. Later through the movement, the extension velocity of the hand and fingers increase significantly, resulting in higher ball release velocity. (Blazevich, 2010) An explanation on how the throw like movement results in higher velocity, is due to momentum being  generated in the proximal segments through production of large muscle forces and is transferred to distal segments. (Blazevich, 2012)

For a tennis player to increase the force behind their forehand stroke the push like movement and throw like movements have to work sequentially. Elliot states, efficient function, with maximal performance and minimal risk of injury, requires optimum activation of all the links in the kinetic chain designed for power. (Elliot, 2006) A lot of a tennis player’s strength comes from their torso, shoulder and forehand when executing a forehand stroke.  Normal shoulder biomechanical function requires an intact kinetic chain to create the energy, produce the forces and stabilize the joint in tennis activities. (Kibler, 1995). For a tennis player aiming to increase speed and power behind their forehand stroke the use of push like pattern is required for force and the use of the throw like pattern is required for speed. The movement pattern is essentially push-like improves accuracy and force, and a throw like pattern to increase ball speed while still maintaining exceptional accuracy.

The Magnus Effect

A spinning ball traveling in the air is confronted with the so called magnus effect. (Adrian & Cooper, 1989) There has been a long debate over the exact mechanism responsible for the development of the lift force on spherical objects such as a tennis ball. (Blazevich, 2012) There has been a large amount of research into the flight of a tennis ball and the advantages of placing a spin on the ball.  In 1672 Newton first noticed how a tennis ball flight was affected by spin, Eighty years later Robins showed the rotating sphere, such as a ball was associated with a sideways force. (Blazevich, 2012) The most common explanation of why a ball spins in flight is that the ‘ball’ grabs the air that flows past and because of the friction between the air and the ball; these particles begin to spin with the ball. (Blazevich, 2012) Further research shows that the velocity of a tennis racquet or another striking implement decreases slightly at contact.  In addition there is a short period of time when the ball is struck and can be compressed sometimes to as much as half its normal shape, before it rebounds. (Adrian & Cooper, 1989) If a tennis player wanted to hit the ball as hard as they could from one side of the court to the other, they would have to hit the ball in an upward direction to get it over the net. It would travel along way before gravity eventually would bring it back down the earth. (Blazevich, 2012) Therefore if a tennis player puts spin on a ball, where the top of the ball spins over the bottom of the ball (this is known as topspin) the air on top would slow down and the air underneath would move relatively quicker. (Blazevich, 2012). Due to the pressure being higher on the top of the ball, the Magnus force will direct the ball down with speed and spin.

 

 

The Coefficient of Restitution

The coefficient of restitution describes the proportion of total energy that remains with the colliding objects after the collision. (Blazevich, 2012) for example when a tennis ball is dropped, it almost always bounces back to the height that it was dropped, this is high coefficient restitution. The higher the speed of the ball and the higher the speed of the bat when they collide results in a higher coefficient restitution. For a collision between two objects, the coefficient of restitution is the ratio of the relative speed after to the relative speed before the collision. The coefficient of restitution is a number between 0 (perfectly inelastic collision) and 1 (elastic collision) inclusive. (Physicsforums.com,2014)

 Factors that need to be considered for tennis players to assist them in increasing speed are:

Increasing bat speed: Increasing bat speed results in higher total momentum of the system, but also make it more likely that the ball will continue to move forward after the collision.  (Blazevich, 2012)

Increase mass of the Racquet: Increasing the mass of the racquet increases total momentum of the system as long as the mass of the bat doesn’t compromise your ability to swing quickly.

Increase coefficient resolution: this will reduce energy lost in collision of the racquet with the ball; it will reduce slightly as the ball speed increase and increased as ball temperature rises.

Angular Kinetics

Tennis players are able to hit the ball with their racquet as they apply force through the swing and onto their racquet. The amount of speed a ball can reach depends on the amount of force applied by the racquet. Factors that make up the aspect of angular kinetics include.

Moment of inertia: tendency for a rotating body to remain in its present state of motion; equal to the product of the mass of an object and its radius of gyration. (Blazevich, 2012) Applies to Newtons 1^st law of inertia and can be translated to An object will remain at rest or continue to move with a constant angular velocity as long as the net forces causing rotation equal zero. (Blazevich, 2012) To move the player needs to overcome the resting inertia by using force (muscular contraction) or gravity.  (ITF Coaching, 2007) Simply moment of inertia is an objects resistance to change in its angular motion. (McGinnis, 2013)

Moment of force: lift force acting on a spinning object. (Blazevich, 2012) A force is simply push or pull and it changes the motion of a body segment or the racket. Motion is created and modified by the action of force. When Force rotates a body segment or the racket, this effect is called torque or moment of force. (UTSA, 2014)

Torque is able to alter the rotation of an object in any given moment where inertia is present. Applies to Newtons 2^nd Law. The angular acceleration of an object is proportional to the net torque acting on it and inversely proportional to the inertia of the object. (Blazevich, 2010)

Angular momentum: Product of the moment of inertia and angular velocity. (Blazevich, 2012) Angular momentum is the relationship between function of mass and velocity. The mass of the racquet remains the same for the match, so the greater the velocity, the greater it’s momentum. (ITF Coaching, 2007)

In summarising for a tennis player to have a quicker forehand they need to swing their arm from the backwards to forwards position (to follow through on the shot) quicker. Increasing the torque developed by the shoulder, and decreases the mass throughout the arm and racquet. Increasing the force behind the hit is vital.

The Answer

All racquet sport skills have a biomechanical base. Successful achievement of any stroke is therefore greatly influenced by technique in which the player hits the ball. (Elliott, 1995)  As mentioned a tennis player working on increasing the force and speed behind their forehand or ground stroke may need to  focus on few biomechanical aspects.  To improve speed and force looking at the way a players kinetic chain functions is vital. When executing a skill that requires both speed and accuracy like the forehand stroke, a progress from a push to throw like movement is needed. (Blazevich, 2012) Elite tennis players use extreme throw like patterns to increase ball speed, while still managing exceptional accuracy. Having the correct stance and movement technique is also vital.  “Majority of elite players emphasize trunk rotation and use an open stance in forehand stroke production in preference to stepping into the ball” (Elliott, 1995).   In addition to have the correct body posture and stance, Moving the limbs (arm) in a simultaneously sequence from the shoulder to the hand transferring force through the racquet will increase the speed applied to the ball.

As explained through the Magnus effect if a tennis player can manipulate they way in which the ball is hit, influencing spin, they should have no problem in trying to get the ball as far as they can. The common accepted explanation is that a spinning object creates a sort of whirlpool of rotating air about itself. On the side where the motion of the whirlpool is in the same direction as that of the wind stream to which the object is exposed, the velocity can be enhanced. (Briggs, 1959). For the ball to move quicker a player needs to put a top spin on the tennis ball, where the top spins over the bottom. The air on the top will slow down and the air on the top will move quicker. (Blazevich, 2012)

If a tennis player can influence various factors for example increasing speeds of the bat, increasing the mass of the bat and increasing the coefficient of restitution. Adjusting these factors will allow for a tennis player hitting the ball with extreme force.  A key factor would be finding a racquet in which the weight maximizes momentum during the swing, which still allows for a high swing velocity.

Finally understanding the principles that underpin angular kinetics can advance a tennis players forehand stroke. By holding the racquet at the end of the handle, but still gaining control will reduce the radius of gyration, and therefore the moment of inertia is decreased.  (Blazevich, 2012) By learning how to manipulate the bodies segments during the game provides the possibility of rotation to create stability of our body’s segments at any point during the execution of the forehand stroke. (Blazevich, 2012)

A tennis player can improve by influencing these factors, they will have no problem in increasing the force and speed behind their forehand stroke.

How Else Can We Use This Information?

All this information and biomechanical knowledge can transfer into many other sporting fields. For example, the use of the kinetic chain can be used in numerous sporting fields. For instance, in baseball or cricket a player who is batting may want to increase the speed or distance on their shot. Basketball and Netball players can also use the kinetic chains, throw like and push like patterns when passing a ball. The Magnus effect can be used with any sport in which there is a bat and ball, or some kind of lever. For example golf, cricket, table tennis and softball. Being able to control the spin on a ball can influence where players can place the ball. Coefficient restitution theory can be used by many coaches across all fields. Adjusting the speed in which the ball hits the bat or the speed in which any other two objects travel when they collide can be useful in game situations. Finally the knowledge of angular kinetics and the way in which the body moves can be used in all sporting environments. In particular track and field events. A runner with a higher angular momentum will require greater angular impulse or they will not be able to change direction as quickly.  (Blazevich, 2012) All of these examples are just minimal ways in which the biomechanical aspects of the kinetic chain, Magnus effect, coefficient of restitution and angular kinetics can be transferred into other sporting contexts. 

Reference List 

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