Introduction to the Kinetic Energy Chain
If you have ever wondered how that little kid on the court next to you can hit the ball harder than you, take a quick look at the fence around the tennis court. A person with a powerful forehand has a lot more in common with that fence then you probably realize.
If you have a weak forehand you probably have more in common with this broken fence.
For a world class forehand the racquet will accelerate from 0 miles per hour at the start of the forehand swing to around 90 miles per hour at the time the ball is impacted. Now most players will never have a world class forehand, but by following the same techniques of generating power that the pro’s use a player can greatly increase the power of their forehand. There are two main forces that help to accelerate the racquet during a forehand swing. A force is defined as something that causes an object with mass to change its velocity, direction or shape and is described in much greater detail here. The more an object accelerates, the more force it creates as force is equal to mass (or weight) times acceleration. The more force the racquet has, the faster a tennis ball can be hit and the easier it is to change the balls direction.
The first source that helps generate power is gravity (The effect of gravity on the swing is discussed in greater detail in the gravity section.) For every foot the racquet drops, gravity helps to accelerate it approximately 6 mph. Most racquets will drop between three and four feet during the forehand swing if using a loop swing. This will generate about 18-24 miles per hour of racquet head speed. This leaves approximately 70 miles per hour of speed still needing to be generated for a world class forehand. The other 70 miles per hour of racquet head speed for the forehand is generated from the body. In order to have a powerful forehand the whole body must be used (not just the arm) to swing. Using the whole body to swing, besides being a greater source of power, also limits the shock to the shoulder and arm. People who swing mostly with the arm have a much greater chance of sustaining an arm injury.
The Kinetic Energy Chain
The various parts of the human body act like a system of chain links. The force or energy generated from one part, or segment of the body, as it decelerates will transfer into the next part of the body. This transfer will increase the energy in the new segment and cause that part of the body to accelerate. As each new segment starts to decelerate it transfers its energy (and the energy from all the previous segments its acquired) into the next segment causing that segment to accelerate even more than the previous segment. This process is known as the kinetic energy chain. Kinetic energy is defined as the energy which an object possesses due to its motion.
Once the chain is broken all that previous energy that had been built up cannot be regained. This will severely limit the amount of power in the shot. The only way to make up for some of this power loss will be to recruit muscles not normally needed to hit the ball. This can lead to many potential problems for the swing including a less fluid swing, a less powerful swing, a swing that is much more difficult to control and, if done long term, can lead to injuries. The two biggest causes for losing kinetic energy are only using the arm to swing instead of the whole body and not decelerating a part of the body so another part can accelerate.
Segments – Forehand Kinetic Energy Chain
The various parts of the body that make up the kinetic energy chain are called segments. For the forehand there are four segments. The power generated during the kinetic energy chain starts from the ground and works its way up the body. The first segment of the kinetic energy chain goes from the ground to the knees. For a right handed player it is the left foot and the left knee that segment one encompasses.
This segment will generate reactionary forces from the ground and be the first segment of the body to decelerate. Once this segment starts to decelerate it will transfer the energy its built up into segment two which will accelerate segment two.
Segment 2 goes from the knees to the hips. For a right handed player it is the left knee to left hip.
This segment will take the energy from segment one, generate energy of its own and transfer all the accumulated energy into segment three as it decelerates.
Segment 3 goes from the hips to the shoulders. The third segment goes from the left hip up through the right shoulder on a right handed player.
The energy obtained from the first two segments is transferred to segment three. This accelerates the third segment even faster then the previous segments. Once it starts to decelerate it transfers the accumulated energy into the final segment.
The final segment includes the arm and racquet. For a right handed player it is their right arm.
With all the previous segments energy plus the added energy from this segment the racquet will end up with a lot of energy transferred into it and be accelerating at a much faster rate then if the body was not used. Below is an image of the overall kinetic energy chain flow.
When hitting a forehand the body will decelerate first at the knee, then the hip then the shoulder and then the arm. The first two parts of the kinetic energy chain stop in relatively quick succession. These segments all start and end during the racquet drop stage of the forehand.
The third segment starts to accelerate at the racquet drop stage,
and decelerates during the forward swing stage.
The fourth segment starts accelerating during the forward part of the swing
and finishes at the end of the follow through.
Kinetic Energy Chain – Stretch-Shortening Cycle
The transfer of energy from segment to segment is also enhanced by the stretch-shortening cycle of the muscles. A stretch-shortening cycle is basically what it says, as a muscle stretches it is followed by a shortening or contraction of the same muscle. The more the muscles are stretched the more energy they will build up which will be released once the muscles shorten.
Think of the stretching of the muscles like a rubber band.
The more the rubber band is stretched, the more potential energy that is in the rubber band. Potential energy is just what it implies, its energy that is possible to use, but not guaranteed to be used. The scientific definition of potential energy is the energy of an object or a system due to the position of the body or the arrangement of the particles of the system. Using the rubber band analogy this basically means that if someone wants to shoot a rubber band really far, they pull it back far. If they want to shoot it a shorter distance they don’t pull it back as far.
Once the rubber band is let go, it converts the potential energy into kinetic energy. The further it is stretched the more potential energy the rubber band has and the further it will fly through the air. This happens because the rubber band is taking the potential energy built up by stretching it and turning it into kinetic energy when its let go.
The less it is pulled back the less the potential energy and the less the kinetic energy it has once it is let go. The less stretched rubber band shown below will not fly near as far as the fully stretched rubber band above. Even though its the same rubber band the less stretched rubber band flies a less distance because the potential energy of the rubber band was not used to its fullest extent as it was never stretched all the way.
Potential energy can also be wasted. Once again using the rubber band analogy. If the rubber band is pulled back really far it has great potential energy, but an unknown amount of kinetic energy,
The kinetic energy is unknown because someone can slowly “unstretch” the rubber band until it is barely pulled back and then lets it go. This will make the rubber band fly a much shorter distance then if it was let go when it was pulled back far.
So even though there was a great amount of potential energy when the rubber band was fully stretched, the amount of kinetic energy was diminished by “unstretching” it before releasing it. A majority of the potential energy went unused when the rubber band was unstretched.
A good example of the stretch-shortening cycle when hitting a forehand is the stretching of the chest and shoulder muscles as the trunk of the body rotates into the shot. This stretching of the chest and shoulder muscles occurs because the arm and racquet do not want to change the direction that it is moving (down and towards the back fence) . This unwillingness to change direction is known as inertia. Inertia is defined as the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. As the body starts to rotate in a different direction than the arm and racquet, the arm and racquet will lag behind since it doesn’t want to change direction. This causes the muscles to stretch as the initial inertia forces on the racquet are greater than the forces of the muscles. In other words the arm and racquet are creating stronger forces then the rotating chest and shoulder muscles and the greater forces always win. Eventually the inertia force of the racquet and arm is less than the force of the stretched muscles. When this happens, first the arm and then the racquet are forced in the direction that the chest and shoulders are moving, The contracting of the muscles starts and this whips the arm and racquet forward causing a tremendous increase in racquet and arm speed. This is most obviously seen with the naked eye by looking at the butt of the racquet, The below images shows this in action.
In the first three images the chest and shoulders are starting to uncoil while the hitting arm and racquet are staying relatively in the same position. Hover over the below image to see how far the shoulders have started to uncoil.
You can tell the hitting arm hasn’t moved much by looking at the hitting hand. At this point the inertia of the arm is still greater than the kinetic energy of the uncoiling shoulders so it has not moved much. The muscles in the arm are being stretched by the uncoiling shoulder and chest and will start to contract once the stretching of the muscles causes the energy in those stretched muscles to be greater than the inertia force of the arm. This contracting of the muscles will start the arm moving. This is starting to happen in image 4 as the hand is starting to move away from the body.
In images 5 thru 8 the arm is moving out away from the body as the muscles in it start to contract and the kinetic energy is transferring into it. This is really obvious by looking at the hitting hand. By image 8 it is away from the body. The racquet head is still not moving in the direction that the rest of the body is headed as its inertial force is still greater then the forces of the body that are trying to act on it. Not much kinetic energy has been transferred into the racquet yet. This is what causes the butt of the racquet to point towards the net as the racquet handle starts to move in the same direction as the rest of the body before the head of the racquet does.
Image 5 shows that the head of the racquet is to the right of the butt of the handle. By image 8 the head of the racquet is to the left of the butt of the handle. Hover over the below image to see the butt of the racquet moving away from the body while the head of the racquet stays relatively stationary.
This is caused because the kinetic energy is working its way up the racquet allowing the handle to move before the head. Also notice how much farther away from the body the hand is by image 8. By image 8 the shoulders have pretty much decelerated and won’t move much until after ball contact. This will greatly increase the kinetic energy transfer into the arm and racquet and cause it to quickly start accelerating.
Image 9 shows the first real movement of the racquet. By now the inertia force of the racquet is less then the kinetic energy force of the arms so the racquet head starts to follow the path of the arm. The heavier the racquet the more the muscles will stretch. This is one reason why heavier racquets can be more powerful, they cause the muscles to stretch more because they have more inertia force because of the added weight. This creates more potential energy that is eventually turned into kinetic energy.
By image 10 the racquet has moved a substantial amount. This shows the effect of the stretch shortening cycle. The muscles are forcefully contracting bringing the racquet quickly forward.
This also shows the kinetic energy chain in action really well as the upper body has stopped moving, except for the arm and racquet. Also the knees are still lifting the body up which is great for getting the ball high over the net. Hover over the below image to see that the shoulders have pretty much stopped moving before the racquet moves forward.
Kinetic Energy Chain – Timing
It is important that the timing of the kinetic energy chain is correct. If the timing is off there is a good chance the chain will not flow efficiently which can lead to a less powerful shot. Each segment must have the proper amount of time to build up energy and to transfer it to the next segment. If the chain is started too early the built up energy will start to diminish as the body will need to be slowed down early in order to hit the ball. Very much like stretching the rubber band far and then slowly “unstretching” it. If the chain is started too late, there will not be enough time for each segment to build up the maximum amount of energy and speed. Proper timing of the segments in the kinetic chain and an efficient stretch-shortening cycle maximize the transfer of energy and generates the greatest racquet speed. The greater the racquet speed, the more powerful the swing.
The kinetic energy chain is the reason some little kids can hit the ball harder then adults.
Its the reason the pro’s swing generate so much power with what appears to be little effort.
Its also the reason you do not see super muscular people playing tennis.
Most muscular people are not very flexible. Without flexibility there cannot be much of a stretching of the muscles which means there cannot be much power.
Think back to the fence analogy at the top of this page. A fence that is not broken is much stronger then a broken fence. Push on a broken fence and see how much more give the fence has then a fence that is unbroken. So keep your body together like an unbroken fence to unleash the strength within you. The kinetic energy chain is the key to a powerful forehand!