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Sources of Power — Primary and Secondary

Joe Dinoffer
USPTA and PTR Master Professional
President, www.OncourtOffcourt.com

Biomechanics and the Kinetic Chain

This article is about the different possible axes of rotation used during a tennis stroke. As we progress through this discussion, we will divide tennis strokes into the following two groups: primary and secondary axes of rotation.  Which one of these has the most impact on each stroke is often based on whether a player is hitting on balance (primary and secondary axes can be employed) or completely on the run (secondary axis can be employed).

For the purpose of this article, the primary axes include the legs, hips, core, and upper body, while the secondary axes would be the joints or hinges of the shoulder, elbow, and wrist.

The generation of energy is further divided into categories called “linear energy” such as using the legs to push off on the serve or groundstrokes and “rotational energy” which most commonly comes from the circular movement of the hips, torso, and shoulders on various shots.

Before describing these points further, let’s identify that the principle of the axis of rotation can best be described as a radius (of a circular motion) that is called the swing arm.  A major point of understanding is that the longer the swing arm or swing radius, the more racket head speed is generated and therefore more power is efficiently transferred to the ball. The common experiment to prove this point is to tie a rock to a string and swing the rock in a circle around a central point. If the rotation point (or axis) is moving at a constant speed, then the longer the string, the faster the rock will be traveling. 

Click photo: Example 1: Kim Clijsters runs wide with the majority of her energy expended moving to the ball. Since she cannot get on balance, the primary kinetic chain links are contributing little to racquet speed. Using the wrist as the axis of rotation is the key.

It should also be understood that all the elements of what we are calling primary and secondary axes of rotation are all part of the kinetic chain. The highly visual analogy of a bull whip has often been used as a comparison to a perfectly and fully executed tennis groundstroke or serve, since the energy delivered to the whip’s target originates and accelerates from the handle outwards towards the final powerful and fast cracking of the tip of the whip. In tennis, the kinetic chain is understood to begin with the ground forces and legs and move up through the body and then outwards towards the racquet arm and, ultimately, to the racquet and into the point of contact.

To further elaborate on this point, here are four different examples of how the various primary and secondary radius possibilities on a forehand can be employed:

  1. The first example is a squash shot. In this case, the wrist would be the center of rotation on this defensive emergency shot when reaching wide. While the wrist is being called a “secondary axis” in this article, it would be the primary rotational joint used to hit this shot.

  1. Click photo: Example 2: Tomas Berdych runs down a wide forehand and clearly cannot get on balance or use much of his legs, hips, or torso as the primary kinetic chain links contributing to racquet speed. In this example, his hitting arm shoulder is the axis of rotation to create power.
    Second, the shoulder and wrist would be the main axes of rotation when hitting on the run with little or no coiling of the trunk or hips. While both the wrist and shoulder are each being called a “secondary axis” in this article, these would be the primary rotational joints used to hit this shot.
  1. Third, the spine would be the axis of rotation on an open stance forehand where the weight remains on the right leg for a right-handed player. The spine is being called a “primary axis” in this article, but note that the secondary axes of the shoulder, elbow, and wrist are also used to hit this shot.
  1. And, finally, the left leg for a right-handed player would become the primary axis of rotation on a semi-open stance forehand. Note that the longest radius is produced when the opposite leg (left leg for a right-handed forehand) absorbs the weight transfer prior to contact. Therefore the ideal stroke would be a semi-open stance forehand which optimizes both the radius length and coiling and uncoiling potential.  We say that the semi-open stance forehand also maximizes the coiling and uncoiling potential. Note that a closed stance forehand utilizes the same leg but the freedom needed for an optimally fluid swing is restricted due to a more limited follow through because of the sideways stance. 

Click photo: Example 3: Nikolay Davydenko hits a forehand from the middle of the court. He clearly gets on balance and uses his legs, hips, and torso in addition to his swinging arm as the primary kinetic chain links contributing to racquet speed. But, notice how much of each. Let's just compare the rotation of his hips versus his torso or spine. In this example, his hips only rotate about 45 degrees. However, from the preparation phase through the finish, his torso rotation appears to be nearly 120 degrees, significantly more. He also falls backwards during his shot, making it difficult for him to optimize all the axes of rotation to generate optimal and efficient power.

Primary Axes of Rotation

For the purpose of this article, we are defining the “primary” axes of rotation as the larger linear and rotational forces of the following moving parts of the body: legs, hips, core, and upper body. Here’s just one example of how one or more of these primary forces (all part of the kinetic chain) can work.

On a properly hit semi-open stance forehand the front leg acts as the primary axis of rotation and maximizes the radius for hitting the open stance. For a right-handed player, the front leg would be the left one. The major point of understanding that was mentioned earlier in this article is that the longer the swing arm or swing radius, the more racket head speed is generated with more power efficiently transferred to the ball. This shouldn’t lead us to believe that players should therefore always swing at their normal fast speeds to get more power. They can also swing slower to improve their timing and, through proper biomechanics, would be able to hit with optimum efficiency alongside the power that this efficiency permits. This is particularly helpful if you ever feel out of rhythm and need to slow down your swing to get back in the flow!

Secondary Axes of Rotation

As mentioned earlier, we define “secondary” axes of rotation as the hinges or joints of the arm: shoulder, elbow, and wrist.  We are referring to these as “secondary” axes or rotation points because they typically function after the primary axes have done their job. But don’t be fooled. These “secondary” rotation points are just as important to a fluid yet powerful swing as the “primary” axes discussed.

Click photo: Example 4: David Nalbandian hits a forehand groundstroke and appears to have all the time he needs to set the complete kinetic chain in motion. He clearly gets on balance and appears to fully use his legs, hips, and torso in addition to his swinging arm to generate racquet speed and therefore ball speed. One interesting side note is to compare this forehand by Nalbandian to Davydenko in the previous clip. Look at both and focus on the shoulder rotation of each. While Davydenko has a shorter rotation in the backswing phase of the swing, his rotation is greater in the follow through phase. Generally speaking, with players of similar body types and flexibility, this would not be the case. But, considering that Nalbandian has a very stocky build compared to the slender Davydenko, it is not surprising.

Try a little test for yourself. Hold a racquet and take a practice swing with a loose grip and arm. It should feel loose and fluid. Then squeeze the grip as tight as you can and also feel your shoulder, elbow, and wrist tighten up as well. Now swing again. No matter how well you use your legs and your hips to rotate and explode into your shots (groundstrokes or serves), you will feel tight and “blocked” unless you use the axes in your arm as well.

Summary

Superhuman players, like the top players in the world, regularly hit with near-perfect efficiency. But the rest of us have also experienced close to that level of efficiency, albeit these memorable shots may be few and far between. For those who may be unfamiliar with how that “dream shot” feels, let’s try and describe it. You are loose and confident. You are on balance and moving at least a little bit forwards into the court.  Your legs feel strong and balanced while your upper body and arm are extremely relaxed and fluid. At point of contact, everything seems to “click” perfectly. The ball pops off your racquet with unusual power and speed. Simply put, it feels perfect! Hopefully, by better understanding what technical and bio-mechanical ingredients produce this perfection, you can experience it more often for yourself.

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