The Kinetic Chain Wave

The term "kinetic chain" has been used to describe the body mechanism by which power is transferred from the legs, through the trunk, shoulder, forearm and wrist and ultimately to the ball. Kinetic chain is a useful term to the extent that it emphasizes the need to get power from the lower body, not generate it in the upper body. It also points out that there is a sequence of energy transfers; legs to hips to spine to shoulders to arms to forearms to racket to ball, and that they must occur in order over time. Finally, the chain concept points out that if any one of these "links" is broken, the energy is lost, and bodily injury at that broken link will often result.

All of that good stuff notwithstanding, I still do not like the term. It tends to lead us less-than wonderfully talented folks to think of ourselves as rigid mechanical contrivances. It encourages us to try to push the energy along at each joint with muscular activity rather than just letting it flow through us insensibly. It also encourages us to keep rotating our hips and shoulders through the stroke long after the energy has passed through these components. The legs should produce energy, transfer it to the hips then stop moving before they throw you completely off balance. The same with the hips and shoulders. They should do their thing, then get out of the way. We need to make our body more of a whip than a chain. Our bones, joints and muscles need to be a fluid medium through which a "kinetic wave" of power freely propagates.

Why Waves?

Waves are a much more efficient way of moving energy and momentum from place to place. Firstly, you don't have to worry about displacement. A chain or gears model of tennis stroking leaves one all twisted up in a knot at the end since the rotation of the body is all in one direction. A wave is bidirectional - to and fro, forward and back, up and down. The net displacement of the parts is zero in the end - like a cork on the sea because waves don't move, they propagate through a medium. They momentarily disturb the medium through which they propagate but leave it unchanged unless the medium absorbs power from the wave.

The Power Wave of the Topspin Forehand: Begins with a push-off against the earth and weight transfer from the front foot to the back foot (gold) during the unit turn which powers the lock (backswing proper). It propagates through the muscles as the first trough of the wave, rippling up the trunk, through the chest and shoulders and down the arm to the hitting hand where it is transferred to the racket, throwing the racket back away from the ball. This reverse force on the racket sets the tone of the forearm and stretch-shortens the forearm muscles to store directional control and spin. Next one pushes off the back foot throwing the bodyweight to the front foot (green) which creates the main peak of the power wave. That wave carries the forward momentum needed to accelerates the racket head toward the ball. Finally, a weight transfer from the front foot to the back creates a final trough in the power wave which opposes the peak of the wave, interrupting the forward acceleration and thereby releasing the forces stored in the forearm. There is a delay between the production of power in the legs and the use of that power in the upper body, resulting in a troubling but unavoidable syncopation of the rhythm of the stroke.

The kinetic wave in the forehand starts in the backswing with a gentle clockwise (if looking down) reverse-rotation. At this point one sets the back foot by transferring body weight onto it. Immediately the weight begins to be transferred back to the front foot. At the moment that weight is evenly distributed and both feet have solid purchase on the court there is a quick series of two rotations by the hips - first counter-clockwise in the direction of the stroke starting the rotation of the hips then clockwise in the opposite direction slowing them down. Both movements contribute energy to the shot, but the net rotation of the hips is minimal. The dance move that creates the wave is also over quickly. Both the small net displacement and discrete time interval help maintain balance and leaves the upper body alone so that it can coordinate the rest of the stroke. That brings us to the second advantage of waves over chains, gears, and levers. The energy can be created early and used later. That means you don't have to do everything - add power, get control of the ball, address the ball, add spin, etc. all in one motion at a single point in time. As we shall see, strokes consist of a series of tasks that occur in sequence, each one with a limited set of objectives.

As the power wave courses through the body towards the racket and ball, energy is harvested from the wave by each component of the stroke, independent of what has happened earlier or will happen after. Thus the racket can be accelerated to build up racket head speed for pace during a "lag " phase without interfering with the storage of control and spin forces that occurs earlier during the a "load" phase.

The Forehand Power Wave: The power wave reveals itself as rotational acceleration and deceleration apparent the hips, chest, right shoulder, right arm and finally the racket. The orange and blue arrows represent rotational acceleration. The wave is bidirectional, but there is some incidental net rotation of the body which is greatest in the follow through. Ignore that and focus on the way the rotations speed up and slow down - that is evidence of power propagating through the body. Note the very compact rotations of the shoulders and hips leading up to the moment of contact despite massive acceleration.