The Stretch Shortening Cycle

Note: All due credit to the tennis teaching community - they had discovered the power of applying intimidating, technical sounding terms to instinctive behaviors that Animalia had employed before the first salamander jumped out of the slimy ooze onto dry land. I say credit is due and I mean it because although every athletically talented human I know applies this principal whenever practicable without having to name it, those of us who are less-than-amazingly-coordinated struggle with this technique. Speaking for myself, I found a basic understanding of the Stretch Shortening Cycle (SSC) helped my game immensely. Otherwise, I would not presume to impose on your valuable time and attention.

There have been many excellent attempts to explain the Stretch Shortening Cycle SSC and its relevance to tennis online and if you read all of them you may get a picture of what it is and why you might want to pay attention to it. I like things simple, and I have found during 40 years of teaching radiology to medical residents and students that if I use fancy technical terms, they end up very impressed with me but never really get what I am trying to teach them. Conversely, if I boil tough, complex ideas down to the point that they are soft and chewy, my residents go "Oh...I knew that! I already do that myself!" As you can imagine, I don't get much credit for the teaching people things so well they believe they already knew it, but I have observed that ultimately all my residents end up knowing their jobs and how to do them. So forgive me if you come to feel I am telling you stuff you already know.

So, imagine you want to jump up onto a stool that is waist high. You have to crouch first because you can't jump from a position where your legs are extended. So, do you squat down on your haunches, wait until your legs relax, then jump? No, you crouch and immediately "spring" up. Why?

Stretch-Shortening Cycle: Think of each muscle fiber as a motor and a spring in series. The motor represents the myofibril complex that is responsible for active muscle contraction. The spring represents the elastic fibers surrounding the myofibrils, the muscle's fibrous capsule and tendons which passively stretch and rebound. In stretch-shortening, forced extension of a joint due to some outside force (gravity or a power wave) causes the entire muscle unit to lengthen as the myofibrils are contracting, stretching and storing force in the elastic fibers. This stored force can be released more quickly than one could apply force using muscle contraction alone.

The difference between squatting and crouching is subtle. In both of them your quadriceps muscles, the "main motivators" of the jump, are elongated or "stretched". The difference is inside the muscle. Think of the muscle as a motorized winch and a spring attached end-to-end or in series (see model above).

The motor represents the myofibril complex that is responsible for active muscle contraction. The spring represents the elastic fibers surrounding the myofibrils, the muscle's fibrous capsule and tendons which passively stretch and rebound. The winch doesn't drive the joint directly - it just tightens the spring. This tightened spring can then be used to store force from an outside power source, such as the power wave of a tennis stroke, which acts on the extremity to further stretch the spring. That stored force can be completely released in a handful of milliseconds, delivering power much more quickly than can ever be delivered by direct muscle contraction. The speed of transfer of force is important in tennis because you only have .05 seconds of contact with the ball during which time all of the pace, directional control, and spin forces must be transferred from the racket to the ball.

The main reason our muscles have evolved this winch and spring design of our muscles is that the winch is pretty slow to develop tension.

The main reason our muscles have evolved this winch and spring design of our muscles is that the winch is pretty slow to develop tension. If you have to snap a limb into action, as you must in virtually every tennis stroke to effectively control the ball, the winch (myofibrils) are just too slow. It is much more effective to use the winch to "preload" the spring, load the spring with power from some outside source such as the legs, hips, and shoulders, then release that energy from the spring at the right moment.

You load the spring through the process of "Lock and Load". First, you relax your hitting shoulder, forearm, and wrist into a configuration appropriate to the stroke you are trying to hit. You then pull the racket gently away from the ball. This is the "lock" backswing proper. At the end of the lock, you shorten the myofibril units of the muscles important to control and spin (agonist muscles) by consciously resisting the backward momentum of the racket head. The beginning of the load is triggered by a sudden change in rotation of the shoulders from away from the ball to towards the ball, dragging the wrist forward. As the wrist changes direction suddenly from back to forward, the inertia of the racket head stubbornly resists this new acceleration and unwinds the arm into a position wherein the agonist muscles are stretched. Thus at the end of the load, the agonist muscles will be fully stretch-shortened. This is the fully "loaded" state. Just before you bring the racket head into contact with the ball, you quench the forward acceleration, and the all of that pent-up force is transferred to the ball resulting in both pace and control. The result of this force transfer is called impulse which is defined as force X time, and it results in an increase of the momentum exclusively in the direction of the force. The only way to deliver sufficient force to take control of the ball over such a short time and deliver it precisely when it is needed is to store that force through stretch shortening. The talentoid's ability to use stretch shortening on every strike is key to their superior hegemony over the ball. Luckily, like Prometheus, we can steal this fire from the Gods of tennis and turn its power against them.