Setting the Tone
Why do professional tennis players miss? They do everything right. They have grooved their strokes with thousands of hits. They have the best coaches in the world. They boast the most perfectly designed and conditioned tennis bodies. Still, even the greatest can slap an easy forehand into the center of the net. The persistence of stroking fails in the pro ranks implies that the knowledge, ability, and intention to hit the ball correctly is not enough. Somewhere in the process of stroking there has to be a slider bar.
I hate slider bars. A slider bar is a computer interface tool that lets you set a parameter to any value between two extremes. In tennis, there are many implicit slider bars. How hard do I want to hit this next forehand? Somewhere between way too hard and way too soft. OK, where? Halfway in-between? Three-quarters of the way on the way to too hard? How much spin should I put on my first serve? You get the point. There is no time for such contemplation in a tennis match. That is why pros try to give themselves as few decisions as possible. They almost always hit their forehands with the same hardness and their first serves with the same amount of spin. There is one slider bar adjustment that is unavoidable, however, and it occurs at the heart of every stroke, every time. It is setting the tone of the forearm musculature.
Tone refers to muscle tone or tonus. Job one of the forearm muscles in every stroke, from the drop-shot to the serve, is to store and deliver impulse force to the ball to create directional control and spin. The physiological technique by which these forces are stored is called the stretch-shortening cycle. An outside force stretches a muscle that is already partly contracted (shortened). In tennis, the outside force is usually the inertia of the racket head pulling against the forward acceleration of the shoulders during the load phase of a stroke.
In the topspin forehand, for example, the racket rotates and flips back behind the wrist during the load, stretching the flexor-pronator muscles which will later deliver both linear directional impulse and a rotational spin impulse to the tennis ball. The flexor-pronator muscles here are acting like adjustable tension springs, not like contracting muscles. When muscles act like springs, they can deliver force instantly. Muscle contraction takes considerably longer, and in tennis, the ball is only on the strings for .05 seconds. Thus, employing the stretch-shortening cycle to deliver impulse is the only way you can deliver control to a tennis ball.
Forearm Tone and the Stretch Shortening Cycle
To turn the forearm muscles into springs for force storage one must first adjust the tonus of the muscle. Tonus is the degree of tension in a muscle that is not actively contracting. When one flexes one's biceps or sucks in one's gut to impress a potential mate, one is consciously adjusting the tonus of those muscles. In those cases, however, one is balancing the tone of one set of muscles, the agonists, against the opposing set, the antagonists. For our purposes, we want the tonus of the agonist muscles, the group that will contribute to control and spin, to be unopposed. Increasing the tonus of a single, unopposed muscle group is tricky business, and requires some counter force to balance the force generated by the contracting agonist muscles. That counter force is the inertia of the racket as we drive it into the ball. Since muscles do not contract instantaneously, however, we must first supply a counter force to trigger the forearm agonist muscles to begin tightening, again without waking up the antagonist muscles which we need to be relaxed. We create this smaller counter-force by throwing the racket head away from the ball momentarily.