A tennis ball returns only 55% or so of its impact energy, but strings return more than 90%. When a ball collides with strings, both deform to some extent. The more the strings store the energy of the collision by deforming like a trampoline, the less the ball stores energy by flattening. To get the most energy return out of the collision, we want the strings to store as much of the total energy as possible, because they will give back more than 90% of it, whereas almost half of any energy stored in the ball will be wasted. Looser strings deform more easily, thus storing more of the energy of the collision and minimizing the amount wasted by the ball.
At this point, looser strings sound ideal. We should all know better than to waste energy, after all. So, why do looser strings cause a loss of control?
Although it's generally true that looser strings reduce control, this principle needs one important qualification. If you shorten your strokes to take advantage of a more powerful string bed, you might improve your control, because it's easier to time your swings and pinpoint your racquet position on impact when you use a shorter swing.
If your swings stay the same, though, you get less control with looser strings. Different experts argue over a number of reasons for this effect, but two main reasons are clear:
1. As the looser string bed compresses more, the ball stays on the strings longer, during which time any tiny changes in your racquet position can change the path of the ball. The ball isn't on your strings long enough for you to consciously do anything to it. Your brain can't execute any actions in the few milliseconds available, but that few milliseconds is enough time for unintended movement to occur, especially when an off-center hit exerts a turning force on the racquet head. 2. If you hit fairly hard, you get an inconsistent response from the string bed. The strings do not compress in direct proportion to the force of the impact. They will compress most easily as they first meet the ball, then their rate of compression can drop off sharply or even approach zero as the ball presses farther in. This means that the amount of energy returned from the ball-string collision can be unpredictable. Hitting 20% harder might not produce 20% more ball speed, because once the compression of the strings is maxed out, the ball compresses more, and a higher percentage of the total energy is thus lost. A tighter string bed always forces the ball to take up much of the energy, whereas looser strings vary greatly in the extent to which they do so. This makes looser strings less predictable.
If you don't hit particularly hard or with much spin, and you want more power or your arm hurts, looser strings make sense. When looser strings compress more, the impact of the ball occurs over a longer period of time, thus making the shock less severe at any one moment.
If you like to hit quite hard or with a lot of spin, tighter strings are probably your better choice. As noted, you'll have more control, and you'll get more spin because the ball will flatten out more on your strings, giving each string a better bite on the ball.
Look for a feature coming soon that will discuss the role of string materials and thickness.
Dr. Howard Brody's Tennis Science for Tennis Players (see books below) is a wonderfully readable resource for detailed explanations of the type of questions explored in this article.