When just beginning in tennis, a racquet is simply a necessary tool to play the game. With experience, this tool becomes more essential and the player begins to understand the subtle differences. Racquet weight, frame material, and grip are all important to the experienced tennis player. Another, sometimes overlooked, factor is string tension.
String tension can change the way a player plays the game. It is more than a minor factor in power and control. Aside from swing speed, string tension may be the most important factor in power. Control is directly impacted by tension, as well. The average range of string tensions, for well made racquets, is 58 to 68 pounds. High tension would be near the 68 pound limit, with low tension creeping down toward the 58 pound range.
Low string tension (within the 58 to 68 pound range) increases the power of the hit without adding additional stress on the arm. The trade off is less control. Strings in the higher tension range give more control, but some of the power is lost. The impact on the strings and additional force needed to gain the extra power may also lead to slightly more arm strain. That is the simple answer, but it is important to understand the basics behind these concepts, as well.
A tennis ball, when impacted, dissipates about 45% of the energy applied to it. This means that it only absorbs and returns 55% of that energy. That is why in the test standard for tennis balls, dropping it from 100 inches, it only rebounds 53 to 58 inches. It is returning that much energy. The strings of the racquet, however, return 90% of the energy they are impacted with. So, when the tennis ball hit's the strings and they stretch (absorb the energy), they will return 90% of that force in the return of the ball. When a ball hits strings with a higher tension, the force is exerted on the ball, instead of the strings, and the energy return is closer to the 55% range.
This stretching of the strings is also what causes the decrease in control. The longer the ball stays on the strings, the more effect minor angle changes will have on the balls trajectory. A slight wrist flexion or extension, or lateral rolling of the wrist will cause the ball to release at a slightly different angle. The ball naturally leaves the racquet at a little higher trajectory in lower tension racquets. When a ball is hit by strings with higher tension it will come off at the angle of the racquet head at the time of impact.
When an experienced tennis player decides to experiment with string tension for their game, it is important to look at a few factors. If the player already has good power they may want higher string tension to improve their control. A player with good ball placement, who lacks some power, may choose a racquet with lower tension. The key, no matter which tension is chosen, is to practice with the new racquet to get used to the changes. Once the basics of the game are learned, a change in string tension could be the next step in improving game play.