String tension is the final piece in the racquet-string-tension triad. It’s also the least understood by most recreational players. Let’s start with the basics - lower tensions provide more power, tighter tensions provide more control. This is a very general rule of thumb and assumes a certain level of player ability (especially the control part). A beginning player may need more control but tighter string tensions aren’t the solution. This player needs a soft, forgiving string bed that lower tensions provide due to the frequency of off-center hits. Advanced players who swing fast and hit hard usually need more control and will, therefore, benefit from tighter tensions. There are, of course, always exceptions but these generalisations apply to the majority of players.
Each racquet has a recommended tension range. This range has been determined by the manufacturer as a result of extensive play-testing by real players. If a player doesn’t have a specific need (more power, arm problems, etc.), he should start at mid-range and make any adjustments from there.
Note: It has to be mentioned that the tension range is only a 'recommendation'
Otherwise, here are some specific guidelines for selecting a string tension.
Power - as stated above, if a player is seeking more power from their racquet, they should try dropping tension a few pounds. The string-bed will deflect more (and the ball less), returning greater energy to the ball. There is a point of diminishing returns where the string-bed turns into a butterfly net, but it’s well below any racquet’s recommended tension range.
Control - a tighter string-bed deflects less and deforms the ball more, providing less energy than looser strings. This means the ball won’t fly as far when you hit it. Beginners who are shanking the ball in every direction won’t gain any advantage by increasing tension, but intermediate and advanced players who are hitting a lot of balls long, will be able to reduce the depth of their shots without changing their swing. It is also generally accepted that spin potential is enhanced with higher tensions, which provides even more control for topspin and slice players.
Arm Injuries and Tennis Elbow - A player with a temporarily sore shoulder, elbow or wrist needs their racquet to absorb some of the game’s physical punishment. Your string job can add that measure of relief and hasten his/her recovery. String toward the lower end of the recommended tension range, using a thin gauge of that string. A looser string face will absorb more of the ball’s impact, give a larger sweet-spot and reduce the amount of shock and vibration transmitted to the hand and elbow. It will also add ball depth to shots without a full-powered swing. Select the most elastic string possible, such as natural gut or a nylon multi filament. Also try some customisation by adding lead tape to increase overall mass. This added weight would help reduce the amount of shock transmitted to the hand, wrist and arm. One should not count on these measures alone to cure tendinitis. The advice of a doctor should be sought. A choice of strings for Tennis elbow & other arm issues can be found here.
Switching Rackets - too many players are stuck on a tension (“I always string my racket at 60 pounds”) and don’t make allowances when changing rackets. Whether changing head sizes, brands, or buying a new racket, a player will need to make the corresponding tension change. If 60 pounds was mid-range on his old racket and the new racket’s tension range is 50-60 pounds he should start at 55 pounds with the new racket.
Switching Strings - if a player changes from a standard nylon or synthetic gut string to a Kevlar composite or hybrid, I suggest to reduce the tension to compensate for the added material stiffness. With composites, a 5% reduction is recommend . Kevlar hybrids should be strung 10% looser to approximate the same feel of nylon strings. On the other end of the spectrum, players switching from nylon strings to polyester may want to increase tension 5-10% to compensate for polyester’s high initial tension loss.