Professional players restring rackets constantly and specify tension precisely. The number determines how the string bed behaves on impact, which changes both power and spin.
The string bed acts as a spring
On contact the strings deflect, store energy and return it to the ball. A looser bed deflects further and returns more energy, producing more speed for the same swing.
A tighter bed deflects less, so more of the impact energy is lost into the ball's own deformation and less is returned. The shot is slower but leaves the racket more predictably.
This is why tension is described as a trade between power and control rather than as a setting that is simply better or worse.
Dwell time explains the control difference
A ball stays on a loose string bed marginally longer and sinks deeper into it. That extra dwell increases the influence of any imperfection in the swing path.
With a tight bed, the ball leaves sooner and along a direction closer to the racket face angle, which is what players mean by a more predictable response.
The differences are small in absolute terms and enormous in effect, because a fractional change in launch angle becomes a large error over the length of a court.
Spin depends on how strings move and return
Modern polyester strings slide laterally on impact and snap back, brushing across the ball as they return and adding rotation beyond what the swing alone generates.
That snap-back works best within a tension range. Strung too tightly, the strings barely move; too loosely, they may not return to position before the ball departs.
Players therefore choose tension partly to optimise spin rather than power, particularly those whose game depends on heavy topspin to control depth.
Conditions move the target tension
Warm air is less dense and lets the ball travel further, so players commonly string tighter in heat to retain control. Cold conditions produce the opposite adjustment.
Altitude has a stronger version of the same effect, and matches at elevation often see tensions raised substantially compared with sea level.
Humidity affects natural gut more than synthetic materials, which is one reason hybrid setups combining different strings in main and cross positions became common.
Tension falls from the moment stringing finishes
All strings lose tension over time, most rapidly in the first hours after stringing and then more slowly through use.
Polyester loses it fastest, which is why professionals may use several rackets in a match and rotate them at fixed intervals rather than when a string breaks.
The rotation keeps the string bed inside a narrow window of behaviour, so the racket a player picks up late in a match responds like the one used at the start.
