How do the players launch those guided missiles? It starts with their strings.

Birgit Ziener may be the busiest person in the tennis business. As the tour manager for Luxilon, her job is to supply the pros with the brand’s popular string. How popular? Wilson, the global distributor and marketer of Luxilon, estimates that more than 3 million feet of it was distributed to the pros at the Grand Slams and in Miami alone last year. At the time of the Cincinnati tournaments in August, 70 percent of the Top 20 ATP tour players and 40 percent of their WTA tour counterparts were using Luxilon, either as a full set or as a hybrid mix with gut.

You may have heard Luxilon’s most popular model, Big Banger Alu, referred to as a polyester string. That isn’t entirely accurate. Basic polys have been around for ages, but companies like Luxilon have messed around with the configurations, adding proprietary polymers. Luxilon, for one, guards its formula as religiously as KFC protects its secret recipe.

What we do know is that next-generation polyesters are here to stay. Of the six non-Luxilon men in the Top 20, Rafael Nadal, Andy Roddick and Fernando Gonzalez string with their racquet company’s version of poly, Babolat Pro Hurricane, and Nikolay Davydenko uses Poly Star Energy polyester. That leaves Gilles Simon and Radek Stepanek as the only poly holdouts at the top. The majority of the top women players also string with Luxilon or other brand’s poly-based strings, while Kim Clijsters and Serena and Venus Williams use gut.

Of all the brands, Luxilon is clearly king. Unlike the racquets, clothing and shoes they’re paid big bucks to endorse, many pros use the string without being paid to do so. Wilson players get Luxilon for free, a nice deal for someone like Roger Federer, who goes through 400 sets of Luxilon a year. Others, like Head racquet user Novak Djokovic and Yonex player Ana Ivanovic, pay the going rate or get their sponsors to buy it, according to Nate Ferguson, owner of Priority One in Tampa, Fla., and the personal stringer and racquet customizer to top pros such as Federer, Djokovic and Andy Murray.

The advent of polyester has done more to change the men’s game than anything else, including better fitness and souped-up racquet technology. It’s a key reason that returning serve has become easier and serving and volleying has become tougher.

Whether it’s Luxilon or other popular strings, like Babolat Pro Hurricane, Tecnifibre Pro Red Code or Isospeed Hybrid Spin, polyester-based strings allow the top players to whack the fuzz off the ball and keep it in the court because of its dead feel. But Ferguson says his clients contend that Luxilon does the rest of the pack one better: It helps them impart more spin.

Fewer WTA players use poly because many prefer the inherent power and feel of gut. Ferguson says some should consider switching. “I think [gut user] Serena Williams would cut her errors way down,” Ferguson says. “I’d put her in a Lux in the mains for control with gut crosses for comfort.” But he says Venus should stay clear of poly, which is stiff and hard on the arm, because of her history of wrist problems.

Ferguson thinks even more players will switch to Luxilon because of the introduction of Luxilon M2, a softer version that’s gentler on the arm, in 2009. The only top-rated player using it so far is Maria Sharapova, who had rotator cuff surgery in 2008.

If M2 catches on, that will make Ziener, Luxilon’s tour manager, even busier.