Are women athletes catching the men?

Serena Williams' quest for her third consecutive US Open title remains well and truly on track after she breezed past Flavia Pennette in straight sets to move into the semi-finals.

On fire ... Sabine Lisicki serves to Maria Sharapova at the US Open. Source: AFP

A MONTH ago at the Bank of the West Classic, Germany's Sabine Lisicki served a ball 211km/h — the fastest ever recorded in women's tennis.

That's not just fast for a woman. Lisicki's record-setting bomb — which broke Venus Williams' mark of 207km/h — is faster than any serve Roger Federer has hit in 2014, according to statistics provided by the Association of Tennis Professionals.

Lisicki, who lost in the third round of the US Open to Maria Sharapova, stuck around long enough to blast the speediest serve of any woman at this year's US Open (199km/h). That's barely slower than the best effort in the Open of top men's seed Novak Djokovic (201km/h) and equal to or better than the fastest serves of at least 29 men, including seeded players Kei Nishikori, Richard Gasquet, David Ferrer, Guillermo García-López, and Mikhail Youzhny.

Sabine Lisicki is the fastest ever women's server. Source: AFP

Serena Williams' top serve at this year's Open (196km/h) has also outpaced that of Ferrer (191 km/h), García-López (190km/h), and Youzhny (188km/h).

And women can compete with the men on average first-serve speed as well.

Serena Williams' average (174km/h) in her quarter-final against Flavia Pennetta was better than that of No. 12 men's seed Gasquet (167km/h) in his third-round loss to Gaël Monfils.

And No. 10 seed Nishikori, a US Open semi-finalist, has an average first-serve speed of 174 km/h — the same as Serena's highest — in three of his five matches thus far.

Maria Sharapova returns a shot against Caroline Wozniacki at the US Open. Source: AFP

In a recent column on hard-throwing Little Leaguer Mo'ne Davis, the Washington Post's Sally Jenkins argued that cross-gender comparisons risk making the world's best female athletes look like failures.

Indeed, it would be silly to argue that Lisicki and Serena Williams' fast-serving feats aren't all that impressive because certain men, like Milos Raonic and John Isner, can serve a whole lot faster.

But what's fascinating about the tennis serve is that the gender gap is so narrow by comparison with other athletic endeavours.

And sometimes, as in the cases of Sabine Lisicki and world No. 5 David Ferrer, the gender gap gets flipped completely.

Serena Williams returns a shot against Flavia Pennett during their quarterfinal at the US Open. Source: AFP

In his excellent book The Sports Gene, David Epstein points out that there's long been speculation that women might catch up to men in athletic performance — for instance, a 1992 article in Nature asked, "Will Women Soon Outrun Men?" Given that women have only recently been encouraged to participate in sports, it was natural that elite female athletes would improve at a rapid clip.

Epstein argues, however, that after the initial boom in women's sports, "the biological gap is expanding".

In track and field events, he writes, "the top 10 men at any distance … are about 11 percent faster than the top 10 women ... [and] in the long jump, women are 19 percent behind men. The smallest gap occurs in distance swimming races. In the 800m freestyle, top women are within six percent of top men."

And then there's tennis. The overlap between men's and women's serving speeds in the ball-and-racquet sport isn't totally unique.

Lisicki has been serving faster than Roger Federer. Source: AP

In golf, the top three women on the LPGA tour in average driving distance — Lexi Thompson, Gerina Piller, and Brittany Lincicome — all outdrive the last man on the PGA Tour's driving distance scoreboard, Paul Goydos.

But there is far more overlap with tennis serves, where someone like Lisicki can crank it faster than many of the world's best male players.

Golf and tennis have something in common: Players impart force to the ball by swinging a piece of equipment. Do the racquets explain the rocketing serves of top female pros?

In a piece on ESPNW.com, the global tour manager for Wilson tells Johnette Howard that he thinks improved racquet technology isn't a huge factor — that improved training and technique "are contributing to higher ball speeds more than a racquet change has".

Or is it the balls?

Forbes' Allen St. John claims women are serving so well at the US Open because they're playing with a different ball than the men are, one that flies more quickly.

But the data doesn't support that conclusion — or at the very least doesn't support the idea that the ball explains everything.

At Wimbledon, where men and women do play with the same ball, 20 women served 178km/h or faster.

At the US Open, 20 women have served 180km/h or faster. The fastest serve by a woman at Wimbledon in 2014 (Madison Keys' 198km/h strike) was just 2km/h slower than the fastest serve by a woman at this year's US Open. And Serena Williams' fastest average first-serve speed in any Wimbledon match in 2014 (170km/h) was just 3km/h slower than her fastest average speed so far at the U.S. Open.

Lexi Thompson is among the longest drivers in women's golf. Source: AFP

If it's not the ball, then what is it about the tennis serve that makes for such gender parity? Timothy Olds, a professor at the University of South Australia who has studied tennis serves extensively, offers a mechanical explanation.

Olds said a good serve depends on two things: the height at which the racquet meets the ball and the speed of contact.

On average, men are taller than women, which provides a natural advantage as they're driving the ball down toward the service box. The speed of contact depends on racket speed, which is a function of what Olds called the "kinetic chain connecting the torso, upper arm, lower arm, and wrist." This kinetic chain depends on muscle strength and arm length, the latter of which helps the player generate angular velocity. While men tend to have longer and stronger limbs, exceptional servers such as the Williams sisters and Lisicki are quite muscular and long-limbed.

This analysis largely holds for golf swings as well. Olds said golf's kinetic chain is similar, involving whiplash movements dependent on high angular velocities, as is the one employed by baseball pitchers.

Brittany Lincicome of the USA during the third round of the Canadian Women's Open. Source: AP

So why, then—at least after puberty sets in and Little Leaguers move on to high school — is the gender gap so much wider when it comes to throwing a baseball?

In a study of 11 male and 11 female elite baseball pitchers co-authored by surgeon to the stars Dr. James Andrews, the women were found to have a few significant differences in their baseball deliveries.

Female pitchers had "shorter and more open strides" and produced "lower peak angular velocity" with smaller forces at the shoulder and elbow joints.

In some ways, this tells us what we already know: women tend to be shorter (shorter stride), have shorter limbs (lower angular velocity) and less muscle mass (smaller shoulder and elbow joint forces).

But it also tells us something critical: these elite female pitchers are more representative of typical gender differences than those in tennis and golf.

Elite female tennis players are exceptional outliers, whereas, to this point at least, female baseball pitchers are not.

Mo'ne Davis took the Little League World Series by storm. Source: AP

This is part of what made 13-year-old Mo'ne Davis, with her 113km/h fastball, a unique figure. Elite girls are naturally steered toward tennis, soccer, basketball, softball and other sports that might lead to a college scholarship or professional contract.

In baseball, the potential financial payoff for a woman is non-existent.

In June, Emma Span wrote an op-ed for the New York Times laying out the various ways in which girls are steered away or outright rejected from playing baseball.

One Arizona girl told Span that, despite being good enough for her middle school baseball team, the coach instructed her to play softball, simply because she's a girl. Sadly, the most anomalous aspect of Davis' performance is that she was allowed to play at all.

This selection effect creates a surfeit of female talent in a small number of sports.

The world's top women's tennis players, particularly the Williams sisters, are likely among the best female athletes alive.

The best male athletes, by contrast, aren't concentrated in a particular sport — they might play basketball or soccer or football or run track, depending on their country of origin, socioeconomic status, and any number of other factors.

Still, this doesn't fully explain why women serve so well yet fall behind in throwing.

In a joint study by the University of North Texas and the University of Western Australia, scientists tested Aboriginal Australians for throwing ability.

The girls had been trained just like the boys but still threw with much less velocity.

Clearly, then, differences in throwing aren't just cultural and aren't just the result of a selection effect.

David Ferrer has one of the weakest serves in the men's game. Source: AFP

There's something biological when it comes to throwing ability that doesn't have as strong of an effect on serving a tennis ball.

Like so many inquiries into nature vs. nurture, the explanation for what's going on in tennis certainly lies somewhere between those two poles.

On the one hand, the kinesthetics of a tennis serve are heavily dependent on technique and skill, for which gender has little bearing.

Olds mentioned a third component to the serve — spin — which is mostly determined by wrist movement, another factor that's not all that dependent on gender.

In comparison to many other sporting feats, a tennis serve is less reliant on the traditional male strong suit of muscle mass.

Perhaps no player exemplifies this more than Aleksandra Krunic, the 21-year-old Serbian qualifier who served up to 188km/h in her fourth-round US Open match against Victoria Azarenka, this despite her 1.62m, 53kg physique.

Although that kind of velocity isn't close to any world record, it matches many serves on the male circuit.

If someone like Krunic can smack the ball at world-class speeds, then serving must be about a whole lot more than just brute strength.

Aleksandra Krunic of Serbia serves to Victoria Azarenka at the US Open. Source: AFP

This article was written by Aaron Gordon from Slate and was legally licensed through the NewsCred publisher network.