The FIFA Under-20 Women’s World Cup got underway in Canada this week amid increasing criticism of the turf the organization used to surface some of the fields. Instead of grass, some of the venues feature FieldTurf, an artificial alternative to sod, that some say can alter the game.

“The game plays differently on artificial surface, not only because of the fear of injury but because it’s a different surface,” American forward Abby Wambach told Sports Illustrated earlier this year. “I’m feeling like this is the women’s game taking a step back.”

Not all the FIFA U-20 matches, which are set to end Aug. 24, will be played on FieldTurf surfaces, but next year, when the women’s 2015 World Cup takes place in Canada, all six venues will feature the artificial grass.

Wambach isn’t the only athlete who’s upset. A group of about 40 female soccer players from around the world retained legal counsel this week to send a letter, according to The Equalizer. Copies of the letter, organized by U.S. law firm Boies, Schiller & Flexner LLP and Canadian firm of Osler, Hoskin & Harcourt LLP, were sent to Canada Soccer and FIFA last week.

The letter (via The Equalizer) calls the turf “a second class surface” and says its use “is gender discrimination that violates European charters and numerous provisions of Canadian law, including human rights codes and the Canadian Charter of Rights and Freedoms.”

Both FIFA and Canada Soccer officials confirmed to The Equalizer that they received the letter, but neither offered an official response. Earlier this week, however, the Toronto Sun reported FIFA President Sepp Blatter dismissed concerns over the artificial surface, noting turf technology has come a long way in recent years and FieldTurf in particular meets its list of lengthy replacement-grass requirements, which are laid out in the 103-page document “The FIFA Quality Concept for Football Turf.” It begins:

Artificial turf has been around now for several decades. It can be argued that artificial turf was originally developed to address the limitations of natural grass. However, the earliest versions were not designed for football and changed the game dramatically. Therefore, football never thoroughly embraced the idea of high-level competition matches on artificial surfaces. The breakthrough came when manufacturers started to develop surfaces specifically designed for football. Manufacturers have now developed a turf that mirrors real grass. In order to get away from the short, tightly packed matting of the earlier generation, nowadays, the concept is to produce longer and more thinly spaced tufts and most of the systems are infilled with sand for support and rubber granules to give bounce. This newest generation of artificial turf has proven to be the most favourable for football to date.

The document continues, meticulously outlining how FIFA tests everything about artificial turfs — from their durability to their ball rebound abilities — to classify the surfaces as either FIFA 1-star or FIFA 2-star turfs. The latter rating is the highest. FieldTurf, the artificial grass being protested by Wambach and about 40 other players, is rated FIFA 2-star, according to the company.

FieldTurf is not new to soccer or other sports, however. It’s currently used in several American football/soccer stadiums, including Gillette Stadium, where the New England Patriots New England Revolution play, and CenturyLink Field, where the Seattle Seahawks and Seattle Sounders play.

Also, not every player or coach hates it. Sounders FC head coach Sigi Schmid seems satisfied with the surface, which was installed in February in the team’s home stadium. He said in a FieldTurf press release:

“The new turf field is a big plus for us from the standpoint that it’s going to play much truer to grass. It slows the ball down at times, which is important for a team like us that likes to keep possession. There’s more bounce to it and more give. It’s a spongier surface so it plays a lot better than it did last year.”

But while one can debate whether the differences that exist between artificial turf and real grass are good or bad for the game, it seems evident that some sort of difference does exist. FIFA, so far, does not seem too concerned, though. Blatter is confident about the outlook of artificial turf and insists players just need more time to get used to it.

“This is for the future,” Blatter told reporters (via The Equalizer) on Monday before the 2014 U-20 World Cup kicked off. “If now there is a category of players or coaches, they are not used to this new technology, which is this turf. They say at the first instance, they say it’s not good. But it’s good.”