On International Women's Day in 2019, all 28 players on the top-ranked U.S. women's national soccer team filed a lawsuit against the United States Soccer Federation (USSF) alleging gender-based discrimination and arguing for equal pay. Now, eye-opening court documents filed Tuesday in Los Angeles reveal the grounds on which the USSF is arguing the lawsuit should be thrown out.

One of its arguments is headlined: "WNT [Women's National Team] and MNT [Men's National Team] Players Do Not Perform Equal Work Requiring Equal Skill, Effort, and Responsibility Under Similar Working Conditions." The document goes on to say, "The overall soccer-playing ability required to compete at the senior men's national team level is materially influenced by the level of certain physical attributes, such as speed and strength, required for the job."

U.S. Soccer argues that paying its female players less than its male players is justified because of inherent physiological differences between men and women in things like skeletal structure, muscle composition, heart and lung capacity, and even "the absolute ability to process carbohydrates." The filing references an article by a Duke Law School professor, entitled "Sex in Sport," as proof of those claims.

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It is not "a 'sexist stereotype' to recognize the different levels of speed and strength required for the two jobs, as Plaintiffs' counsel contend," USSF's lawyers argue in the court filing. "On the contrary, it is indisputable 'science.'"

And that's not the only point that's likely to enrage fans.

The U.S. Soccer Federation also argues that, even though men and women may both play soccer professionally for a living, its female players and its male players work "materially different jobs" because the men have to contend with more hostility from fans when they compete around the world.

"Opposing fan hostility encountered in these MNT road environments, especially in Mexico and Central America, is unmatched by anything the WNT must face while trying to qualify for an important tournament," lawyers for the USSF argue. "Even the hostility of fans at home crowds for the MNT in some friendlies can be unlike anything the WNT faces."

Alex Morgan, Megan Rapinoe and their USWNT teammates are seeking more than $66 million in damages under the Equal Pay Act and the Civil Rights Act of 1964.

In the documents, however, USSF says the women's argument not valid because the women's and men's teams do not work in the same physical building, which it claims is a necessary condition for comparison under the Equal Pay Act.

"Plaintiffs and MNT Players Do Not Work in the Same Establishment," the USSF document reads. "Plaintiffs ignore the portions of that regulation stating that an 'establishment' under the EPA ordinarily 'refers to a distinct physical place of business rather than to an entire business or 'enterprise' which may include several separate places of business.'"

The U.S. women are ranked No. 1 in the world and have won four World Cup titles. The U.S. men's team is ranked 22nd and has never won a World Cup. The men failed even to qualify for the 2018 World Cup. In the last World Cup, the women's final attracted more American TV viewers than the men's.

Despite this, the female players — many of whom have become household names — are paid significantly less than their male counterparts. Now, it seems the United States Soccer Federation is arguing that the discrepancy merited because they are not actually counterparts.

In a statement to the Associated Press, Molly Levinson, a spokesperson for the women who are suing, said the USSF's argument belongs in "the Paleolithic Era."

"It sounds as if it has been made by a caveman," she said. "Literally everyone in the world understands that an argument that male players 'have more responsibility' is just plain simple sexism and illustrates the very gender discrimination that caused us to file this lawsuit to begin with."

The trial is scheduled to begin on May 5, but both sides have asked U.S. District Judge R. Gary Klausner to decide in their favor without a trial. Regardless of what he ultimately decides, however, many people in the court of public opinion seem to have already sided with the women.