International track officials are revisiting controversial territory with a new rule that defines female athletes by their body chemistry.
The standards, issued by the International Assn. of Athletics Federations early Thursday morning, were characterized as a means of keeping women with naturally high testosterone levels from holding an unfair advantage.
"As the international federation for our sport, we have a responsibility to ensure a level playing field," IAAF President Sebastian Coe said in a statement.
The rule will apply to all races between 400 meters and the mile, which could present a challenge for Caster Semenya, the reigning Olympic champion who has long dominated at 800 meters.
The South African runner has been a lightning rod for complaints, facing gender tests as opponents questioned her winning times. Results of her testing have not made public.
"I am 97% sure you don't like me," Semenya tweeted on Thursday, "but I'm 100% sure I don't care."
The IAAF had previously addressed the issue of hyperandrogenism with a policy that recommended therapeutic treatments for athletes who tested above a specific level.
That rule was successfully challenged in the Court of Arbitration for Sport in 2015, but the IAAF has now returned to the issue with new research.
Athletes with a "difference of sexual development" or DSD — meaning they have a circulating level of testosterone at five nanomoles per liter or greater and are androgen-sensitive — have two options.
They can run against men. Or, they must be recognized as female or "intersex" by law and must take medications to alter their chemistry, the federation stated.
"The revised rules are not about cheating, no athlete with DSD has cheated," Coe said.
Dr. Stephane Bermon, of the IAAF's medical and science department, said: "The latest research we have undertaken, and data we have compiled, show that there is a performance advantage in female athletes with DSD over the track distances covered by this rule."
With the DSD standard set to take effect next fall, Coe defended the change, saying: "We want athletes to be incentivised to make the huge commitment and sacrifice required to excel in the sport."