AP Explains: Ruling Against Olympian Caster SemenyaIn one of the most ethically and scientifically challenging cases in modern sports law, two-time Olympic champion Caster Semenya lost her appeal of a rule that requires women to medically reduce their natural levels of testosterone in order to compete in certain track events. Here is a look at the case that sports' highest court acknowledged in its 2-1 ruling Wednesday was discriminatory, but said the discrimination is “necessary, reasonable and proportionate” to maintain fairness in women's track: The rules In April 2018, the governing body of track and field published eligibility rules in its races for women runners with “differences of sex development (DSD).” Those women must reduce their testosterone to comply with IAAF rules for at least six continuous months before competing at top-level events. The rules apply only to “conditions where the affected individual has XY chromosomes” — male chromosomes — it was explained Wednesday by the Court of Arbitration for Sport. Those athletes “have testosterone levels well into the male range.” The IAAF has said the rules “exist solely to ensure fair and meaningful competition,” and not to judge or question any athlete's sex or gender identity. Put on hold by Semenya's appeal, those rules now take effect next Wednesday. The IAAF's first attempt to address the issue in 2011 was rejected by a previous Court of Arbitration for Sport panel in 2015. That ruling challenged the IAAF to produce better evidence because it had not proven that hyperandrogenic women — women with excessive amounts of male hormones — gained a significant advantage. Semenya’s defense Semenya's lawyers argued that “her genetic gift should be celebrated, not discriminated against.” Although testosterone — a hormone that strengthens muscle tone and bone mass — is a doping product if injected or ingested, no one has suggested Semenya's natural physiology is a form of cheating. Semenya's case was championed by United Nations human rights experts and women's sports activists, including Billie Jean King, who saw potential for abuse and discrimination against women. South Africa's ruling African National Congress party linked Semenya's cause to the liberation struggle of Nelson Mandela and, on Wednesday, accused the IAAF of “acting in a prejudicial manner.” The judges The judges from Canada, Australia and Switzerland scrutinized evidence, including testimony from Semenya, over five days in February in Lausanne, Switzerland. The nuanced verdict was detailed in a two-page court statement expressing concern about some evidence underpinning the rules and how they will now be applied. The judges believe some women who choose treatment with medication will have difficulties staying within with the limit and face bans for “unintentional non-compliance.” The IAAF said Wednesday it “will keep all practical matters of implementation under periodic review.”  Behind the ruling The judges' full 165-page verdict remains confidential, and contains sensitive medical information. A six-page executive summary published Wednesday clarified the judges' majority opinion without detailing the evidence. The ruling derives from the premise that separate men's and women's races were created due to “insuperable performance advantages derived from biology.” From there, “it follows that it may be legitimate to regulate the right to participate in the female category by reference to those biological factors rather than legal status alone.” The case appeared as if it would likely sway on scientific evidence rather than a concept of fairness, and one expert witness called by Semenya's team questioned the data. ”In the absence of really compelling evidence, something discriminatory is not justifiable,” South African academic Ross Tucker told The Associated Press. “I don't see good science having been followed here.” Semenya’s future The CAS judges paid tribute to “Ms. Semenya's grace and fortitude throughout this process.” She will potentially run her final international 800-meter race on Friday in Doha, Qatar, when the Diamond League series opens. If she wants to defend her 800-meter world title in September — also in Doha — she must begin medicating within days. The IAAF requires affected athletes to give a blood test by next Wednesday within the set limits. With the world championships opening on Sept. 28, the IAAF said it will waive the 6-month compliance rule for that event. Semenya can run without medication in the 5,000 meters — or possibly in the 1,500 if the IAAF follows the judges' guidance to defer applying the rules to that event. Semenya won a bronze medal in the 1,500 at the 2017 worlds in London. There is no indication the IAAF will do that. What next? Semenya and the South Africa track federation have 30 days to appeal to Switzerland's Supreme Court in Lausanne. Federal judges rarely overturn CAS decisions but can intervene if it's determined the legal process was abused. Semenya could request an interim federal ruling to freeze the CAS decision pending a full appeal. With the IAAF rules under permanent review, other female athletes could one day challenge them. “I can say this decision is certainly not the perfect one,” CAS official Matthieu Reeb said, “but is there a perfect decision in this situation?”