It is a case described by the court of arbitration for sport as “one of the most pivotal” it has ever heard. And after months of legal wrangling and highly charged debate, Olympic 800m champion Caster Semenya has lost her landmark legal case against athletics’ governing body, the IAAF, which means she will have to take medication to reduce her testosterone if she wants to keep running on the international stage.

Why did the case come about?

In April 2018 the IAAF introduced new rules requiring female athletes with differences in sexual development (DSDs) – usually because they were born with testes – to lower their testosterone if they wanted to compete internationally in events between 400m and a mile. Semenya challenged those in the court of arbitration for sport (Cas) in February, claiming these rules discriminated against her.

What was the IAAF’s argument?

It insists that the rules are needed to protect female sport, as DSD athletes such as Semenya have a large advantage. As the IAAF president, Sebastian Coe, puts it: “This is an incredibly sensitive subject but it is the responsibility of the federation to create a level playing field in female sport.”

How does the IAAF back up its case?

It points out that, while most females have testosterone levels ranging from 0.12 to 1.79 nmol/L, DSD athletes – who are often born with testes and get similar increases in muscle size, strength and haemoglobin levels as a male does after puberty – are usually in the normal adult male range, which is from 7.7 to 29.4 nmol/L. Therefore it wants DSD athletes to reduce their testosterone to below 5 nmol/L for at least six months in some events.

What was Semenya’s argument?

Her legal team have argued her advantages are no different from other genetic variations celebrated in sport – and that “her genetic gift should be celebrated, not discriminated against”. They also maintain: “Her case is about the rights of women who are born as women, reared and socialised as women – [to] be permitted to compete in the female category without discrimination.” Finally they say the IAAF’s requirement for DSDs to take hormone suppressants to reduce testosterone is ethically wrong and potentially poses a health risk.

What does the science say about testosterone?

Good question. Almost all scientists agree that testosterone is a key factor as to why elite men are typically around 10-12% faster and stronger than women. However Semenya’s team have argued DSD women with high testosterone may not get the same performance benefits from the hormone because their bodies do not convert the testosterone into a fully active form.

And there are other contentious issues, right?

Yes. In 2015 when Cas heard the case of Dutee Chand, it told the IAAF to show the extent to which elevated testosterone gave DSD women a competitive advantage. In 2017 the IAAF then published research which claimed elite women runners with high testosterone performed as much as 3% better than those with lower levels. However, that paper has since been heavily criticised as being flawed by the academics Roger Pielke, Erik Boye and Ross Tucker, who say 17-32% of the data was erroneous. The fact that the proposed IAAF policy applies only from the 400m to a mile is also odd, given testosterone has more of an effect in power events. However, there are not enough DSD athletes in events such as the javelin or the shot for the IAAF to make its case in those disciplines.

What can we expect from Semenya now she has lost?

Tucker, who was part of her team of experts at Cas, believes she will run the 800m around seven seconds slower if she reduces her testosterone – essentially turning her from a world beater into an also-ran.

Why has the case been so difficult for Cas?

Partly because it was being asked to balance the rights of Semenya and other DSD athletes with the broader need to protect women’s sport. Partly because this also touches on the inherent tensions between fairness and inclusivity, as well as ethical questions. A huge question remains: if it is accepted that there should be separate categories for men and women in elite sport, can it be done in a manner that satisfies everybody?