Picture the lineup of athletes along a start line as they wait eagerly for the sound of the gun or gymnasts as they perform flips on a balance beam with unfathomable precision.
Although tremendous skill, hard work and thousands of hours of practice are involved in them molding their bodies for sporting excellence, nature is what brought these athletes to the front line.
Each competitor will vary in a multitude of ways, including height, weight, age and proportion. There are also differences in less-visible characteristics such as lung capacity, recovery rates and even hormone levels, with this variability generally accepted as a standard part of sporting culture.
But that hasn't always been the case when it comes to the hormone testosterone.
Differences in testosterone, specifically higher levels of testosterone among female athletes, have been a topic of debate in the Olympics for many decades, with officials stating concerns that it could provide an unfair advantage.
But given the sea of attributes giving each individual an edge, should levels of testosterone matter?
The right to run
Until recently, women with naturally high levels of testosterone -- known as hyperandrogenism
-- were not allowed to race without undergoing medical interventions to lower their levels of the hormones if they were measured to be within the range typically associated with men. This was a prerequisite as part of the International Association of Athletics Federations' Hyperandrogenism Regulation, which required athletes to prove that they
derive no advantage from their relatively high testosterone levels, or else not compete.
The regulations were introduced in 2011 after an 18-month review by an association expert working group in conjunction with the International Olympic Committee
and were "based on strong scientific consensus that the clear sex difference in sports performance is mainly due to the marked difference in male and female testosterone levels," according to a statement
. They were stated to have the aim to "preserve fair competition."
But in 2015, things changed.
That year, India's fastest woman, Dutee Chand, challenged the association
, arguing that she had a right to run and compete without artificially changing her body's hormones.
The Court of Arbitration for Sport
ruled in the 100-meter sprinter's favor, arguing that the association had not proved that testosterone gives athletes an unfair advantage.
The court then gave the International Association of Athletics Federations two more years to find evidence of the degree of performance advantage provided by increased levels of the hormone, which the association is now addressing.
Until then, the regulations would be suspended.
Competing without drugs
For 20-year-old Chand, the ruling meant she could compete on the world stage at the Rio Olympics and live her dream of representing her country in the 100 meters.
The decision also meant that South African runner Caster Semenya -- a fellow athlete with naturally high levels of testosterone -- was able to compete without being forced to take testosterone-suppressing drugs. She would also escape further invasive screenings.
In 2009, when Semenya was 18, she won gold in the women's 800 meters at the 2009 World Athletics Championship in Berlin. Her victory was quickly marred by widespread scrutiny of her sex, with the International Association of Athletics Federations launching an investigation hours after the race finished. Her sex and testosterone levels were tested, and although she was allowed to keep her gold medal, the association ultimately enforced the Hyperandrogenism Regulation in 2011.
have compared racing against Semenya to "running against a man."
What is hyperandrogenism?
Although commonly associated with men, testosterone is also produced naturally by women, but to a lesser extent. The hormone helps regulate the menstrual cycle and plays a role in the development of muscle tissue. As with anything in nature, some women have slightly higher levels than others; this is known as hyperandrogenism.
The condition is estimated to occur in 5% to 10% of women.
The most common symptoms of hyperandrogenism include excess hair growth, acne, alopecia and irregular menstrual cycles, which often appear gradually over a period of years.
The majority of women with the condition have a syndrome called polycystic ovary syndrome, according to David Ehrmann
, professor of medicine at the University of Chicago. The syndrome can cause ovaries to have large, underdeveloped follicles that may be unable to release eggs and ovulate.
"It's one of the most common hormonal disorders in women. Normally, it starts around the time of puberty, but it can manifest in the late teens and early 20s as well," Ehrmann said.
When a woman has polycystic ovary syndrome or hyperandrogenism, her body is less able to regulate levels of testosterone produced in the ovaries.
"Their testosterone levels are usually two to three times the normal limit of an average woman," Ehrmann said. But they can go higher, which is when the regulations kick in.
To add to the ambiguity of defining sex, there are many chromosomal variations beyond the typical XY and XX. According to the World Health Organization, one in 400
people has chromosomes that differ from the standard XY or XX formation.
During the 1964 Olympics, Polish runner Ewa Klobukowska
helped set a world record in the 4x100 meter relay. However, her efforts were discounted in 1967 after the International Association of Athletics Federations found her to have "one chromosome too many." Klobukowska was found to have an extra Y chromosome -- in addition to the traditional XX female chromosomes -- during a sex test.
For many years athletes, women's rights advocates and geneticists criticized the sex tests conducted by the association, as they ensured that only women with XX chromosomes were allowed to compete, rather than account for natural variation.
Their battle was won in 1992, when the International Association of Athletics Federations abandoned
all forms of systematic sex testing.
The pressure to be feminine
The spectrum of biological traits among both men and women means it can be problematic to force humans to fit into two distinct sex groups, particularly when it comes to sports
. Some people consider themselves to be intersex, non-binary
or other sexual identities.
When the International Association of Athletics Federations introduced the testing of athletes for hyperandrogenism, female athletes felt they had to conform to feminine ideals, according to Silvia Camporesi
, lecturer in bioethics and society at King's College London. This is highlighted by the fact that during testing, focus was placed on physical characteristics typical of being female, such as (lack of) body hair, voice pitch and the size and shape of breasts, as part of the process, which also includes testing blood serum for hormone levels. This put female athletes under pressure to look more feminine, because if they didn't, they would face the scrutiny of the International Association of Athletics Federations.
"Women were recommended to take androgen-suppressing therapy but also to do other kinds of feminizing procedures," Camporesi said.
Between 2011 and 2015, four unnamed athletes agreed to procedures that included partial clitoridectomy -- the excision of the clitoris -- in order to meet International Association of Athletics Federations eligibility standards. The association's regulation stated that an enlarged clitoris could be an indicator of severe hyperandrogenism, but researchers argued that the operations were unnecessary and that they raised serious ethical concerns.
The International Association of Athletics Federations did not respond to CNN's requests for comment.
"This is outside of medical necessity, and what does that have to do with competing?" Camporesi asked.
She adds that the International Association of Athletics Federations relies too heavily on a binary distinction of sex in terms of males and females, which does not reflect nature.
"Elite athletes have all kinds of genetic or biological traits that make them what they are ... that we as normal people don't have," Camporesi said. "I don't think hyperandrogenism should be singled out."
Celebrate the difference
Camporesi believes that these traits -- combined with mental capacity and training -- are how athletes become truly elite.
The International Olympic Committee recently commissioned research
into the genetic and molecular aspects of sports performance. The studies celebrated athletes with naturally occurring genetic or biological variations as being a source of inborn greatness. But to celebrate biological diversity on the one hand while punishing it on the other is inconsistent, says Camporesi.
Either way, athletes like Chand and Semenya are clear of testing at Rio 2016. They'll have only the starting line to worry about -- and the chance to showcase their extraordinary athletic talent.