Tennis legend Martina Navratilova has voiced her support for trans women being excluded from competing in female elite matches.
The ex-world number one revealed in a piece for The Times that said the recent move by World Athletics was 'a step in the right direction'.
"I have long been vocal on this matter, as I have educated myself on what the realities are for trans women competing against biological females," she said.
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Navratilova, who is a member of the Women’s Sport Policy Working Group, went on to add that lung capacity, skeletal structure, skin thickness, and muscle mass.
"The other one nobody talks about is the airway through which we breathe, which is 25 to 50 per cent larger in men," the 59-time Grand Slam winner said.
"If you can breathe faster and deeper, that is a massive advantage in top-level sport.
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"That’s before you touch upon what could be something like five inches of height difference between somebody who was born male and a biological female, with a difference in arm length of two inches."
That means that if women were to compete against men in tennis or sports like, say, basketball, women would need to jump seven inches higher than their male competitors to break even.
"It’s no accident that we don’t hear complaints about trans men competing against men," Navratilova said.
"They don’t have the physical advantages of a male who has gone through puberty.
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"But the advantage trans women have is obviously unfair."
She added that 'testosterone is the biggest driving force' in the gender divide in sport and even with transition medication, 'the effects of male puberty are irreversible'.
"You cannot simply turn back the clock," she said.
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Navratilova, who is widely considered to be one of the greatest tennis players of all time, went on to call for an 'open category' to even the playing field.
"It would be a category for all-comers: men who identify as men; women who identify as women; women who identify as men; men who identify as women; non-binary - it would be a catch-all," the 66-year-old said.
"Biological females are most likely to compete in the biological female category, as that’s their best shot at winning and it maintains the principle of fairness.
"With an 'open' category there are no question marks, no provisos, no asterisks, no doubts."
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She added: "It’s a simple solution."