Since 2003, the International Olympic Committee have permitted transgender athletes to compete in the Olympic Games under the gender with which they identify. According to their 2015 guidelines, transgender women don’t need legal recognition of their gender, but must limit their testosterone levels.


Some elite female athletes, including Paula Radcliffe and Dame Kelly Holmes, have recently questioned this and similar policies, calling for more research into whether transgender women have inherent athletic advantages over their cisgender competitors.

This idea stems from the fact that men are, on average, taller than women, and have higher testosterone levels and greater muscle mass. On top of this, men tend to have less fat mass, greater bone density and a higher capacity to carry oxygen in the blood.

In total, an elite male athlete performs, on average, ten per cent better than a female equivalent. Some athletes believe, therefore, that transgender women retain a performance advantage after transitioning. However, there is not yet enough evidence to draw a conclusion either way.

Yannis Pitsiladis, Professor of Sport and Exercise Science at the University of Brighton, explains: “We need the evidence to be able to say that having transgender females competing against cisgender females is fair – or at least, we can make the argument that it’s fair.”

Take, for example, muscle mass. During the hormonal treatment in the male-to-female transition, the hormone testosterone is suppressed. Testosterone, whether naturally occurring or introduced artificially, plays a role in building muscle, so suppressing it causes muscle mass to die away.

However, it is possible that the muscle retains its capacity to grow to its former size. “Clearly the muscle atrophies – there’s no doubt about that,” Pitsiladis says. “But does the muscle still have the potential to grow like a male muscle, given that it’s been in that environment for a long time?”

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The capacity for growth in a muscle lies in the contents of its cells. Muscle cells are unusual in that each one can contain multiple nuclei, known as myonuclei. The number of these myonuclei determines how big the muscle can grow.

“It’s the myonuclei that permit the laying down of more muscle fibres and therefore bigger muscle cells,” says Pitsiladis. “If one goes to the gym and does exercise, the myonuclear number goes up.”

A high testosterone level also results in a high myonuclear number. “It appears, though, that that number, once it goes up, stays up: it doesn’t go down,” Pitsiladis says. As a result, muscles appear to have a long-term ‘muscle memory’ of their capacity to grow, even after testosterone levels are lowered.

Jamaica's 4X100 relay gold medalist Usain Bolt (R) and Britain's 5000m gold medalist Mohamed Farah pose after the athletics event during the London 2012 Olympic Games on August 11, 2012 in London © Johannes Eisele/AFP/GettyImages

Pitsiladis is keen to point out, however, that the effect of muscle memory has so far only been observed in mice: “Let’s see what we find, in terms of human studies.”

What’s more, muscle mass on its own is far from a deciding factor. While a transgender woman might have a larger skeleton than a cisgender woman, that’s not necessarily a benefit. “From a biomechanical point of view there may be advantages, but there could also be disadvantages,” says Pitsiladis.

For example, oxygen-carrying capacity decreases over the transition with the decreasing levels of haemoglobin. “You have a bigger body, and you have a smaller engine to move that vehicle around,” he says. Even so, a bigger frame may be an advantage for basketball players or long-jumpers, but it could prove cumbersome for gymnasts or jockeys.

Studying these individual factors on their own is not enough to determine the fairness of competition between transgender and cisgender women. More than muscle mass or haemoglobin levels, research needs to be directed at the changing athletic performance over the course of transition. “Until we do those kinds of studies in humans who are transitioning, it’s really all conjecture,” Pitsiladis says.

There really is very little evidence either way. In 2016, a review of the research into transgender athletes found no papers that provided evidence of transgender women having any athletic advantage over cisgender women.

Whether this is because there is no advantage or because of the lack of research in the area is not clear. “There is very little science there,” Pitsiladis says. “We are desperately trying to get our studies underway.”

One key study was carried out in 2004 by Louis Gooren and Mathijs Bunck of VU University Medical Center Amsterdam, and looked at physical changes over the course of the transition. They found a significant overlap in muscle mass between transgender and cisgender women and concluded that the two groups could reasonably compete. However, the participants in the study were not athletes, and none of the physical changes studied directly measured athleticism.

The only study to date which has measured the changing athleticism in transgender athletes was performed by medical physicist, long-distance runner and transgender woman Joanna Harper. She compared the race times of herself and seven other transgender female distance runners before and after transition using a comparative method called ‘age grading’.

The age grading method assesses a runner’s performance by assigning a score relative to the best race time from a runner of the same age and gender. Harper’s study found that the runners all performed to a similar level, relative to the appropriate competitors, before and after transition. It’s worth noting that the small group of participants in this study were not elite athletes, and that conclusions about other sports cannot be drawn from this one.

Pitsiladis believes that in-depth research into the performance of transgender athletes is well worth doing. “We need to do those kinds of studies and look at how best to integrate in a way that is also fair on cisgender athletes,” he explains, “and in a civilised, modern society, we should be able to deal with these matters in a non-emotive way. We should get rid of emotion and just deal with the facts.”

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Sara RigbyOnline staff writer, BBC Science Focus

Sara is the online staff writer at BBC Science Focus. She has an MPhys in mathematical physics and loves all things space, dinosaurs and dogs.