NEW YORK – A new study financed by the International Olympic Committee (IOC) found that transgender female athletes showed greater hand-grip strength – an indicator of overall muscle strength – but lower jumping ability, lung function and relative cardiovascular fitness compared with women whose gender was assigned female at birth.

That data, which also compared trans women with men, contradicted a broad claim often made by proponents of rules that ban transgender women from competing in women’s sports. It also led the study’s authors to caution against a rush to expand such policies, which already bar transgender athletes from some Olympic sports.

The study’s most important finding, according to one of its authors, Yannis Pitsiladis of the IOC’s medical and scientific commission, was that, given physiological differences, “trans women are not biological men”.

The authors warned against the presumption of immutable and disproportionate advantages for transgender female athletes who compete in women’s sports, and they advised against “precautionary bans and sport-eligibility exclusions” that were not based on sport-specific research.

Outright bans, though, continue to proliferate.

Twenty-five American states have laws barring transgender athletes from competing in women’s sports, according to the Movement Advancement Project, a non-profit that focuses on gay, lesbian, bisexual and transgender parity.

And the National Association of Intercollegiate Athletics, the governing body for smaller colleges, in April barred transgender athletes from competing in women’s sports, unless their sex was assigned female at birth.

Two of the most visible sports at this summer’s Paris Games – swimming and track and field – have effectively barred transgender female athletes who went through puberty as males.

Those in other sports permitted to compete often face stricter requirements in suppressing their levels of testosterone.

In general, the argument for the bans has been that profound advantages gained from testosterone-fuelled male puberty – broader shoulders, bigger hands, longer torsos, and greater muscle mass, strength, bone density and heart and lung capacity – give transgender female athletes a competitive edge.

The new laboratory-based, peer-reviewed and IOC-funded study at the University of Brighton, published in April in the British Journal of Sports Medicine, tested 19 cisgender men (those whose gender identity matches the sex they were assigned at birth) and 12 trans men, along with 23 trans women and 21 cisgender women.

All of the participants played competitive sports or underwent physical training at least three times a week.

And all of the trans female athletes had undergone at least a year of treatment suppressing their testosterone levels and taking estrogen supplementation, the researchers said.

The study found that transgender female participants showed greater hand-grip strength than cisgender female participants but lower lung function and relative VO2 max, the amount of oxygen used when exercising. Transgender female athletes also scored below cisgender women and men on a jumping test that measured lower-body power.

The study acknowledged some limitations, including its small sample size and the fact that the athletes were not followed over the long term as they transitioned.

And, as previous research has indicated, it found that transgender female athletes did retain at least one advantage over cisgender female athletes – a measurement of hand-grip strength.

But it is a combination of factors, not a single parameter, that determines athletic performance, said Pitsiladis, a professor of sport and exercise science.

Athletes who grow taller and heavier after going through puberty as males must “carry this big skeleton with a smaller engine” after transitioning, he said.

He cited volleyball as an example, saying that, for transgender female athletes, “the jumping and blocking will not be to the same height as they were doing before. And they may find that, overall, their performance is less good”.

But Michael Joyner, a doctor who studies the physiology of male and female athletes, said that, based on his research, science supports the bans in elite sports, where events can be decided by the smallest of margins.

“We know testosterone is performance enhancing,” he said.

Additionally, he added, declines in performance by trans women after taking drugs to suppress their testosterone levels do not fully reduce the typical differences in athletic performance between men and women.

While there will always be two viewpoints in this hugely debatable topic, Outsports, a website that reports on LGBTQ+ issues, hailed the IOC-funded study as a “landmark” that concluded that “blanket sports bans are a mistake”.

But some scientists and athletes also called the study deeply flawed in an article in The Telegraph, which labelled the suggestion that transgender women are at a disadvantage in sports a “new low” for the IOC.

So heated is the debate that Pitsiladis said he and his research team have received threats. That, he warned, could lead other scientists to shy away from pursuing research on the topic.

“This is no longer a science matter. Unfortunately, it’s become a political matter,” he said. NYTIMES