Trans Women in Sports: The Truck With the Compact-Car Engine

Überladener LKW

Women are compact cars, men are trucks. Reduced to its simplest form, though not entirely unfairly, that is the usual logic of the debate about trans women in sports.

The problem: A trans woman on hormone therapy is not driving a truck with a truck engine. She is driving a truck chassis with the engine of a compact car.

And that is exactly where the genuinely interesting question begins. How much horsepower is actually left under the hood when we measure instead of merely looking?

The discussion about trans women in sports begins surprisingly often with body structure. Taller. Broader shoulders. More muscle mass. Larger bones. After all, male puberty cannot be reversed.

That is even partly true.

It just does not answer the crucial question.

Because in sports, the body with the largest bones does not win. Nor does the person with the greatest muscle mass automatically win. What matters is what that body can actually perform in a particular sport.

And this is exactly where the scientific evidence becomes considerably more complicated than the political debate.

First of All: Yes, Male Puberty Makes a Difference

We do not have to pretend that biological differences do not exist.

During male puberty, testosterone levels increase substantially. This changes muscle mass and strength, hemoglobin, and various cardiovascular and anatomical factors, among other things. In sports where strength, speed, power, or endurance are decisive, this is why substantial average performance differences emerge between cis men and cis women after puberty.

A 2023 consensus paper from the American College of Sports Medicine places these differences at roughly 10 to 30 percent, depending on the athletic demand. Timing matters as well: performance differences increase significantly with the onset of puberty and are closely associated with rising sex hormones, particularly testosterone [4].

So if someone says that going through male puberty is completely irrelevant to the discussion about sports, that is difficult to defend scientifically.

But it does not follow that a trans woman after years of feminizing hormone therapy is physiologically equivalent to a cis man in athletic terms.

And that is precisely the conceptual leap that happens surprisingly often.

What Happens When Testosterone Levels Drop?

Feminizing hormone therapy does not change the body cosmetically. It changes some of the systems that are relevant to athletic performance.

A systematic review by Joanna Harper and colleagues examined a total of 24 studies in 2021 on the effects of testosterone-suppressing hormone therapy in trans women. The changes in blood were particularly striking. After only about four months, hemoglobin and hematocrit levels were within the range found in cisgender women [2].

Hemoglobin is hardly irrelevant in sports. It transports oxygen in the blood and is therefore one of the factors that influence aerobic performance in particular.

Other changes take longer.

After twelve months of hormone therapy, the studies reviewed by Harper and colleagues found reductions in lean body mass, muscle cross-sectional area, and strength. At the same time, some of these values remained, on average, above those of cisgender women even after longer treatment. This finding became a major part of the argument that an advantage created by male puberty might not disappear completely [2].

A later review in the Journal of Clinical Endocrinology & Metabolism presents a more differentiated picture. After roughly one year of feminizing hormone therapy, fat mass increases while muscle mass decreases. Absolute lean body mass may remain higher than in cis women. However, when measures such as muscle strength or maximal oxygen uptake are adjusted for body or muscle mass, some of these differences disappear in the studies reviewed. At the same time, the authors emphasize how limited the evidence remains, especially when it comes to actual athletes [3].

And that brings us to the core of the problem.

A Physical Difference Is Not Yet an Athletic Advantage

A trans woman may be taller than the average cis woman. She may have longer limbs. She may also retain more absolute lean body mass after hormone therapy.

Those are measurable differences.

But a difference only becomes an athletic advantage if it actually contributes to better performance in the relevant discipline.

That sounds obvious. Yet in the debate about trans women in sports, this distinction is skipped surprisingly often.

Height means something different in basketball than it does in gymnastics. Body mass matters differently in weightlifting than in marathon running. Long arms may be useful in swimming and have an entirely different biomechanical effect in another movement.

And even muscle mass cannot simply be equated with athletic performance. A larger muscle also has to be supplied with oxygen and moved. Absolute strength, strength relative to body weight, and sport-specific performance are different measures.

That is precisely why the question "Do physical differences resulting from male puberty remain?" is scientifically not identical to the question "Do trans women have an unfair athletic advantage after hormone therapy?"

For the first question, we now have a relatively large amount of data.

For the second, surprisingly little.

What Happens When We Measure Actual Performance?

At this point, a systematic review and meta-analysis published in 2026 becomes particularly interesting.

Sofia Mendes Sieczkowska and colleagues evaluated 50 studies involving a total of 6,099 people. They examined body composition and physical performance parameters in trans and cisgender people.

For trans women compared with cis women, they found no statistically significant differences in relative fat mass, relative lean mass, upper-body strength, lower-body strength, or maximal oxygen uptake. Under feminizing hormone therapy, fat mass increased, while lean mass and upper-body strength decreased over a period of one to three years [1].

That is a remarkable finding.

Especially because it makes visible exactly the distinction that is so often lost in public debate: A body can differ from another in certain anatomical or physiological characteristics without therefore being superior in the performance parameters being measured.

The authors therefore conclude that the current evidence does not support the assumption of an inherent athletic advantage for trans women over cis women [1].

At first, that sounds fairly definitive.

It is not.

Because the same authors explicitly point out that the certainty of the evidence for many of the parameters studied is low or very low. The studies differ considerably in design, participants, and duration of hormone therapy. And most importantly, many of the people studied were not competitive athletes [1].

"An advantage could not be demonstrated" is scientifically different from "It has been proven that there is no advantage."

That distinction matters.

And There Are Findings Pointing in the Other Direction

Anyone who wants to discuss the research seriously must also include findings that suggest remaining advantages.

One case study, for example, examined the competitive performance of a trans swimmer before and after approximately two years of feminizing hormone therapy. Her absolute times became slower, but the loss in performance varied by freestyle distance and was smaller than the difference between male and female elite performances used by the authors for comparison. Relative to her respective NCAA competition, her ranking improved after moving into the women's category. The authors interpreted this as an indication of a remaining performance advantage [5].

That is relevant evidence.

But it is also a case study involving exactly one athlete.

It cannot tell us that all trans women possess such an advantage, nor that the same effect would occur in track and field, cycling, weightlifting, soccer, or karate.

That is exactly why we need better data.

What Happens to the Same Athlete Before and After Transition?

One particularly interesting approach is not to compare different groups with one another, but to follow the same athletes across their transitions.

In 2025, Joanna Harper and colleagues published such a study. Nine trans runners and one trans swimmer provided verifiable competition data from before and after beginning hormone therapy.

Among the runners, absolute race times worsened by an average of 14.6 percent after an average of 31 months of hormone therapy. The researchers then did not simply compare the raw times. Instead, they compared age- and sex-adjusted relative performance before and after hormone therapy.

Among the nine runners, there was no statistically significant difference in relative performance before and after hormone therapy. The result remained unchanged after changes in training volume were taken into account [6].

Put simply: The runners studied did not suddenly become unusually more successful athletes within their new comparison group as a result of their transition.

This study is not definitive proof either. Nine runners are a tiny sample. The swimmer also cannot meaningfully be generalized from. But the study shows how important actual performance data are.

Because in the end, sports are not only about how much muscle mass someone has.

We care about how fast she runs.

This Is Exactly Where Our Truck Metaphor Reaches Its Limits

The metaphor of the truck with the compact-car engine works as an introduction because it exposes a flaw in the way people think about the issue.

But it is not physiology.

Feminizing hormone therapy does not simply convert a human body from one performance class into another. Some characteristics change quickly, some slowly, and others hardly at all. Hemoglobin responds differently from muscle mass. Muscle mass is not the same as strength. Strength is not the same as running speed. And running speed tells us nothing about how someone swims, jumps, or lifts weights.

So this is exactly where we should forget the metaphor again.

Because the scientifically interesting question is not: Does a trans woman still have characteristics that developed during male puberty?

Of course she may.

The relevant question is: Which of those characteristics still produce a measurable and competitively relevant advantage after feminizing hormone therapy, and in which sport?

Science does not yet have one universally applicable answer to that question.

And Yet Universal Rules Are Being Created

While research is still trying to understand these differences on a sport-specific basis, some sports governing bodies have already made very far-reaching decisions.

World Athletics introduced new rules for its female category on September 1, 2025. For competitions relevant to the world rankings, the organization requires a one-time SRY gene test as part of its eligibility rules. Trans women therefore fall under a framework in which participation in the female elite category is no longer determined solely by current testosterone levels or the duration of hormone therapy. One piece of information published by World Athletics when introducing the rules is particularly notable: Under the previous rules, no trans woman was competing at international elite level at that time [8].

World Aquatics also follows a restrictive eligibility policy. Its Eligibility Policy, adopted in 2022 and still maintained, regulates participation in men's and women's categories according to its own biological criteria [9].

The US NCAA went one step further in February 2025. Since then, athletes who were assigned male at birth have been barred from NCAA women's competitions regardless of hormone status or duration of transition. The NCAA explicitly stated that the change was adopted following an executive order from the US government at the time [10].

We therefore now have a remarkable situation.

The scientific literature emphasizes uncertainty, small samples, differences between sports, and the need for more sport-specific research.

Some rulebooks nevertheless draw extremely far-reaching and categorical boundaries from that evidence.

The two should not be confused.

A governing body's rule is not a scientific finding.

Perhaps We Are Asking the Wrong Question

The debate usually asks: Do trans women have an advantage?

But even that question is too broad.

Which trans woman? With what pubertal history? After how many months or years of hormone therapy? In which sport? At what level of competition? And what kind of advantage do we even mean? Height? Absolute strength? Relative strength? Explosiveness? Oxygen uptake? Endurance? Reach? Body mass? Technique?

A recent review of trans inclusion in sports reaches a cautious conclusion at exactly this point: Existing studies show physiological changes resulting from gender-affirming hormone therapy, but they are limited by small samples, short observation periods, differing treatments, and measurements that are rarely sport-specific. According to the authors, the available evidence is therefore insufficient to scientifically justify uniform rules across all sports [7].

That seems much closer to reality to me than the certainty with which this topic is so often discussed.

Fairness Does Not Mean Denying Differences

I do not want to claim that male puberty is irrelevant.

That would be wrong given the existing research.

Nor do I want to claim that we can already prove that no relevant advantage remains in any conceivable sport after feminizing hormone therapy.

The evidence is not sufficient for that either.

But body height, bone structure, or remaining muscle mass also do not automatically prove that a trans woman has an unfair advantage in athletic competition.

The most comprehensive meta-analysis to date finds no statistically detectable difference from cis women across several fitness parameters examined. Individual studies, by contrast, find remaining differences or advantages. Others observe that the relative competitive performance of trans athletes after longer hormone therapy ends up roughly where it had already been within their previous comparison group. At the same time, the quality of the overall evidence remains limited [1, 6, 5].

Perhaps a little less certainty would therefore be appropriate.

And a little more science.

Because there may indeed still be a fairly large truck standing there.

The only question is whether that means it wins the race.

Yours, Lizbeth

Sources

1. Sieczkowska, S. M. et al. (2026): Body composition and physical fitness in transgender versus cisgender individuals: a systematic review with meta-analysis. British Journal of Sports Medicine, 60(3), 198 ff. Die Analyse umfasst 50 Studien mit 6.099 Personen. Eine Korrektur des Artikels wurde im Juni 2026 veröffentlicht. Studie im British Journal of Sports Medicine

2. Harper, J. et al. (2021): How does hormone transition in transgender women change body composition, muscle strength and haemoglobin? Systematic review with a focus on the implications for sport participation. British Journal of Sports Medicine, 55(15), 865-872. PubMed-Eintrag

3. Cheung, A. S. et al. (2023): The Impact of Gender-Affirming Hormone Therapy on Physical Performance. Journal of Clinical Endocrinology & Metabolism. PubMed-Eintrag

4. Hunter, S. K. et al. (2023): The Biological Basis of Sex Differences in Athletic Performance: Consensus Statement for the American College of Sports Medicine. Medicine & Science in Sports & Exercise, 55(12), 2328-2360. PubMed-Eintrag

5. Senefeld, J. W., Hunter, S. K. & Coleman, D. (2023): Case Studies in Physiology: Male to female transgender swimmer in college athletics. Journal of Applied Physiology. PubMed-Eintrag

6. Harper, J. et al. (2025): Longitudinal Performance Changes in Transgender Women Athletes Pre and Post Gender Affirming Hormone Therapy. European Journal of Sport Science, 25(9), e70036. PubMed-Eintrag

7. Cheung et al. (2026): Trans-Inclusion in Sports: History, Scientific Evidence and Future Directions. Aktuelle Übersichtsarbeit zu Evidenz und Regelwerken für trans Athlet:innen. PubMed-Eintrag

8. World Athletics (2025): World Athletics introduces SRY gene test for athletes wishing to compete in the female category. Regeländerung mit Wirkung zum 1. September 2025. World Athletics

9. World Aquatics (2022, aktuelle Regelseite): Policy on Eligibility for the Men's & Women's Competition Categories. World Aquatics Competition Regulations

10. NCAA (2025): Participation Policy for Transgender Student-Athletes. Gültig seit 6. Februar 2025. Aktuelle NCAA-Regelung