A review finds no differences in physical performance between trans women and cis women, even though they have more muscle mass

The work by Sieczkowska and colleagues, to be published in the British Journal of Sports Medicine (part of the BMJ group), consists of a systematic review followed by a meta-analysis of transgender people who underwent gender-affirming hormone treatment for at least three years, comparing them with cisgender men and women using various measures including fat-free body mass and upper and lower limb strength.

In my opinion, the study has two significant flaws, which are only detectable after a thorough and careful reading. Firstly, the study presents a significant discrepancy between the conclusions drawn in the abstract and the limitations acknowledged in the discussion, which is worth highlighting. In the abstract, the authors state that their results ‘do not support theories of inherent athletic advantages in trans women versus cis women’.

This formulation suggests to the reader a strong and general conclusion, with clear implications for public debate. However, in the discussion, the authors themselves explicitly acknowledge that the lean mass and physical performance data do not necessarily come from the same cohorts or study designs, which prevents direct correlations from being established between muscle mass and strength or functional performance.

This acknowledgement invalidates any robust inference about the existence or absence of athletic advantages. From a methodological point of view, the evidence presented does not allow for conclusions in either direction. What the data show is, at most, an absence of detectable differences in certain performance metrics in the short and medium term, but not a refutation of the hypothesis of inherent athletic advantages.

The problem is not the quality of the analysis—which is cautious in its discussion—but the rhetorical shift between sections: a strong conclusion in the abstract, which is what most readers and journalists read, and an admission of substantial uncertainty in the body of the article. This type of formulation can lead to simplified or overgeneralised interpretations that are not fully supported by the data.

In a scientifically complex and socially sensitive field, the distinction between “finding no evidence” and “demonstrating absence of effect” is not a minor nuance, but a basic principle of scientific reasoning. In this case, the study provides relevant information, but does not settle the question that the abstract itself suggests it has resolved.

Secondly, both the introduction and the discussion lack a developmental perspective that is essential in these cases, specifically the three-phase action of testosterone (during gestation, mini-puberty after birth and from puberty onwards), nor do they mention recent studies on the effects of feminisation treatment (estradiol + cyproterone acetate) on the epigenome.

The article attempts to shed light on an important social debate in which science has yet to have its say: the participation of transgender people in competitive sports, especially transgender women.

It is a systematic review of the literature to date, including a total of 52 studies. The article highlights a series of data that are of interest when it comes to justifying or not the participation of transgender people in sport.

It shows that there is no significant difference in lean body composition or upper and lower body strength between cis women and trans women after 1-3 years of hormone treatment. There are also no differences in cardiorespiratory fitness. Therefore, there is no evidence that trans women are stronger or have greater muscle mass or endurance, as has been suggested in the public debate.

Furthermore, in comparisons between trans women undergoing hormone treatment and cis men, trans women do have lower results in terms of lean mass, upper and lower body strength, and cardiometabolic performance than cis men.

Trans men do have greater strength and muscle mass than cis women, but they do not reach the values achieved by cis men.

Although the review has some limitations, such as the lack of studies on elite athletes and the lack of studies on specific sports events, it gives us two clear messages: there is no evidence to support the claims that trans women have greater strength and endurance than cis women, and that exposure to testosterone prior to hormone treatment in trans women does not lead to better physical conditions for sport.

Therefore, contrary to the opinion that trans people should not compete in their gender categories, this systematic review shows us that there is no scientific justification for this discrimination in sport.

In my opinion, the article confirms that transgender athletes have greater muscle mass and, therefore, with proper training, would have a performance advantage over cisgender women. With greater baseline muscle mass and higher testosterone levels until puberty, the body has memory and, with proper stimulation, would be able to develop more functional capacity.

The study presented takes into account different systems and all types of transgender populations to measure functional capacity. It is not specific to transgender women who participate in sports and, therefore, in my opinion, cannot be extrapolated to professional sports.

SEMED-FEMEDE has already issued a statement indicating that, in our opinion, perhaps the fairest solution would be to establish an open or third gender category for competition.

Of course, the research—which can be considered high quality—raises doubts where, apparently, there were none. As it points out, just one to three years after starting hormone treatment, there is a clear convergence in the functional performance of transgender women with that of cisgender women, particularly in strength and aerobic capacity. At the very least, this should lead to a rethinking of certain maximalist positions in the field of competitive sport.

On the other hand, it is worth highlighting the significant limitations of the study, mainly relating to the small number of transgender athletes in elite sport, which makes it difficult for this type of work to achieve the statistical power necessary to offer categorical results. Therefore, further research is needed.

The problem, from my point of view—more that of a field technician than a scientist, even though I have participated in research on this topic—lies more on the social than the physiological level. Sports federations are not doing enough to promote policies for the inclusion of transgender people in competition, which deepens the distress these individuals may suffer, with unpredictable consequences for their mental health. Science will take time to resolve this issue; however, there are young people who are missing out on opportunities to compete and feel discriminated against as a result.

Therefore, beyond the need for further research, I believe it is urgent to explore ways for transgender athletes to participate that do not violate the principle of “physiological equality” that is supposed to govern sport. The solution may not be easy in some sports, but I am convinced that in others it is not so difficult.

This article provides an updated summary of physical fitness and body composition in transgender individuals compared to cisgender individuals, integrating data from observational and longitudinal studies that analyse key variables such as fat and lean mass, muscle strength, and maximum oxygen consumption (VO₂max).

The results show that, in transgender women, relative fat mass (%), upper and lower body strength, and VO₂max are similar to those observed in cisgender women, although absolute lean mass (kg) is higher in some cases. However, this higher lean mass does not seem to translate into significant differences in overall physical fitness, underscoring the importance of distinguishing between body composition and functional performance. Analysis in transgender men shows higher fat mass (before or in the early stages), while muscle mass and strength data remain below those observed in cisgender men.

The article highlights the role of gender-affirming hormone therapy (GAHT) as a modulator of body composition and VO₂max. In transgender women, longitudinal evidence indicates that hormone therapy is associated, over the first three years, with an increase in fat mass and a reduction in lean mass and strength, especially in the upper body, progressively placing their physiological profiles within the ranges described for cisgender women. However, we must bear in mind that transgender women have also shown lower muscle mass and strength compared to cisgender men. In parallel, transgender men have been observed to have a physiological pattern consistent with the effects of GAHT, with a reduction in fat mass and an increase in lean mass and strength after its initiation.

The article contributes significantly to clarifying the current state of scientific evidence in a field that requires cautious, evidence-based and contextualised interpretations. We must therefore bear in mind that physical fitness is not a single or static variable, but is determined by the interaction of multiple factors: endocrine, nutritional, training, health, age, etc. Therefore, in order to reach more solid conclusions, further studies are needed that assess these factors together and not in isolation.

From a methodological point of view, the authors rigorously acknowledge the limitations of the available evidence, pointing out that most studies have low certainty and heterogeneous quality, with small sample sizes and considerable variability in designs, variables assessed and instruments used. This transparency strengthens the interpretative validity of the work and avoids overly generalised conclusions. With these conclusions, the data provided should be subject to review as more robust and methodologically homogeneous longitudinal studies become available.