Trans athletes are not a single group with a single answer. The research shows a mixed picture: hormone therapy closes some biological gaps significantly, but not all of them, and the degree of remaining difference depends on the specific physical trait and the sport in question. The most studied group is trans women (male-to-female), where the question centers on whether male puberty leaves lasting athletic advantages even after testosterone suppression.
What Hormone Therapy Changes
Some biological markers shift quickly and completely. Hemoglobin, the protein in red blood cells that carries oxygen to muscles, drops to typical female levels within about four months of hormone therapy and stays there. This matters because hemoglobin concentration is one of the key drivers of aerobic endurance, and this particular advantage effectively disappears early in transition.
Muscle mass also decreases, but more slowly and less completely. After 12 months of estrogen therapy, studies report roughly a 5% loss in lower limb muscle mass and about 9% loss in total muscle mass. That’s a meaningful decline, but the average difference in muscle mass between males and females before any intervention is considerably larger, often in the range of 30% to 40% depending on the muscle group. So while hormone therapy moves the needle, it doesn’t fully close the gap within the first year or two that most policies require.
What Hormone Therapy Doesn’t Change
Skeletal structure is permanent. Bone length, shoulder width, hip geometry, and hand size are all set during puberty and do not reverse with hormone therapy. A trans woman who went through male puberty will retain longer limbs, larger hands, and a different hip-to-shoulder ratio. Whether these structural differences translate into a competitive edge depends on the sport. In swimming, longer arms and bigger hands improve stroke mechanics. In running, longer legs can mean a longer stride. In sports where leverage and reach matter, like combat sports or volleyball, the structural difference is more relevant than in, say, distance cycling.
Lung capacity is another factor that persists. A cross-sectional study of trans women athletes who had been on hormone therapy for at least a year found they had a lower ratio of air expelled to total lung capacity compared to cisgender women, reflecting the larger ribcage and airways developed during male puberty. Bigger lungs can move more oxygen, which feeds into endurance performance.
Grip Strength and Power
One of the clearest findings in the research involves grip strength. In a study comparing trans women athletes (all on hormone therapy for at least one year, with testosterone levels equivalent to cisgender women) to cisgender women athletes, trans women averaged 40.7 kg of grip strength compared to 34.3 kg for cisgender women. That’s roughly a 19% advantage in absolute hand strength despite similar testosterone levels.
Interestingly, the picture flips for explosive lower-body power. In the same study, trans women had significantly lower relative jump height (0.7 cm per kg of body weight versus 1.0 cm/kg for cisgender women) and lower relative aerobic capacity (45.1 mL/kg/min versus 54.1 mL/kg/min). Trans women in this cohort tended to carry more body mass, which reduced their power-to-weight ratio. So the advantage isn’t uniform across all physical qualities. Upper body strength appears to be retained to a greater degree than lower body explosiveness relative to body weight.
How Race Times Actually Change
The most direct evidence comes from tracking the same athletes before and after hormone therapy. A longitudinal study published in the European Journal of Sport Science found that trans women runners saw their race times increase (slow down) by an average of 14.6% after about 31 months of hormone therapy. The slowdown was larger in longer events: sprint times increased by about 11%, middle-distance by roughly 15%, and long-distance by about 17%. For context, the performance gap between elite male and female runners is typically 10% to 12%, which means the hormonal transition may bring some trans women runners close to, or even below, the female performance range depending on their starting point.
Swimming told a different story. One trans woman swimmer’s times increased by only 5.2% on average after 34 months of therapy, with substantial variation across distances. Short races barely changed (0.5% in the 100-yard event), while the 1,650-yard race slowed by 7.3%. The smaller overall slowdown in swimming may reflect the greater role of structural factors like arm span, hand size, and height in the water, traits that hormone therapy cannot alter.
The Puberty Question
Nearly all existing research involves trans people who began hormone therapy after going through their natal puberty. This is an important limitation. The physical traits being debated, including skeletal size, lung volume, and baseline muscle development, are largely products of puberty. A trans girl who uses puberty blockers before male puberty and then transitions with estrogen would theoretically develop a body much more similar to a cisgender girl’s. But as of now, no published studies have measured athletic performance in people who transitioned before puberty. The science simply hasn’t caught up to this group yet.
Trans Men in Competition
Trans men (female-to-male) receive far less attention in this debate, partly because testosterone therapy moves their physiology toward male norms. Testosterone increases muscle mass, red blood cell production, and bone density. The concern with trans men competing in the men’s category is typically about safety and competitiveness rather than unfair advantage, since they are generally moving into a more physically demanding category. Research specifically measuring athletic performance changes in trans men remains extremely limited.
How Sports Governing Bodies Draw the Line
Policies vary widely and have tightened in recent years. World Athletics updated its regulations in March 2023 to require trans women to maintain serum testosterone below 2.5 nmol/L for at least 24 continuous months before competing in the female category. That’s a significant shift from the previous policy, which required only six months of suppression at a higher threshold. The testosterone must be measured between 8 and 10 a.m. after two hours of rest, reflecting the hormone’s natural daily fluctuation.
Other organizations have gone further. World Aquatics (swimming’s governing body) effectively barred trans women who experienced any part of male puberty from elite women’s competition, while creating an “open” category. The International Olympic Committee, by contrast, moved away from setting a universal testosterone threshold in 2021, instead leaving eligibility decisions to individual sports federations. The result is a patchwork: a trans woman might be eligible to compete in one sport but not another, depending on which federation’s rules apply.
Why the Debate Stays Complicated
The core tension is that hormone therapy reduces but does not eliminate all sex-linked physical differences, and the remaining differences vary by trait and by sport. Hemoglobin normalizes quickly. Muscle mass shrinks but retains a partial advantage. Skeletal dimensions don’t change at all. Whether these residual differences amount to a meaningful competitive edge depends on the sport, the level of competition, and how “advantage” is defined. Elite sport is already a contest of biological outliers, and the margins that separate winners from losers are often very small.
The research base is also still thin. Most studies involve small sample sizes, short follow-up periods, and recreational rather than elite athletes. Longitudinal data tracking the same individuals over time is especially scarce. What the evidence consistently shows is that the answer isn’t a simple yes or no. It’s a question of how much advantage, in which traits, in which sports, and after how long on hormone therapy.