Discussion about allowing only members of the female sex to participate in girl’s and women’s sports may lead to questions about the effects of puberty blockers on physical fitness and athletic performance in male children and adolescent who identify as girls (i.e. transgirls). However, there is considerable controversy regarding the quality of evidence supporting the use of puberty blockers (12), and there are sufficient concerns regarding the untoward health consequences of using puberty blockers that some countries prohibit the use of puberty blockers except in clinical research. Unfortunately, there is limited research on the effects of puberty blockers on factors affecting physical fitness and athletic performance, including no data on the effects of puberty blockers on muscle strength, running speed, or endurance capacity.
Klaver et al. (25) examined the use of puberty blockers on body composition and demonstrated that in Tanner stage 2-3 teenagers body fat was increased and lean body mass was decreased in transgirls, but the use of puberty blockers did not eliminate the differences in body composition between transgirls and comparable female teenagers. Specifically, before the start of puberty blockers the transgirls had ~75% lean body mass and comparable female teenagers had ~63% lean body mass. After ~2.5 years of puberty blocker use, the transgirls had ~69% lean body mass while comparable female teenagers had ~61% lean body mass. By 22 years of age, after ~8 years of puberty blocker and cross sex hormone use, the transgirls had ~66% lean body mass while comparable females had ~59% lean body mass. Two other papers indicate the use of puberty blockers (33) and cross sex hormones (32) in transgender teenagers does not eliminate the male sex based advantages in lean body mass. Another recent study reported that height in adulthood is relatively unaffected by prior treatment with GnRH analogs and estradiol during adolescence, implying that transgirls grow taller than reference females (7). This height advantage could confer athletic advantages in various sports, not least because height in general is also strongly correlated with total lean body mass. Therefore, while there is very limited information on the effects of puberty blockers and GAHT in children, the current evidence suggest that male children retain sex-based advantages in body height and lean body mass which may allow for retained male athletic advantages.
Summary
In summary, there are clear sex-based differences between males and females in physical fitness and athletic performance even before puberty. Boys run faster, jump farther and higher, and have greater muscle strength than comparable girls. These pre-pubertal sex-based differences are smaller than the differences between post pubertal males and females, which increase significantly with the rise in circulating testosterone in males during puberty, but are likely meaningful in competition. Shortly after the onset of puberty and throughout adulthood, males outperform females by ~10-60% in measures of physical fitness and during athletic performance. Once puberty has occurred, the suppression of testosterone and the administration of estrogen fails to eliminate acquired male biological traits (e.g. greater body mass and height) and minimally reduces measured performance differences (e.g. greater muscle strength and faster running performance), with the likely implication that sporting performance advantages are retained in transwomen despite testosterone suppression. Currently, there is insufficient evidence to determine what effects puberty blockers have on physical fitness and athletic performance in children, but the limited evidence that exists suggests that male growth is not entirely suppressed which may confer athletic advantages on transgirls.
The question of what constitutes fair competition is challenging. Historically sports have been separated by sex to allow girls and women a level competitive playing field because of the 10-60% advantages provided to boys and men by male biology. Anabolic-Androgenic steroids provide a 5-20% enhancement in strength and are almost universally considered to be unfair. In 2008 non-textile swimsuits were released which were reported to improve swimming performance by 2-4% and were deemed to be unfair and banned in 2010. Research to date indicates that identifying as a transwoman with or without the use of GAHT does not eliminate the male physiological athletic advantages. Whether the male athletic advantage remaining after GAHT is unfair is a question that is currently being debated by scholars, sport governing bodies, and legislators.
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