Are there sex based difference in athletic performance before puberty?
In the past few years most state legislatures have considered laws stating that only members of the female sex can participate in girl’s and women’s sports (37 states in 2021 alone), and as of April 20, 2022 fifteen states have adopted such legislation (1). There have also been several well publicized instances of transwomen competing for championships in women’s sports (for example see 2, 3, 4). The International Olympic Committee, the NCAA, and other sports governing bodies have also recently revised their policies regarding the inclusion of transwomen in women’s sports (5, 6). All of this has resulted in students in my exercise physiology classes commonly asking questions about sex-based differences in sports performance and the inclusion of transwomen in women’s sports.
In a previous PECOP Blog (7) I briefly summarized the sex-based advantages men have in athletic performance in adults, and the research evaluating the effects of testosterone suppression and cross sex hormone use on factors that influence athletic performance. In this PECOP Blog, I will briefly summarize the sex based prepubertal differences in athletic performance and touch on puberty blockers.
A 2012 report from the CDC indicated there were no differences between 6–11-year-old boys and girls in performance on physical fitness tests (8). Many sports leagues for pre-pubertal children are not separated by sex since the focus is developing basic sports skills rather than competition (9). Furthermore, some scholars have stated that there are no differences in athletic performance between boys and girls prior to the onset of puberty, and that it is only the increased testosterone secretion during puberty that causes males to outperform females in athletic competition (10, 11).
On the other hand, evaluations of fitness testing in children as young as 3 years old shows that boys perform better than girls of the same age on tests of muscular strength, muscular endurance, and aerobic fitness (12-17). For example, Tomkinson et al. (17) observed that at age 9 boys are running an average of 3.2% faster than girls of the same age during the last stage of a 20 m shuttle run (Figure 1). In a separate evaluation Tomkinson et al. (16) reported that at age 9 boys have a bent arm hang time that is an average of 48.1% longer than girls of the same age (Figure 2).
Furthermore, youth records from USA Track & Field (18) in the 8-and-under age group and in the 9-10-years-old age group (who can reasonably be assumed to be pre-pubertal) show that boys outperform girls in all events (Table 1). The smallest difference in track and field records between boys and girls is 0.94% in the 8-and-under 100 m run, with the largest difference being 38.42% in the 8-and-under javelin throw. We recently analyzed top 10 data for national performance from Athletic.net in 100 m, 200 m, 400 m, 800 m, 1500 m, and 1600 m running events for children in the 7-8 and 9–10 year-old age groups for the years 2019-2021 and found that across all events 7-8-year-old boys were 4.4 ± 1.9% faster than girls, and 9-10-year-old boys were 5.4 ± 1.8% faster than girls (figure 3; not yet published data). Youth records from USA swimming also show that in 19 out of 23 events the national records for 10 and under boys are faster than girls by an average of 1.72% (19). It is important to note that in competition the difference between first and second place often comes down to as little as 0.02% difference in speed (Data to be presented at the 2022 ACSM Annual Meeting).
There is no question that the differences in running performance between prepubertal boys and girls is less than the 10-13% difference in running performance observed between post-pubertal boys and girls, and between adult men and women (10, 11, 20). And there is no question that the large increases in circulating testosterone experienced by boys during puberty is responsible for most of the differences in athletic performance between post-pubertal boys and girls, and between adult men and women (21). But the existence of differences in athletic performance between prepubertal boys and girls is well demonstrated (12-19). Juxtaposing the statements of no pre-pubertal athletic differences between boys and girls (8, 10, 11) and the evidence demonstrating that there are pre-pubertal athletic differences between boys and girls (12-19) can facilitate an interesting discussion about data collection, sample size, data analysis, and other factors that may contribute to these contradictory findings.
When explaining the biological causes of the prepubertal athletic advantages in boys, a good starting point is to discuss the differences in growth and development between boys and girls and to explain the processes of sex determination and sex differentiation (22). Sex determination occurs at conception with the conferral of sex chromosomes. Six weeks later, sex differentiation begins to become apparent and during the remainder of development the gonads and genitalia acquire male or female characteristics. During sexual differentiation, the presence of the SRY gene on the Y chromosome along with androgen exposure and anti-Müllerian hormone cause the internal and external genitalia to follow the male developmental pathway. In the absence of the SRY gene on the Y chromosome, lack of androgen exposure, and lack of anti-Müllerian hormone the female developmental pathway occurs. Of course these few brief sentences fail to cover the myriad of complex interactions of genes, primordial stem cells, and hormones that regulate sex development, and the possible differences and disorders that can occur. But it is remarkable that with all of the possible missteps that can happen during sexual differentiation and development, sex can be accurately and easily identified at birth 99.83% of the time (23).
Further substantiating the important role of sex in growth and development are the World Health Organization fetal growth charts (24), which indicate small but meaningful sex-based differences with male fetuses being consistently larger than female fetuses. Similarly, substantiating the important role of sex in growth and development, the Centers for Disease Control and Prevention have different growth charts for boys and girls from birth through adolescence with boys having consistently higher values for body mass and body height (25).
With an eye towards physical fitness and athletic performance, starting at birth and continuing throughout youth girls have more body fat and less fat-free mass than boys. For example, Davis et al. (26) in an evaluation of 602 infants reported that at birth and age 5 months, infant boys have larger total body mass, body length, and fat-free mass while having lower percent body fat than infant girls. In an evaluation of 20 boys and 20 girls ages 3-8 years old, matched for age, height, and body weight Taylor et al. (27) reported that the boys had less body fat, lower percent body fat, and a higher bone free lean body mass than the girls, such that the girls’ fat mass was 52% higher than the boys, while the bone-free lean tissue mass was 9% lower. In an evaluation of 376 prepubertal [Tanner Stage 1] boys and girls, Taylor et al. (28) observed that the boys had ~22% more lean mass, and ~13% less body fat (when expressed as percent of total body mass) than did the girls. In a review of 22 peer reviewed publications on the topic, Staiano and Katzmarzyk (29) concluded that girls have more total body fat than boys throughout childhood and adolescence. It is a tenet of exercise science that having more lean body mass provides athletic advantages, so it is reasonable to conclude that having more lean body mass contributes to the prepubertal sex-based male athletic advantages.
It is worth noting that serum testosterone concentrations in boys are higher for the first 5 months after birth than in girls (30). Testosterone concentrations are then similar between boys and girls until the onset of puberty, when testosterone concentrations increase 10-20-fold in boys. Given the well know anabolic and androgenic effects of testosterone, the higher testosterone levels in newborn boys likely contributes to the sex related differences in body size and composition in newborns. It is unknown how much the lingering sex-linked differences in body size, body composition, physical fitness, and athletic performance are due lasting effects of the higher testosterone levels in newborns, and how much the differences are due to Y chromosome or other sex-linked effects.
Strongly suggesting that sex linked differences in physical fitness and athletic performance in children before puberty are due to biological factors, Eiberg et al. (13) measured body composition, VO2max, and physical activity in 366 Danish boys and 332 Danish girls between the ages of 6 and 7 years old. Their observations indicated that absolute VO2max was 11% higher in boys than girls, while relative to body mass the boys’ VO2max was still 8% higher than the girls. Accelerometry based measurements of physical activity indicated that when boys and girls regularly participated in the same amount and intensity of physical activity, the boys had higher measured physical fitness than the girls. When the findings of Eiberg (13) are taken collectively with the findings of large scale school based physical fitness testing in children that also shows pre-pubertal boys outperforming girls in measurements of aerobic fitness, muscular strength, and muscular endurance (12, 14-17), the youth records from USA Track & Field (18) showing that pre-pubertal boys outperform girls in all events, and the 10 and under records from USA Swimming showing that boys outperform girls in 19 out of 23 events (19), there exists strong evidence that there are differences in physical fitness and athletic performance between boys and girls before puberty.
And finally, this discussion arising from laws stating that only members of the female sex can participate in girl’s and women’s sports can lead to questions about the effects of puberty blockers on physical fitness and athletic performance in prepubertal children. Puberty blockers are correctly known as gonadotropin-releasing hormone agonists (GnRHa), which disrupt the normal pattern of secretion of as gonadotropin-releasing hormone causing the pituitary gland to stop producing follicle-stimulating hormone and luteinizing hormone. Unfortunately, there is minimal research on the effects of puberty blockers on factors that influence physical fitness and athletic performance.
To the best of my knowledge, there is no research on the effects of puberty blockers on muscle strength, running speed, or other measures of athletic performance. Indeed, Klaver et al. (31) is the only published research that I am aware of that has evaluated the use of puberty blockers on any athletic performance related factor, and this is only on body composition. Klaver et al. (31) demonstrated that the use of puberty blockers in Tanner stage 2-3 teenagers increased body fat and decreased lean body mass in transgirls, but the use of puberty blockers did not eliminate the differences in body composition between transgirls and comparable female teenagers. Roberts and Carswell (32), concluded that there is no published research that sufficiently characterizes the impact of puberty blockers on growth or final adult height. Thus, the effect of prescribing puberty blockers to a male child before the onset of puberty on the physical components of athletic performance is almost entirely unknown. This becomes a great point in a discussion to remind students of the ever-evolving nature of science. Any further discussion on this topic becomes speculation or can be removed from the realm of physiology and into metaphysical discussions of what is or is not fair. Such metaphysical discussions can be fascinating, and also heated, so caution is advisable when proceeding outside of the realm of physiology in a physiology classroom.
In summary, there is strong evidence that even before puberty there are sex-based differences in physical fitness and athletic performance with boys running faster, jumping farther and higher, and demonstrating greater muscle strength than girls of the same age. These pre-pubertal sex based differences are smaller than the differences in post pubertal teens and adults, but the differences are likely meaningful in terms of competition. There is currently insufficient evidence to determine what effects puberty blockers have on physical fitness and athletic performance in children.
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