Male Participation in Female Sports: How Sports Performance is Impacted by Sex

Male Participation in Female Sports: How Sports Performance is Impacted by Sex

ABSTRACT: In the midst of society’s and mainstream medical organizations’ accepting if not promoting (i.e., “affirming”) false sexual identities, the American College of Pediatricians (ACPeds) is concerned that medical and public policies are denying the importance of biological sex and truth. Sex is a dimorphic, innate trait defined in relation to an organism’s biological role in reproduction.

Males have inherent anatomical and physiological advantages compared to females that impact male and female sports performance beginning in early childhood and continuing through adulthood.  The use of puberty blockers and cross sex hormones in males does little to change the inherent advantage they have in sports that require muscular strength, power, aerobic endurance, and speed.  Therefore, males, regardless of gender identity, should not be allowed to participate in most women’s sports. This statement is not applicable to the rare individual with a disorder of sexual differentiation.

Genetics is the primary driver of sex differences

In humans, primary sex determination occurs at fertilization and is directed by a complement of sex determining genes on the X and Y chromosomes. This genetic signature is present in every nucleated somatic cell and is not altered by drugs or surgical interventions. Sex differences arise from at least four different genetic mechanisms, in addition to the actions of sex hormones and environmental influences.1

Genes found on the sex chromosomes determine the developmental origin of many differences between males and females. Sex hormones, which are ultimately determined by these genes, are the second most impactful factor. Fluctuations of sex hormones exert significant effects during the prenatal period in males, during the reproductive years in females, and during puberty and following midlife in both sexes.2

There are at least four genetic mechanisms that contribute to sex differences.3 The first mechanism involves sex chromosome effects. Genes present on the Y chromosome influence male development and function in multiple organs within and beyond the reproductive system. Among females, randomized inactivation of half of their X chromosomes exerts a genetic influence males do not experience. A second mechanism involves sex-dependent genetic liability thresholds. For instance, the predominance of pyloric stenosis in male infants (a thickening of the pylorus muscle that prevents the stomach from emptying into the small intestine) falls into this category.4

Two additional genetic mechanisms underlying sexual dimorphism include gene–environment interactions and sex-differential gene expression.5   Regarding the latter, at least 6500 shared genes have been identified that are expressed differently in males and females.6 Even the development and strength of skeletal muscles are the result of sex-differential gene expression and studies have identified over 3,000 genes that are differentially expressed in male and female skeletal muscle.7

In 2001, the Institute of Medicine (IOM) concluded that the genetics of sex contributes significantly to males and females having different propensities for diseases, sex-dependent responses to pain, drugs and toxins, sex differentiated cognitive and emotional processes, sex variation in behavior and more.8

The ACPeds paper, Sex is a Biological Trait of Medical Significance – American College of Pediatricians, highlighted some of the sex differences that have been identified in the fields of neuroscience, pharmacology, cardiovascular health, and sports medicine in the decades since this landmark IOM review was published.  The current paper will focus more specifically on scientific research in sports medicine to help answer the following questions:

  1. Should males ever be allowed to participate in women’s sports?
  2. Should prepubertal boys be allowed to participate in girls’ sports?
  3. Should males who have used puberty blockers and/or cross sex hormones to minimize their anatomic and physiological differences be allowed to participate in women’s sports?
  4. Are there sports in which males and females can fairly and safely compete against one another?

Role of Sex Steroid Hormones in Sex Differences

The two major sex steroid hormones that influence athletic performance are estrogen and testosterone.  Because of the variations in estrogen levels throughout the menstrual cycle and the changes that are induced with the use of oral contraceptives, the role of estrogen has not been as well researched and is not as well understood as the role of testosterone.  Estrogen appears to be associated with some changes in cardiovascular and respiratory function and progesterone with thermoregulation, but the varying levels throughout the menstrual cycle do not appear to convey advantages or disadvantages to the athletes.9  A systematic review and meta-analysis of 51 studies confirmed the small impact of varying estrogen levels leading authors to conclude, “exercise performance might be trivially reduced during the early follicular phase of the menstrual cycle, compared to all other phases.”10

Testosterone, however, greatly influences athletic performance, so it is important to understand how testosterone levels fluctuate during the life cycle and how this hormone impacts development. Testosterone production begins in the male testes by 8-9 weeks after fertilization.11 This rise in testosterone contributes to the development of the male reproductive organs, impacts the development of other organ systems, and helps organize the male brain.   The female fetus is exposed to testosterone, but at much lower levels.

Research on the impact of testosterone during fetal life is in its early stages. Notable differences between male and female fetal brains have been documented with magnetic resonance imaging (MRI).  There is increased interconnectedness in males in the areas of the brain involved with perception-action coordination, auditory / visual spatial awareness and cognitive processing.  Females have increased connectivity in areas related to memory, social cognition and non-verbal reasoning.12 These differences may influence athletic performance later in life.

During the developmental stage titled “minipuberty,” there is an increase in serum testosterone in infant boys with a peak at 1 – 3 months after birth, followed by a decline to prepubertal levels by 6 – 9 months.13 To the contrary, infant girls experience increases in estradiol, and therefore have a slower growth velocity than boys. By mid-childhood, girls have accumulated more body fat than boys, and this persists and increases during puberty.

During puberty, testosterone levels greatly increase in males so that by age 18 years, boys have experienced a 30-fold increase in this sex hormone.14 The exact mechanisms for pubertal timing are unknown but the final process involves initiation of pulsatile gonadotropin releasing hormone secretion which stimulates pituitary LH and FSH secretion.  These work in combination to stimulate testosterone secretion, spermatogenesis and increase testicular size. The increased testosterone in turn stimulates the development of primary (penis) and secondary (body and facial hair, muscle mass, bone and cartilage structures, etc.) sexual characteristics. Females also have increases, albeit much smaller, of testosterone during puberty that affect acne and some stimulated hair growth without significant effect on their sex organs (ovaries, fallopian tubes, uteri). Their puberty is also influenced by cortical/hypothalamic influences that start the cycle of progesterone and estrogen interaction with their ovaries and uteri. Estrogen also decreases tendon and ligament stiffness.15

Testosterone plays an important role in regulating bone mass and shape, fat distribution, muscle mass and strength, and the production of red blood cells leading to higher circulating hemoglobin. After puberty, male circulating testosterone concentrations are 15 times greater than those of females at any age. The result is a clear male advantage in regard to muscle mass, strength and circulating hemoglobin levels even after adjusting for sex differences in height and weight.16 The impact of testosterone on muscle mass, size, and strength appears to be dose dependent, and therefore females, with significantly lower levels of testosterone, experience lower muscle mass, strength, and athletic performance.17

Testosterone can impact the anatomical and physiological sex differences in two ways.  Exposure to testosterone in early life causes changes that tend to be permanent, while altering testosterone levels after puberty can modify some systems in the body, including hemoglobin levels.

Changes induced by testosterone early in life

Anatomical differences exist between the sexes, and, from infancy on, these differences impact the body’s response to acute exercise, training, and ultimately athletic performance beginning in infancy and continuing through adulthood.18  Testosterone’s impact on the brain, skeletal structure/muscle mass, and muscle fiber type and muscle memory, as well as cardiorespiratory system (heart size, stroke volume and lung capacity) are architectural.  In other words, these features are permanent and not modifiable, and therefore they do not change with estradiol supplementation or testosterone suppression.

  1. Males are generally taller than females with greater lean body mass, lower percentage of fat, and longer upper and lower limbs with larger and denser bones.19 Although most of the size differences between males and females occurs during puberty, even at birth, the average male is heavier and longer (taller) than the average female, and there are sex differences in body composition with males having greater total mass and fat free mass at birth and at 5 months.20
  2. Differences in the bone mass of the axial skeleton are present prior to puberty, with boys having greater cross-sectional area in vertebrae than girls of the same height, weight, and age.21
    1. Muscle mass in comparison to body mass is generally greater in men (38.4%) than in women (30.6%).22
    2. Females have lower percentages of muscle mass in the upper body (39.7%) compared with males (42.9%), but a higher percentage in the lower body (57.7%).23
    3. These anatomical differences provide greater leverage (greater torque) for muscular limb power to be exerted in jumping, throwing, and other explosive power activities.24
    4. “The muscle mass and limb power of males can be twice that of females across many age groups with larger sex differences in muscle mass and strength in the upper body than the lower body.”25 This leads to women having only 50% – 60% of the strength in upper arm muscles compared with males due to decreased cross-sectional areas of the muscles.
      1. Males have on average a skeletal muscle mass over 12kg greater than age‐matched females at any given body weight.26
      2. Women have 65-70% of male’s thigh muscle cross-sectional area with a comparable reduction in strength.27
  • Researchers evaluated the impact of isometric training on 342 women and 243 men. Their initial results before training demonstrated that males had 57% greater muscle size and 109% greater isometric strength, confirming that males have increased muscle mass and strength in their upper bodies.28
  1. Skeletal muscle fiber composition differs between men and women resulting in a difference between the sexes in skeletal muscle fatigue recovery and endurance testing
    1. There are two distinct types of muscle fibers – Type I and Type II. Type II fibers have increased myosin chains and have twice the maximal shortening velocity of Type I fibers.29
    2. Males have a higher proportion of Type II (fast-twitch) fibers.30
  • Since males have larger and more type II fibers, they have faster contracting muscles than females.31
  1. The increase in Type II fibers and contraction velocity as well as increased strength provides an advantage for males in sports such as cycling where females demonstrate peak power that is 63-67% that of males.32
  1. “Muscle power is the product of strength (force or torque) and contraction velocity, both of which are greater in males than females.” 33 Muscle force is also related to the ability and speed of activation of the motor unit, but this does not differ significantly between males and females. So, it is “the sex differences in muscular strength and power…due to larger skeletal muscle mass and faster contracting skeletal muscle” that gives males an advantage over females.34
  2. Another measure of muscular work is the evaluation of anaerobic power – a reflection of energy-output capacity of muscular use of high energy phosphates such as ATP.
    1. Anaerobic power is highly related to the amount of muscle mass, as well as the glycolytic capacity of skeletal muscles, both of which are generally greater in men.35
    2. Men, for example, demonstrate 15% – 50% greater maximal anaerobic power of their lower limbs than women. This means that when tasks require short bursts of exercise and energy, men will outperform women.36
  3. Aerobic power, on the other hand, is a predictor of endurance required in distance sports such as running, swimming and cross-country skiing. “The maximum rate of oxygen consumption attainable during intense maximal exercise is termed VO2 max and is considered a gold-standard index of aerobic capacity and maximal cardiorespiratory function correlating strongly with performance in endurance-heavy sports such as running, swimming, or cycling or long stop and start sports such as soccer or basketball.”37
    1. “The average sex differences in body composition favors higher relative aerobic capacity among males whose greater muscle mass and lower fat mass allows for greater uptake of oxygen per kilogram of total body mass during physical activity.”38
    2. Men have higher measures of aerobic power. Elite male runners have a VO2 max that is 10% – 14% higher than female runners39, much of which is due to higher cardiac output because of increased ventricular wall thickness and higher left ventricular end diastolic volume.40
  • Even when corrected for fat-free mass, VO2 max in males averages 5 – 10% higher than in women.41 This means “males utilize more oxygen at greater rates and thus move large muscle groups faster than females.”42
  1. While numerous genes and environmental factors such as physical activity and diet contribute to muscle mass, the major cause of the sex difference in muscle mass and strength is the level of circulating testosterone. Taken together, these discrepancies render females, on average, disadvantaged against males in power‐based or endurance‐based sports.43
  1. Higher hemoglobin levels (12%) due to increased testosterone allow males to have increased oxygen levels that lead to respiratory advantages, so the VO2 max of an elite male athlete is approximately 70 – 85 mL/kg/min, whereas female levels are 15%-30% lower at 60-75 mL/kg/min.44 (This is modifiable with changing testosterone levels.)
  2. Bone structure, length, and shape are impacted by early life testosterone and can impact athletic abilities.
    1. Females have wider width in their pelvis which is beneficial for childbirth, while males have a narrower pelvis that allows for a smaller Q angle between the quadricep muscles and the patellar tendons at the knee joint. Males can therefore generate a greater force during extension that improves their ability to stand from a squat, kick a ball, or pedal.
    2. Males also have a smaller angle between the humerus and the ulna at the elbow that benefits their throwing and hitting abilities.45
  3. Sex differences in lung development are seen as early as 16 – 24 weeks gestation, with females having fewer bronchi but faster maturation.46
    1. Prior to puberty, females have a smaller lung volume than males, even when age and height are considered. There are also sex differences in airway growth.47
    2. In addition, female newborns have higher expiratory flow rates than males and this remains true throughout adulthood.48
    3. After growth is completed, vital capacity, total lung capacity, and peak flows are greater in males, while females maintain larger expiratory flow rates.49
  4. There are also significant inherent cardiovascular differences between males and females.
    1. Although the female heart is 5% larger at birth, it ends up approximately 26% smaller by adulthood.50
    2. The adult female heart weighs on average one fourth less than that of males,51 or stated differently the female heart mass is 70-85% that of males.52
    3. Other differences include a thinner ventricular wall in females as well as a thinner septal thickness.53
    4. Females have a cardiac output and stroke volume that is approximately 23% less than that of males.54 This difference remains in athletes.
    5. There are other significant differences in ejection fractions, heart rate, and blood pressure, as well as in the cellular makeup of the heart muscle. So, one author states, “Sex differences between female and male hearts are too complex to be ignored: The female heart is not just a small version of the male heart.” 55

 

Age of Divergence

Handelsman utilized three sources to determine the age at which boys and girls begin to diverge in their athletic performance and compared that data with the timing of testosterone surges obtained from a fourth source. In the introduction to the research paper, it states, “It is well known that men’s athletic performance exceeds that of women especially in power sports because of men’s greater strength, speed and endurance.”56

Handelsman used data from the US Age Group Swimming time standards for all boys and girls events from 1981 to 2016 as well as the world records for boys and girls between 5 and 19 years for track and field running events.  He also used a study on hand-grip strength in nonathletic children and adolescents from Canada and the United States with data on 5676 males and 5489 females between 1966 and 2009.  Testosterone levels were obtained from a study of over 100,000 serum samples over a 7-year period. He found “The onset and tempo of gender divergence were very similar for swimming, running and jumping as well as the hand-grip strength in nonathletes.”  He concluded “The gender divergence in athletic performance begins at the age of 12 – 13 years and reaches adult plateau in the late teenage years with the timing and tempo closely parallel to the rise in circulating testosterone in boys during puberty.”57

Because some females are hyperandrogenic (having elevated levels of testosterone), the International Association of Athletic Federations adopted a regulation that stated female athletes must maintain serum testosterone levels less than 10 nmol/L.  This rule was challenged for lack of data, so Handelsman presented research on testosterone levels in males and females after puberty.58  The authors provided evidence that testosterone levels explain most of the sex differences in athletic performance and recommended that women should have a testosterone level no higher than 5.0 nmol/L, which is lower than the 10.0 nmol/L previously used.

Sex Differences in Athletic Abilities

With the understanding that there are many variables that are difficult to control when researching sports performance, the following studies from the scientific literature demonstrate distinct differences recognized between males and females in their athletic performance.

Sex Differences in Prepubertal Children’s Athletic Abilities

Arguments supporting prepubertal coed sports participation

Historically, boys and girls have been viewed as having no differences in athletic abilities prior to puberty. This concept is based upon the fact that after “minipuberty” until actual puberty, testosterone levels are similar between boys and girls, so boys and girls were thought to have comparable athletic abilities.

Even though research on prepubertal children was very limited, the Centers for Disease Control and Prevention in 2012 reported “no differences between 6 and 11-year old males and females in performance on physical fitness tests”.59  (Brown quoting Ervin, 2013).

The Women’s Sports Foundation states, “Prior to puberty, females and males should compete with and against each other on coeducational teams. Prior to puberty, there is no gender-based physiological reason to separate females and males in sports competition.”60 They go on to promote, “Girls and boys possessing similar skills should be playing with each other and against teams consisting of boys and girls who are similarly skilled.”

Even the American College of Sports Medicine stated in 2023, “Before puberty, the sex difference in athletic performance is minimal.” 61

Research demonstrating advantages of prepubertal boys in competitive sports

Recent research, however, is showing there are differences in athletic abilities even before puberty.  Brown, et al. evaluated finalist times from the USA Track and Field National Youth Outdoor Championships and the National Junior Olympic Championships for the years 2016-2023 for various running distances in the 8 and under age group as well as the 9-10-year-old age group.62 In the younger age group, males were faster than females in all events by 4.0% to 6.7%). Specifically, males were faster in the 100 m (4.0%), 200 m (4.7%), 400 m (5.3%), 800 m (6.7%), and 1500 m (6.1%).  Similarly, the boys were faster than the girls in all events in the 9-10-year-old age group.  “In each distance and age group between 2016 and 2023, the individual fastest male was faster than the individual fastest female by 3.7 +/- 2.3%.”

Another research project in 2025 evaluated “sex-based differences in aerobic running performance at 1600 m for children aged 6 – 12 yr” while also studying whether sex-based differences in participation affected the results.63 Utilizing the runnercard.com website, they evaluated the running velocities and participation for 3621 children and found “male children were faster than female children at every grade level with an average difference of 7.7%.”  Their conclusion stated, “innate physiological sex differences may be responsible.”

A study of over 85,000 Australian children between 9 and 17 years demonstrated differences between young boys and girls, with 9-year-old boys running faster than girls in short sprints (9.8%), running faster in the longer distance one mile (16.6%), and jumping farther from a standing start (9.5%).64  In addition the boys could complete 33% more push-ups in 30 seconds and had a more powerful grip by 13.8%. The advantage of 6-year-old boys in running and jumping activities was confirmed in another study in Greece.65

Researchers in Denmark measured maximum oxygen uptake (VO2max) in 366 boys and 332 girls 6 – 7 years of age.66 Boys had higher VO2max (1.19) compared to girls (1.06), an 11% difference.  This difference was maintained (although smaller) even when body mass and lean body mass was considered, and authors state, “Most of the difference in VO2max relative to body mass was explained by the larger percentage of body fat in girls.”

A cross-sectional study of 312 prepubescent children’s physical fitness (aerobic fitness, strength, flexibility, speed, agility, and balance) revealed that boys had higher scores in all tests, except balance and flexibility.  The greatest sex differences were found in the explosive strength of upper and lower limbs.67

Two large studies from Europe demonstrate the athletic advantages of boys over girls that are present before puberty. A study of 108,295 third graders evaluated physical fitness of boys and girls using five different tests of endurance, coordination, speed, power of lower limbs, and power of upper limbs. The researchers found “Boys outperformed girls in all four physical fitness components (cardiorespiratory endurance, coordination, speed, power [LOW/UP]).[lower limbs and upper limbs].”68

A systematic review of 98 papers from 30 European countries identified datasets on 2,779,165 children ages 9-17 years who had participated in one of nine Eurofit tests.  Results were segregated by age and sex, so data from prepubertal children was available. Again, the researchers found “Boys performed substantially (standardised differences >0.2) better than girls on muscular strength, muscular power, muscular endurance, speed-agility and CRF (cardiometabolic) tests, but worse on the flexibility test.”69

Since there are around 6500 genes that are expressed differently between males and females, an estimated 3000 of which likely influence the composition and function of skeletal muscle,70 the “minipuberty” of males soon after birth may be responsible for these prepubertal sex-related differences.

Given these differences in athletic performance between prepubertal boys and girls, there are concerns that biological males who have their puberty blocked may still have innate advantages over females in athletic performance.

 

Sex Differences in Post Pubertal Athletic Abilities

Long-term research on elite athletes has consistently shown that when matched for training and age, males outperform females in regard to speed, strength, endurance, and power.71 So, historically most sports teams were segregated by sex beginning in middle school or high school based on the well-known differences in athletic abilities of males and females.

Research into sex differences includes the following:

  1. Evaluation of world records and best performances of events that rely on endurance and muscle power shows that males have a 10 – 30% advantage over females.72 The authors state, “The largest sex differences are apparent for sports and events relying more on muscular power such as in weightlifting, jumping events, and short distance swimming.”
  1. Demonstrating one of the striking differences between male and female runners, a comparison of the fastest running times in the 400 meter in 2019 found “over 10,000 men (including boys <18 yr) ran faster than the three fastest recorded women in that year (2019), illustrating no overlap in the performance of men and women at the top level.”73
  2. Bartolomei, et al, compared male and female athletes “in strength and power performance relative to body mass (BM) and lean body mass (LBM)” as well as an evaluation of muscle architecture’s relationship to strength.74 Utilizing tests for lower body power (jumping) and upper body power (bench press)  as well as repetitions in bench press, deadlift, and squat, the researchers found, “Significantly greater (p <0.05) results in all performance assessments adjusted for MT (muscle thickness) of the specific muscles were detected in males compared to females.”
  3. Hallam and Amorim used the World Athletics public database over two decades to evaluate the winning times for the top 20 best male and female Olympic runners for the 100 m, 200 m, 400 m, 800 m, 1,500 m, 5000 m, and 10,000 m.75
    1. They evaluated the physiological, anatomical, neuromuscular and biomechanical explanations for the sex differences in events that were short distances (sprints), middle-distances and long distances.
    2. Sprints were affected by the sex differences in muscle anatomy and physiology as male runners have larger muscle volumes, more skeletal muscle mass, and larger muscle fiber cross sectional areas.  (Hallam and Amorim)
    3. Middle-distance running is affected by both aerobic and anaerobic demands. They found some advantages for males in their biomechanical and neuromuscular abilities, as well as in their anaerobic speed reserves, however this middle-distance running has not been as well studied as sprints and long-distance running.  (Hallam and Amorim)
    4. Long distance running is mainly affected by VO2max which is 10% higher in elite male marathon runners than in females, due mainly to less fat mass and greater skeletal muscle mass in males as well as higher hemoglobin concentrations and larger hearts and lungs.  (Hallam and Amorim)
    5. The authors state, “It is apparent that females are the disadvantaged sex in sport.”  In the paper, the authors suggest ways to minimize the sex performance gap and encourage additional research.76
  4. A literature review on sports performance differences between men and women elite athletes found the smallest performance gaps were seen in rowing, swimming, and running (11 – 13%)”. Performance gaps increased to 16% in cycling and to 18% in jumping events.77
    1. The authors note there is an “increasing performance gap between males and females as upper body strength becomes more critical for performance”
    2. This is mainly due to the greater strength males have in their upper bodies compared with females, along with the longer arms and greater torque production seen in throwing, punching, or pushing.
    3. Competition records show a male advantage of >20% in activities that involve upper body strength.  Specifically, researchers have found a 20% male advantage in the speed of tennis serves and > 50% difference in the speed of pitched baseballs.
    4. Male puberty has such a great effect on performance that 14- and 15-year-old schoolboys hold records in sprints of 100m, 800m, and 1500m, as well as in the long jump and discus throw that are superior to adult elite female athlete records.
    5. After enumerating the many advantages males have due to the influence of testosterone, the authors conclude, “Of course, different sports select for different physiological characteristics—an advantage in one discipline may be neutral or even a disadvantage in another—but examination of a variety of record and performance metrics in any discipline reveals there are few sporting disciplines where males do not possess performance advantage over females as a result of the physiological characteristics affected by testosterone.”
  1. Researchers utilized a data set of world records from 82 events from 5 Olympic disciplines beginning at the start of the Olympic Games and compared results for men and women in track and field (jumping and running), swimming, speed skating, track cycling, and weightlifting.78  “The gender gap ranges from 5.5% (800-m freestyle, swimming) to 18.8% (long jump). The mean gap is 10.7% for running performances, 17.5% for jumps, 8.9% for swimming races, 7.0% for speed skating

and 8.7% in cycling. The top ten performers’ analysis reveals a similar gender gap trend with a stabilization in 1982 at 11.7%,…”  The authors concluded, “These results suggest that women will not run, jump, swim or ride as fast as men.”  (Thibault)

There are, however, sports in which males and females are more evenly matched – those events in which skill is more important than muscle power, as is the case in archery and shooting.79

Research on Sex Steroid Hormone Supplementation and Suppression

In order to help answer the question of whether males should be allowed to participate on female sports teams, this section will focus solely on the effects of estrogen supplementation / testosterone suppression in males.  Because of the inherent differences between males and females, it is extremely rare to see females requesting to play on men’s sports teams at a competitive level. So, the effects of testosterone on females will not be included in this paper.  However, it is well recognized that testosterone supplementation will increase lean mass and muscle strength in a dose dependent manner,80 and this is the reason that anabolic steroid supplementation is not allowed in Olympic sports.

  1. Physiology of Estrogen
  1.  There are four different estrogens that differ in their modes of action as well as the locations where they are synthesized in the body.  All four are synthesized from cholesterol and the various estrogens can be formed in the ovaries, liver, adrenal glands, uterus, mammary glands, brain, skin, and placenta.81
  2. E2 (estradiol) is the main female sex hormone that increases during puberty and fluctuates the most during the menstrual cycle.  Levels are usually four-fold higher in premenopausal women compared with men.82
  3. Estrogen helps maintain glucose homeostasis and bone and muscle health in both men and women, but it does not promote muscle growth and strength as does testosterone.
  1. General Effects of Estrogen Supplementation
  1. Males who participate in women’s sports often take estrogen supplements to promote the external appearance of being female.  Estrogen supplementation in biological males can cause estrogen levels to reach the normal values seen in biological females.
  2. Supplementation with estrogen also causes testosterone levels to decrease in most, but not all, studies.83
  3. While the goal of estrogen regimens is to lower serum concentrations of testosterone to levels normally seen in biological females (<1 nmol/L), this hormone therapy has “limited success in suppressing testosterone levels, with many transgender women failing to achieve the desired level.”84
  4. One study found only 49% of biological males who received estrogen as well as antiandrogen supplementation for at least 6 months lowered their testosterone levels.85
  5. Bodily changes include redistribution of fat, reduction in muscle mass, reduction in facial and body hair and decreased libido.  Changes are not generally seen in voice pitch or skeletal size,86 nor are changes seen in bone mass.87
  6. There may well be changes in brain size and function after estrogen supplementation, but research seems to show that the males receiving estrogen maintain their male dominance in some skills that may be important in athletic competitions.
    1. Researchers have noted male dominance in spatial ability, visual memory, and perception.88
    2.  A study of 103 biological males who received estrogen supplementation were evaluated before and after intervention and the results showed there was no change in the three skills listed above.89
  7. Risks to estrogen supplementation include an increased risk of venous thromboembolism, weight gain, infertility and increased risk of cardiovascular disease and cerebrovascular disease.90
  1. Effects of estrogen supplementation on specific organ systems
  1. Cardiopulmonary function and strength
    1. One of the first studies to evaluate the effects of estrogen supplementation in MtF (male to female) transgender individuals compared the cardiopulmonary responses and strength of nonathletic women and males to individuals who had been on long-term hormone therapy for 14.4 +/- 3.5 years.  The researchers found the “absolute mean VO2 peak of non-athlete transgender women while performing physical exertion was higher than that of non-athlete cisgender women and lower than that of cisgender men.”91  When the researchers corrected for fat-free mass there were no differences noted; however, in each category studied, the MtF individuals who had been on long term estrogen supplementation were found to be stronger and have higher cardiopulmonary function than women and were weaker and had lower function than men.
    2. Another small study of cardiopulmonary function and strength similar to the above, but researching athletes, found that males taking estrogen maintained higher absolute handgrip strength.92
  2. Muscle strength
    1. A systematic review of 24 studies evaluating body composition, muscle strength and hemoglobin in males taking estrogen found after 4 months the males had hemoglobin levels similar to those of females.  After one year of supplementation, the males had significantly less strength, muscle mass and lean body mass, but these levels remained higher than females even after 36 months of hormone therapy.93
    2. A study of muscle strength, size, and composition in nonathletes taking cross sex hormones (without a control group) found that after one year of hormonal intervention, men taking estrogen had a 5% decrease in muscle volume as measured by MRI and CT imaging, as well as a 4% decrease in muscle cross sectional area. However, these individuals “generally maintained their strength levels,” demonstrating hormonal intervention does not equalize the sex differences between males and females.94
    3. Another study evaluating testosterone suppression in 19  males found that testosterone levels were reduced within the first year of supplementation to 1 nmol/L (normal female level) and thigh muscle area decreased by 9% as documented by MRI.
      After three years of suppression, the thigh muscle mass had decreased by a total of 12%,
      but the thigh muscle mass area was still 13% greater than that of females.95
    4. Hilton and Lundberg evaluated 12 longitudinal studies that evaluated the results of testosterone suppression on muscle size in males and found 12 months of testosterone suppression resulted in approximately 5% loss of lean body mass or muscle size.  No study found muscle loss greater than 12%. They stated, “Thus, given the large baseline differences in muscle mass between males and females (Table 1; approximately 40%), the reduction achieved by 12 months of testosterone suppression can reasonably be assessed as small relative to the initial superior mass. We, therefore, conclude that the muscle mass advantage males possess over females, and the performance implications thereof, are not removed by the currently studied durations (4 months, 1, 2 and 3 years) of testosterone suppression in transgender women.”96
    5. A study of 249 males who took 12 months of testosterone suppression showed a decrease of 4% in grip strength.  However, these individuals maintained a significant advantage over females as their handgrip strength was in the 25th percentile for males but was in the 95th percentile for females.97
  3. Bone mass – Males taking estrogen and antiandrogens over a median of 12.5 years  continued to have significant bone mineral density advantages over age-matched biological females.98 Hilton and Lundberg conclude, “height and skeletal parameters remain unaltered in transgender women, and that sporting advantage conferred by skeletal size and bone density would be retained despite testosterone reductions compliant with the IOC’s current guidelines.”99
  4. Hematologic function – The one area in which consistent use of cross sex hormones does fully reverse the male advantage is in hemoglobin concentration.
    1. Testosterone is known to induce hematopoiesis and will therefore produce an increase in the circulating hemoglobin, while estrogen supplementation tends to decrease hemoglobin levels.100
    2. A study from three Kaiser Permanente facilities between 2006 and 2014 identified transgender individuals and these were each matched with 10 male and 10 female controls.  Individuals receiving testosterone had higher hemoglobin levels and reached levels commonly seen in biological males, while those who received estrogen had lower hemoglobin levels consistent with biological females.101
    3. In a prospective study of 625 individuals entering care to receive cross-sex hormones, serum hematocrits reached their new levels in 3 months.102
  5. General athletic abilities
    1. A study of Air Force individuals who began cross-sex hormones (29 women and 46 men) found that the males performed more push-ups (31%) and more sit-ups in 1 minute (15%) and ran faster (21%) than their female colleagues.   “After 2 years of taking feminising hormones, the push-up and sit-up differences disappeared but transwomen were still 12% faster.”  (Roberts) This led the authors to conclude, “The 15-31% athletic advantage that transwomen displayed over the female counterparts prior to starting gender affirming hormones declined with feminising therapy.  However, transwomen still had a 9% faster mean run speed after the 1-year period of testosterone suppression that is recommended by World Athletics for inclusion in women’s events.”103
    2. A follow up study by Chiccarelli evaluated 374 Air Force transgender individuals for their performance values for the Air Force physical fitness tests.104 This study has major limitations due to the majority of individuals lost to follow up (346 lost to follow up with only 28 completing the study). Testing on sit-ups, push-ups, and the time on the 1.5 mile run were measured 1 year before and up to 4 years after cross sex hormones were initiated.   “Transgender females’ performance showed statistically significant better performance than cisgender females until 2 years of GAHT in run times and 4 years in sit-up scores and remained superior in push-ups at the study’s 4-year endpoint.” (Chiccarelli) This study implies that the 12 months of testosterone suppression recommended by the IOC for male involvement in female sports is not sufficient to equalize the athletic abilities of males with biological females.
    3. In their review of the literature on sports performance differences between male and female athletes, Hilton and Lundberg also performed a “systematic search of the scientific literature addressing anthropometric and muscle characteristics of transgender women.”105

After evaluating testosterone suppression and its effects on muscle mass, strength, hemoglobin and endurance, they concluded, “The data presented here demonstrate that superior anthropometric, muscle mass and strength parameters achieved by males at puberty, and underpinning a considerable portion of the male performance advantage over females, are not removed by the current regimen of testosterone suppression permitting participation of transgender women in female sports categories. Rather, it appears that the male performance advantage remains substantial.”106

Brief History of Olympic Regulations on Males Inclusion in Women’s Sports

Professional athletic associations have traditionally been extremely careful to assure that men did not participate in women’s sports, to the point that participants were required to undergo genital inspection and chromosome testing.  After World War II, the International Association of Athletics Federation (IAAF) required women competing in track and field events to submit a physician’s letter confirming their sex.  The International Olympic Committee (IOC) instituted the same rule in 1948.  However, because of the possibility of forged physicians’ letters, chromosome testing was required in the early 1960s.107

The IOC first confronted the concept of transgender athletes in 2004 when it was decided athletes could participate in their self-identified sex category if they had completed a full surgical transition and had received two years of postoperative hormonal intervention.  However, in 2015, the IOC eliminated the surgical requirement and allowed males to compete against females if their testosterone levels were below 10 nmol/L for a specific period of time.

Prior to 2021, the IOC had additional stipulations that had to be met by males in order for them to compete in women’s sports.  The men must have identified as women for at least four years and maintained a testosterone level below 10 nmol/l for at least one year prior to and during the competition.

In 2021, the IOC allowed individual sports to develop their own rules with the goal of promoting fairness and nondiscrimination.  In 2024 the IOC added a requirement to help eliminate the advantage of male puberty and determined that males must have completed their transition prior to age 12 years of age.

However, the United States Olympic & Paralympic Committee (USOPC) in July 2025, changed its policy on transgender athletes to comply with President Donald Trump’s executive order.  Under the USOPC athlete safety policy, the committee added “The USOPC will continue to collaborate with various stakeholders … to ensure that women have a fair and safe competition environment consistent with Executive Order 14201.”  That Executive order from President Trump is titled, “Keeping men out of women’s sports”, so the USOPC appears to have banned males from participating in women’s events. It is not yet clear exactly how this new policy will be implemented.108

Conclusion

There are anatomical and physiological differences between males and females, beginning in early fetal life and continuing through puberty.  These differences provide males with a distinct advantage over females in many sports, especially those involving strength, power, endurance, and speed. Research is now showing that there are significant differences seen even in young children.  The differences between the sexes increase with the onset of puberty, and most are not totally eliminated by estrogen supplementation or testosterone suppressing medications.  Therefore, it is not physiologically appropriate or fair-minded to allow male participation in female sports.

Primary Author: Jane E. Anderson, MD, FCP

November 2025

The American College of Pediatricians (ACPeds) is a national association of licensed physicians and healthcare professionals who specialize in the care of infants, children, and adolescents. The mission of the ACPeds is to enable all children to reach their optimal physical and emotional health and well-being.

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American College of Pediatricians | November 2025

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