There are biological differences between men and women.


Muscle Mass:



Body Fat:


Height: On average, men tend to be 10-15 cm taller than women. It is thought that testosterone release during puberty is responsible for the average greater height in men.
Courant 2010
Muscle Mass: On average, men tend to carry 30-40% more lean body mass than women (this includes muscle, organ, bone, water, and all other non-fat mass).Janssen 2000
However, height and weight influence muscle carrying capacity for an individual regardless of sex or gender.
Heymsfield 2015,Gallagher 1997
Interestingly, when correcting for height and total body weight, the gap narrows to an approximately 10% average difference in lean body mass between men and women across the entire lifespan. Kirchengast 2010
Men and women exhibit a similar relative increase in muscle hypertrophy in response to resistance training as well.
Abe 2000
Strength: Muscular strength can be defined as force production measured in a specific context. Based on small data sets, it is reported that men have 48% and 34% greater strength than women in the upper and lower body muscles, respectively. Miller 1993
However, no differences in strength between men and women are seen when normalized for muscle cross-sectional area. Miller 1993, Bishop 1987, Hannah 2012
Additionally, men and women exhibit similar relative responses to resistance training, with no gender-specific responses noted after 10-weeks of resistance training. Gentil 2016
Finally, a number of other gender-independent factors contribute to the regulation of adult muscle mass, including genetics, race, adiposity, training, diet, and birth order. Heymsfield 2015
For example, data from twin studies suggests up to 50% to 60% of the difference in muscle mass and strength may be due to genetic factors without clear gender influences. Silventoinen 2003, Beunen 2004, Silventoinen 2008.­
Said differently, the amount of muscle mass and strength displayed by an individual may be influenced by non-sex genetics to a level that rivals or exceeds the influence of gender.
Power: Muscular power can be defined as high-velocity force production. As described above, men tend to be taller, heavier, and carry more muscle and less body fat than women.
Despite men having more muscle mass however, there does not appear to be any difference in average muscle fiber type composition (e.g. Type I- and Type II fibers) between genders.
Miller 1993
Type I muscle fibers are classically referred to as “slow-twitch” muscle fibers and tend to produce low amounts of force for long periods of time, e.g. they are very fatigue resistant.
In contrast, Type II muscle fibers, or “fast-twitch” muscle fibers, tend to produce high levels of force for short periods of time and are tend to fatigue quickly.
Interestingly, muscle fiber type composition seems to be more strongly correlated with training history and athlete caliber than gender. Serrano 2019
For example, women who compete in Olympic weightlifting at the World or Olympic level were found to have 71% fast-twitch type IIa fibers compared to the 63% seen in men competing at the National level. Serrano 2019
While absolute power output appears to be greater in men, when we normalize existing power data for fat-free mass and fat-free cross-sectional area (approx the amount of skeletal muscle mass) there are few, if any, gender-specific differences in power. van den Tillaar 2004
For example, Slawinski et al. looked at peak acceleration after 1 second in 100 world-class sprinters (50 women and 50 men). After normalizing for body mass, men and women had approximately the same peak acceleration power. Slawinski 2017
This study is unique given the relatively large sample size of 100 and the findings. Nevertheless, the generalizability of these results be limited given that all subjects were already world-class sprinters and thus, not generalizable to untrained individuals.
Additional evidence from researchers in Spain provides insight here: 155 college-aged, untrained individuals (123 men and 32 women) tested their anaerobic power output using a Wingate cycling test,....
which showed no differences between men and women when normalized for fat-free mass.
Perez-Gomez 2008
Body Fat: As mentioned above, men tend to have 10% greater lean body mass than women of the same height and weight, while women have about 10% greater body fat than men. This gap also tends to be preserved over the entire lifespan.
Kirchengast 2010
Hemoglobin: Hemoglobin is responsible for carrying oxygen around to active tissues. On average, hemoglobin levels are 12% higher in males than females, which is thought to be due to the effect of testosterone.Shahani 2009
Testosterone: At this point, it is important to point out that testosterone is not exclusively a male hormone.
It is produced by both males and females in the testicles and ovaries (and also in the adrenal glands), however males need to produce it in higher quantities as they have XY androgen receptors.
XX androgen receptors are highly sensitive to testosterone, requiring much less testosterone to equate the same level of health.
However, both male and female elite athletes also produce testosterone in higher quantities than the ‘normal’ population, and there is also a significant amount of overlap between testosterone levels in male and female elite athletes.
The maintenance of endogenous testosterone levels is essential to basic health in both men and women.
An XY male who transitions to XY female has had their major source of endogenous testosterone (the testicles) taken away, so needs to take synthetic testosterone in order to maintain basic health. (current policy does not allow the transitioned woman to be granted a TUE for T).
The US University of Rochester’s online medical library states that the normal ‘male’ range for testosterone is between 9.7nmol/l and 38.1nmol/l; and between 0.5nmol/l and 2.4nmol/l for ‘females’.
The US government’s National Library of Medicine agrees with the University of Rochester’s ranges with regards to ‘women’, but puts normal ‘male’ levels at between 10.4 nmol/l and 34.7nmol/l.
So individuals with XY chromosomes need a higher level of testosterone those with XX chromosomes to maintain basic health, due to their androgen receptor response.
As these analyses show, these levels can vary with age but also due to occupation.…
A 2014 study,‘Endocrine profiles in 693 elite athletes in the post-competition setting’, found that 16.5% of the ‘male’ athletes had low testosterone levels, and 13.7% of the ‘females’ had high testosterone levels, ‘with complete overlap between the sexes’
The report concluded: ‘The IOC definition of a woman as one who has a ‘normal’ testosterone level is untenable’. In other words, elite athletes have testosterone levels that differ widely from those that might be considered ‘normal’ in the general public population.
We know that the testosterone levels of high-performance female athletes go up from years of competing that this makes conception during competition or after retirement very hard,..
until those bodies come back to their natural state to be able to conceive a child, this very common.
We also know that an XY chromosome body takes six to 10 times the level of testosterone to equate the same levels of ‘health’ as somebody who is XX. This is completely separate to the body’s sex.
The problem has been that sport failed to realise that XY women continue to need XY levels of testosterone in order to maintain basic health following transition. Instead, it required XY females to peg their testosterone levels to the much lower XX female levels,...
due to the myth that XY females would enjoy a performance advantage due to their extra endogenous testosterone.
In effect, sport is blinded by its history with exogenous T, which has historically been used to enhance performance. This requirement to drop their natural, endogenous testosterone levels to XX female levels is what has physically harmed Kristen Worley, myself & others.
This is where everybody got it wrong and continues to get it wrong. The surgically transitioned XY woman, we are the only athletes in the world that can medically illustrate, physiologically, how the body breaks down over time and what happens to the human physiology...
when testosterone values get down to a certain level or is removed, and the body is no longer able to generate any hormones. For all other athletes that have been discussed in this thread, their testosterone levels are their ‘normal’, and they are healthy.
Testosterone has over 200 different functions in the human body, and its removal caused Worley, myself, @CarolineLayt & others a number of serious health complications.
For example, as well as removing the body’s ability to regulate itself and inducting menopause, Worley & myself & others endured complete muscle atrophy (failure of muscle development and recovery), making sport impossible.
Far from removing the competitive advantage that sport claimed trans women held, sport’s regulations removed Worley’s mine & other women ability to compete in sport completely, and damaged our health. And we are far from alone.
The XY females, when they transition, are the only ones that are put into spontaneous menopause. This has over 24 different complications concerning the human physiology.
When an athlete goes down to zero, the impact is massive. I can tell you that at 9.6nmol/L, I go into spontaneous menopause.

The value of 10nmol/L, which the rules are suggesting, is impossible.
Overall, the size and sources of the performance gap in sport are complex and not-yet fully characterized. While it appears that there are real differences in biological inputs such as testosterone, muscle mass, height, and hemoglobin levels (among others), .....
it is also clear that gender-independent factors such as genetics and training, in addition to non-biological factors like equality in participation opportunities, & the negative attitudes against women in strength sports play substantial roles in performance.

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More from @KirstiMiller30

9 Jun
Former elite athlete Kristen Worley, a New Zealander based in Canada, has been working with the International Olympic Committee on its transgender guidelines for the past 18 months and is considered an expert in trans experience in sport.…
She said the reduction of hormones and “sex reassignment” can have a “catastrophic impact” on the body of transgender people.

@PaulSimon1971 this is what others argued with me ✊
“Under the current rules ... this is just above the level [of testosterone] where the body starts to fail. It’s like taking the gas out of the car,” she told Stuff.

@CydZeigler, @CarolineLayt what we were discussing & what I have advocated for years now with very few listening
Read 26 tweets
9 Jun
@Quizical15 @AnnaVolodova @madina_wane @DrJoGrady @XineYaoPhD Despite the many assumptions about the relationship between testosterone and athletic advantage, there is no evidence showing that successful athletes have higher testosterone levels than less successful athletes.
@Quizical15 @AnnaVolodova @madina_wane @DrJoGrady @XineYaoPhD Still, the IAAF continues to use testosterone levels to determine who is eligible for the women’s division. In the recent case between the IAAF & Caster Semenya, the IAAF produced data suggesting that women with testosterone levels in the highest tertile (one third...
@Quizical15 @AnnaVolodova @madina_wane @DrJoGrady @XineYaoPhD had a competitive advantage compared to women competitors with testosterone levels in the lowest tertile.
Read 26 tweets
9 Jun
Hemoglobin is responsible for carrying oxygen around to active tissues. On average, hemoglobin levels are 12% higher in males than females, which is thought to be due to the effect of testosterone.
New research paints a clearer picture that trans women do lose significant performance in just a few short months of commencing HRT.…
Since 2019, Joana Harper has been based at Loughborough University in the U.K. as part of a project using science and research to aid policy making.
Read 10 tweets
9 Jun
A paper by Wiik et al, looked at strength and muscle mass levels in 11 MTF (trans women) and 12 FTM (trans men) individuals over 1 year of transitioning.
As seen in Figure 8, the authors compared the transgender individuals’ muscle torque and muscle volume to cisgender controls.
It is not clear if these controls had similar activity levels, dietary intakes, and/or completed the same strength testing protocols as the transgender subjects.
Read 11 tweets
8 Jun
Despite the many assumptions about the relationship between testosterone and athletic advantage, there is no evidence showing that successful athletes have higher testosterone levels than less successful athletes.
Still, the @WorldAthletics continues to use testosterone levels to determine who the women’s division. It must be noted the @WorldAthletics does not regulate the testosterone levels of any XX women even though their own evidence showed XX women are more sensitive to androgens.
In the case between the IAAF & Semenya, the IAAF produced data suggesting that women with testosterone levels in the highest tertile (one third) had a competitive advantage compared to women competitors with testosterone levels in the lowest tertile.
Read 13 tweets
8 Jun
Australia under Morrison now gaols adults at the highest rate since the convict days.
The return to mass incarceration in Australian prison population does not due to crime rates, which have mostly declined over the past generation.
Instead, failing to implement the majority of the RC recommendations for 30yrs, higher reporting rates, stricter policing practices, tougher sentencing laws, and more stringent bail laws appear to be the main drivers of Australia’s growing prison population.
Read 9 tweets

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