I glossed past something in my last post. Emphasis mine:
[9:18] You see, I absolutely understand why we have and still do categorize sports based upon sex, as it’s simply the case that the vast majority of males have significant athletic advantages over females, but strictly speaking it’s not due to their sex. It’s due to factors that heavily correlate with their sex, such as height, width, heart size, lung size, bone density, muscle mass, muscle fiber type, hemoglobin, and so on. Or, in other words, sports are not segregated due to chromosomes, they’re segregated due to morphology.
I think it’s time we had a look at his science on this. Of the eleven scientific studies I counted in RR’s citations, only two dealt with muscle fibre composition:
Oertel, Gisela. “Morphometric Analysis of Normal Skeletal Muscles in Infancy, Childhood and Adolescence: An Autopsy Study.” Journal of the Neurological Sciences 88, no. 1 (December 1, 1988): 303–13. https://doi.org/10.1016/0022-510X(88)90227-4.
Staron, Robert S., Fredrick C. Hagerman, Robert S. Hikida, Thomas F. Murray, David P. Hostler, Mathew T. Crill, Kerry E. Ragg, and Kumika Toma. “Fiber Type Composition of the Vastus Lateralis Muscle of Young Men and Women.” Journal of Histochemistry & Cytochemistry 48, no. 5 (May 2000): 623–29. https://doi.org/10.1177/002215540004800506.
From that, we can extract the key charts on fibre composition. I’ll dim the irrelevant sections.
… Huh. That second study only found a statistically significant difference for one type of muscle fibre for one sex, out of twelve possibilities. Assuming there’s no true correlation between sex and muscle fibre type, and our failure rate is 5% (it’s not), the odds of getting this finding due to chance are 34%.[*] The first study doesn’t list statistical significance, but if we convert the standard deviations to standard errors we find the 95% confidence intervals around the mean overlap. Under Fisherian frequentism standards, we’d fail to reject the hypothesis that muscle fibre composition is identical between the sexes. A Bayesian analysis would probably wind up supporting the hypothesis that there’s no difference in fibre composition, but I haven’t run the numbers.
And that’s just the start of the problems here. For seconds, the first study only has two muscle fibre types, while the second has six. Physiology must have advanced a fair bit in the twelve years between these two, so why should we trust the results of the older study? For thirds, the authors’ claimed the difference in sexes were not present before age 15. Puberty finishes around age 15, however, so if the difference is due to puberty then why doesn’t the difference show up during puberty? For fourths, let’s say there is a genuine difference and it is due to puberty. If puberty is characterized by a flood of hormones, isn’t it possible the change in muscle composition is due to hormone levels? Because if it is, then transgender people on HRT would adjust their fibre types to match their cis counterparts. For fifths, that first study claimed no difference in muscle fibre composition from birth, and yet later on that composition changed. If it is possible to change your composition due to internal factors, it may be possible to change it due to external ones like athletic training.
Finally, neither study looked at athletes. The first doesn’t mention them at all, and the second explicitly declares these people are “untrained young men and women” right in the abstract. Rationality Rules references no other studies covering muscle fibre composition, so how is he able to make this claim?
[14:35] The one thing that gives me slight pause, however, is just how large the gap is between pubertal boys and girls in events such as the one hundred metre, as 1: fast-twitch muscle fibers play a vital role in such events; and 2: there’s conflicting studies on just how the muscle fibers are developed, and to what degree male puberty has a role.
Serrano, Nathan, Lauren M. Colenso-Semple, Kara K. Lazauskus, Jeremy W. Siu, James R. Bagley, Robert G. Lockie, Pablo B. Costa, and Andrew J. Galpin. “Extraordinary Fast-Twitch Fiber Abundance in Elite Weightlifters.” PLOS ONE 14, no. 3 (March 27, 2019): e0207975.
It finally provides some scientific references to an advantage for slow-twitch fibres in endurance sport, but immediately points out methodological flaws in that research.
This work provided an important foundation, but used sub-elite participants and/or laboratory methods that failed to accurately resolve the highly prevalent hybrids —which compromises measurement fidelity and produces erroneous FT% [fibre type percentage] conclusions. More precise techniques were developed in the early 1990’s that allowed proper quantification of FT% by analyzing each SF [single fibre]. […]
Although the differences in FT% between our three groups did not reach statistical significance, large effect sizes were evident and MHC [myosin heavy chain] IIa frequencies of 74%-89% occurred in 66% of WCF [world-calibre female athletes] but only in 44% and 33% of NCF [national-calibre female athletes] and NCM [national-calibre male athletes], respectively. Thus, scientists should further examine how FT% may separate World from National-level athletes as it would enhance our understanding of the physiological factors determining maximal human performance. For example, the only published report on a world-record holding anaerobic athlete found a FT profile remarkably different from our study or any other previous research in elite sprinters. The minimal exploration in this area makes it difficult to determine if such a separation in FT profile between elite subgroups is a true and consistent phenomenon or merely an artifact of too little research.
This study took tissue biopsies of elite weightlifters within days of their competition, and used two different tests to assess muscle fibre composition. If you read that last paragraph carefully, you picked up on the same thing that Rod Clark did: elite female weightlifters had a greater proportion of fast-twitch muscle fibres than male weightlifters! I want to highlight something else their research suggests, though.
Our groups differed in two other important characteristics; sex and years competing in the sport. Sex comparisons between athletes remain tenuous because nearly all investigations utilize non-gold standard FT% methods and sedentary or “recreationally active” individuals . Not only do our findings contradict the claim that women possess more slow-twitch myofibers than men, they illustrate the opposite when accounting for talent level (WCF < NCF = NCM). WCF had also been competing in the sport for ~5 years longer than both NCF and NCM. The current cross-sectional study-design precludes direct analysis, but extensive research affords strong support for exercise history as a critical determinate of FT%.
Naturally, I had this finding in mind when I dropped the possibility of external factors influencing muscle fibre composition. If training has a much greater impact on fibre composition than hormones or sex assigned at birth, transgender athletes have no significant advantage or disadvantage relative to their cis peers.
[16:48] Which is to say that the attributes granted from male puberty that play a vital role in explosive events – such as height, width, limb length, and fast twitch muscle fibers – have not been shown to be sufficiently mitigated by HRT in trans women.
 LEE, PETER A., Shumei S. Guo, and Howard E. Kulin. “Age of Puberty: Data from the United States of America.” Apmis 109, no. S103 (2001): S156–S163.
[HJH 2019-07-19: Whoops, should have put the images under the fold.]
[HJH 2019-07-19: * Also, I just realized I may have misinterpreted the second study’s graph. Rather than being significant only for one type and one sex, it could be significant for BOTH sexes, but because the authors assumed a sex binary they figured that both statements were equivalent. Fortunately that doesn’t change the argument much, the odds only drop to 23%. Because so little changes, I’ll leave the original intact and mention the potential mix-up down here.]