Transgender Athletes: To Be Or Not To Be

A Cheat Sheet for the debate about fairness and level playing fields

(Fluorescent marker images for calcium ion release by testosterone in a rat myotube)

From California to the New York Island, few aspects of transgender rights seem to elicit more misgivings (not to say hysteria) than participation in organized athletics. To a great many people, it simply isn't “fair.” As an earnest Long Island parent put it: ““It’s wrong. It’s not about hate, it’s about fairness. Imagine you practice, practice, practice, and then this kid comes along who is biologically a boy and dominates because he has a physical advantage. It’s not fair.”

Critics of transgender athletes love to appeal to science. To paraphrase an eloquent lawyer: science matters. So (not even a paraphrase): “Just the facts, ma'am.” Consider a few of them.

1) Men (or boys) have little if any physical advantage over women. Some men have such an advantage.

“Physical advantage” has to be defined. Physiologists often measure “strength” in an arrangement where the subject can push consistently against the force of a spring. (A typical setup is illustrated in the linked paper.) Then that force is divided by the total muscle fiber area. This may be obtained using computed tomography (effectively the gold standard for accuracy).

The result may be called “specific strength,” in newtons per square meter or equivalent. In the experiments just cited, the difference between untrained men and women was not statistically significant: the average for men is 6% greater than for women, but the standard deviation is 21% (women) to 24% (men).

In other words, the variation among individuals of one sex is three to four times greater than the difference between the two. Furthermore, the expected random variation of muscle diameter measurement is close to the 6% variation of specific strength. As the authors state explicitly: “The variation between subjects is such that strength is not a useful predictive index of muscle cross-sectional area.” (And of course vice versa.)

(Strength vs. leg muscle cross-sectional area for men and women (25 each). Not corrected for total body mass; when this is done, the vertical axes are almost identical. From R. Maughan, et al., J. Physiol. 338, 37-49 (1983)

These data relate to slow-motion exertions. What about fast ones? Maximum running speed is determined mostly by weight, height, and the force (called “ground support force”) that the legs apply during each step, which in turn is a function of the total muscle strength. These treadmill measurements show that at the highest speed for each, the men exert only slightly more force (~3%) relative to their body mass.

A great deal of the athletic “gender gap” of roughly 10% certainly comes from different body sizes, but not from intrinsic strength. If “fairness” is the goal, segregation would have to be based on body size, not sex or gender.

2) Higher testosterone does provide a chemical advantage (though not in the way that's popularly believed).

Everyone “knows” about testosterone and its anabolic (muscle-building) effect. It's at the heart of doping, and hence, presumably, of athletic advantage; at least this is the story often told. Because fat-free mass (mostly muscle) in trans athletes typically doesn't decay completely to cisgender female averages, it is often asserted that this testosterone suppression is not “leveling the playing field” between trans- and cisgender women.

Testosterone actually is essential for all people, however, and has many functions. One of them is to control the release of calcium ions. Each of the millions of molecular cross-bridges along each muscle fiber requires one. No calcium: no contraction. Testosterone in male-average physiological concentrations increases calcium levels in proximity to those cross-bridge receptors by several-fold. This happens as fast as a microscope can detect the fluorescent indicator change, in both striated and heart muscle, and it goes away when the hormone goes away.

Fluorescence signal (a marker for released calcium ions) after injection of testosterone in rat myotubules. From M. Estrada, et al., Endocrinology 144, 3586–3597 (2003),

Direct extrapolation from these lab animal observations to humans in active competition is not easy, and technology won't be available to do it soon. However, the expected effect on the known biochemical reactions in muscle cells is consistent with those few percent sex differences that were just described, both for slow and fast exertions. Testosterone affects muscle power, not just mass.

3) Cross-species evidence further underscores how testosterone acts directly (nothing to do with muscle mass).

Why can cheetahs run so much faster than humans? Most studies have focused on physics: muscle fiber type. But chemistry rules again. Unlike humans, it turns out that female cheetahs run faster than males, by about 45% on average (67% for the fastest of each – 100 vs 60 mph). Yet this is not due to muscle mass, because (like humans) they're smaller than males: in one population, 36 ± 5 vs. 43 ± 6 kg.

Measuring hormone concentrations in cheetahs is harder than in athletes. It's done by correlating fecal measurements with blood values under controlled conditions. The result is that female testosterone values are within less than 10% of the males. This number is only for captive animals, but since the speed comparison is also for captives, oranges are being compared to oranges. Since fiber types differ little between sexes, we're left with a very difficult-to-escape conclusion that testosterone concentrations have a profound effect on “athletic ability” of the cheetah, and very likely also the human.

After this short excursion into what many readers may consider arcane and abstruse science, where are we? There is plenty of research yet to come; we're far from knowing all that we'd like about the effects of gender-affirming hormones. And as Erin Reed's recent post underscores, there are many other aspects to consider. But these three observations are definitive: in athletics, transgender women resemble cis women far more than men. I hope they can allay some of the misgivings that trouble those skeptical but sincere bystanders.

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