A Swedish study of people undergoing hormone therapy showed transwomen lost some of their muscle mass but maintained or even increased strength. This could reinforce the argument against pitting them in sports with cisgenders.

People who believe that transgender athletes should not be allowed to compete in women’s sports, because their initial life as biological males gives them an unfair advantage, may have received a new argument in their favor.

A study conducted at two Swedish universities, the Karolinska Institute and the Linkoping University, found that hormonal therapy during the gender adjustment process does little to reduce the strength of transwomen. The natural difference in physique is the key reason why there are separate competitions for men and women in the first place.

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The paper was pre-printed this week and is currently awaiting peer review. It details a study of 11 transwomen and 12 transmen as they underwent hormonal treatment. The scientists measured their muscle volume and density as well as their leg strength, in particular the strength of knee flexors and extenders.

The therapy had affected the muscles as expected, making transmen bulkier and transwomen slimmer, but did so to a different degree. Only transmen gained some muscle density while transwomen’s muscle measured the same after 12 months of treatment.

Most interestingly, while transmen gained in strength as they got their higher level of testosterone, the reverse was not true for their counterparts. Not only did transwomen stay more robust than transmen, their strength maintained a baseline level or even became higher.

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The researchers say the increase of strength they observed may be explained by the learning effect from repeating the same test on four different occasions or by the anabolic effect of the estrogen treatment the transwomen had received. The subjects of the study were also not professional athletes.

Nevertheless, the authors say their result indicates that “that after 12 months of hormonal therapy, a transwoman will still likely have performance benefits over a cis-woman.”

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