I thought I'd chime in just because I find the science of it fascinating where hormones are concerned.
I can see why people would be up in arms over all this, especially with the idea that there's an unfair advantage at play. I'm a data person, and I like statistics, so I usually like to go there to base my opinion. Transgender individuals make up approximately 1% of the population, which makes studying the effects of cross-hormone treatment difficult with relation to the cisgender population. As far as I could tell, there's not been a lot of cross-examination data between the two populations, but there have been same-group data comparisons - specifically related to transwomen pre- and post-transition, because that's the group that's caught the most flak. That said, those studies have their own bias as it was run by someone who is transgender herself. Not to say they're disingenuous - just that that's something to keep in mind when considering a single study where there's not a large consensus yet.
Where running is concerned, according to a single study, it showed that transwomen do indeed have significantly slower race times than before they transitioned, which is fascinating that estrogen has that sort of impact. According to that study, there was something called an 'age range differentiator' meant to calculate how different that post-transition race time stacks up against cisgender women. It said that there wasn't too much difference. However, that was a single study from a pop science article where I couldn't see methodology for how they calculated this age differentiator. As far as I could tell from that study, there's not a terrible amount of difference, but that is not to say a difference couldn't exist - and science is based on a huge basis of data, not just a single study. So there's room for interpretation.
When I started to dig into stuff like weight, it got even fuzzier. There just isn't a whole lot of data! Again, it's a very niche group, and a niche group INSIDE of a niche group. It's still a really cool field of study because we get to see just how much hormones can change the way the body 'orders' itself. Seeing as there wasn't much data, I decided to look at specifically what HRT does to the body and kinda form an opinion based off that. Muscle mass does significantly begin to drop, as well as fat intake increase, after starting estrogen treatment, alongside development of other tissues. What doesn't change, though, are the bones, the ligaments, the general integumental connective tissue, enough that some transwomen will have their bones shaved down for a more feminine appearance. In fact, estrogen maintains bone density in both men and women, so the skeleton you had when you're born is the one you just gotta live with. Here's the kicker that's interesting, and where more of my personal knowledge comes in, as I did a (very) short stint in forensic anthropology.
Humans are a rather sexually dimorphic - sexually differentiated - species of mammals. Men and women have very different skeletal structures, meant to leverage different muscles in different ways. From an evolutionary perspective, men were the larger sex with a real emphasis on long-distance-running and long-distance-chucking-things and in general throwing themselves at whatever is trying to eat the tribe. Women, on the other hand, developed with childbearing in mind, as well as long-term survival and so there was an emphasis on storing fat for lean times, for milk production, as well as keener color differentiation and peripheral vision, better peripheral senses in general, as women tended to hunker down in a single spot foraging with babies and toddlers while keeping on the lookout for a saber toothed cat looking for easy pickings.
How does that relate to sports? At lot of this is why we have segregated sports to begin with. Women have wider hips with a very different center of gravity, sitting lower, as well as a ligament system that wasn't intended for intense, hard running over long periods of time. Men, on the other hand, have very narrow hips and a ligament system meant for the opposite - intense running, long-distance, and just more heft in general on average. That system is set up and maintained via genetics, rather than hormonal control, so even with less muscle to work the same skeleton, the leverage system is different. It's the difference between using a single pulley for a fifteen pound weight versus five pulleys for a fifteen pound weight. I might pull at the same PSI, but I will definitely be pulling that fifteen pound weight up farther and faster with the five pulleys than the one just by physics alone. My supposition is that your average transwoman will be at very least as fast or as strong as your athletic cisgender woman. Of course, that's a hypothesis. There's a really in-depth cohort study looking into this very thing in Sweden with DEXA scans and CT scanning and blood samples pre- and post-transition, except that they haven't completed it yet...
That's not to say that there isn't room for the possibility that there is no difference, but it seems that transwomen athletes have had a pretty good track record (ha!) so far on average in whatever sport they apply themselves to. It would be interesting to see a category of only transwomen athletes and compare their statistics to their cisgender counterparts in a larger, longitudinal way. This is, of course, not to say that women shouldn't be in sports period because of biology, or that transgender individuals have no place in a competitive arena - but these are things to consider, especially where the safety of the participants are concerned going both ways, for transmen and transwomen. Personally, contact sports would be where I draw the line until there's more data as there is a real potential for harm in things like competitive wrestling, boxing, and MMA. Rowing, track and field, and other non-contact sports, it becomes less of a safety issue.
Sorry for the wall of text, biology gets dense....
