What's the psychological reason why a man might be attracted to a woman with muscles?

Is something that has been in my head for a while now. Maybe something related with a childhood experience (being raised with very dominant women)? The look of confidence and strength on a (muscular) woman?

Maybe someone can come up here saying "there isn't need to be a reason", but... idk, I've always thought that there's a reason for everything, and is the first time I ask myself the origin of this particular fetish

I've got 2 "origins" for me that I always go with if this get asked

1) the teacher in grade school that everyone had a crush on, ours was a former track star with amazing legs
2) my mom played a lot of volleyball, so I spent a lot of time as a kid around asian women with big calves

Seeing as how I love legs more than anything else, I figure those are my reasons

Good question!

I have to go back to my teens, something like 35 years ago, I had a neighboor, mother of a friend my age, who was really strong with muscular legs. Once I clearly remember she lifted my pulling one arm from the balcony, she litterally carried me first floor, I was probably more than 100 lbs, she easily did it and I felt excited.

In general, probably muscular development is a general signal of health. Even in in mainstream society a woman is considered attractive if she has developed body, (physically fit, abs, etc) and the reason high heals are sexy is because they make one's calves look larger. Muscle girl appreciators probably just take that natural preference to the extreme because we have modern medicine, advanced training, and roids. So, now we have female body-types that couldn't exist pre-1950.

I had similiar experiences while growing up starting with a summer camp I attended and we would have athletic competitions with a nearby "all girls" camp 
each summer.  One girl in particular who could "out do" any of us boys however hard we tried.  Swimming, running, etc. it was just not fair   But as I now think about it I have come to the conclusion that we're just "wired that way" and I for one am happy that I am!!!

For me it's a combination of many factors.
I think it's in part associative. I like it when women wear athletic gear. "Sexy" outfits are generally less sexy on these women, not because of a poor fit or anything, it just detracts from a major aspect of the attractivity.
The performance aspect, with the sweat and the effort, I find attractive. Some old and forgotten associations (PE, sports situations) with early experiences of women's bodies and their smells I guess.
There is an appreciation of high strength for sure. When I see someone like Loredana Toma moving heavy weights with extreme speed, I can't help but find that very attractive.
But the extreme mental strength I know it requires is also attractive (extreme ability is often attractive in other domains too, like arts, music etc.).
And then the aesthetics of a very fit body, it seems to trigger in part the 'normal' side that likes curves (mostly when it comes to the legs), and on the other hand perhaps some sort of unchecked (but fundamentally natural) appreciation of fitness.
Perhaps it is not that we have some 'thing', perhaps it is that the normal situation has the presence of some instinctual sensation that a woman who is very brawny has too much T, or is a threat?
I don't think non-muscular women are unattractive, but I do think I have more of a problem than most with e.g. thin limbs. I find that it seems frail, which - all else being equal - feels unattractive.
It might be in part genetic. I am naturally quite lean and find it relatively easy to build muscle, so maybe that's an expression of having descended from other males that choose fit women. Who knows.
As for women being stronger than me, it's completely irrelevant to my attraction, except that I want to be as strong as possible to 'deserve' being with a strong woman.