I feel like I've always been to muscular women...
I remember my first experience that really woke me to lust after muscular women was in kindergarten when I was like 5-6 years old. One of my female teachers had this green shirt with Donald Duck flexing his bicep. I one day said something about her shirt which made for some reason flex her bicep as well, and seeing that bicep bulge beneath her skin felt like magic, and I couldn't put my finger on it. I remember trying to ask her to flex her bicep for me again, which she complied to and I remember having a serious attraction her. I don't know what it is, but seeing a woman flex her biceps is just so friggin hot. She had some great arms as well...
But then something that really woke my, arousal for muscular women was stumbling onto female muscle growth transformation animations on youtube, back in like... 2012 or something when age restriction wasn't as strict or those videos didn't have that many views.
But yeah that's about it...