This isn't exactly about the "writing muscle", but it affects how the "writing muscle" may be used. To explain: I belong to several writers' e-mail lilsts, and from time to time, quiestions about POV(point of view) come up. Also questions --- always important --- of what agents and publishers will accept in certain genres.
For example, it has been asserted that, in historical novels, a female POV main character is preferred, because women are the main readers of historical novels. No, not historical romances --- historical novels. This may or may not be true. OTOH, there are writers, both male and female, who prefer to write male POV's for various reasons. One author I've corresponded with, has "fallen in love" with certain (male) historical characters, and while she started out writing things with female POV's, she is presently doing a lot of writing from the "male" POV. Theoretically, there is nothing wrong with that; in some periods, it's just a lot easier to find "interesting" male characters without "digging", especially if you insist, if this author does, for a variety of reasons I won't go into here, that the characters must reflect the "thinking" of whatever historical period they'rw writing about. Again, there's nothing wrong with that --- up to a point. There is nothing potentially more annoying than having some female character acting like a "feminist"(whatever that may be), 500 years before feminism was "invented". OTOH, there's also nothing wrong with "bending" the social rules a little, as long as you don't have your character "bending" them too much. But again, some writers insist on not "bending" what they imagine to be the "rules" of a past period, this, despite the fact that, in reality, past and present, social rules are "bent" all the time, with varying consequences. The people who "bent" them weren't always famous enough or noted enough to get written about, though. This applies, to both male and female characters.
Going a little farther with this, how do you even choose whether or not to have a "male" or "female" POV in any piece of writing? As a general rule, most male writers tend to stick with a "male" POV, though I hasten to add that there are plenty of exceptions here. Women --- as a general rule --- again, there are notable exceptions, which I'll come to in a moment --- tend to write in either POV, depending on the kind of story they're writng. The exception is romance, although even here, there may be "switches" from male to female POV's. But then, romance is almost exclusively a woman writer's domain; other genres, with the exception of thrillers, are created by both men and women(and even that may be breaking down; just recently a woman wrote a thriller).
In general, it seems to me that people write in the POV that seems to best fit the story. If it's more "action oriented", the writer may decide a male protagonist is the best choice. If there are lots of scenes where emotion and interpersonal relationships are involved, female POV's may work better, but again,t here is lots of overlap. OTOH, some people can't comfortably write novels featuring the "opposite" sex. Bernard Cornwell's novels seem to be of this type. They are very good and well-written and interesting, but his female characters tend to be rather interchangeable and "flat", and his emphasis is on lots of "action". Novels like this are what I call "guy books". Again, nothing wrong with these; they are basically the male equivalent of romance(and there's nothing wrong with well-written romance, either).
But unless we are Bernard Cornwell wannabes writing "guy books", or romance writers, or people who are just plain uncomfortable writing about the "opposite" sex, there will always be a question of who the protagonist should be. And this is where the "writing muscle" comes in. What kind of a story is it? Thriller/novel with lots of battles and actions? Romance? Science fiction? Mystery? Or something else, perhaps a combination of all of these, or "literary" fiction? Most of these genres or story types could use a protagonist of either sex, but which one is chosen, would depend on the type of story being told. If it's about someone with a "past", it may well be more conceptually easy to imagine the protagonist as female, and the writer will then go with a female POV, though there may well(or, in my view, should)be a strong male character or two. If it involves lots of action, particularly if it's historical action, a male POV may work better, though this, too, can be a mixed bag. I can speak from my own experience here. My first attempt had an alternating male and female POV. Both were strong characters, and both(eventually)came together to fight a "bigger problem". I'm still working on this one; it's kind of "on the shelf for now". The second attempt was about one of the characters from the first novel, who was a child at the time; now she is fifteen years old. And I felt it had to be a female protagonist, because it was what is sometimes called a "coming of age story". But I never thought of it that way; I just tried to remember my own experiences of being that age, and projected it into the near future. This too has not quite jelled. The third attempt is the one I'm writing now. There is a main female protagonist and two "strong secondary" females, who may get their own stories later on. And there are two main male characters, both of them emotionally involved, though in different ways, with the main female character. Plus there are two "secondary" male characters. This is the most "balanced" of my attempts, and it's in historical time, and has what one friend of mine calls a "cast of thousands". Originally, this was going to be a mainly "female" POV, but as I wrote it, certain characters just grew and grew, and I had to add more strong male characters. Again, I don't think there is anything wrong with this process. For a writer, it just is.
So, when it comes down to it, I don't really have any "advice". I just used my "writing muscle", which, in me, is quite well-developed. But if you, dear reader, want to write, and you don't think yours is very well-developed, don't despair. There are ways to strengthen "writing muscles", just as there are ways to strengthen and tone your body(as I have found out over the past week, and continue to find out). I'll suggest some in a later post. Nothing profound, just things you can try. And even if you don't think writing something creative is for you, you still need not despair, because your "writing muscle" is basically nothing more than a "creativity muscle", directed in certain ways. And everyone has a creative side!