I'm not an expert but we should probably clarify with what I know so we have a point of reference- I was definitely a finger writer for years and years before my accident (and before calligraphy). I became an arm writer largely out of necessity (my injury was a pretty severely crushed right hand complete with multiple broken fingers and torn tendons). That said, if you're doing manuscript styles with broad edge it's typically accepted that most of the practitioners out there are finger writers- however, I was able to produce fairly decent, or at least I think so, foundational and italic with arm movements.
I eventually moved into pointed pen where I am now and, over the last half year or so, practicing very regularly, I've come to really love pointed pen scripts (in my case mostly Spencerian and Copperplate) in a way that I'm not quite sure I ever did with broad edges. Muscular movement, as I understand it (from reading multiple books and watching lots of videos) is based on a movement that starts with the pectorals and shoulders and sort of ripples down into the forearm with the fleshy part of the forearm acting as the primary pivot point. There are, of course, lots of little tricks and quirks that individual practitioners have come up with on their own (I've seen really interesting pivots on knuckles and pen twists among other things) but the theory is very much the same- that your motion is generated further up on the arm and shoulder and therefore it saves your fingers and hand from fatiguing.
If you want some of my sources or just to talk shop about the theory of how it works feel free to PM me as this is a topic I've become very invested into for my calligraphy pursuits. Like I said, I'm not an expert by any stretch, but I think I'd at least be able to point you in a direction.
