Isn't it interesting that we all have personal ways of expressing ourselves through handwriting? Some people who grew up in the middle of the 20th century keep the classic models handed (oy--no pun intended) to them by their early training, some evolved through the stylized fads of adolescence, and some have morphed into a scrawl that can only be understood by a select few. Some people have very small handwriting, and some people have very large handwriting. People use their fingers to hold writing instruments in different ways: one grip doesn't work for everybody.
Penmanship often depends on the quality (or type) of pen (or pencil) we use. For some handwriting geniuses the tool doesn't matter. I only write well when I use one of a few specific fountain pens or one of a few specific kinds of pencils. Everyone has a spectrum of neatness. Some people (even the very young) can be both neat, quick, and elegant without appearing to think about it, and some adults simply cannot write in a hand that looks "grown up."
As the title of this post suggests, I think there is a direct correlation to the way a string player uses the bow. With bowing there is a also continuum of neatness. It is possible to bow mindlessly, paying attention only to whether the bow is going up or down, and it is possible to bow with extreme mindfulness, using microscopic differences of speed and pressure to make nuances and generate efficient musical energy. The bow itself also matters a great deal. A great bow can offer worlds of musical motivation simply by the way it moves, the way it contacts the string, the kind of sounds it can produce, and the way it feels in the hand. A pen or pencil can offer a similar kind of motivation for expression: the way it moves, the way it contacts the paper, the way ink flows from it, and the way it feels in the hand.
The best bow arms look and "feel" like they are generating Spencerian script.
Habits of good "bowmanship" can be taught to very young children (but they can still play mindlessly), and extremely mindful use of the bow can still result in playing that is less than elegant if the motions are not yet integrated into a player's musical physiology. It takes time and work to develop the muscles that constitute a good bow arm, and it takes constant attention to keep that developed bow arm behaving properly.
Then there's the question of having something to say!