After a Berlin Philharmonic concert at Carnegie Hall a few years ago, I ran into a musician from the New York Philharmonic whom I happened to know. Berlin, I think, is all but universally acknowledged to be the world's beset -- and most inspiring -- orchestra, an institution run by its musicians, who show great commitment and great autonomy while they play, not least in the way they move, putting their entire bodies into every note.
"Did you see that?" the Philharmonic musician asked me, almost levitating (as, I think, we all were, from how wonderfully the musicians played). "Did you see how they move? If I moved like that, I'd be reprimanded."
I play much better standing up, whether guitar and singing or flute or horn. Besides being able to breathe better and more naturally, standing up allows me to move as I play. Working with music therapy groups over the years, and now fronting the Kenwood Players, I don't conduct, but lead through gesture, along with verbal instructions half sung with the music.
When I see players sitting stock still when making music it makes me think the music is too abstract and too far removed from the motions and gestures that make us human. Cerebral music, like say a Bach four part organ fugue, usually doesn't engage me (The Art of the Fugue being a major exception).
On the other hand, I think we've all seen people making music with far more physical gestures than needed, as if those gestures can make up for the lack of technique needed for the music being performed. Like everything else in music, a balance needs to be found.