It's the interosseous muscles that adjust intonation. Not only are they the only muscles practically available for the job they are also the most sensitive - Arnold Schultz would smile!
That the physics of shortening the strings to change the pitch overrides consideration for placing fingers comes as a little revelation. The fingers can never sit comfortably on the fingerboard (Chopin called it conformation) - all must be placed. I mean, unlike the piano, there is no position where the interosseous muscles are not in use. This is fascinating - who was a great violinist? Bach. How do you play Bach on keyboard? Lots of interosseous involvement!
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