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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Discover the best of Violinist.com in these collections of editor Laurie Niles' exclusive interviews.
Violinist.com Interviews Volume 1, with introduction by Hilary Hahn
Violinist.com Interviews Volume 2, with introduction by Rachel Barton Pine