dtoh wrote:P.S. Per Gray's Anatomy, "Each interosseous has a considerable ability to rotate the digit at the [MCP] joint.
Thanks for this - I found the passage and it is talking about the palmar interossei, i.e. the ones that adduct the fingers or bring them back into contact with one another. Obviously if you have to adduct the fingers in order to get them to rotate it defeats the object - but maybe if you have really strong dorsal interossei they will hold the fingers apart against the action of the palmar interossei, which will then produce rotation only (as they do if you hold the fingers apart with your other hand while trying to pull them back together). Maybe that is what you've managed to achieve through your exercises.
I also found an extract saying that the inability to spread the fingers with the hand fully closed is due either to the interossei not being able to shorten (because their ends have been brought too close together by the flexion and they can't get taut) or to the line of pull changing due to the changed hand position. This suggests that it is not to do with ligaments and tendons. At any point where there is some ability to spread the fingers, strengthening the dorsal interossei is likely to help. Since the basic action of these muscles is to abduct straight fingers, the best way to strengthen them may be to practice making the Spock sign (and variants with other finger) with elastic bands around the fingertips.
This by the way from Yates, on the question of positioning the fingers diagonally to the frets: "A rotated hand position may be regarded as the default for playing in the lower position (a parallel position being used only when absolutely necessary)". The main reason given is that it reduces the need to separate the fingers laterally.