As much as I've avoided the prospect of making a video of myself talking, the time has come in which my verbal explanations of things are not cutting it. In this video, I go through four of the common violin/instrumental stances and how they affect the hips and upper body.
A few notes- I've exaggerated some of the movements just to get the point across. In addition, the pelvis is sort of like Grand Central Station-it's a point of structural and muscular connection between your lower body and your upper body. The tilt of the pelvis affects everything above it, and the way you place your feet underneath the knees affects the pelvis. Thus foot position affects the entire spinal column. As a human being, you want to have the ability to turn your foot (hips!) inwards and outwards- this is important movement to maintain! The externally rotated position itself isn't purely "bad" or "good," but isn't great for consistent standing and walking, as it will begin to affect the spine, gait, knees, etc., and if done asymmetrically, will have huge repercussions for the body.
Which are your most common positions, even if you sit while playing? Which foot do you favor?