I use blend shapes a lot for my muscle deformations now. For those, I'll usually set up as simple of a polygon cylinder or sphere as I can use, and then shape that into the appropriate shape for the area. An important thing I learned, is to make sure and try to have the geometry for your blendshape base shape to be as close to the skin as possible. Otherwise, if you have a gap between your blend shape and your skin, it starts to make the skin move not quite how you would expect.
Like if you have a 2 cm gap between the muscle and the skin, when that muscle rotates, the gap between that and the skin will cause the skin verts to move even further along that arch, possibly making them collide with like the neck verts, for example on the deltoid blend shape.
But yeah, I'll either rigid bind the muscle shape to a joint, like say the bicep to the upper arm joint, and then create a simple flex and relaxed blend shape. Then just have that driven through a set driven key by the rotation of a joint above or below it.
Otherwise, sometimes I'll soft bind a poly influence to 2 or 3 joints, and adjust the weights so it is more or less bending with the joints the way I want it. Of course, it is still going to be collapsing in on itself some and all of that, so I'll go back in and make a blendshape to make the muscle thicken up and seem to contract or flex or whatever, so that it isn't bending over on itself so much...
Like with the deltoid, for example, I'd skin it to the upper arm bone, the clavicle, and the top ridge of the scapula, as it inserts to all of these points. Of course, when you rotated the upper arm bone like the character is raising his hands, the deltoid shape is going to collapse in on itself a bit on the top, so to fix that, I'll just make a blendshape target, move the joints into that position, crank your blendshape slider all the way up to one, and start tweaking on your targe geometry and watch the actual blend influence verts move as you move the verts on the target object, which has been moved off to the side.
Lemme know if you'd like a scene example of this Paul and I can maybe show you a bit more clearly what I'm talking about if you want.
