Wow, I can’t believe I missed this thread the first time around. Especially since I based my picking around the great M.M.
Firstly,… I think most guitarists who use a lot of finger movement don’t just rely on the finger muscles alone, since finger are quite weak… and will fatigue very quickly. Mostly, we use the thumb to do the majority of the exertion. The thumb can either work with the index finger, or in my case, the the index finger can act almost as a resistance to the thumb, helping it return to it’s un-flexed position.
But for me, and a few other players here… there is actually more to it that this. The thumb can actually be ‘helped’ by the motion of the wrist & forearm. My forearm rotation actually helps to push the thumb, which then ‘pushes’ the finger. I don’t think M.M. does this… but I’ve seen others who do this. It requires a bit of finesse, and a very loose mechanic. The advantage of this goes back to minimizing fatigue at high speeds.
As far as the tracking from the shoulder vs elbow/above vs the side, I think both have their advantages. I used to use the shoulder from above… and found it very controlled for doing sweeps and for doing ascending/descending scalar stuff. But after switching to X-picking, I have switched to the tracking from the side, more like Andy Wood. I’ve found that shoulder tracking from above is tougher to use for really advanced 1NPS hopping around. For example, string-skipping alternations is awkward to do using my shoulder but works well with just the elbow. I can’t say this for certain, but think pure X-pickers are more likely to pick from the side.