I think we're doing it wrong.
Like many folks on this forum, I've been struggling with barre chords for months. I've gotten to the point where I can do about two 10-minute sets of barre exercises in a two hour period, and then my thumb is DONE.
So, last night as I was massaging the web between my thumb and index finger (I've never had my thumb turn black, fortunately) I got to thinking about what I was doing. When I first unpacked this guitar (Gretsch 5120), I laid my fingers on the neck in an E-form barre at the third fret (Gmajor), and it rang so effortlessly that I was stunned. I knew that I had found MY guitar. So why couldn't I do that all the time?
I decided to just put my fingers on the fretboard as lightly as I could to get a good ring. To minimize ingrained bad habits, I stood up. When I laid my fingers on the fretboard, I found myself not sqeezing with my thumb so much as pulling back with my upper arm and shoulder (which had to be counterbalanced by my right forearm on the body of the guitar). The chord rang, just as it had that first time. I ran through my entire ten minute set of excersises--no cramps. I took a short break to think about what was happening, and did it again-- no cramps. I ran through a set of power chords, some blues rhythms, and did the barres AGAIN-- no cramps.
Finger precision is now the whole ball of wax, since trying to muscle the chord into sounding is out; but that's just a matter of practice.
Boys and girls, I feel like I've found the Holy Grail!