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More of a singer-guitarist, and not much of a guitarist, but over many years, I have tried to figure out lots of issues regarding muscle tension.
Muscles can work for an extended period of time, provided one gives the muscles a chance to relax. That is, if one plays guitar with hands closed, and then doesn’t fully open the hands for an extended period of time, the muscles and tendons for the hand become overly stressed over time, resulting in diseases such as carpal tunnel syndrome. Another example, if one holds bend of left arm for an extended period of time, and later does not extend the left arm fully, then the left arm muscles and elbow tendons and ligaments can become overly taut, later causing injury.
How to stretch these bends or taut hands is a bit more complex. If one has held a bent elbow for 2 hours, the solution is not just to straight stretch the elbow for 5 minutes. If one’s hand is taut for 2 hours, the solution is not just to backstretch the fingers for 2 minutes.
I’ll later describe how to do these at my somewhat currently inactive (7/13/17) blog, VocalPosture.com. Please subscribe, if interested.
As for placement of thumb– my current view is that much depends on how one plays the guitar and the degree of one kinesiology memory. As a singer-guitarist, I’m having enough challenges trying to remember lyrics, finger-picking pattern, vocal methodology, audience engagement….. So, what I think works best is a consistent thumb position, because there is then far less thinking as to what to do in wrist when shifting from chord to chord. That is, whether playing an open A or a bar B, the wrist stays in the same position (not wrapping the wrist thumb on open A and then placing opposite bar on bar B.)
Singing and posture--see Categories, About, Launch 4-2014 