Here is my post on 'Fretted Hand Muscles'.
Several months before I began playing after a long absence, a hand muscle between my left thumb and index finger, just disappeared overnight. Went to a Neurologist who didn't give a reason for this but prescribed a rubber ball to squeeze to rebuild my remaining hand muscles. The muscle that disappeared is gone, so I'm stuck with that. I recently bought a hand exerciser online that allows me to exercise 4 fingers individually by squeezing buttons attached to 4 little springs. This has helped me a lot.
I am still having a lot of difficulty doing a barre and I believe it's the weakness in my left thumb. I am still working with the hand exerciser and playing as much as I can and both things help. These hand exercisers are available in different strengths (springs have different stiffness) and I'm on version #2 (extra light) and ready to move to #3.
Hope someone finds this useful.