Whether I'm comping with jazz chords or playing power chord rock, my hand always cramps. I don't get cramps, however, when playing leads or open chords. Specifically, I get cramps in the muscle/tendon/fat area between my thumb and index/pointer finger.
Online, I've seen recommendations to supplement with potassium and/or magnesium, stretching beforehand, and playing through pain to build muscle. I tried all of these, but I'm still cramping.
Is there any advice on how I should fret chords? For example, where do I place my thumb and at what angle? How hard should I press between my thumb, fretboard, and fingers? How can I make notes sound clear while minimizing hand pressure? Etc.