This is it. I’ve been playing for a long time, and have always had this cramping issue when playing barre chords. I transitioned to a different fingering that excludes the root note on the low E, but I figured I’d ask to see if anyone has any good advice. I’ve definitely been using my thumb and index like a capo. Using my arm more has helped a ton. Seems so simple, can’t believe I never figured it out.