mr dave

a capo isn't really going to help as that's not the purpose of the device / limits the range of the neck / alters the key of the instrument.

the vast majority of the time cramps like this are due to a single thing - POSTURE. the fact that it cramped up into a claw sounds like the culmination of potential years of neglect and poor form. especially if it still hurts a few days later. lots of questions coming your way now....

how many days ago was this gig?
has the pain changed at all since? either in intensity or localized area.

have you had any other issues with your hands or wrists? what kind of work do you do? lots of repetitive motions with little variances?

when you play bar chords is your thumb parallel to the frets (good) or angled out in line with the neck (very bad)?

how far into the set were you when this happened?
how long was your set?
how often do you practice?
when's the last time you practiced before this gig?
what sort of warm-up / stretches do you do prior to performing?
how low do you wear your guitar?

if it's continual pain you should probably check with a doctor or some other health care professional before finding out what a bunch of strangers on the net have to say.

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