The muscles there that are used to barre, which is much more demanding on your arm than a power chord, aren't strong enough to do it for prolonged periods of time; you have carpal tunnel, or your technique is off. Those are three possible explanations that I'd look for.
Do you feel warm within your cage?

And have you figured out yet -

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Quote by Hydra150
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im sure that your wrist muscles arent used to it, it took me a while too, just keep playing it and keep making your hand get that exercise burn and eventually you WILL get it
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