Just like the thread name my right arm always flexing both standing and sitting and its really hard for me to downstrokes and alt picking. I flex really hard but not hard like my arm would get cramps or sth like that
Rest palm on bridge for support & quiet unwanted string noise. Arm rested on guitar body.
The short answer is stop flexing it

Tensing your arm isn't an involuntary action, it's something you're consciously doing, probably in an attempt to play faster. Long term it's something that could potentially cause lasting damage to your arm so yes, it's a habit you want to correct.

Slow down and take a good look at your picking action, I'm gurssing what you're doing is tensing your arm and kind of oscillating it up and down - that's bad. You don't pick with your elbow but your wrist so you need to learn to relax your arm and start training yourself to use the wrist instead. It'll take time to adjust but it will be worth it in the long run.
Actually called Mark!

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