Hammer curl

I think it's down to isolation and correct form. One arm at a time will make you concentrate on just doing it correctly - the isolation of the bicep muscle - and with 2, there's the potential to have a little swing from the hips to move the weight up..

Tell you what though, my younger sister is the expert on this stuff...she swears by this thing called an arm blaster. Apparently, Arnold used to use it. It forces the arms to stay still.

Check it out!

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Probably not but personal preference is one arm

That depends on how you plan to utilize your efforts, doesn't it? Hmu and let's figure this out.

i do it one arm at a time.

I do both at the same time. It's really just down to preference.