Taking a narrow grip on the bar (shoulder-width or a little inside of that) will more strongly activate the biceps during the movement.
This happens because the narrow grip forces the biceps into a more supinated position than wider grips (supination is the movement where the hands rotate to a palms-up/forward position).
Supination is the other primary function of the biceps and forcing maximal supination increases tension and muscle activation in the biceps.
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