You’ve done every curl variation known to man (of course you have) and now your biceps won’t budge off their current plateau to give you that final coveted inch you’ve been working so hard for.
Banded barbell curls. Loop an exercise band around the center of a barbell. Step on the loose end with one foot and perform a set of curls. The accommodating resistance provided by the band (it’ll get harder to complete the top of each rep) will provide a new stimulus.
Start with one foot; increase difficulty with two.