You’re Going Too Heavy
Not happy with your biceps? Drop the heavy loads for a while and do 100 reps of empty barbell curls.
Most guys that struggle with biceps growth use really heavy weights and a lot of momentum to curl. Then they do reps in the low ranges and, ultimately, the biceps really don’t end up doing a lot of work or getting a lot of tension. The upper back helps with the initial cheat to start the curl, then the negative portion of the rep is basically just the barbell dropping back down.
Do This Instead
For several weeks, drop all your normal biceps work and try this: grab an empty barbell and shoot for 100 reps. (Or use whatever weight allows you to barely hit 100 reps or close to it.) Do this twice per week on nonconsecutive days.
Even if you get sloppy with 100 empty-bar reps, just making it to the 100 rep mark will have your biceps screaming. If you can’t do the 100 reps nonstop then work towards that goal.
Doing this twice a week, with no other biceps work, caused my biceps to grow a lot in only two months. But like any training stimulus, it’s going to have its effective time limit. Once I could do 200 reps nonstop I didn’t see the same type of growth that I did over the first 8 weeks. But if your biceps growth has been stuck, try it.