Even though barbell curls put more stress on the biceps when they’re stretched out, they don’t make your biceps grow bigger than cable curls — so stretching the muscle under load isn’t enough to...

After ten weeks of preacher curls using either a cable machine or a barbell, young adults who had never trained before gained the same amount of biceps muscle thickness, regardless of whether the exercise placed more resistance at shorter or longer muscle lengths.

Whether you do preacher curls with a cable machine (harder when your arm is bent) or a barbell (harder when your arm is straight), your biceps grow about the same amount after 10 weeks of training.

If you do preacher curls with a barbell (harder when your arm is straight), you get stronger at keeping your arm slightly bent than if you use a cable machine (harder when your arm is fully bent).

If you do preacher curls with a barbell — where it’s hardest when your arm is almost straight — you’ll get stronger specifically at that position better than if you use a cable machine where it’s hardest when your arm is bent.

Whether you use a cable machine or a barbell to do preacher curls for 10 weeks, your biceps will grow about the same amount — the exact way you apply resistance doesn’t matter much for muscle size.