The best exercises for a muscle group are the ones that place the targeted muscle group in a loaded stretch position prior to the concentric (lifting) portion. A stretched muscle is a recruited muscle. This is especially true for biceps.

We can also increase biceps activation by increasing its stabilization role while an arm flexion movement is being performed. An example of this is a single-arm barbell (not dumbbell) curl. The long bar increases the need for stabilization, and that action is provided by the biceps which will act as a static supinator.

Based on that, the best arm flexor exercises are:

Incline Dumbbell Curl

Incline dumbbell curl. Fully stretch the biceps at the bottom position. Don't rotate your arms. Start in a supinated (palms up) position and curl that way too.

Incline Hammer Curl

Incline hammer curl. Again, fully stretch the biceps at the bottom position. Don't rotate the arms. Start and end in a hammer grip position.

Low-Pulley Curl

Low-Pulley Curl

Low-Pulley Hammer Curl

Low-Pulley Hammer Curl

Behind back low-pulley curl. Again, aim for that maximum stretch. This can also be done with the hammer grip.

One-Arm Barbell Curl

One-Arm Barbell Curl

One-Arm Barbell Preacher Curl

One-Arm Barbell Preacher Curl

One-arm barbell curl. Very important to keep the bar perfectly parallel to the floor. Try this one on the preacher bench too.

You'll notice that in all of these exercises (except the hammer variations) the wrist is either extended or neutral, to focus more of the stress on the biceps. That's not to say that there aren't any other great biceps exercises, but from experience, these are the best ones out there.

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