Sit on a preacher bench, holding a dumbbell in each hand with an underhand grip (palms facing up). Position your upper arms on the preacher bench pad and fully extend your arms.
Curl the dumbbells upwards towards your shoulders by bending your elbows.
At the top of the movement, rotate your wrists to turn your palms downward (overhand grip), then lower the dumbbells back to the starting position with control.
Rotate your wrists back to the underhand grip at the bottom, and repeat for the desired number of repetitions.
Form tips
Keep your upper arms pressed against the pad to isolate the biceps.
Focus on a full range of motion for maximum muscle activation.
Perform the exercise slowly to increase time under tension.