Sit on bench. Grasp dumbbell between feet. Place back of upper arm to inner thigh. Lean into leg to raise elbow slightly.
Raise dumbbell to front of shoulder. Lower dumbbell until arm is fully extended. Repeat. Continue with opposite arm.
The long head (lateral head) of biceps brachii is activated significantly more than short head (medial head) of biceps brachii since short head enters into active insufficiency as it continues to contract.