Sit on curl machine placing back of arms on pad. Grasp lever handles with underhand grip. Align elbows at same pivot point as fulcrum of lever.
Raise lever handles toward shoulders. Lower handles until arms are fully extended. Repeat.
Seat should be adjusted to allow arm pit to rest near top of pad. Back of upper arm should remain on pad throughout movement. If machine has secondary lever (as shown), it is not as essential to position elbows in-line with the primary fulcrum, as would otherwise be required. 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.
Long head of Triceps Brachii is only significant activated as antagonist stabilizer when it approaches passive insufficiency as elbow nears full flexion. Also see movements performed on machine with no seat and arms positioned higher.
Antagonist Stabilizers (see comments)