Machine curls

Sit at the machine and grasp the bar with an underhand grip, arms extended, and resting on the support:

– Inhale and raise the forearms.
– Exhale at the end of the movement.

This is one of the best exercises for working the biceps brachii. Fixing the arms against the support makes it impossible to “cheat.”
At the beginning, the muscle tension is intense, so be sure to warm up properly using light weights. To avoid the risk of tendonitis, do not completely extend the arm.
This movement also works the brachialis and, to a lesser extent, the brachioradialis and pronator teres.

Comment: When training the biceps brachii using a barbell, take into account variations in each person's physical structure.
In the anatomical position (arms hanging alongside the body, palms facing forward, and thumbs pointing laterally), the angle at the elbow between the upper arm and the forearm varies from person to person. 
Someone whose forearm hangs distinctly away from the body in a valgus position must break excessively at the wrist when performing a curl with a straight bar, which is painful. Therefore, these people should work with an E-Z bar to spare their wrists.