Arms Stretching - Preacher Bench Passive Arm Extension.
Muscles involved: Biceps brachii, brachioradialis
While on the preacher bench, hold the bar with a supinated grip (palms facing upward), lean your elbows against the pads and let the arms extend until they reach the point of maximum extension.
To finish, do not simply return the weight up by flexing the elbows, but rather get up from the bench completely, so that you are more comfortable and it is less compromising for the joints.
Remember, this is a stretching exercise and you should not load the bar with too much weight. The extension should be slow and controlled, because otherwise the joint could be damaged at the bottom of the movement; perhaps not the biceps itself, but the olecranon, the joint capsule, the humeral artery or certain ligaments.
For those people who find that the regular bar (~ 8 to 10 kg) is too heavy, look for a lighter bar, but never use dumbbells because it would be much harder to maintain full supination of the forearms, which is necessary for stretching the biceps.