The underhand-grip lat pulldown improves strength in the back, biceps, and forearms while engaging the biceps more than a traditional overhand-grip does.
Sit on a bench facing an adjustable cable machine with a lat pulldown bar positioned on a high setting. With your back straight, grab the bar with an underhand grip, your arms fully extended and your hands shoulder-width apart.
Pull your shoulder blades down and back, bringing the bar to your chest. Pause and then return to the starting position.