The largest muscle in the back is scientifically known as the latissimus dorsi. You might see it referred to as the "lats" or "wings." The lats are used any time you pull something. For example, when you open a fridge or a car door, you are primarily using your lat muscles. The lat pulldown is a great way to target the back muscles. The following are instructions for performing this exercise:
If the machine has leg pads, adjust them as needed. First, grasp the lat pulldown bar. Your grip should be about shoulder width. Sit down in the seat with your arms extended. Pull the bar down to your upper chest/neck area. Be sure to keep an erect posture (chest high with lower back arched). Slowly allow the bar to ascend to the starting position. Be sure to control the speed of the bar going up. Stop the momentum of the bar just short of allowing the elbows to fully extend.
Grasp lat bar with shoulder width grip. Sit down in seat.
Maintain erect posture with chest high. Pull lat bar down to neck/chest area.
Allow bar to ascend. Control momentum.