What is a lat pulldown?

In one line

A lat pulldown is an exercise where you sit at a machine and pull a long overhead bar down toward your chest, mainly using the latissimus dorsi in your back.

What is a lat pulldown?
The machine where you pull an overhead bar down
Among the back machines at the gym, there is one where you sit and pull down a long handle hanging above your head. That exercise is the lat pulldown. This post explains what the lat pulldown is and which muscles it uses.
In a nutshell
Sit and pull the overhead bar toward your chest
A lat pulldown is an exercise where you sit at a machine, grab a long handle (bar) above your head with both hands, pull it down toward your chest and let it back up. The bar is connected to the machine's weight by a cable.
What the name means
'Lat' is the latissimus dorsi, 'pulldown' is pulling down
'Lat' is short for latissimus dorsi, the English name of a back muscle. 'Pull-down' means pulling something downward. True to its name, it is an exercise where you pull a bar down to train the latissimus dorsi.
What is the latissimus dorsi?
The muscle that covers the lower back widely
The latissimus dorsi starts around the pelvis and lower spine, spreads widely over the lower back, and narrows to attach to the upper arm bone. Its job is to pull a raised arm back down.
How to measure how hard a muscle works
The electrical signal muscles give off: EMG
Muscles give off a very weak electrical signal when they work. Recording this signal with sensors on the skin is called 'electromyography' (EMG). Studies compare the size of this signal to see how hard a muscle worked.
The muscles it uses
The latissimus dorsi, and the biceps
EMG showed that in the lat pulldown, the latissimus dorsi and the biceps, the front upper-arm muscle that bends the elbow, worked hardest. While the bar goes back up, the latissimus dorsi worked even harder than the biceps.
How it differs from a pull-up
You move the bar instead of your body
In a pull-up or chin-up, you hang from a fixed bar and pull your body up. In a lat pulldown, your body stays seated and you pull the bar down. So you can choose a load lighter or heavier than your own body weight.
Where does the bar go?
In front of the chest, or behind the neck
You can pull the bar down in front of your chest or behind your neck. A review that gathered studies comparing the two found the front-of-chest version was the one most consistently linked to the latissimus dorsi working harder.
A common misconception
Does a wider grip work the back more?
Many believe a wider grip works the latissimus dorsi more. But in studies that changed hand width from 1 to 2 times shoulder width, how hard the latissimus dorsi worked over the whole movement did not differ. Over the whole movement, only the smallest and oldest study found the wide grip ahead. On the way back up alone, it worked harder with the wide grip. With the widest grip, the heaviest weight people pulled down actually dropped a little.
Summary
Pull the overhead bar down toward your chest
A lat pulldown is an exercise where you sit at a machine and pull an overhead bar down toward your chest; it works the latissimus dorsi and the biceps hard. Over the whole movement, a wider grip did not make the latissimus dorsi work more. Related terms: pull-ups and chin-ups, latissimus dorsi, grip.