The widest muscle on your back
Latissimus dorsi is Latin for 'widest of the back.' Most people just say 'lats.'
It starts low on your back, near your hips. Then it fans up across your back, under your armpit, and ties onto your upper arm bone.
When someone's lats are working hard, they can look a little like wings. That's why we call them the back wing muscles.
Pull down, pull back
Try it: reach both arms up high. Now pull your elbows down to your sides, like you're pulling down on a rope.
Feel the sides of your back, under your armpits, get firm? Those are your lats.
Your lats pull your arms down and back. That's the move for climbing a ladder, pulling yourself up on the monkey bars and rowing a boat.
Swim, climb, pull
Every time a swimmer pulls an arm through the water, the lats do much of the pulling.
Climbers use them to pull their bodies up the wall. You use them when you pull a wagon or tug a rope in tug-of-war.