A bowling ball on top
A grown-up's head weighs about 10 to 11 pounds. That's about as heavy as a bowling ball!
Your neck holds your head up all day long. Lots of muscles in your neck and upper back share the job, so you hardly notice.
When you fall asleep sitting up and your head drops forward, that's your neck muscles relaxing.
A new baby's neck muscles are still weak. That's why grown-ups hold a baby's head when they pick it up.
The long name, split in three
Sternocleidomastoid is a big word, but it's really three small words stuck together. Each one tells you where the muscle goes.
Sterno means your breastbone, the flat bone in the middle of your chest. Cleido (KLY-doh) means your collarbone. Mastoid means the hard bump behind your ear.
So this muscle runs from your breastbone and collarbone up to the bump behind your ear.
Try it: feel just behind your earlobe for a hard bump. That's your mastoid, where this muscle ties on.
Turn and nod
Try it: turn your head to the left. Now put your fingers on the right side of your neck. Feel a strong strap stand out? That's your sternocleidomastoid.
When the muscle on one side squeezes, your head turns the other way.
When both sides squeeze together, your chin tips down toward your chest, like a nod. Other muscles in the back of your neck tip your head up to look at the sky.
Neck muscles and screens
When you look down at a tablet or phone for a long time, your neck muscles have to hold your head out in front of you. That's extra work.
Your neck can get stiff and tired. Hold screens up higher, sit back and take breaks to look up and around.