The champion bone
Your femur is the longest bone in your body. It's also the strongest. It goes all the way from your hip to your knee.
It's about one quarter of your height. If you are 4 feet tall, your femur is about 1 foot long!
Try it: sit down and press along the top of your thigh. Under the big muscles, you may feel the hard bone. Then feel the bony bumps on each side of your knee. That's the bottom of your femur.
A ball at the top
The top of your femur is a smooth ball on a short neck. The neck angles in toward your hip, and the ball fits into a deep cup in your hip bone.
This ball and socket joint lets your leg swing forward and back, out to the side, and turn.
Near the top, on the outside, is a big bump where strong muscles hold on. You can feel it on the side of your hip when you stand.
Strong for its weight
Bone is a clever material. It's hard like stone but a little bendy, and it's light. For how much it weighs, it's amazingly strong.
Your femur is shaped like a tube, with a hard outer wall and soft marrow in the middle. A tube is strong without being heavy. That's why bike frames are made of tubes too!
Every time you jump and land, your thigh bones take a push much bigger than your body weight. They handle it just fine.
Muscles all around
Some of the biggest muscles in your body wrap around your femur. Your quadriceps, on the front, straighten your knee. Your hamstrings, on the back, bend it.
Your glutes, in your bottom, pull on the top of your femur. They push you forward when you run and climb stairs.
Your bones and muscles grow stronger together when you use them.
Growing longer
Your femur has growing spots called growth plates, one near each end. That's where it gets longer as you grow.
The growth plate just above your knee is one of the busiest in your body. It does a big part of your leg's growing.
Your femur keeps getting longer until around the end of your teen years.