Dr. Andy Galpin

The patella tendon inserts on the front of the lower leg bone, which causes the lower leg bone to lift up and for your lower leg to extend.

That quickly is how muscles work and how they actually cause human movement.

It's through, one, the activation of the muscle, two, the contraction of the muscle, and then three, the muscle pulling on connective tissue, which makes bones move.

Now, the amount of muscle fibers you've got in each muscle varies pretty highly, and it's somewhere in the neighborhood of 1,000 to 300,000 individual fibers.

Interestingly, this number will double during the first few months of your life and then basically stabilize by the time you reach adulthood.

Now, as I mentioned, you've got somewhere between 100,000 and 300,000 fibers per muscle.

One example, the biceps brachii has been shown to have about 250,000 muscle fibers.

And so if you ran some quick math there, that would mean most of us probably have somewhere in the neighborhood of between 125 to 250 million muscle fibers throughout our entire body.

And these fibers are extraordinarily unique in all of biology.

For a couple of reasons.

Number one, they are absolutely huge.

They are some of the biggest cells by volume in all of biology.

It is common to have a muscle fiber that is up to two to three centimeters long.

And in fact, many have been shown to be up to 10 centimeters or so long.

Think about the sartorius muscle, this muscle that goes from kind of the inside of your hip bone, that front side there, and goes all the way to the inside of your knee.

It is theorized, and some folks will say in physiology lore, that you can have a single muscle fiber that runs the entire length of that.

I don't actually know whether or not that's true, but it would not be rare to see a fiber that is, again, four to five to up to six inches long, which is enormous in terms of the width.

Same kind of idea.

You'll see these things as extremely large, right?

Somewhere between four to five micrometers in terms of cross-sectional areas, micrometer squared.