All About Fiber Type

by Kris Pitcher

If you took about 40 pencils and held them in a bundle between your hands, then placed a few rubber bands around them to keep the bundle together...that would be a crude example of what a muscle looks like. That bundle we would call a muscle belly. It's made up of fibers, or in this case, pencils. Are you with me?

There are different types of fibers in that bundle. And that my friends is our science geek topic for today - muscle fiber type! There are two main types of fibers.

Slow twitch, or type I, fibers and fast twitch, or type II fibers. We'll start with slow twitch. Slow twitch fibers are very efficient at using oxygen to create energy and they can contract for a long time. They fire slowly and don't tire out easily. Think about your marathon runner, and your Lance Armstrong's of the world. These fibers are all about endurance.

Fast twitch, or type II, fibers use anaerobic (without oxygen) energy to metabolize fuel. They generate short bursts of strength or speed, but these guys tire out quickly. These are the fibers that help us lift heavy and grow big (er).

It gets a little more exciting though. Fast twitch has a further breakdown into fast twitch type IIa, and type IIb. The IIa fibers can use both anaerobic and aerobic metabolism for energy equally - these guys are like chameleons. The IIb fibers use anaerobic metabolism to create energy, they produce quick bursts of power, fire rapidly and tire out quickly. These guys have the highest rate of contraction of all the fiber types. They're like the power kings/queens.

The exciting thing about the subgroup of fast twitch fibers...this is why we train! We genetically have a predetermined number of each type of fiber, slow and fast twitch. And for the average Joe and Jane it's about 50/50. An elite athlete...more like 80% leaning one or the other. Jay Cutler the current reigning Mr. Olympia - fast twitch. Lance Armstrong - slow twitch.

Our propensity for things gives us an indication which way we lean. Our fiber type influences what we're good at. I'm good at lifting weights. I love to train my fast twitch fibers, and you can bet I'm trying to recruit every one of those IIa & IIb fibers to get with the program.

But, if I were training for a triathlon for instance (hey, stop laughing!) I would be trying to recruit my fast twitch type IIa fibers to help me with my endurance sports of swimming, biking, and running. Those chameleon fibers can train either way. Get it? I am standing up typing - I AM THAT EXCITED!!!

This is why we train with purpose, with specificity for our sport, our goals. It's all about fiber type! Whew, how can you not be turned on a little by science? So, get excited about your workouts, you've got fibers to train!