Feeling Twitchy? The Difference Between Fast and Slow Twitch Muscle Fibers

A topic that's interested me since taking up cycling several years ago has to do with the muscles used in different sports, and more interestingly, how the very same muscles can be used quite differently. Prior to rediscovering cycling I was a pretty serious soccer player, playing on my college team and continuing at a competitive level after college and graduate school.

After my first year back on the bike -- having started a regular commuting regimen along with longer weekend rides -- I noticed an interesting impact on my performance on the soccer field. On the one hand, I was in better cardiovascular shape than ever and my weight was down (about 20 pounds from the year before and even 10 pounds below my college soccer days). I typically play in center midfield, a position that involves the most running in a sport where you’re constantly moving. Now I felt like I could run forever; like I could play two 90-minute games back to back!

The problem was I felt like I had lost my quickness. My first step seemed slow, and I was getting beat by feints I would have stopped a year earlier (even when I was carrying 20 pounds of extra girth).

What was happening?

A friend of mine suggested I add some short sprints and jumping to my training, and when I did a little research to find some drills I found he was right. Apparently my cycling had done an excellent job developing slow-twitch muscle fibers, whereas soccer demands both slow and fast-twitch fibers.

What’s the difference?

Physiologists now believe that muscle fiber types can be broken down into two categories: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers.

So what?

These distinctions apparently influence how muscles respond to physical activity, with each fiber type capable of contracting in a certain way. Slow Twitch (Type I) are very efficient at using oxygen for continuous and extended muscle contractions over long periods of time. They fire more slowly than fast twitch fibers and take longer to fatigue. These fibers are great for cycling for hour after hour or for running a marathon.

Fast Twitch (Type II) fibers operate anaerobically (without oxygen) to supply short bursts of strength or speed. Think of weight lifters. They don’t typically breathe hard (at least not for long) because the muscles they’re using (1) don’t require much oxygen, (2) fire quickly and are (3) are done doing their work sooner. Fast twitch fibers produce the same amount of force as slow muscles for each contraction, but they fatigue more quickly. Having more fast twitch fibers can be an asset to a sprinter since she needs to quickly generate a lot of force.

But there’s another interesting wrinkle.

Physiologists suggest that human muscles possess a genetically determined mixture of both slow and fast fiber types (around 50% of each). One important question that apparently has not been definitively answered is “can training help an athlete change the fiber type and composition of their muscles?”

There is an interesting sub-class of fast twitch muscles often referred to as Type IIa, also known as intermediate fast-twitch fibers. They can apparently use both aerobic and anaerobic methods to create energy. In this way, they are a combination of Type I and Type II muscle fibers. So, theoretically, an endurance athlete can "recruit" these dual purpose fibers to act more like slow twitch muscles, while a sprinter can use them for fast twitch power and speed.

It's interesting to think about how these types of muscle fibers affect cycling performance at the highest levels. Great all-around cyclists (and team leaders) need to be able to do everything reasonably well -- climb, sprint and endure mile after mile in long multi-stage races. They need both slow and fast-twitch muscles. But the very best sprinters (those who specialize in the last 100 meters of the race) must have explosive power, and therefore well-developed fast twitch muscles. You can see this directly in their body types. Sprinters are often "bulkier" than generalists, who tend to be lean and sinewy looking.

In the end, although training may or may not actually change the fiber type from slow to fast twitch (or vice versa) it’s clear that it does help you more fully realize the potential your muscles have for work (either intense bursts of power and speed, or long, grueling periods of activity).