Everyone's heard of fast twitch and slow twitch muscles, but what does that really mean? A muscle, like your rectus femoris (one of your quadriceps) is made up of long linear fibers that are packed with contractile proteins that can change shape in such a way as to cause the fiber to shorten in length, when enough fibers shorten at once, the muscle contracts. Muscle fibers must be stimulated electrically in order to contract, and the body accomplishes this in a very efficient manner. In a large muscle that isn't responsible for a lot of subtle movements, up to 2000 fibers can be hooked up to a single nerve cell. This is called a motor unit, and all fibers of a motor unit contract simultaneously with essentially the same force. The degree of strength of any given muscle contraction is based on the number and size of the motor units recruited by the brain for the task at hand.

But all motor units are not the same. Some are relatively small, and can be recruited with a lower threshold stimulus. The fibers associated with these motor units are "slow twitch" or type I fibers and they are rich in myoglobin (muscle's version of hemoglobin), mitochondria and the enzymes that are used in oxidative (aerobic) energy pathways. Because these motor units have a low threshold stimulus, they are called to duty first, and if not much strength is needed for the given task, no further recruitment will happen.

However, if you are calling on your body to do something that requires strength greater than your slow twitch fibers can muster, your larger stronger motor units will be called to action. This will bring your fast twitch fibers or type II into the mix. Fast twitch fibers tend to be larger in diameter and they reach full tension in about half the time of slow twitch fibers. Here is where it gets a little more complicated, fast twitch fibers are also divided into two categories, type IIa and type IIb. Type IIa are fast twitch but they contain moderate levels of myoglobin, mitochondria and oxidative enzymes. The type IIb are low in these aerobic metabolic ingredients and instead are rich in glycogen and glycolytic (anaerobic) enzymes for generating energy without oxygen. In general your individual composition of slow vs. fast twitch fibers is genetically determined. Most evidence points to the idea that no amount of training will change slow to fast or visa versa. However, there is strong evidence to show that training does in fact influence the ratio of type IIa to type IIb.

So what? Slow fibers are "fatigue resistant" due to their dominant use of aerobic metabolism (utilizing fats and not producing lactic acid) and fast fibers are "strong" thanks to their ability to generate great force quickly. So how cool is it to have a big strong fiber that can perform for a long time? It's good, believe me. It's very, very good.

There are many different aspects of fitness to consider , and while the nature of one's musculature is a significant part of the equation, changing the composition of muscle fibers is an adaptation that transpires over years of training. Depending on the type of training you do, you can; 1) increase mitochondrial density and oxidative enzymes 2) increase fiber size 3) increase glycolytic (anaerobic) enzymes and glycogen stores 4) and ultimately change the ratio of type IIa and IIb fibers in your muscles. Most of these changes (except fiber size) happen slowly but are relatively long lasting adaptations, so try to think of them in terms of how you progress as an athlete from year to year, as opposed to the more volatile seasonal changes associated with the type of high intensity riding one does to get in shape for crits or even a Grasshopper. Consider the length and intensity of riding you do in terms of both time frames and remember that every year builds on the previous year.

-Erika Floric