By: Wayne Mercer

Most of you have heard the terms type one and type two muscle fibers, also known as fast twitch and slow twitch, but do you really know what those two terms are referring to? In general, the two terms refer to the two types of muscle fibers present in every mammal, which are responsible for the composition of the muscle. The two types of muscle fibers are as inverse in physiological properties, as they are in the performance that the two yield. By now you are probably wondering which type of muscle dominates your physique. There is actually a very simple way to tell whether you are made up of dark or white meat.

So What Are You Talking About?

I refer to the two as white and dark meat because, that's actually what the two are. Perhaps one of the main differences is the difference in the rate of full contraction for the two different types of muscle. It takes about two and a half times longer for a type one fiber to fully contract, when compared to the full contraction rate of type two fibers. The full contraction rate for a type one muscle fiber is about 150 milliseconds, while that of a type two is about 65 milliseconds.

I will explain a little more about type two muscle (fast twitch) first. Type two fibers are able to exert more force and greater energy, but with much less endurance than type one muscle. Fast twitch muscle fiber is also more complex than type one. There actually two different types of type two muscle. The names of the two different types are designated as Type I I a, and Type I I b. The only difference between the two is that Type II a have more endurance than the Type II b variety.

Nevertheless, no matter what version of type two muscle you are talking about, type two muscle fibers are also larger than type one, and respond better to resistance training than the type one fiber. This is why many people are able to train and become stronger, but stay literally identical in size. So if you excel at short burst or sprinting type activities, then you are probably a white meat bird.

Type One

Type one muscle (slow twitch) is not capable of the type of force exerted by type two fibers, but retains endurance well beyond that of the type two (fast twitch) muscle fiber. Type one muscle is the darker of the two types because of the additional oxygen, mitochondria, and myoglobin (oxygen carrier in muscle) it contains. These are the three factors, which are responsible for the performance characteristics of the muscle fiber as well. This is also the reason a bird possesses both white and dark meat; they have two types of muscle fibers as well.

Humans, not unlike our feathered friends, are made up of mostly type one fibers in the areas that are responsible for doing endurance type of work, the most common area of type one fibers being the calves and lower legs. The type one fiber is also smaller than that of the type two. I personally didn't realize how much type one muscle fiber I have, in my arms of all places, until my recent workout partner of 175 lbs began to stack on more plates than I do while doing curls. I even have an extra 1.5" in my arms than he does! So if your endurance shadows over your strength, then you are more than likely a dark meat bird.

The fact of the matter is that genetics plays the primary role in what type of fiber makes up your muscle composition, and we are actually neither a white meat bird nor a dark meat bird. What type of fiber dominates your muscle is as dependent on genetics as height, eye, or hair color. There is some hope though. Research leads us to believe that participating in exercises or sports that rely on a certain type of fiber can lead to an increase of that particular fibers development. Although the increase in fiber development isn't equal to the amount of effort put into the cause, you will gain some of the fiber type desired. Once again, there is always a genetic plateau.

As always, I hope that you have enjoyed this rather intriguing article, or at the very least educational article, and if you have any questions let me know. All e-mails are welcomed, and I will get back to you with all possible speed.

Thanks,

Wayne Mercer