There are different types of skeletal muscle

Up until now, we have been treating all skeletal muscle as the same. But it turns out that there are different types of skeletal muscle fibers. We are only going to be learning about two types (although more have been characterized). These two types are the same in many ways, like they are both made up of myofibrils like you have already learned. Where they differ is in some smaller details, which allow some to produce force quicker while others maintain contractions longer. You have actually already heard about these two types of muscle...

... think about Thanksgiving. Out comes the turkey. Which part of the turkey do you want? White meat or dark meat? A-ha! White meat is muscle made up mostly of fast muscle fibers, while dark meat is muscle made up mostly of slow muscle fibers.

Your book covers this material on pages 293 - 294.

Fast Muscle Fibers (also called White Muscle Fibers)

	Designed for quick movements (like the muscles in your hands & for moving your eyes)
	More SR in these fibers, so they are better equipped for quick release and re-uptake of calcium ions
	Myosin heads have a slight molecular difference that makes them faster and more efficient at hydrolyzing ATP. Because of this, they can run through the cross-bridge cycle faster.
	More likely to fatigue (due to lactic acid build up)

Slow Muscle Fibers (also called Red Muscle Fibers)

Designed for sustained movements (like in postural muscles)

More mitochondria & myoglobin & a better blood supply. Two major advantages of this:

	rather than making ATP and having lactic acid build up, the pyruvate is better able to enter the mitochondria and be broken down further there... all you need is plenty of mitochondria and plenty of myoglobin.
	slow muscle fibers are less likely to fatigue (because of a slower build-up of lactic acid)

Note that since myoglobin is a red pigment molecule (like hemoglobin is), these fibers tend to look redder (or darker) than fast muscle fibers.

How are these muscle fiber types arranged in muscles?

Muscles have hundreds or thousands of muscle fibers. The muscle fibers that make up a given muscle are typically of both types. Some muscles are really mixed, and are about 50 : 50 in these two types. Other muscles need to be more of one type or another. For example, muscles used in posture, like those in our backs and in our legs for extension (or in chicken legs), need to be able to provide force for extended periods of time. If they fatigued and stopped working quickly, we'd collapse. Therefore, these postural muscles need to be made up mainly of slow muscle, that doesn't fatigue easily. Other muscles, like our finger muscles that allow me to type so quickly, have to have more of the fast muscle fibers so that I can command them to move speedily, but for shorter durations.

Another A&P web page has some information on muscle fibers... if you go to this A&P page from Northland Community College, you can select the slide show presentations on muscle. Slide show "Muscles, part 3" has some information on this topic. You may even just want to browse through some of the other muscle slide shows for a review if you have time.