Fast Running Blog

I would like to learn more about muscle fibers. Some questions:

I have heard that fast twitch fibers tend to have a wider cross-section than the slow twitch ones. To what extent is that true?

Is there a correlation between the shape of individual fibers and the shape of the muscle they compose? If yes, to what extent?

I've heard of a study demonstrating that a runner has a distinct fiber shape compared to a biker, but cannot find it anywhere on the Internet. Does anybody have a reference?

I'll have to go get out my old anatomy and physiology book to see.  I hope I can find them, but if not I'm sure someone knows the answer.

Skeltal muscle shapes are based on skeletal muscle fiber shapes.

Fusiform muscles' fibers run parallel with the length of the muscle
-these muscles create greater Range of Motion but less force (contains less muscle fibers for total area).

Pennate muscles' fibers run at angles to the muscle tendon (much like a fibers of a feather).
-contain more fibers for the area, and create greater force but less range of motion.

examples of muscles, their shapes, and how the fibers run:
-longitudinal (sartorius)
-unipennate (tibialis posterior)
-bipennate (gastrocnemius)
-multipennate (deltoid)
-radiate (gluteus medius)

You can look online to get some images of each of these to get a better idea.

Sasha, This isn't directly related to your question. However, beyond the biomechanics and shape is everything that is going on at the molecular level with fuel utilization in the different fiber types. Specifically, I published a paper on Friday in the Journal of Biological Chemistry (Wende et al. JBC 2007; 282: 36642-36651) that shows that when you induce the expression of a single protein (PGC-1α) known to control muscle fiber type composition as determined by the myosin heavy chain isoform distribution, you actually completely change fuel utilization. Specifically, muscle with high levels of this protein have previously been shown to mimic a slow twitch or oxidative fiber with increased mitochondrial content and fatty acid utilization, very similar to that of a trained endurance athlete. What my study added to the field was that this muscle actually reduced the capacity to utilize sugar. In fact glycogen stores were more than doubled and in response to high intensity exercise, known to recruit glucose oxidation, subjects performed worse despite increased mitochondria, glycogen, and fat stores. I bring this up only to point out that a lot more is going on than just the physical structure.
As a caveat, keep in mind that in my study the induction of this gene was not within physiologically normal levels so the blockade may be artificial. In response to normal exercise the expression of this gene is induced about 10-fold and quickly goes back to normal (~6hrs later). In my model the expression was much higher and was always turned on. Despite these limitations, the results point out that a muscle type that looks like it should be a super athlete will not always equate. -Adam

Adam RW, What is your job or what are you studying?

The paper I mentioned was actually for my thesis work at Washington University in St. Louis. It took me that long to get the final chapter published. My “official” title here at The U is American Heart Association Post-doctoral Fellow. It is the Ph.D. equivalent to an M.D.’s residency… I’m currently working for Dr. E. Dale Abel and I’m mostly focused on glucose delivery to the heart. I’m taking three broad approaches to look at this. I’m looking at diabetes, hypertension, and exercise. By comparing these three models and how the heart changes metabolic status in response to each of them we hope to determine treatments that will help normalize sugar and fat utilization in the diseased heart. We are mostly focused at the molecular level and are trying to figure out how changes in mitochondrial substrate utilization affect overall cardiac metabolism and contractile function. I hope to eventually return to studies in skeletal muscle but it will probably be a few more years before it is my mine focus again…

what were your degrees in and from what schools?

I study exercise science at BYU (mostly biomechanics, functional anatomy), and should be starting on a masters somewhere this fall in something related (ex phys, motor control, etc)

Adam, It is funny that the two scientists posting here both have the same name. I'm sure you know more about exercise and physiology than I do. My undergraduate degree was in Biochemistry/Biology double major (Knox College Galesburg) and my undergraduate thesis was actually in plant biology, membrane fluidity in response to temperature changes and how this affects protein translation. My Ph.D. was in Molecular Cellular Biology at Washington University in St. Louis. My understanding of the exercise field is just enough for me to explain my studies in relation to something the general public would actually care about. My real interest is molecular interactions. The majority of my graduate thesis focused on transcriptional regulation (so protein-DNA interactions) I really didn't get into the physiologic relevance of the changes in regulation until my last two years. So I know relatively little about physiology, that is why I'm in Dale's lab now. I wanted to learn more physiology and endocrinology to complement my understanding of muscle at the molecular level... Who are you planning on doing your masters with?

I thought the same thing about the names...my last name starts with an R too, so when I see your RW it is only the W that lets me know somebody isn't stealing my name!

I'm looking at a few different options with the masters right now.

BYU is the most affordable, especially for me, since I don't actually have a state of residence. I would do the MS in ex phys and probably do most of my work in biomechanics (since my research as of late has been biomechanics centered). But I may decide to get into a more biochem related physiology. Dr. Gary Mack is here at BYU and is amazing with it. So it may be fun to work with him a bit.

U of South Carolina, MS in motor control. They have a good program and it is interesting to me.

Western Carolina University, Masters of Physical Therapy. I figure on going on to doing a PT anyway, so I may just get a jump on it now, and then go on to a Ph.D in rehabilitation or sports med. It all depends on where I'm accepted, but these are some of the schools I've applied to or started applications with.