In explaining the variation of the percentage of slow-twitch
and fast-twitch muscle fibers in humans, Shenk notes how, although on average
there might be a nearly equal ratio of fast- to slow-twitch muscle fibers the quadriceps
muscle, some people can have “as low as 19 percent and as high as 95 percent”
slow-twitch muscle fiber compared to fast-twitch fiber, citing Anderson et al.,
“Music, Genes and Athletic Perfmorance.” Considering the relationship between
structure and function of both fast- and slow-twitch muscle fibers, how might
the disparity between the types of muscle fiber support or refute Shenk’s claim
of GxE?
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ReplyDeleteThe structure and function of both fast-twitch and slow-twitch muscle fibers relate to the type running a person does. “Slow-twitch fibers (also called Type 1 muscle fibers) are slow to contract, but they are also very slow to fatigue. Fast-twitch fibers are very quick to contract and come in two varieties: Type 2A muscle fibers which fatigue at an intermediate rate, and Type 2B muscle fibers which fatigue very quickly“ (http://web.mit.edu/tkd/stretch/stretching_2.html). Given that slow-twitch fibers take longer to fatigue, long distance runners will be more likely to have a higher ratio of slow-twitch fibers to fast-twitch fibers because they need muscle endurance to run long distances. In relation to our biology theme of structure and function, slow twitch-fibers have a higher density of capillaries that surround the muscle and more mitochondria within the muscle, which function to help long distance runners because the denser capillary network allows more oxygen to be delivered to the muscle fibers and the high number of mitochondria produce more ATP. Similarly, because fast-twitch fibers are quicker to contract, short distance sprinters will be more likely to have a higher ratio of fast-twitch fibers to slow-twitch fibers for running faster. This also demonstrates the biology theme of structure and function because the structure of fast-twitch fibers, large diameter and dense myofibrils, function to increase the speed of contraction, which would help sprinters.
ReplyDeleteThe disparity in the types of muscle fiber does support Shenk's claim of GxE. As also stated in footnote 107, “the muscles we are born with are merely default muscles – ready to be re-created in one or another particular direction by active use”(309). Therefore, the ratio of fast-twitch and slow-twitch muscle can change depending on the environment because “the DNA responds with instructions that will lead to the strengthening and enlarging [hypertropy] of each fiber” (310). For example, if a long distance runner created the environment where hypoxia occurred in muscle cells due to a lack of a sustainable oxygen supply for a long period period of time, then in support of Shenk's claim of GxE, genes would be activated that start angiogenesis, which would create more capillaries that supply blood to the muscles that experienced hypoxia. Ultimately, the difference in the ratio of muscle fibers supports Shenk's claim of GxE because depending on the environment, sprinter versus long distance runner, different genes are expressed that create fast-twitch or slow-twitch muscle fibers.
-Edward Wu (edwardwu0@gmail.com)
The structure of different types of muscle fibers relates to their function in an individual as well as among different athletes. Athletes that participate in explosive activities such as sprinting usually have high percentages of fast-twitch muscle fibers, while endurance athletes such as marathon runners usually have high percentages of slow-twitch muscle fibers. In order to generate explosiveness, fast-twitch fibers rely on anaerobic metabolism consuming glycogen stored in muscles. Thus, the muscle cells do not possess many mitochondria are usually light-colored. The fibers are also wider in diameter to allow for more forceful contractions. In contrast, in order to maximize oxygen usage and prevent fatigue, slow-twitch fibers rely on efficient aerobic metabolism consuming myoglobin stored in muscles. These muscle cells require lots of mitochondria for aerobic cellular respiration and are darker and redder than fast-twitch fibers (Campbell 1111). Slow-twitch fibers are also usually surrounded by a lot more blood vessels to supply oxygen. Note that there are also intermediate fiber types that can behave like fast-twitch or slow-twitch fibers (http://www.livestrong.com/article/454134-difference-between-slow-twitch-and-fast-twitch/). The different muscle fiber types relate to the biological theme of structure and function because each type possesses just the necessary components to either fire quickly and generate rapid movements, or fire slowly and use energy as efficiently as possible. To elaborate on a specific example, the slow-twitch fibers need as many mitochondria as possible to maximize the amount of aerobic respiration performed in order to prevent having to resort to less efficient anaerobic respiration; however, the lengthy aerobic respiration process also prevents rapid contraction, which is why slow-twitch fibers are used for less explosive, more time-consuming activities such as walking long distances.
ReplyDeleteWhile I agree with Edward that the disparity in the muscle fibers could support Shenk's claim of gene-environment interaction, I do not believe this arises strictly from the evidence between distinct muscle structures and functions. Rather, as noted by Edward, it is the fact that “the muscles we are born with are merely default muscles – ready to be re-created in one or another particular direction by active use” that supports Shenk's argument (309). A scientist that simply recognizes the differences between fast- and slow-twitch fibers would not immediately decide that different percentages of the fibers among explosive and endurance athletes can be attributed solely to training and non-genetic explanations. For instance, a researcher may notice that a sprinter has 95% fast-twitch fibers in his quadriceps muscle and decide the athlete became a sprinter because his genes endowed him with such a fortunate combination of muscle types. While most scientists agree that muscle fibers can change from one type to the other (especially from fast-twitch to slow-twitch), little is known whether the ratio between the two can actually be changed. In fact, many scientists believe the ratio is genetically-determined, although this may be based on a misrepresentation of the gene-environment relationship. Nonetheless, these scientists argue with evidence that high percentages of one muscle type are due to the behavior of intermediate fiber types that may act like fast- or slow-twitch fibers (http://books.google.com/books?id=qo-vs5-SQC4C&pg=PA57&lpg=PA57&dq=change+slow-twitch+to+fast-twitch&source=bl&ots=2joTXsE0_M&sig=gl5O0Lh6rfUA7mmKYm9I5AW7RdE&hl=en#v=onepage&q=change%20slow-twitch%20to%20fast-twitch&f=false). Thus, the three types of skeletal muscles support Shenk's claims of gene-environment interactions because intermediate fibers behave to meet environmental demands but also counter his claims because scientists believe the relationship between strictly fast- and slow-twitch fibers cannot change after birth.