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Muscle Fibers - Slow Twitch, Fast Twitch, What's The Deal?
Posted by: Jon
One evening, while looking for new bodybuilding magazines, I ran across the September 2000 issue of Scientific American. What caught my eye was the cover which said "Muscles & Genes - Are Star Athletes Born, Not Made?" Inside I found an article (by Jesper L. Andersen, Peter Schjerling and Bengt Saltin) discussing the different types of muscle fibers. This is definitely vital information to understanding your body. Unfortunately, most people do not even begin to try understanding why this stuff matters. I'm not a scientist, so I will just give a brief synopsis of the article. Below you will find links to find more information on this subject.
The first step to understanding your muscles is to know how they are made up. Muscle is simply a bundle of cells being held together by a connective tissue called collagen tissue. Each muscle cell (also known as a muscle fiber) has an outer membrane. Scattered around under the membrane are some nuclei which contain the genes. Also in each muscle cell are thousands of strands known as myofibrils. The myofibrils are what cause the muscle cells to contract.
Now down to the real nitty gritty, keeping in mind we are down to the inner strands of a single muscle cell. The myofibrils are made up of filaments, linked end to end, called sarcomeres. The sarcomeres are made of two filamentary proteins known as myosin and actin. These two proteins interact to cause the muscle contraction. One of the components of the myosin molecule (the heavy chain) makes up the characteristics of that muscle fiber. This heavy chain can exist in 3 different "isoforms". These are I (slow fibers), IIa and IIx (fast fibers). The contraction speed of the type IIx fiber is approximately ten times that of the type I fiber. The type IIa fibers' contraction speed is somewhere between the other two. Slow fibers rely on efficient aerobic metabolism such as cardiovascular exercise. Fast fibers rely on anaerobic metabolism such as weight training. In reaction to rigorous exercise, nuclei are recruited to make additional myofibril in each muscle fiber. Therefore, each muscle fiber becomes larger.
Note: There are also hybrid fibers that contain two different isoforms. However, hybrid fibers are dominated by one of the two isoforms and take on that particular characteristic.
So what does this all mean? Well, the article points out that "The 'average' healty adult has roughly equal numbers of slow and fast fibers..." However, the extraordinary athletes are found to have a domination of one fiber type over the other. For example, sprinters may have up to 95% fast fibers in their legs, whereas marathon runners may have up to 95% slow fibers in their legs. As a bodybuilder you obviously want to train to make all of your muscle fibers larger, thus gaining the maximum amount of muscle size possible. So you need to train so that ALL of your muscle fibers will react with this growth.
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