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Muscle Cells are not all the same ...

Are there different types of muscle fibers? Researchers refer to skeletal muscle cells as fibers because they are thin and long. In fact, some muscle fibers can extend the entire length of a muscle group, such as in the biceps. That is several inches! In addition to their unique design, skeletal muscle cells are not all the same. In fact, humans are blessed with more than one type of skeletal muscle cell, which vary in performance and metabolic properties (see Table). This allows us to efficiently perform a broad range of activities or sports that vary in nature. This includes sports that are longer duration/lower intensity and short duration/higher intensity.


What are the different classes of muscle cells?

Muscle cells into grouped into two general categories or “types” (Type I and II). Type II muscle fibers are often classified as IIA and IIX. For this text, it is enough to distinguish between the two main types. Skeletal muscle is actually bundles of a mixture of Type I and II muscle fibers. In fact, the average person will tend to have about a 50/50 mixture of Types I and II muscle fibers. Meanwhile, highly successful athletes tend to have a significant imbalance one way or the other which allows them to excel at a particular sport.

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What are Type I muscle cells?

Type I fibers (sometimes called “slow-twitch” or “slow-oxidative” fibers) are better designed for prolonged exercise performed at a lower intensity. In comparison to Type II fibers, Type I fibers will have more mitochondria and rely more heavily on the aerobic generation of ATP. The primary energy molecules used to generate ATP in these muscle cells will be fatty acids and glucose. Since ATP production in mitochondria requires oxygen, proper function of these muscle fibers is very dependent upon O2 supply via the blood. Based on that need, Type I muscle cells always have capillaries around them to deliver O2-endowed blood. In addition, Type I fibers contain a substance called myoglobin which is an iron-containing protein that binds O2 and serves as an O2 reserve for these cells during exercise.


What are Type II muscle cells? Type II muscle fibers, which are sometimes called fast twitch or fast-glycolytic fibers, can execute a much faster speed of contraction than Type I muscle fibers. This is to say that Type II muscle fibers are designed to generate force more rapidly, thereby allowing them to be more powerful as they will allow a job to be performed in a shorter amount of time. Meanwhile, Type II muscle fibers are relatively limited in their ability to generate ATP by aerobic means. So, when these cells break down glucose to generate ATP in the process, much of the pyruvate that is formed will then be converted to lactic acid (lactate). This is because these muscle cells have less mitochondria and receive less O2 as they are served by fewer blood vessels.


How does the brain know which type of muscle cells to use for different sports?

This is a no-brainer for the brain. This is because the brain will always call upon Type I muscle fibers first and then Type II. The major factor will be the required force to perform the exercise. For instance, when an exercise requires less force (e.g., jogging, fast walking, and casual cycling) the brain will for the most part call upon Type I muscle fibers. However, as the necessary force to perform an exercise increases (e.g., running, cycling fast, weightlifting), the brain will also call upon Type II muscle fibers to generate force to support the force generated by Type I fibers.

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How does recruiting different muscle fibers relate to performance?

Calling upon Type II fibers is sort of a win/lose situation for performance. It is a winner in that it will allow us to generate a lot more force to perform an exercise. However, it is a loser in that the exercise will become fatiguing as more lactic acid, heat and are other by products are produced within Type II fibers which is tied to fatigue or a dramatic reduction in performance potential. This is why 5K runners cannot sprint the entire race. What they will do is run at the highest level that they are able, but that also keeps them from fatiguing before the end of the race. Incremental to type I muscle fibers, their brain will call upon (recruit) enough Type II muscle fibers to generate more force to allow them to run faster, however not enough Type II muscle fibers that would generate critical levels of lactic acid and other factors that would result in fatigue before they cross the finish line.


Do successful athletes have an imbalance of muscle fiber type?

Successful athletes seem to have an imbalance in muscle fiber types that favors excelling in a sport. For instance, successful sprinters often have a higher percentage of Type II fibers, allowing them to generate more force in a very brief period of time. This then allows them to be more powerful, generate more speed, and complete a sprint distance more quickly. Conversely, successful endurance athletes tend to have a greater percentage of Type I muscle fibers. This allows them to generate more force through aerobic energy systems in muscle cells. They can perform at a higher intensity during runs, cycling events, etc before their muscle fatigues.


Are athletes born or developed?

Often the question is asked whether top athletes are born or bred. Said differently, does the athlete pick the sport or does the sport pick the athlete? The answer is both. Most highly successful athletes are born with the genetic propensity to excel physically at a particular sport and training can then improve that potential. This is mostly true for sports that are more extreme in either sport type such as high-endurance based (distance cycling, running) or involve extreme power outputs (100-400 meter sprinters). An athlete’s genes direct the distribution of more of either Type I or Type II muscle cells and body design and potential for skill development to excel at a particular sports. Then to truly excel at a sport, the athlete must train and practice to optimize that performance.


Can training allow muscle fibers to change type?

We do know that training results in changes in muscle metabolism which may make us think that it is possible for Type I fibers to change into Type II fibers and vice versa. However, whether or not this happens definitively is still debated. For instance, endurance training can lead to changes associated with Type II muscle fibers that will make them more aerobic. The fibers will adapt to have an increased ability to generate ATP by using O2. However, they don’t adapt to the point where we would classify them as Type I. Oppositely, we all know that resistance training (e.g., weight lifting) improves the strength and power of a muscle group and some of the adaptation would be measureable in Type I muscle fibers since they will be recruited too. Said differently, the Type I muscle fibers adapt to become stronger but still retain their base properties. 

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