Types of Muscle Fibers
Imagine that it is Thanksgiving and as you are passing around all of the delicious calorie filled food you are asked the ultimate question ... Would you like white or dark meat? The choice is obvious but have you ever wondered why there is a difference between the two and what is responsible for that difference. Animals have specific patterns of movement characteristic to a particular species. Dark meat is composed of muscle cells that are built for endurance type activities; white meat contains cells built for short powerful movements. Let's think about the turkey. Turkeys and chickens primarily walk/run everywhere they go. While these birds can fly they can only sustain that type of activity for short periods of time. Based on this behavioral pattern, as anatomists we can infer that we will see cells built for endurance in the muscles that are used for prolonged periods of time; these cells compose the dark meat in legs and thighs of these birds. We can also infer that since flight is an activity that cannot be sustained for long periods of time, we will see cells built for quick bursts of energy in the muscle that cause flight; these cells compose the white meat in the breast of these birds. Now, think about a bird of flight like ducks or geese. What kind of meat would you find in these birds?
Now that we have solved that question it brings up another very interesting one, if animals' muscles are classified as dark and white meat, as humans do we have the same classification. We are all animals after all. But unlike the animals we just discussed, humans do not have specific behavioral patterns. You can use one muscle for a variety of actions - you can pick up a feather, then a couch, then hold a book out to someone, then drive, etc. This is why whole skeletal muscles like the biceps brachii are not made up of only one type of skeletal muscle cell, there are in fact 3 different types of skeletal muscle cells that are present in one whole skeletal muscle. There are two criteria to consider when classifying the types of skeletal muscle cells: how fast some cells contract relative to others and how cells produce ATP. The speed of contraction is dependent on how quickly myosin's ATPase hydrolyzes ATP to produce cross-bridges. Fast fibers hydrolyze ATP approximately twice as quickly as slow fibers, resulting in much quicker cross-bridge cycling (which pulls the thin filaments toward the center of the sarcomeres at a faster rate). How cells produce ATP is dependent on the primary metabolic pathway that the cell uses and will determine whether the cell is classified as oxidative or glycolytic. If a cell primarily produces ATP through aerobic (with oxygen) pathways it is oxidative. More ATP can be produced during each metabolic cycle, making the cell more resistant to fatigue. Oxygen debt is the amount of oxygen needed to compensate for ATP produced without oxygen during muscle contraction. The oxidative fibers contain many more mitochondria than the glycolytic fibers, because aerobic metabolism occurs in the mitochondria. Glycolytic fibers primarily create ATP through anaerobic (without oxygen) glycolysis, which produces less ATP per cycle. As a result, glycolytic fibers fatigue at a quicker rate. Using these criteria, the three main types of skeletal muscle cells are: slow oxidative, fast oxidative - glycolytic, and fast glycolytic.
Slow oxidative (SO) fibers contract relatively slowly and use aerobic respiration to produce ATP. The SO fibers possess a large number of mitochondria and are capable of contracting for longer periods because of the large amount of ATP they can produce, but they have a relatively small diameter and do not produce a large amount of tension. SO fibers are extensively supplied with blood capillaries to supply O2 from the red blood cells in the bloodstream. The SO fibers also possess myoglobin, an O2-carrying molecule similar to O2-carrying hemoglobin in the red blood cells. The myoglobin stores some of the needed O2 within the fibers themselves (and gives SO fibers their red color). All of these features allow SO fibers to produce large quantities of ATP, which can sustain muscle activity without fatiguing for long periods of time. The fact that SO fibers can function for long periods without fatiguing makes them useful in maintaining posture, producing isometric contractions, stabilizing bones and joints, and making small movements that happen often but do not require large amounts of energy.
Fast oxidative - glycolytic (FOG) fibers have fast contractions and primarily use aerobic respiration, but because they may switch to anaerobic respiration (glycolysis), can fatigue more quickly than SO fibers. FOG fibers are sometimes called intermediate fibers because they possess characteristics that are intermediate between fast fibers and slow fibers. They produce ATP relatively quickly, more quickly than SO fibers, and thus can produce relatively high amounts of tension. They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly. However, FO fibers do not possess significant myoglobin, giving them a lighter color than the red SO fibers. FO fibers are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement, such as sprinting. FO fibers are useful for this type of movement because they produce more tension than SO fibers but they are more fatigue-resistant than FG fibers.
Lastly, fast glycolytic (FG) fibers have fast contractions and primarily use anaerobic glycolysis. The FG fibers fatigue more quickly than the others. FG fibers primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white color. FG fibers are used to produce rapid, forceful contractions to make quick, powerful movements. These fibers fatigue quickly, permitting them to only be used for short periods.
Figure 5. Types of Muscle Fibers. Please note that Slow Oxidative Fibers are the darkest in color, while Fast Oxidative Glycolytic Fibers are lighter and Fast Glycolytic Fibers are the lightest. Are you able to explain this, based on what you've learned?
As we stated earlier most skeletal muscles in a human possess a mixture of each cell type, although in varying proportions. The predominant fiber type in a muscle is determined by the primary function of the muscle. This is why you can train a particular muscle for a specific activity. If you plan to run a marathon, you must train your muscles to be more fatigue resistant by running a short distance and gradually increasing that distance as the weeks pass by. This training converts FOG and FG cell types in a skeletal muscle into SO cells. In doing this the whole muscle will be more fatigue resistant as a whole and you can then run the marathon. If you stop running the need for all of these SO cells decreases and the extra SO cells will be converted back into FOG and FG cell types.
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