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THE 3 TYPES OF MUSCLE FIBERS PERFORM DIFFERENT FUNCTIONS

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Question: We have been having a discussion in the weight room regarding how we should lift weights to develop power for football and other sports. Some of the guys say that fast movements help develop fast-twitch fibers and that slow movements work the slow-twitch fibers more. Our coach said he thought that any heavy lifting worked both type of fibers but wasn't sure which type of movement was best. Would you discuss this topic in your column?

Answer: Before I answer your question, I will review the different muscle fibers so that we will all be on the same level of understanding. Basically, there are three fiber types in humans: Type I, Type IIa and Type IIb. The Type I fibers are red in color and contract fairly slowly compared with the Type II fibers but are quite fatigue-resistant because they have many mitochondria and can produce a lot of energy through the oxidative or aerobic energy system.

Type IIb fibers are light in color (white) and can contract with a great deal of force; but they fatigue easily because they have few if any mitochondria and use anaerobic energy systems for contractions.

The red and white meat of a Thanksgiving turkey illustrate good examples of the two types of fibers described. The legs are red (Type I) because the turkey uses them all day long as it goes around the yard. The breast is white because turkeys fly for only a short time and flight requires a large amount of energy but no endurance.

Type IIa fibers are "in between" Type I and Type IIb. You can see this type of fiber in the breast of a pheasant, who flies farther but still requires a lot of power.

In humans, fibers are recruited according to the need of the activity. For normal, low-resistance activity, we use mostly Type I fibers. If we increase the rate we move or the force we apply, we recruit more Type IIa and finally IIb fibers to match the need. Because of this recruitment pattern, many coaches have thought that high-speed movement is necessary to recruit the Type IIa and IIb fibers. In other words, "to become explosive, you must train in an explosive manner." However, it is clear that these higher type fibers are recruited even at low speeds when the resistance is high.

One problem with lifting weights rapidly is that the momentum of the action decreases the resistance during the last stage of the movement because the bar has momentum and doesn't require the same energy as it required during the initial stage.

A second problem relates to injury. Explosive movements with a large weight can be dangerous. Using any given weight the potential force is directly related to the weight's acceleration, so an increased speed of movement increases the stress on the connective tissue between the bony lever and the muscle.

Although there is no research that clearly tells us whether explosive training is best for developing power, I think that any strength training program recruits the Type IIa and IIb fibers that make an athlete powerful. I think the high-speed, explosive movements associated with practicing the activity are the only explosive movements needed for effective training.