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Muscle Fiber Types

Muscles are made-up of many different types of fibers. However, these various types of fibers are typically classified into two broad categories:

Type I (slow-twitch)
Type II (fast-twitch)

We most often refer to “slow-twitch muscles” and “fast-twitch muscles,” but in reality the differing characteristics of muscle fibers aren’t determined by their structure, but rather by the nerves that control the muscle fibers.

If a “fast-twitch” nerve connects to a muscle fiber then it will act as a fast-twitch fiber. Likewise, “slow-twitch” nerves cause muscle fibers to act at slow-twitch muscle fibers. Studies have demonstrated that by simply switching the input, the muscle fibers will take on different characteristics.

Because of this, it is more appropriate to talk about slow-twitch and fast-twitch motor neurons. A motor neuron is made-up of the nerve and all the muscle fibers it connects to.

Type I motor neurons are characterized by a small number of muscle fibers, which makes them ideally suited for activities which require small amounts of force. The muscle fibers of a Type I motor neuron are characterized by a large amount of mitochondria, myoglobin, and aerobic enzymes. All of these characteristics make Type I fibers excellent for endurance.

Type II motor neurons are characterized by a large number of muscle fibers and a considerable myelin sheath along the nerve, which allows for the electrical impulse to be transmitted quickly. The muscle fibers are characterized by lower levels of mitochondia, myoglobin, and aerobic enzymes than Type I fibers. These characteristics make them better-suited to short-duration activities which require force to be developed quickly.

During most activities motor neurons are recruited according to the size principle which says that smaller motor neurons are recruited before larger ones. This means that slow-twitch motor neurons are recruited first and fast-twitch motor neurons are recruited last. This structure makes intuitive sense, as producing too much force could be very detrimental to your health (e.g. stabbing yourself in the mouth with your fork while eating).

In fact, fast-twitch motor neurons are often difficult to recruit at all under normal circumstances due to protective mechanisms in our nervous system. But under extreme circumstances these protective mechanisms are shut-off, allowing for incredible displays of strength. This is the science behind the seemingly frail mother lifting a car off of her trapped child. When these protective mechanisms are turned-off we finally have access to the full potential of our muscles.

For those who want to build fast muscle however, the size principle is not so beneficial. The fast-twitch fibers have the greatest potential for strength and muscle growth, but since it is difficult to recruit these fibers it can be difficult to make them grow.

Differences in the ratio of Type I to Type II fibers are often cited as one of the genetic reasons why some people seem to be predisposed to have high levels of strength and muscle mass. While this ratio likely plays a role, it is also likely that these people have their protective mechanisms set at a lower level, which allows them to a recruit a higher percentage of Type II fibers.

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