A Closer Look at a Marathon Runner’s Muscle Fibers

Do you know the difference between fast-twitch and slow-twitch?

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When it comes to physiological movers and shakers, the musculature system is king. More than 600 muscles in your body work to create motion and force. They allow your heart to beat, your eyes to move, your food to digest, and your legs to run. The three main types of muscle fibers are: cardiac, smooth, and skeletal. While the cardiac muscle makes your heart beat and the smooth muscle lines your intestines, pushing food through your system, the skeletal muscle plays the biggest role in human locomotion. Skeletal muscles make running possible.

Not only are the skeletal muscles responsible for generating physiological movement, they are also where the majority of energy is stored. These muscles include slow-twitch fibers and fast-twitch fibers, the latter of which has several subcategories. Each muscle contains both types of muscle fiber, which are bound together like bundles of cable, each bundle consisting of a single type. Thousands of these bundles constitute a muscle, and each individual bundle is controlled by a single motor neuron. The motor neurons are located in the central nervous system, where they work to control muscles, and in turn, movement.

Altogether, the fibers and the motor neuron make up the motor unit. Since each bundle contains only one type of fiber, a bundle of slow-twitch fibers and a bundle of fast-twitch fibers will receive information from the brain via separate motor units. If one motor neuron is activated, a weak muscle contraction occurs. If multiple motor neurons are activated, however, a more powerful muscular contraction is created.

Why is all this important? Ultimately, the structure of the skeletal muscle system dictates marathon ability. The better understanding you have of your own physiology, the smarter your training will be. Let’s look more closely at the muscle types.

Type I Fibers (Slow-Twitch Fibers)

Your family tree plays an important part in determining your marathon potential. If your parents endowed you with an abundance of slow-twitch muscle fibers, you have a leg up on the competition. These slow-twitch fibers, also called type I fibers, are particularly important for endurance events because of their efficient use of fuel and their resistance to fatigue. Slow-twitch muscle fibers are aerobic, which means they use oxygen to transfer energy. This is a result of their large capillary area, which provides a much greater available supply of oxygen than fast-twitch fibers. Additionally, these fibers have the machinery necessary for aerobic metabolism to take place. Known as the mitochondria, this machinery is often referred to as the “powerhouse of the cell.” Thanks to the mitochondria, you are able to use fats and carbohydrates as fuel sources to keep your body running.

True to their name, the slow-twitch fibers also have a slower shortening speed than the other types of fibers, which serves an important function for endurance runners. While these fibers cannot generate as much force as the others, they supply energy at a steady rate and can generate a good amount of power for an extended period. They also have smaller motor neurons, which require less neural impulse to make them contract, so the slow-twitch fibers are first to start contracting when you begin running. In addition to being slower to contract, type I fibers are only about half the diameter of fast-twitch fibers. Although they are smaller and slower, they are also more efficient and persistent, warding off fatigue during a long haul on the roads.

Type II Fibers (Fast-Twitch Fibers)

Fast-twitch fibers, also known as type II fibers, are also genetically determined and are the slow-twitch fibers’ more ostentatious counterpart. They are bigger, faster, and pack a powerful punch, but they also fatigue rapidly. Since these fibers have very few mitochondria, they transfer energy anaerobically, without the use of oxygen. These forceful contractions use such large amounts of adenosine triphosphate (ATP), basically a high-energy molecule, that they quickly tire and become weak. That is precisely why an Olympic 100-meter champion can run a record-setting pace for the length of the homestretch, while a marathon champion can maintain record-setting pace for 26.2 miles. Two different muscle-fiber types; two different results.

The type II fibers are further divided into subgroups. Two of the most common are type IIa and type IIb, also known as the intermediate fibers. The type IIa fibers share several characteristics with slow-twitch fibers, as they have more mitochondria and capillaries than other types of fast-twitch fibers. As a result, type IIa fibers are considered to be aerobic, although they still provide a more forceful contraction than slow-twitch fibers. By contrast, type IIb fibers contract powerfully, transfer energy anaerobically, and fatigue quickly. See the table below for a brief comparison of fiber types.

Contraction Time Slow Fast Fastest
Fatigue Resistance High Medium Low
Force Production Low High Highest
Mitochondria Density High High Low
Capillary Density High Medium Low
Oxidative Capacity High High Low

A Working System

All humans have both type I and type II muscle fibers, but the distribution varies greatly. Most people, regardless of gender, have a type I fiber distribution of 45–55 percent in their arms and legs. Individuals who are fitness conscious, but not completely devoted to training, can see a type I distribution of around 60 percent. Meanwhile, trained distance runners tend to have a type I distribution of 70 percent, and elite marathoners have an even greater percentage than that. Herein lies the challenge. When it comes to running a marathon, Runner A, who has a high proportion of type I fibers, will naturally be better off than Runner B, who has a low type I and low type IIa distribution. So how does Runner B get around his own physiology?

Luckily for both runners, the body is an amazing machine capable of adapting to myriad stresses. In the field of exercise physiology, “stress” denotes the repeated and intense training that leads to certain physiological adaptations. Researchers have long sought the key to muscle-fiber conversion, hoping they might discover how a person like Runner B could actually change the composition of his or her muscles via training stress. Although much of the research remains inconclusive, it is agreed that elite distance runners have a greater proportion of type I fibers than the average recreational runner, and that those type I fibers are necessary for a fast marathon performance. (See the table below for a comparison among different types of runners.) What we don’t know is if you are genetically bound to a particular muscle-fiber arrangement or if you can change it with physical training through certain training stresses. Although it may be too early to make any definite statements about conversions from type I to type II fibers, it has been shown that transformations can take place within the type II fibers. Even after a relatively short training block of 10–12 weeks, a runner can display a transition from anaerobic, fatigable type IIb fibers to the more aerobic, fatigue-resistant type IIa fibers. This is great for an endurance runner. It shows that training elicits tangible physiological changes that create performance advantages and real improvements. There is much hope for Runner B.

Sprinter 20% 45% 35%
Sedentary 40% 30% 30%
Average Active 50% 40% 10%
Middle-Distance Runner 60% 35% 5%
World-Class Marathoner 80% 20% <1%

Maximizing Muscle Fibers

Regardless of genetics, training remains a vital predictor of running performance. To get your muscles to respond the way you want them to on race day, you must train them to fire in a particular manner. It all starts with a signal sent from the motor units in the central nervous system, which begins by recruiting the slow-twitch fibers. You continue to rely heavily on those fibers unless one of these three things happens:

  • You increase your pace.
  • You encounter a hill or another force that creates resistance.
  • You run long enough to exhaust the slow-twitch fibers.

Depending on fitness level, some runners can go an hour at a modest pace before they begin to recruit the fast-twitch fibers; others can go up to two. It is likely that you’ll rely on type I fibers almost exclusively during the first half or so of the marathon. As those fibers tire, the body will begin to employ the type IIa fibers, those slightly larger, aerobic fast-twitch fibers. If you have trained properly, you’ll have enough leeway to get through the rest of the marathon using these fibers. While they aren’t great for endurance running, they are a good substitute for the exhausted type I fibers. Issues arise when the undertrained runner is forced to go to the third line of defense: type IIb fibers. Remember, these are built for power, and they fatigue quickly. If you are relying on these fibers to get you to the finish line, things will not end well.

What the Hansons Marathon Method seeks to do is teach you how to maximize the use of the type I and type IIa muscle fibers, without having to resort to the type IIb fibers. While genetics dictates what kind of work you may be innately suited for, the right training helps you maximize your individual potential.

Adapted from Hanson’s Marathon Method by Luke Humphrey with Keith & Kevin Hanson, with permission of VeloPress.

Hansons Marathon Method, 2nd Ed.