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The muscular works to control the movement of our body and internal organs. Muscle tissue contains something called muscle fibers.

Muscle fibers consist of a single muscle cell. They help to control the physical forces within the body. When grouped together, they can facilitate organized movement of your limbs and tissues.

There are several types of muscle fiber, each with different characteristics.

Skeletal muscles are made up of individual muscle fibers. And like muscles themselves, not all muscle fibers are the same. There are two types of skeletal muscle fibers, fast-twitch and slow-twitch, and they each have different functions that are important to understand when it comes to movement and exercise programming.

Slow-twitch muscle fibers are fatigue resistant, and focused on sustained, smaller movements and postural control. They contain more mitochondria and myoglobin, and are aerobic in nature compared to fast-twitch fibers. Slow-twitch fibers are also sometimes called type I or red fibers because of their blood supply.

Fast-twitch muscle fibers provide bigger and more powerful forces, but for shorter durations and fatigue quickly. They are more anaerobic with less blood supply, hence they are sometimes referred to as white fibers or type II. Skeletal muscles contain both types of fibers, but the ratios can differ depending on a variety of factors including muscle function, age and training.

Skeletal muscles contain both types of fibers, but the ratios can differ depending on a variety of factors, including muscle function, age and training. If you are a sports performance specialist, it’s crucial to remember the differences between the two muscle types.


The two types of skeletal muscle fibers are slow-twitch (type I) and fast-twitch (type II). Slow-twitch muscle fibers support long distance endurance activities like marathon running, while fast-twitch muscle fibers support quick, powerful movements such as sprinting or weightlifting.


Slow-twitch muscle fibers have high concentrations of mitochondria and myoglobin. Although they are smaller than the fast-twitch fibers, they are surrounded by more capillaries (1,2). This combination supports aerobic metabolism and fatigue resistance, particularly important for prolonged submaximal (aerobic) exercise activities.

Type I fibers produce less force and are slower to produce maximal tension (lower myosin ATPase activity) compared to type II fibers. But they are able to maintain longer-term contractions, key for stabilization and postural control (1,2).


  • Small muscle fibers
  • Low, slow force
  • Fatigues slower than fast-twitch, type II
  • Long-term contractions
  • Supports fatigue resistance for aerobic activities, stabilization and postural control


Fast-twitch type II muscle fibers are further divided into Type IIx and Type IIa.

Typically, these have lower concentrations of mitochondria, myoglobin, and capillaries compared to our slow-twitch fibers, which means they are quicker to fatigue.

These larger-sized fibers are also produce a greater and quicker force, an important consideration for power activities .

Type IIX (also known as Type IIB) fibers produce the most force, but are incredibly inefficient based on their high myosin ATPase activity, low oxidative capacity, and heavy reliance on anaerobic metabolism.

Type IIA fibers, also known as intermediate muscle fibers, are a mix of type I and type IIx, with comparable tension. Able to use both aerobic and anaerobic energy systems, these fibers have a higher oxidative capacity and fatigue more slowly than type IIx.


  • Large muscle fibers
  • Greater and quicker force
  • Fatigues faster than slow-twitch type I
  • Two types: Type IIx and Type IIa
  • Type IIx produces the most force but inefficient (fatigues very fast)
  • Type IIa is a mix of type I and type IIx muscle fibers (fatigues slower than Type IIx)

  • Short-term contractions
  • Supports power activities


So now that we’ve covered the different types, are you wondering what type you are? What type of muscles are in your hands, biceps, legs, chest and arms?

You and your muscles are not comprised of one type of muscle fiber. All of your muscles are a mix of fast-twitch and slow-twitch muscle fiber types (1).

Whether you have more of type I or type II depends on your activity level and age.


Nonathletic individuals have close to a 50/50 balance of fiber types.

When you start looking at highly skilled, top-performing athletes, some differences may begin to appear.

Power athletes have a higher ratio of fast-twitch fibers (e.g., sprinters 70-75% type II), whereas for endurance athletes have more slow-twitch fibers (e.g., marathon/distance runners 70-80% type I) (2).


Age is also a factor for our muscle fibers.

Aging causes a loss in lean muscle mass, with a decline in our fast-twitch fibers, especially the type IIx, but there is also an increase in our slow-twitch fibers.

Recall that the fast-twitch fibers are larger in size than the slow-twitch and are metabolically efficient fibers. Thus, loss of lean muscle mass can contribute to age-related metabolic dysfunctions, body composition changes, even an increased risk of falls.

Resistance training can combat this decline.


You can modify fiber types through exercise.

Type I muscle fibers can be developed through endurance training, such as low resistance, high repetition, or long duration, low intensity.

Type II muscle fibers can be developed through strength training.

Resistance training increases the size of both type I and type II muscle fibers. Greater growth (i.e., hypertrophy) occurs in type II fibers and increases actin and myosin filaments. This results in an increased ability to generate force.

Fast-twitch fibers can also recruit slow-twitch fibers: endurance training at high-intensity intervals can be effective in improving aerobic power.

Tapering during training programs (reducing volume and intensity), can also improve the strength and power of type IIA fibers without decreasing type I performance.

Key takeaways

Fast twitch muscles are optimal for short, quick bursts of energy. Slow twitch muscles are better for long-term endurance activities and can improve your heart health.

Working out both can give you a wide variety of activities to choose from and increase your overall health and strength.

Source / Healthline

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