Video: Skeletal muscle fiber types

We stand, walk, run, and lift weights, thanks to our muscles. Actually, thanks to our muscle fibers, the individual cells that make up our muscles. While they share a common basic architecture, muscle fibers have some features that make them more suited for specific movements. And in this tutorial, we'll explore these different types of skeletal muscle fibers. Skeletal muscles have fibers of different types in varying proportions depending on their primary action.

Muscle fibers are often characterized based on speed of contraction and metabolic pathways. In humans, these characteristics range along a continuum. Fibers on opposite ends of the spectrum are called Type I and Type II fibers. The Type II fibers are further classified into Type IIa and Type IIx fibers. Based on the speed of contraction, muscle fibers are classified as slow twitch and fast twitch muscle fibers.

This variation in contraction speed is due to various isoforms or versions of the enzyme myosin ATPase. Myosin ATPase is an enzyme that breaks down ATP to use the energy for muscle contraction. ATP is the chemical energy that powers cross bridge cycling. The higher the myosin ATPase activity, the faster the muscle contracts. Type I fibers have the lowest myosin ATPase activity and are thus known as slow twitch fibers.

They have the slowest contraction speed and the longest twitch duration. On the other hand, Type II fibers are known as fast twitch fibers as they have a high myosin ATPase activity contracting faster and for a short duration. In addition to their speed of contraction, muscle fibers are also classified based on their metabolic pathways. The slow twitch Type I fibers use the slow process of aerobic metabolism via oxidative phosphorylation. Because of this, they are also known as slow oxidative fibers.

Aerobic metabolism takes place in mitochondria and requires oxygen. Muscle fibers get oxygen via diffusion from capillaries and from myoglobin, a molecule that binds and stores oxygen in the muscle sarcoplasm. Thus, Type I fibers have many mitochondria, a large amount of myoglobin, and a dense capillary network. The high myoglobin gives these fibers a dark red color. At the opposite end of the spectrum, Type IIx use the faster process of anaerobic glycolysis, hence their classification as fast glycolytic fibers.

Glycolysis is the breakdown of glucose, and one source of that glucose is stored glycogen in the muscle fiber. This anaerobic metabolism does not require oxygen and takes place in the sarcoplasm. Thus, compared to Type I fibers, these fibers are paler, have less myoglobin, have fewer mitochondria, have a sparse capillary network, and have much more glycogen. In between these two extremes, there are the fast twitch Type IIa fibers, also known as the fast oxidative glycolytic fibers. They use oxidative phosphorylation as their ATP source and can switch to anaerobic glycolysis when required.

They are pinkish red, with more myoglobin, mitochondria and capillaries than the Type IIx fibers, but less than Type I. They also have moderate glycogen stores to use when needed. These structural and biochemical characteristics affect how much tension the different fiber types can generate and their resistance to fatigue. Type I fibers are the smallest in diameter, generate the least tension, and are usually the first to be recruited by the central nervous system. As the force increases, larger fibers that generate higher tension are recruited, Type IIa, and eventually Type IIx fibers.

Of the three types, we've established that Type IIx fibers are the fastest, but that speed comes with a price. They fatigue quickly, unlike Type I fibers, which can be active for long durations without fatiguing. Thus, Type I fibers are primarily used for maintaining posture and for endurance activities, while Type IIx are better suited for short duration, high intensity activities like weight lifting and sprinting. Type IIa fibers are in between the two, useful for activities like walking. Repeating a specific activity over and over can trigger muscle plasticity, meaning that the fibers adapt to changing demands by altering their physiological characteristics, effectively shifting along the fiber type continuum.

For example, with endurance activities like long distance running, muscle fibers can develop a denser capillary network and more mitochondria adapting to the requirements of oxidative phosphorylation. This concludes our tutorial. Learn more about skeletal muscle fiber types with our study units and quizzes.