Muscle Fiber Types: Fast Twitch & Slow Twitch

We all know that we use our muscles to exercise and to keep our bodies functioning. But, did you know that you’re born with certain types of muscle fibers in certain amounts?

By understanding how our muscles are actually constructed, you can better understand why certain exercises and workouts work well and why some people are genetically predisposed to building muscle.

What are Muscle Fibers?

The human body is made up of many different muscles, but they can all be categorized into three main groups: cardiac, smooth, and striated.

Cardiac and smooth muscles are both involuntary, meaning they function without conscious control. You would find cardiac muscle in the heart and smooth muscle in your other organs. Striated skeletal muscle is voluntarily controlled, and as the name suggests, is attached to the skeleton.

Made up of myocytes (muscle cells or more commonly known as muscle fibers), muscles contain long rod-like structures called myofibrils, composed of different types of protein. These proteins are grouped into thin and thick portions called filaments. Your muscles are able to contract when these thick and thin portions slide along each other. When skeletal muscles contract they cause a movement at a joint. They are able to do so because they are attached to bones by tendons. While all skeletal muscles share these properties, they can be further categorized by muscle fiber type.

What are the Different Muscle Fiber Types?

Beyond the cardiac, smooth, and striated, muscle fibers can also be divided by type: Type I, Type IIa, and Type IIb. These are divided based on differences in the amount of mitochondria (the powerhouse of the cell) they have, how quickly they contract, color, and other factors. ¹

Type I

• Red in color due to high concentrations of myoglobin (the compound in muscles that carries oxygen)
• Very resistant to fatigue
• Contains large amounts of mitochondria
• Contracts slowly
• Produces a low amount of power when contracted
• Used in aerobic activities such as long distance running
• Also called slow twitch fibers

Type IIa

• Red in color due to high concentrations of myoglobin
• Resistant to fatigue (but not as much as Type I fibers)
• Contains large amounts of mitochondria
• Contracts relatively quickly
• Produces a moderate amount of power when contracted
• Used in long-term anaerobic activities such as swimming (activities lasting less than 30 minutes)
• Also called fast twitch A fibers

Type IIb

• White in color due to low myoglobin concentrations
• Fatigue very easily
• Contains low amounts of mitochondria
• Contracts very quickly
• Produces a high amount of power when contracted
• Used in short-term anaerobic activities such as sprinting and lifting heavy weights (activities lasting less than a minute)
• Also called fast twitch B fibers

Individual muscles in the body are made of a mixture of different fiber types and their composition will vary depending on what the muscle is used for. For example, postural muscles (e.g. spinal muscles, hip flexors, calves) are predominantly made up of Type I fibers because they do not need to produce a lot of power and are very resistant to fatigue. Furthermore, when a muscle contracts, only the fibers that are needed will contract. If a weak contraction occurs, only the Type I muscle fibers will contract. If a strong contraction occurs that requires a lot of power (like in lifting a heavy weight), the Type IIa and IIb fibers will be activated along with the Type I fibers, with the Type IIa and IIb fibers activating last.

Can Muscle Fibers Change?

All of us are born with a set percentage of these muscle fibers. However, some theories claim that you can change the properties of your muscle fibers based on what type of exercises you do.

For instance, someone born with a certain amount of fast twitch and slow twitch fibers can have slow twitch muscle fibers that exhibit some characteristics of fast twitch muscle fibers through training such as sprinting or heavy weightlifting.

So, while you may not be blessed with the slow twitch muscle makeup of an Olympic marathon runner or the fast twitch fiber makeup of a sprinter, it is possible to improve your performance through proper training and hard work.