Medically reviewed by Alana Biggers, M.D., MPH — Written by Jill Seladi-Schulman, Ph.D. on May 12, 2020

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

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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. Keep reading to learn more about these different types, what they do, and more.

You have three types of muscle tissue in your body. These include:

skeletal musclesmooth muscle

Each of these types of muscle tissue has muscle fibers. Let’s take a deeper dive into the muscle fibers in each type of muscle tissue.

Skeletal muscle

Each one of your skeletal muscles is made up of hundreds to thousands of muscle fibers that are tightly wrapped together by connective tissue.

Each muscle fiber contains smaller units made up of repeating thick and thin filaments. This causes the muscle tissue to be striated, or have a striped appearance.

Skeletal muscle fibers are classified into two types: type 1 and type 2. Type 2 is further broken down into subtypes.

Type 1. These fibers utilize oxygen to generate energy for movement. Type 1 fibers have a higher density of energy-generating organelles called mitochondria. This makes them dark.Type 2A. Like type 1 fibers, type 2A fibers can also use oxygen to generate energy for movement. However, they contain less mitochondria, making them light.Type 2B. Type 2B fibers don’t use oxygen to generate energy. Instead, they store energy that can be used for short bursts of movement. They contain even less mitochondria than type 2A fibers and appear white.

Smooth muscle

Unlike skeletal muscles, smooth muscles aren’t striated. Their more uniform appearance gives them their name.

Smooth muscle fibers have an oblong shape, much like a football. They’re also thousands of times shorter than skeletal muscle fibers.

Cardiac muscle

Similar to skeletal muscles, cardiac muscles are striated. They’re only found in the heart. Cardiac muscle fibers have some unique features.

Cardiac muscle fibers have their own rhythm. Special cells, called pacemaker cells, generate the impulses that cause cardiac muscle to contract. This typically happens at a constant pace, but can also speed up or slow down as necessary.

Second, cardiac muscle fibers are branched and interconnected. When the pacemaker cells generate an impulse, it spreads in an organized, wavelike pattern, which facilitates the beating of your heart.

The types of muscle tissue have different functions within your body:

Smooth muscle. Smooth muscles are involuntary, meaning that you can’t control them. They’re found in your internal organs and eyes. Examples of some of their functions include moving food through your digestive tract and changing the sizes of your pupil.Cardiac muscle. Cardiac muscle is found in your heart. Like smooth muscle, it’s also involuntary. Cardiac muscle contracts in a coordinated way to allow your heart to beat.

Muscle fibers and muscles work to cause movement in the body. But how does this occur? While the exact mechanism is different between striated and smooth muscles, the basic process is similar.

The first thing that occurs is something called depolarization. Depolarization is a change in electric charge. It can be initiated by a stimulatory input like a nerve impulse or, in the case of the heart, by pacemaker cells.

Depolarization leads to a complex chain reaction within muscle fibers. This eventually leads to a release of energy, resulting in muscle contraction. Muscles relax when they stop receiving a stimulatory input.

You may have also heard about something called fast-twitch (FT) and slow-twitch (ST) muscle. FT and ST refer to skeletal muscle fibers. Types 2A and 2B are considered to be FT while type 1 fibers are ST.

FT and ST refer to how fast muscles contract. The speed at which a muscle contracts is determined by how quickly it acts on ATP. ATP is a molecule that releases energy when it’s broken down. FT fibers break down ATP twice as fast as ST fibers.

Additionally, fibers that use oxygen to produce energy (ATP) fatigue at a slower rate than those that don’t. So as far as endurance is concerned, the skeletal muscles listed from highest to lowest are:

type 1type 2Atype 2B

ST fibers are good for long lasting activities. These can include things like holding a posture and stabilizing bones and joints. They’re also used in endurance activities, such as running, cycling, or swimming.

FT fibers produce shorter, more explosive bursts of energy. Because of this, they’re good in activities involving bursts of energy or strength. Examples include sprinting and weightlifting.

Everyone has both FT and ST muscles throughout their body. However, the overall amount of each varies greatly between individuals.

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FT versus ST composition can also influence athletics. Generally speaking, endurance athletes often have more ST fibers, while athletes like sprinters or power-lifters often have more FT fibers.