Muscle Classifications

By CFCF (Own work) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

Muscles can be classified or thought of in several ways. Classifications may refer to the muscle shape, size, function, use, or orientation. We will cover some of the most common classifications of muscles. Individual classifications are neither very complex nor long.

Muscle Type

There are three types of muscle in the human body. These are:

  • Skeletal Muscles
  • Smooth Muscles
  • Cardiac Muscles

Most muscles in the body are skeletal muscles. These muscles comprise about 40% of the human body, whereas smooth and cardiac muscles combined represent about 10% of the mass of the human body.

Skeletal muscles typically attach to one or more bones and are often used to move bones or other parts of the anatomy. These are usually muscles that you can consciously control.

Smooth muscles line organ and blood vessel walls. These are generally fair weak muscles and perform their movements in response to signals from the autonomic nervous system. You cannot consciously control the actions or movements of these muscles. In the digestive system these muscles provide the peristalsis (rhythmic waves) that move chyme through the digestive tract.

The cardiac muscles are found only in the heart. These muscles are again not subject to conscious control. Unlike smooth muscles, cardiac muscles have striated rather than smooth muscle fibers. This enables the heart to produce significant contractions that provide efficient arterial blood flow.

Muscle Shape

Muscles are often classified by their general shape. Common shapes include:

  • Fusiform
  • Parallel
  • Convergent
  • Unipennate
  • Bipennate
  • Multipennate
  • Circular

Fusiform muscles are long and narrow muscles that often connect between two bones, helping to form a lever. Most of the muscles that control extension and flexion functions are Fusiform muscles. The biceps are a commonly cited example of a Fusiform muscle.

Parallel muscles, also commonly called strap muscles, are long and thin muscles containing many parallel fibers. These are not particularly strong muscles, but do have substantial endurance. Some academic sources classify Fusiform and Convergent muscles as Parallel muscles, but many other sources maintain these as separate classifications. The Sartorius muscle in the diagram above is an example of a Parallel muscle.

Convergent muscles are also commonly called triangular muscles because they attach to a single point at one end and to a broader surface area at the other. The deltoids (shoulder muscles) are examples of convergent muscles. This muscle type is also commonly found around hinge joints where a broad surface area must pull against a distant bone surface.

Unipennate muscles look like a feather one might find on an arrow or dart. Many muscle fibers attach to a long tendon at an angle creating a strong muscle.

Bipennate muscles look like a full feather with diagonally arranged muscle fibers attaching to either side of a long central tendon. This arrangement creates a very strong and powerful muscle.

A Multipennate muscle has multiple central tendons with diagonally arranged muscle fibers attached on both sides of each tendon. The tendons all merge to form one large tendon at one end of the muscle, but have separate attachment points at the other end of the muscle.

Circular muscles surround an orifice and support its opening and closing. The lips are an example of a circular muscle.

Muscle Function

Muscles are often classified by their function or purpose. A flexor muscle causes a joint to contract or flex. An extensor muscle moves the joint in the opposite direction. For example, when you extend your pointing finger extensor muscles cause your finger to straighten.

Abductor muscles pull a limb away from the core of the body. If you raise your arm directly outward and upward from your side then abductor muscles are performing that function. Adductors pull a limb in toward the core of the body. Pulling the same arm down again would involve use of adductor muscles.

Elevator muscles cause a body part to rise. A depresser causes a body part to lower. Elevator and depresser muscles are involved in raising and lowering the jaw. Shrugging and lowering the shoulders also involves muscles of these types.

Pronator muscles move a body part in pronation. In the forearm pronation muscles turn the palm inward and down. Supinator muscles move the palm in the opposite direction (the palm turns outward and up).

Sphincter muscles narrow or close an aperture. Dilator muscles widen an aperture to open it.

Finally, rotator muscles cause a body part to rotate. Medial rotation occurs when the body part is rotated toward the center of the body. Lateral rotation occurs when the body part rotates away from the center of the body. If you raise your straight right leg and then move the toes inward, then your hip (and leg) is moving with medial rotation. Moving the toes in the opposite direction involves lateral rotation.

Control Method

The method by which muscle movement initiates is also a form of classification. There are two control methods employed. The first is voluntary control. This involves muscles that you can consciously control. Moving your left thumb involves a conscious act. You can choose to move or not move your thumb.  Therefore, these are voluntary movements.

Involuntary movement involves muscles that you cannot consciously control. The muscles that control sphincters in the digestive system or that open and close the iris of the eye are involuntary actions. These happen without any conscious action and you cannot mentally control these movements. Yes, you can look suddenly into a bright light and that will cause your iris to narrow. So you might think this is a voluntary action. But it is not. Try to look at the bright light and keep your iris from narrowing. You have no direct mental control over the muscles that open or close the iris.

The autonomic nervous system controls involuntary muscles. This systems controls the actions of your heart, digestive system muscles, and a host of other functions you cannot consciously control and of which you are largely unaware.

There are a few exceptions of note to voluntary and involuntary muscular action. Breathing is an action performed either voluntarily or involuntarily. Blinking is another example. These actions are normally considered involuntary because they will normally occur if you do not consciously think to avoid them.

Reflexes are another special case consideration. In a reflex response the nervous system sends signal to the spine which in turn activates normally voluntary muscles in an involuntary manner to induce the reflex action.

Striation

Muscles with striation have repeating sections of muscle tissue called sarcomeres. This causes the muscle form parallel bundles of muscle tissue. Smooth muscles do not have these repeating patterns. Striated muscles include the voluntary and cardiac muscles. Cardiac muscles behave somewhat differently than voluntary muscles so the cardiac muscles are generally classified as a separate class of striated muscle.

Fiber Direction

Muscles are often classified according to their muscle fiber orientation relative to the central core of the body.

Vertical muscles have vertically oriented fibers. Transverse muscles have fibers oriented across the body. Oblique muscles have fibers oriented in a direction other than vertical or horizontal.

The rectus abdominus muscles (number 1 in the diagram at right) are vertical muscles while the abdominal external oblique muscles (number 3 in the diagram at right) are examples of, surprise, oblique muscles.  The underlying transverse abdominus muscles (number 9 in the diagram at right) are clear examples of transverse muscles.

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