More than 600 muscles provide movement throughout the body. Cinical anatomy places muscles in three categories: skeletal muscle, cardiac muscle, and smooth muscle. Some muscle movements require a bit of thought on your part, whereas others happen automatically.


Skeletal muscles are responsible for making the skeleton move. They’re voluntary muscles because you can decide whether the muscles move or not. Skeletal muscle is also subject to reflex actions. Skeletal muscles have some basic roles when it comes to moving and positioning the body:

  • A prime mover or agonist is the muscle that’s mainly responsible for the movement.

  • A fixator is a muscle that holds the proximal part down so the movement can occur distally.

  • A synergist muscle helps the agonist.

  • An antagonist muscle opposes the contraction of the agonist.

Each muscle has at least two attachments. The attachment that stays in place during movement is called the origin. The attachment that moves the most is called the insertion. The origin is usually proximal to the insertion.

During development, individual myoblasts fuse together to form long, cylindrical, multinucleated skeletal muscle fiber. Each skeletal muscle is comprised of a contractile portion made up of muscle fiber bundles called fascicles (the fibers have a striped appearance, so sometimes these muscles are called striated muscles;). This part is called a head or belly of the muscle. Each muscle also has a portion that doesn’t contract, which is made up of collagen fibers. The noncontractile portions are called tendons when they’re rounded or aponeuroses when they’re flat.

Skeletal muscles come in the following different shapes:

  • Circular: These muscles surround an orifice that is constricted when the muscle contracts.

  • Convergent: Muscles of this type have a wide attachment at one end. The fascicles start at that attachment and converge into a single tendon.

  • Digastric: These muscles have two heads in series that share a tendon.

  • Fusiform: These muscles are shaped like spindles — thicker in the middle with tapered ends.

  • Parallel: In these muscles, the fascicles lie parallel to the long axis of the muscle.

  • Pennate: This type of muscle resembles a feather because the fibers run obliquely to the tendon (in other words, at an angle to the tendon). Unipennate muscles have fibers running only on one side of the tendon, and bipennate muscles have two sets of fibers, one on each side of the tendon. Multipennate muscles are groupings of several bipennate muscles.

Skeletal muscle can atrophy when a limb isn’t able to move for a long period of time, like when a person is wearing a cast while a bone heals. Some disorders of the nervous system can result in atrophy as well. Hypertrophy, the opposite of atrophy, refers to muscles increasing in size. That may sound like a good thing, but in the case of the heart muscle, it most definitely is not good because it reduces the size of the chambers of the heart.

The muscle of the heart, or myocardium, is similar to skeletal muscle because it also has a striated appearance. But unlike skeletal muscle, you have no control over the heart muscle; it’s all controlled by the autonomic nervous system. Cardiac muscle also has intercalated discs, which are regions where the muscle cells connect. These connections help maintain the structure of the muscle and provide electrical connections between the cells so impulses can travel rapidly throughout the heart.

Smooth muscle doesn’t have the striated appearance of the other types of muscle; instead, it has a smooth-looking surface. The fibers are spindle shaped and form sheets of closely apposed cells. Smooth muscle is found in the walls of blood vessels and the digestive tract, in ducts, in the iris of the eyeball, and attached to the hair follicles in the skin.

Like cardiac muscle, smooth muscle is under the control of the autonomic nervous system, so it isn’t under voluntary control. Smooth muscle doesn’t tire out like your arm and leg muscles do; it can stay partially contracted for long time periods.