More than 600 voluntary, or skeletal, muscles are in the human body, and those muscles account for about 40 percent of a person's weight. Voluntary muscles also are called striated muscles because of their striped appearance under the microscope. Unlike involuntary muscles, which are called cardiac and smooth muscles and operate on their own, voluntary muscles are controlled at will and are responsible for creating movement in all parts of the body.
Voluntary muscles are attached to one or more bones of the skeleton with some support from bundles of collagen called tendons. Each muscle has two distinct points of attachment. The point at which the muscle is anchored to the bone is called the origin. The point at which the muscle is attached to the bone that it moves is called the insertion. In general, during muscle contraction, the insertion bone moves toward the origin bone, and therefore, is responsible for a greater amount of movement.
Most of the voluntary muscles have Latin names that describe some feature or function of the muscle. For example, some muscles were given names that describe their shape, such as the deltoid muscle in the shoulder, which is shaped like a triangle. Delta is Latin for triangle. Latissimus, which means wide, are the muscles on either side of the back. Other muscles were named for their number of origins, such as the biceps, which means two heads, and triceps, or three heads. Quadricep means four heads. Still others were named for their function in the body. An abductor moves parts of the body away from each other, such as when a person lifts his leg out to the side, but an adductor moves parts toward the body, such as bringing that same leg back down and in front of the body. A flexor causes a joint to bend, as is demonstrated in a bicep curl, and an extensor causes a joint to straighten, such as when a person arches his back.
Two distinct types of voluntary muscle fibers are called slow-twitch fibers and fast-twitch fibers. Slow-twitch fibers can go for longer periods before fatiguing and involve longer, more sustained activities, such as marathon running or bicycling for an extended amount of time. Fast twitch fibers, however, fatigue faster but are better at providing a runner with short bursts of energy used in sprinting. Everybody is born with a certain ratio of slow- to fast-twitch muscle fibers, however, that ratio can vary from body part to body part. For example, a person with a higher number of slow-twitch fibers in his quadricep will be a better endurance runner than a person with a higher number of fast twitch fibers, who likely will be a better sprinter.
Voluntary muscles are under the control of the somatic nervous system. In addition to creating movement in all parts of the body, the voluntary muscles also have several other roles, such as protecting vital organs, stabilizing joints and maintaining posture. Voluntary muscles also are responsible for helping us perform routine tasks such as eating, turning pages in a book, raising our hands or brushing our teeth. Voluntary muscles are also located in the face and are responsible for chewing and displaying facial expressions.
Voluntary vs. Involuntary
Involuntary muscles line the internal organs of the body and do not have the ability to be consciously controlled. These muscles consist of those in the stomach, intestines, mouth, bladder, uterus and blood vessels. Though it has certain characteristics of voluntary muscles, the beating heart is classified as an involuntary muscle in a category of its own called the cardiac muscle category. Involuntary muscles are also known as "smooth" muscles, which describe their appearance under the microscope, except for the heart, which has a striated appearance similar to that of voluntary muscles.
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