Muscular System

Athletic strength and skill depend on muscle development. Take, for example, gymnasts, dancers, and football players. All must train their muscles to perform powerfully and precisely.

More important, muscles power essential bodily functions. Heart muscles pump blood. The diaphragm powers breathing. Smooth muscle moves food through the digestive system.

In all, the body contains over 600 different muscles. Each performs a specific task. Some work together. Others operate on their own. Their main function is motion. By contracting and relaxing, muscles make body parts move. Some of the smallest muscles open and close eyelids. Some of the largest move the legs. Muscles also protect the body’s organs.

Types of Muscles

The body contains three kinds of muscle. These are skeletal, smooth, and cardiac. Each has a different function and structure.

Strong fibers make up skeletal muscle. These fibers contract and relax to produce movement. Most skeletal muscles attach to bones. For the most part, people consciously control the movement of skeletal muscles. So, skeletal muscles are also known as voluntary muscles.

Smooth muscles form broad sheets. They line many organs. The smooth muscle of the stomach, for example, churns food. Smooth muscles operate without conscious control. So they are also called involuntary muscles.

Cardiac muscle pumps the heart. Like smooth muscle, it operates automatically. Like skeletal muscle, it is made of strong fibers.

Energy for Muscle Action

To move muscles, the body needs energy. Glycogen is the body’s most readily available fuel. The body creates it from sugars and starches in food. When the body runs out of glycogen, it begins breaking down fat for fuel. It can also break down protein.

Burning glycogen is a complex reaction. It takes time. By contrast, movement requires instant energy. The body’s instant fuel is an energy molecule called adenosine triphosphate, or ATP. Muscle cells store ATP for instant action. When it is used up, the body breaks down glycogen to replace it.

Muscle Fatigue

Burning glycogen produces the waste product lactic acid. Oxygen enables muscle cells to eliminate lactic acid. Problems arise when muscle activity uses more oxygen than the blood can deliver. Then, lactic acid builds up. This produces muscle fatigue and soreness. If lactic acid continues to accumulate, it can cause damage. Eventually it can kill muscle tissues.

Control of Muscle Activity

Muscles are controlled by messages from the brain and spinal cord. Threadlike nerve fibers transmit these messages. Each nerve message, or impulse, can cause as many as 100 muscle fibers to contract. The contraction lasts only a fraction of a second. For lasting movement, the brain must continue sending messages in rapid order. When muscle fibers contract, they do so completely. To produce more forceful movement, the brain sends messages to more muscle fibers.

Muscle Motion

Flexor muscles pull adjacent body parts closer together. The biceps, on the front of the upper arm, are an example. Extensors move body parts apart. The triceps, on the back of the upper arm, are examples of extensors. Abductors move limbs away from the rest of the body. Examples include the outer thigh muscles. Adductors move limbs back. The inner thigh muscles are adductors.

Pronator muscles turn body parts upward. Supinators turn them face down. They operate the wrist. Rotators swivel the hips and shoulders.

Muscles Act in Groups

Most movements involve several muscles. Typically, some must relax while others contract. Take, for example, what happens when an elbow bends. The triceps muscle relaxes. At the same time, the biceps contracts. Muscles that pull together during movement are called synergistic. By contrast, antagonistic muscles oppose each other’s movement. The triceps and biceps, for example, are antagonists.

Even widely separated muscles can work together. Pushing a heavy object engages the muscles of the abdomen as well as the arms and shoulders.

Tendons

Tendons are ropelike tissues that attach muscles to bones. They are both flexible and strong. Some, such as the Achilles tendon, are visible. It runs down the back of the ankle. Tendons are different from ligaments. Found at joints, ligaments attach one bone to another.

Muscle Tone

In addition to producing movement, muscles prevent unwanted movement. Even when the muscles appear relaxed, some of their fibers are contracting. This partial contraction is called muscle tone, or tonus. It holds bones in place and keeps the muscles ready to fully contract. Muscle tone is especially important for good posture.

Muscles! (Photo credit: Unlisted Sightings)

Muscles Can Change

Muscles grow when exercised regularly. But they do not gain new fibers. Rather, the existing muscle fibers expand in diameter. Just as exercise increases muscle size, inactivity causes them to shrink.

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When muscles are injured, the fibers cannot divide to repair the wound. The fiber ends can lengthen to replace some lost tissue. But most of the repair involves connective tissue. This produces scarring and can limit muscle strength and flexibility. In some cases, adjacent muscles can take over some of the injured muscle’s work.