General Health

The movements your muscles make are coordinated and controlled by the brain and nervous system. The involuntary muscles are controlled by structures deep within the brain and the upper part of the spinal cord called the brain stem. The voluntary muscles are regulated by the parts of the brain known as the cerebral motor cortex and the cerebellum.

When you decide to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles, causing them to contract. The motor cortex on the right side of the brain controls the muscles on the left side of the body and vice versa.

The cerebellum (pronounced: ser-uh-beh-lum) coordinates the muscle movements ordered by the motor cortex. Sensors in the muscles and joints send messages back through peripheral nerves to tell the cerebellum and other parts of the brain where and how the arm or leg is moving and what position it's in. This feedback results in smooth, coordinated motion. If you want to lift your arm, your brain sends a message to the muscles in your arm and you move it. When you run, the messages to the brain are more involved, because many muscles have to work in rhythm.

Muscles move body parts by contracting and then relaxing. Your muscles can pull bones, but they can't push them back to the original position. So they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint. Then, when you've completed the movement, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint. For example, the biceps muscle, in the front of the upper arm, is a flexor, and the triceps, at the back of the upper arm, is an extensor. When you bend at your elbow, the biceps contracts. Then the biceps relaxes and the triceps contracts to straighten the elbow.

What Are the Joints and What Do They Do?

Joints allow our bodies to move in many ways. Some joints open and close like a hinge (such as knees and elbows), whereas others allow for more complicated movement - a shoulder or hip joint, for example, allows for backward, forward, sideways, and rotating movement.

Joints are classified by their range of movement. Immovable, or fibrous, joints don't move. The dome of the skull, for example, is made of bony plates, which must be immovable to protect the brain. Between the edges of these plates are links, or joints, of fibrous tissue. Fibrous joints also hold the teeth in the jawbone.

Partially movable, or cartilaginous (pronounced: kar-tuh-lah-juh-nus), joints move a little. They are linked by cartilage, as in the spine. Each of the vertebrae in the spine moves in relation to the one above and below it, and together these movements give the spine its flexibility.

Freely movable, or synovial (pronounced: sih-no-vee-ul), joints move in many directions. The main joints of the body - found at the hip, shoulders, elbows, knees, wrists, and ankles - are freely movable. They are filled with synovial fluid, which acts as a lubricant to help the joints move easily. There are three kinds of freely movable joints that play a big part in voluntary movement:

• Hinge joints allow movement in one direction, as seen in the knees and elbows.
• Pivot joints allow a rotating or twisting motion, like that of the head moving from side to side.
• Ball-and-socket joints allow the greatest freedom of movement. The hips and shoulders have this type of joint, in which the round end of a long bone fits into the hollow of another bone.

Things That Can Go Wrong With the Bones, Muscles, and Joints

As strong as bones are, they can break. Muscles can weaken, and joints (as well as tendons, ligaments, and cartilage) can be damaged by injury or disease. The following are problems that can affect the bones, muscles, and joints in kids and teens:

• Arthritis. Arthritis (pronounced: ar-threye-tus) is the inflammation of a joint, and people who have it experience swelling, warmth, pain, and often have trouble moving. Although we often think of arthritis as a condition that affects only older people, arthritis can also occur in children and teens. Health problems that involve arthritis in kids and teens include juvenile lupus, Fracture. A fracture occurs when a bone breaks; it may crack, snap, or shatter. After a bone fracture, new bone cells fill the gap and repair the break. Applying a strong plaster cast, which keeps the bone in the correct position until it heals, is the usual treatment. If the fracture is complicated, metal pins and plates can be placed to better stabilize the fracture while the bone heals.
• Osgood-Schlatter disease(OSD). Osgood-Schlatter disease is an inflammation (pain and swelling) of the bone, cartilage, and/or tendon at the top of the shinbone, where the tendon from the kneecap attaches. OSD usually strikes active teens around the beginning of their growth spurts, the approximately 2-year period during which they grow most rapidly.
• Osteomyelitis. Osteomyelitis (pronounced: os-tee-oh-my-uh-lie-tus) is a bone infection that is often caused by Staphylococcus aureus (pronounced: sta-fuh-low-kah-kus are-ee-us) bacteria, though other types of bacteria can cause it, too. In kids and teens, osteomyelitis usually affects the long bones of the arms and legs. Osteomyelitis often develops after an injury or trauma.
• Osteoporosis. In osteoporosis (pronounced: ahs-tee-o-puh-row-sus), bone tissue becomes brittle, thin, and spongy. Bones break easily, and the spine sometimes begins to crumble and collapse. Although the condition usually affects older people, kids and teens with Repetitive stress injuries. Repetitive stress injuries (RSIs) are a group of injuries that happen when too much stress is placed on a part of the body, resulting in inflammation (pain and swelling), muscle strain, or tissue damage. This stress generally occurs from repeating the same movements over and over again. RSIs are becoming more common in kids and teens because they spend more time than ever using computers. Playing sports like tennis that involve repetitive motions can also lead to RSIs. Kids and teens who spend a lot of time playing musical instruments or video games are also at risk for RSIs.
• Strains and sprains. Strains occur when muscles or tendons are overstretched. Sprains are an overstretching or a partial tearing of the ligaments. Strains usually happen when a person takes part in a strenuous activity when the muscles haven't properly warmed up or the muscle is not used to the activity (such as a new sport or playing a familiar sport after a long break). Sprains, on the other hand, are usually the result of an injury, such as twisting an ankle or knee. A common sprain injury is a torn Achilles tendon, which connects the calf muscles to the heel. This tendon can snap, but it usually can be repaired by surgery. Both strains and sprains are common in children and teens because they're active and still growing.
• Tendinitis. Tendinitis (pronounced: ten-duh-neye-tus) is a common sports injury that usually happens after overexercising a muscle. The tendon and tendon sheath become inflamed, which can be painful. Resting the muscles and taking anti-inflammatory medication can help to relieve this condition.

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