Bones, Muscles, and Joints
The Musculoskeletal System
Every time you sprint through the halls because you’re late for class, score against your opponents during a game, or shoot pool with friends, you’re using your bones, muscles, and joints. Without these important body parts, you’d be seriously sidelined — you’d be unable to sit, stand, walk, or do any of the activities you do every day.
From our head to our toes, our bones provide support for our bodies and help form our shape. The skull protects the brain and forms the shape of our face. The spinal cord, a pathway for messages between the brain and the body, is protected by the backbone, or spinal column. The ribs form a cage that shelters the heart, lungs, liver, and spleen, and the pelvis helps protect the bladder, intestines, and in girls, the reproductive organs. Although they’re very light, bones are strong enough to support our entire weight.
Joints are where two bones meet. They make the skeleton flexible — without them, movement would be impossible. Muscles are also necessary for movement: They’re the masses of tough, elastic tissue that pull our bones when we move.
Together, our bones, muscles, and joints — along with tendons, ligaments, and cartilage — form our musculoskeletal system and enable us to do everyday physical activities.
What Are the Bones and What Do They Do?
The human skeleton has 206 bones. Our bones begin to develop before birth. When the skeleton first forms, it is made of flexible cartilage, but within a few weeks it begins the process of ossification (pronounced: ah-suh-fuh-KAY-shun). Ossification is when the cartilage is replaced by hard deposits of calcium phosphate and stretchy collagen, the two main components of bone. It takes about 20 years for this process to be completed.
The bones of kids and young teens are smaller than those of adults and contain “growing zones” called growth plates. These plates consist of columns of multiplying cartilage cells that grow in length, and then change into hard, mineralized bone. These growth plates are easy to spot on an X-ray. Because girls mature at an earlier age than boys, their growth plates change into hard bone at an earlier age.
Bone building continues throughout your life, as your body constantly renews and reshapes the bones’ living tissue. Bone contains three types of cells: osteoblasts (pronounced: AHS-tee-uh-blastz), which make new bone and help repair damage; osteocytes (pronounced: AHS-tee-o-sites), which are mature bone cells that help form the new bone; and osteoclasts (pronounced: AHS-tee-o-klasts), which break down bone and help to sculpt and shape it. Osteoclasts are very active in kids and teens, working on bone as it is remodeled during growth. They also play an important role in the repair of fractures.
Bones are made up of calcium, phosphorus, sodium, and other minerals, as well as the protein collagen. Calcium is needed to make bones hard, which allows them to support your weight. Bones also store calcium and release some into the bloodstream when it’s needed by other parts of the body. The amounts of certain vitamins and minerals that you eat, especially vitamin D and calcium, directly affect how much calcium is stored in the bones.
The soft bone marrow inside many of our bones is where most of the blood cells flowing through our bodies are made. The bone marrow contains stem cells, which produce the body’s red blood cells and platelets, and some types of white blood cells. Red blood cells carry oxygen to the body’s tissues, and platelets help with blood clotting when someone has a cut or wound. White blood cells help the body fight infection.
Bones are made up of two types of material — compact bone and cancellous bone. Compact bone is the solid, hard outside part of the bone. This type of bone makes up most of the human skeleton. It looks like ivory and is extremely strong. Holes and channels run through it, carrying blood vessels and nerves from the periosteum, the bone’s outer membrane. Cancellous (pronounced: KAN-suh-lus) bone, which looks like a sponge, is inside the compact bone. It is made up of a mesh-like network of tiny pieces of bone called trabeculae (pronounced: truh-BEH-kyoo-lee). This is where red and white blood cells are formed in the marrow.
Bones are fastened to other bones by long, fibrous straps called ligaments (pronounced: LIG-uh-mentz). Cartilage (pronounced: KAR-tul-ij), a flexible, rubbery substance in our joints, supports bones and protects them where they rub against each other.
What Are the Muscles and What Do They Do?
Bones don’t work alone — they need help from the muscles and joints. Muscles pull on the joints, allowing us to move. They also help the body perform other functions so we can grow and remain strong, such as chewing food and then moving it through the digestive system.
The human body has more than 600 muscles. Muscles make up half of a person’s body weight. They are connected to bones by tough, cord-like tissues called tendons, which allow the muscles to pull on bones. If you wiggle your fingers, you can see the tendons on the back of your hand move as they do their work.
Humans have three different kinds of muscle:
- Skeletal muscle is attached to bone, mostly in the legs, arms, abdomen, chest, neck, and face. Skeletal muscles are called striated (pronounced: STRY-ay-ted) because they are made up of fibers that have horizontal stripes when viewed under a microscope. These muscles hold the skeleton together, give the body shape, and help it with everyday movements (they are known as voluntary muscles because you can control their movement). They can contract (shorten or tighten) quickly and powerfully, but they tire easily and have to rest between workouts.
Smooth, or involuntary, muscle is also made of fibers, but this type of muscle looks smooth, not striated. Generally, we can’t consciously control our smooth muscles; rather, they’re controlled by the nervous system automatically (which is why they are also called involuntary). Examples of smooth muscles are the walls of the stomach and intestines, which help break up food and move it through the digestive system.
Smooth muscle is also found in the walls of blood vessels, where it squeezes the stream of blood flowing through the vessels to help maintain blood pressure. Smooth muscles take longer to contract than skeletal muscles do, but they can stay contracted for a long time because they don’t tire easily.
- Cardiac (pronounced: KAR-dee-ak) muscle is found in the heart. The walls of the heart’s chambers are composed almost entirely of muscle fibers. Cardiac muscle is also an involuntary type of muscle. Its rhythmic, powerful contractions force blood out of the heart as it beats.
Muscles and Movement
Even when you sit perfectly still, there are muscles throughout your body that are constantly moving. Muscles enable your heart to beat, your chest to rise and fall as you breathe, and your blood vessels to help regulate the pressure and flow of blood through your body. When we smile and talk, muscles are helping us communicate, and when we exercise, they help us stay physically fit and healthy.
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-BEL-um) 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 their 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 teens:
Arthritis. Arthritis 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 also can affect children and teens. Health problems that involve arthritis in kids and teens include juvenile idiopathic arthritis (JIA, also know as juvenile rheumatoid arthritis, or JRA), lupus, Lyme disease, and septic arthritis (a bacterial infection of a joint).
Fracture. A fracture is 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.
Muscular dystrophy. Muscular dystrophy (pronounced: DIS-truh-fee) is an inherited group of diseases that affect the muscles, causing them to weaken and break down over time. The most common form in childhood is called Duchenne muscular dystrophy, and it most often affects boys.
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, girls with female athlete triad syndrome and teens with eating disorders can get the condition. Exercising regularly and getting plenty of calcium when you’re a kid and teen can prevent or delay you from getting osteoporosis later in life.
Repetitive stress injuries (RSIs). 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 is from repeating the same movements over and over again. 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. RSIs are becoming more common in kids and teens because they spend more time than ever using computers.
Scoliosis. Every person’s spine curves a little bit; a certain amount of curvature is necessary for people to move and walk properly. But three to five people out of 1,000 have a condition called scoliosis (pronounced: sko-lee-OH-sus), which causes the spine to curve too much. The condition can be hereditary, so a person who has scoliosis often has family members who have it.
Strains and sprains. Strains happen when a muscle or tendon is overstretched. Sprains are an overstretching or a partial tear 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. Both strains and sprains are common in teens because they’re active and still growing.
Tendonitis. Tendonitis (pronounced: ten-duh-NY-tus) is a common sports injury that usually happens after overusing 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.
Reviewed by: Yamini Durani, MD
Date reviewed: October 2012