Dancing, bending, writing and wiggling your toes. Your joints perform in harmony with your muscles to give you this wide range of movements. Your joints also give you support, stability, flexibility and protection. They're amazingly rugged and impressively agile, but are susceptible to pain and damage.

Your joints are made of materials designed for a lifetime of faithful service. A joint occurs anywhere that two bones meet. Bones in your joints are capped with shock-absorbing cartilage — a tough, smooth, slippery material that allows bones to glide over each other.

Joints are surrounded and lubricated by the synovial membrane — the inner lining of the joint capsule. The capsule is a tough, fibrous material that attaches to bone on either side of the joint. Ligaments help by contributing to joint alignment and stability. Ligaments are cords of strong fiber that attach bone to bone and support the joint.

The joint is held firmly together by muscles tipped by tendons that attach to bone just outside the capsule above or below the joint. Bursae are friction-reducing, fluid-filled sacs between your muscles, tendons and bones that are present near some of your joints. Each bursa is lined with synovial membrane, which releases a lubricating fluid.

Although the majority of your joints move a lot, some move very little or not at all. Your body has four types of joints:

• Fixed. These joints don't move. They absorb shock. One example of a fixed joint is your skull bones, which are essentially fused together to protect sensitive tissue underneath.
• Hinge. These joints are similar to the hinge of a door. Hinge joints, such as your knees, allow you to move forward and backward.
• Pivot. These joints, such as your elbows, allow for rotating movement.
• Ball-and-socket. These joints, such as your hips, give you the most movement. That's because the large, round end of a long bone fits into a hollow part of another bone, making swinging and rotating movements possible.
  

There are hundreds of joints in the human body. All of your joints are designed to work together to help you move with ease. Here are some of the major joints that help you move throughout the day:

Spine joints
Your spine begins at the base of the skull and extends to the tailbone (sacrum). Your spine is a series of 24 bones — vertebrae — that surround and protect the spinal cord. The joints that interlock between your vertebrae give you the ability to bend and twist, and they also help to stabilize your spine when you lift heavy objects.

Shoulder joint
Your shoulder is a ball-and-socket joint with greater range of motion than any other joint. Most shoulder movement occurs where the head (ball) of the large bone in your upper arm (humerus) fits into the socket at your shoulder blade (scapula). Your shoulder's wide range of motion enables you to perform extremely varied activities with your upper extremity, from rolling a bowling ball to hitting a tennis ball to pitching a baseball.

Hip joint
Your hip allows you to make a variety of movements such as bending (flexion), straightening (extension), moving your legs apart (abduction) and moving your legs together (adduction). Designed to bear weight, it's the largest ball-and-socket joint in your body. The ball is actually the head of your thighbone (femur). Much of the hip's stability is due to the deep placement of the ball into a socket in your hip, called the acetabulum, and to its ligaments and muscles.

Knee joint
Your knee is the largest and heaviest hinge joint in your body. It's also injured more often on average than any other joint. Your knee's main movements are bending and straightening. The knee is made up of the lower end of the thighbone that rotates on the upper end of the shinbone (tibia), and the kneecap (patella), which slides in a groove on the end of the femur. Large ligaments attach to the femur and tibia to provide stability. The long thigh muscles — quadriceps on the front of the thigh and hamstrings on the back of the thigh — give the knee strength. Normally, all of these components work together smoothly. But disease or injury can disrupt this balance, resulting in pain, muscle weakness and decreased function.

Elbow joint
Your elbow is an example of a joint that has both hinge and pivot joints — the ulnohumeral joint and the radioulnar joint. The ends of your upper arm bone (humerus) and the larger of the two bones of your forearm (ulna) form a hinge that allows you to bend your arm — the ulnohumeral joint.

The smaller bone in your forearm (radius) — which is on the thumb side of your arm — and the ulna form the radioulnar joint. This joint allows rotation or pivoting of your forearm so that you can place your palm up or down.

Hand, finger and wrist joints
Your hands and fingers contain bones sheathed by tendons. Joints in your hand allow for activities such as holding and carrying. The bases of your thumb, fourth and fifth fingers have some joints that allow movement. Joints at the bases of your second and third fingers are fixed. The remaining finger joints are modified hinged joints, enabling the fingers to bend over toward the palm but not toward each other. The wrist joint is composed of eight wrist bones (carpals) and the bones of the forearm. Your wrist allows your hand to rotate on the forearm.

Foot joint
Your feet are designed to bear weight. There are 33 joints in each foot that absorb the shock of walking, running and skipping throughout your life. The American Podiatric Medical Association estimates that the average person takes 8,000 to 10,000 steps a day. Over your lifetime, your feet probably transport you more than 100,000 miles — equal to about four trips around the world. Nearly 8 out of 10 Americans experience some foot and ankle problems.

Oh my aching joints! You've probably heard that exclamation or said it yourself. Although they're made of extremely durable materials, your joints may be a source of pain at times. Joint problems are many and varied, and include:

Injury
Trauma, such as a bone fracture or injury to the ligaments, muscles, tendons around the joint or the cartilage within the joint, can lead to joint pain. Ligament sprains, if significant, can impair the stability of the joint and make it susceptible to further damage. Tears of the cartilage in the knee can cause a block to the range of motion of the knee, also called locking of the knee. And fractures or ligament injuries to joints can contribute to joint destruction and osteoarthritis years later. Knees are the joints most likely to be affected by trauma and injury.

Arthritis
Arthritis is one of the most common conditions causing joint pain and disability. Nearly 43 million Americans have arthritis or a related condition.

Osteoarthritis is the most common form of joint disease. It's often called wear-and-tear arthritis because it involves the wearing away of the cartilage that caps the bones in your joints. Over time, if the cartilage wears down completely, you lose shock absorption capabilities and may be left with bone rubbing on bone. This may eventually damage the ends of your bones. Osteoarthritis can affect almost any joint in your body.

Rheumatoid arthritis isn't associated with the overuse or injury of a joint. In rheumatoid arthritis, the synovial membrane that protects and lubricates joints becomes inflamed, causing pain and swelling. Rheumatoid arthritis can be more disabling than osteoarthritis, and it sometimes results in deformed joints. It can affect any joint, but wrists, knuckles and small joints of the feet are among the most common sites.

Avascular necrosis
Avascular necrosis, also called osteonecrosis or aseptic necrosis, results in the death of a section of a bone due to loss of blood supply. Avascular necrosis most often affects the head of the thighbone (femur) — part of the hip joint. The femur loses its blood supply and dies. This causes the collapse of the femoral head and degeneration of the joint. The condition is often caused by a fracture, but can be the result of other factors including alcohol abuse, diabetes, steroid use and lupus. Other bones besides the femur can also be affected.

Gout
Gout is a form of arthritis characterized by sudden, severe attacks of pain, redness and tenderness in your joints. Gout is caused by excessive blood levels of uric acid, a waste product formed from the breakdown of purines. These substances are found naturally in your body as well as in certain foods, including organ meats — such as liver, brains, kidney and sweetbreads — and anchovies, herring and mackerel.

Normally, uric acid dissolves in your blood and passes through your kidneys into your urine. But sometimes your body produces too much or excretes too little of this acid. In that case, uric acid can build up, forming sharp, needlelike crystals in a joint or surrounding tissue that cause pain, inflammation and swelling.

Bursitis
Bursitis is a painful inflammation of the fluid-filled capsule outside your joint. The inflammation is caused by overuse, stress or direct trauma to the joint, such as with repeated bumping or prolonged pressure from kneeling. Common locations of bursitis are at the hip, elbow, shoulder or knee.