By knowing how your joints function when they’re in tip-top shape, you can better understand the problems you face when a joint, or joints, are damaged or in pain.

Joints literally keep the human frame together. From the top of our heads to the tips of our toes, joints link our 206 bones, make our bodies flexible, and enable our muscles to maneuver into thousands of positions.

Some joints, such as those in the cranium where rigidity is desirable, don’t move at all. Others, such as those in the pelvic area, move very little. But most joints move a lot. Hinge joints, as in our elbows and knees, swing back and forth like doors. Ball-and-socket joints, as in our hip and shoulder, enable bones to twist and turn in many directions while remaining firmly connected to each other.

The movable joints in our bodies thrive on use and deteriorate with disease, injury, neglect or excessive overuse. So the adage "use it or lose it" is particularly apt – but in moderation. Even without arthritis, joints tend to stiffen as we age, so regular exercise and stretching become increasingly important.

Jaw: The jaw joints are famous for helping us with two important activities – processing food and talking. Because of their proximity to the ears, eyes, nose, throat, tongue, sinuses and cervical spine, their malfunction (disease of the tempromandibular joint) can have a profound effect on these organs.

Spine: Literally the backbone of the body, the spine begins at the base of the skull and extends to the sacrum. It is composed of 24 vertebrae stacked one on top of another to provide a flexible column of support for the spinal cord. The movement of these vertebrae allows us to stoop, squat, turn and nod our heads, and twist our shoulders and hips.

The spine works on the motion-segment principle: Many small movements add up to big ones. Each individual vertebra can move only a little in relation to its neighbors. But putting all the units together creates awesome movement. Watch when a batter stands at the plate and swings at the ball. The cumulative effect of tiny spinal movements rippling up the back permits a broad, sweeping swing.

Shoulder: The series of joints from the shoulder to the fingertips makes our arms extremely flexible. The shoulder’s ball-and-socket joint enables us to move the arm in almost any direction. The rotator cuff – a common site of injury in baseball pitchers – is composed of muscles and tendons that hold the shoulder joint in place and allow us to lift an arm and reach overhead. Shoulder injuries caused by excessive overhead arm motion are common, and can be caused by activities such as painting or hanging curtains.

Elbow: The elbow operates as a simple hinge that bends and straightens the arm. Although the joint allows some twisting and turning of the forearm, most of the forearm’s movement results from rotation of the radius and ulna, the bones of the lower arm.

Wrist: Two major arteries, three major nerves and 20 tendons pass through the eight-bone wrist. Today, conditions such as carpal tunnel syndrome – in which cumulative micro-injury results in compression of the median nerve – focus considerable attention on the wrist.

Hand: Truly a marvel, each hand is built around a skeletal network of 19 bones connected by a multitude of complex joints. At the base of the fingers are saddle joints, some allowing movement (those at the base of the thumb and the fourth and fifth fingers) and some more fixed (those at the base of the second and third fingers). The remaining finger joints are modified hinges, enabling the fingers to bend over toward the palm, but not toward each other.

For years, engineers have tried to build a robot that can mimic the human hand. But the hand’s movements are too complicated to copy exactly without feedback from the eyes and touch-sensitive skin. Robot hands are used in factory production lines very effectively for single tasks. While human hands can paint, weld, drill, screw, adjust and assemble, a separate robot hand and arm design is required for each of these diverse jobs.

Hip: Watch a ballet dancer perform and you’ll note what an incredible range of movement the hip joints permit. As the junction of the femur with the socket of the pelvis, the hip is the leg’s equivalent of the shoulder, although it’s much stronger. It is also more stable to allow it to withstand the stresses of walking. For example, during normal walking, the force placed on the hip is three to four times our body weight; in running, it is five times.

Although stronger than the shoulder, the hip is much less mobile. Large movements, such as drawing the leg back in order to kick, are possible only because the whole pelvis tilts over the other hip. When a hip is damaged severely enough to limit activity hip replacement surgery can often make it possible to resume a more active lifestyle. In 1996, the most recent year for which figures are available, some 130,000 Americans underwent hip replacement surgery.*

Knee: The knee works like the hinge joint in the elbow, allowing us to fold our legs under when we kneel or stretch them out to take a big step. It can swivel only slightly, helping turn the foot to point the toes out or in. The biggest and heaviest joint, the knee carries nearly half the body’s weight and works like a hinge to move the shin and foot forward and backward. Inside the knee, extra ligaments and cartilage stabilize and support the joint and prevent its moving from side to side. Still, the knee is injured more often than any other joint. In 1996, the most recent year for which figures are available, 245,000 Americans had knee replacement surgery.*

Ankle: The ankle is a hinge joint linking the lower ends of the tibia and the fibula (the bones of the lower leg) to the talus, the topmost of the tarsal bones in the foot, allowing the foot to make up-and-down movements.

Foot: Though the wrist is different from the ankle, the 26 foot bones share the same arrangement as the corresponding bones in the hand. Hand bones are more delicate and their joints more flexible, while the foot is designed for bearing weight. Our feet transport most of us more than 100,000 miles in a lifetime – equal to about four trips around the world. Women generally average 10 miles a day, outdistancing men who walk an average of seven miles a day. With each step taken by a person weighing 130 pounds, the foot absorbs 500 pounds of pressure, which comes to about 5 million pounds of impact on the feet in an average day. It’s no surprise then, that nearly eight out of 10 Americans will experience some foot and ankle problems.

* Source: National Center for Health Statistics