It is a familiar experience for physically active people. The basketball star who aggressively leaps to make the shot and lands on the side of his foot. Or the ballet dancer who twists her ankle while trying to execute a high-speed pirouette.

The sudden, agonizing pain that erupts is usually the sign of an ankle sprain. Although the exact number of people who injure their ankles each year is not known, it is estimated to be in the millions, according to Thomas O. Clanton, an orthopedic surgeon at the University of Texas Medical School in Houston.

One of the most common types is called an inversion sprain -- when the bottom of the foot turns in and the ankle twists out. The resulting pressure causes ligaments -- fibrous bands of tissue that connect bone to bone -- to stretch or tear.

The ligament that is injured most often -- the anterior talofibular -- is located on the outside of the ankle between the leg bone and the first bone in the foot, said Clanton.

When ligaments are ruptured, they can bleed, causing the ankle to swell and throb. Treatment depends on the severity of the sprain, which ranges from a mild injury -- a stretched ligament -- to a more severe one -- a ligament that is completely torn.

Most physicians will treat a sprain with ice to control swelling and may wrap the ankle with a bandage to provide support, said Jay S. Cox, an orthopedic surgeon and director of clinical services at Pennsylvania State University's Center for Sports Medicine.

To help prevent ankle injuries, sports medicine experts suggest exercises to strengthen supporting muscles and improve the flexibility of the joint.

Clanton recommends using rubber tubing to provide resistance. Loop the tube around one foot and attach the other end to the leg of a chair. Pull the foot against the tubing, first to the inside to exercise the invertor calf muscles and then to the outside to work the evertor muscles.

People with a tight Achilles' tendon -- the cord connecting the heel to the muscles of the calf -- also have a tendency to sprain their ankles, Clanton said. To stretch out the tendon, lean against a wall, keeping the heel of the back leg on the floor and the knee straight. Bend the knee of the front leg. Hold this for about 10 to 30 seconds and repeat it three to five times daily.

Not all ankle injuries can be prevented. People who have sprained their ankles once are vulnerable to further injury, said Penn State's Cox. He suggests wearing a brace or taping the ankle while playing a high-impact sport like basketball or volleyball.