The tough covering of a disk in the spine can tear (rupture), causing pain. The soft, jelly-like interior may then bulge out (herniate) through the covering, causing more pain. Pain occurs because the bulge puts pressure on the spinal nerve root next to it. Sometimes the nerve root becomes inflamed or is damaged. More than 80% of herniated disks occur in the lower back. They are most common among people aged 30 to 50 years. Between these ages, the covering weakens. The jelly-like interior, which is under high pressure, may squeeze through a tear or a weakened spot in the covering and bulge out. After age 50, the interior of the disk begins to harden, making herniation less likely. A disk may herniate because of a sudden, traumatic injury or repeated minor injuries. Being overweight or lifting heavy objects, particularly lifting incorrectly, increases the risk. Often, herniated disks, even ones that appear obviously bulging or herniated on imaging tests such as magnetic resonance imaging (MRI) or computed tomography (CT), cause no symptoms. Herniated discs that do not cause symptoms are more common as people age. However, herniated disks may cause slight to debilitating pain. Movement often intensifies the pain. Where the pain occurs depends on which disk is herniated and which spinal nerve root is affected. The pain may be felt along the pathway of the nerve compressed by the herniated disk. For example, a herniated disk commonly causes sciatica—pain along the sciatic nerve, down the back of the leg. A herniated disk can also cause numbness and muscle weakness. If pressure on the nerve root is great, a leg may be paralyzed. Rarely, the disk can put pressure on the spinal cord itself, possibly causing weakness or paralysis of both legs. If the cauda equina (the bundle of nerves extending from the bottom of the cord) is affected, control of bladder and bowels can be lost. If these serious symptoms develop, medical attention is required immediately. Most people recover without any treatment, usually within 3 months, but often much faster. Applying cold (such as ice packs) or heat (such as a heating pad) or using over-the-counter analgesics may help relieve the pain. Sometimes surgery to remove part or all the disk and part of a vertebra is necessary. In 10 to 20% of people who have surgery for sciatica due to a herniated disk, another disk ruptures.