As people age, bone strength and muscle elasticity and tone tend to decrease. The discs begin to lose fluid and flexibility, which decreases their ability to cushion the vertebrae and causes back pain.
Pain can occur when, for example, one lifts something too heavy or overstretches, causing a sprain, strain, or spasm in a muscle or ligament in the back. If the spine is overstrained or compressed, a disc may rupture or bulge outward. This rupture may put pressure on one of the more than 50 nerves rooted to the spinal cord that control body movements and transmit signals from the body to the brain. When these nerve roots are compressed or irritated, back pain results.
Low back pain may reflect nerve or muscle irritation or a bone lesion. Most low back pain results from an injury or trauma to the back; but pain may also be caused by degenerative conditions such as arthritis or disc disease, osteoporosis or other bone diseases, viral infections, irritation to joints and discs, and congenital abnormalities in the spine. Obesity, smoking, weight gain during pregnancy, stress, poor physical condition, posture inappropriate for the activity being performed, and poor sleeping position also contribute to low back pain. Additionally, the scar tissue created when the injured back heals itself does not have the strength or flexibility of normal tissue. The buildup of scar tissue from repeated injuries eventually weakens the back and can lead to more serious injury.
Occasionally, low back pain may indicate a more serious medical problem. Pain accompanied by fever or loss of bowel or bladder control, pain when coughing, and progressive weakness in the legs may indicate a pinched nerve or other serious condition. People with diabetes may have severe back pain or pain radiating down the leg related to neuropathy. People with such symptoms should contact a doctor immediately to prevent permanent damage.
This answer is based on source information from The National Institute of Neurological Disorders and Strokes.