Backpack overload in children: Low back pain unrelated to injury or other known cause is unusual in pre-teen children. However, a backpack overloaded with schoolbooks and supplies can strain the back and cause muscle fatigue. The American Academy of Orthopaedic Surgeons recommends that a child’s backpack should weigh no more than 15 to 20 percent of the child’s body weight.
Paget's disease of the bone is a condition of unknown cause in which the bone formation is out of synchrony with normal bone remodeling. This condition results in abnormally weakened bone and deformity and can cause localized bone pain, though it often causes no symptoms. Paget's disease is more common in people over the age of 50. Heredity (genetic background) and certain unusual virus infections have been suggested as causes. Thickening of involved bony areas of the lumbar spine can cause the radiating lower extremity pain of sciatica.
When you're working to strengthen the core, you'll want to focus on exercises that don't exacerbate lower back issues. "It's important to find out which movements (flexion, extension, rotation) cause pain or discomfort and to avoid those movements, while continuing to work into ranges that are not provoking," Dircksen says. Crockford suggests focusing on exercises that keep the core stable and avoiding twisting movements to avoid exacerbating pain.
2016 — More editing, more! Added some better information about pain being a poor indicator, and the role of myofascial trigger points. This article has become extremely busy in the last couple months — about 4,000 readers per day, as described here — so I am really polishing it and making sure that it’s the best possible answer to people’s fears about back pain.
Low back pain that lasts at least one day and limits activity is a common complaint.[7] Globally, about 40% of people have LBP at some point in their lives,[7] with estimates as high as 80% of people in the developed world.[22] Approximately 9 to 12% of people (632 million) have LBP at any given point in time, and nearly one quarter (23.2%) report having it at some point over any one-month period.[7][8] Difficulty most often begins between 20 and 40 years of age.[1] Low back pain is more common among people aged 40–80 years, with the overall number of individuals affected expected to increase as the population ages.[7]
Backpack overload in children: Low back pain unrelated to injury or other known cause is unusual in pre-teen children. However, a backpack overloaded with schoolbooks and supplies can strain the back and cause muscle fatigue. The American Academy of Orthopaedic Surgeons recommends that a child’s backpack should weigh no more than 15 to 20 percent of the child’s body weight.
2016 — More editing, more! Added some better information about pain being a poor indicator, and the role of myofascial trigger points. This article has become extremely busy in the last couple months — about 4,000 readers per day, as described here — so I am really polishing it and making sure that it’s the best possible answer to people’s fears about back pain.
To understand various causes of low back pain, it is important to appreciate the normal design (anatomy) of the tissues of this area of the body. Important structures of the low back that can be related to symptoms in this region include the bony lumbar spine (vertebrae, singular = vertebra), discs between the vertebrae, ligaments around the spine and discs, spinal cord and nerves, muscles of the low back, internal organs of the pelvis and abdomen, and the skin covering the lumbar area.
Hip hikers (also known as the pelvic drop) are great exercises to get your gluteal muscles working in a weight bearing position. To do the exercise, stand sideways with one foot on a step and the other hanging off. Keeping both knees straight, lower down your pelvis on one side so your foot moves toward the floor. Both knees should remain straight; the motion should come from your hip joint. Once your pelvis is lowered down, slowly raise it back up to the starting position. Repeat the exercise for 10 repetitions.

The hip is a common site of osteoarthritis. To help protect the hip joint from "wear and tear," it is important to strengthen the muscles that support it. Your hip also controls the position of your knee, and strengthening your hips may be one component of your rehab program for knee pain. Your physical therapist may also prescribe hip exercises after total hip replacement if you have a hip labrum tear or as part of your hip exercise program for hip pain.
Treatment options include physical therapy, back exercises, weight reduction, steroid injections (epidural steroids), nonsteroidal anti-inflammatory medications, rehabilitation and limited activity. All of these treatment options are aimed at relieving the inflammation in the back and irritation of nerve roots. Physicians usually recommend four to six weeks of conservative therapy before considering surgery. 
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