There are two main exercises that are often prescribed for lower back pain and dysfunctions: sit ups and back extensions. They're both recommended on the assumption that back pain is caused by weak muscles.
Although weak trunk muscles are part of the problem and sit ups and back extension exercises do strengthens some of the trunk muscles, they're also the most destructive exercises for the spinal disc.
Just look at this chart to see how much excessive load there is on the spinal disc.
According to the studies on spinal disc pressure by Dr. Nachemson, there is 210% of your body weight of pressure into the spinal disc with sit ups and 180% of your body weight of pressure with back extension.
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