Dr. Stuart McGill is a professor at the University of Waterloo, U.S.A., and his findings suggest that spinal flexion ( as you do when performing crunches and sit-ups ) could seriously damage your lower back.
McGill discovered that these two exercises place 3300 Newtons ( the equivalent of 340kg ) of compressive force on the spine when bent in flexion. These forces can squeeze the discs in your back to the point that it bulges. This presses on nerves, and will cause back pain, and potentially lead to a herniated disc.
Callaghan JP, McGill SM (2001). Intervertebral disc herniation: studies on a porcine model exposed to highly repetitive flexion/extension motion with compressive force. Clinical Biomechanics, 16, 28-37
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