Obesity affects back pain for at least two reasons:
1. Increased girth of the stomach area causes the spine to arch backward (spinal extension) to compensate for this weight. Spinal extension increases compression of the vertebrae, disks, and nerve roots exiting the spine.
2. Excessive weight overloads the leg muscles. In response, obese individuals lock their knees so the the muscles don't have to work so hard. This tilts the pelvis forward (anterior pelvic tilt) which then causes the spine to arch backward (spinal extension) to compensate. Again this leads to compression of spinal tissues and pain.