An often heard recommendation is to never arch (hyperextend) the lower back. However, this cannot be substantiated. In the neutral spine position, the normal curvature of the lumbar spine is slightly “arched”. In fact, there are four natural curves to the spine with the biggest functional curve in the lumbar area. The spine, and especially the lumbar spine, should never be straight.
The curved spine is what is considered to be the anatomical position or neutral position of the spine. From this position the lumbar spine can be flexed approximately 30 to 45° in forward and backward directions. This is a total range of approximately 60 to 90° depending upon your flexibility.
If the spine is flexed as when you bend forward from the waist, then the lumbar spinal curvature is removed and the spine assumes a rounded shape. If the spine is extended as when you bend backwards you increase the amount of arch (hyperextension) in the lumbar spine.
The full range of motion in flexion and extension/hyperextension is considered to be safe motion. If you feel pain on either the bending forward or backward in this range of motion (60 to 90°), there is probably something wrong in the spine.
The only time hyperextension of the lumbar spine is considered to be potentially dangerous, is when the muscles are completely relaxed and the range of motion is extreme, as for example, when doing a standing backbend. This movement is typically seen in gymnastics.
However, when the muscles are under contraction, as for example when you are horizontal, they maintain stability of the spine as it goes through its dynamic motion. This in turn makes the movement safe.
Thus, when you hear or see the statement, “do not arch your back”, you should look for substantiation of this statement. The only time there is potential danger is when you perform excessive flexion or hyperextension, i.e., going beyond the normal range of motion that you have.
For most individuals, doing movements that involve hyperextension of the spine as for example, leaning backwards 30-40º, is very beneficial as a stretch. When you do strength exercises such as back raises on the Yessis Glute Ham Back Machine you can raise the trunk into hyperextension to work the lumbar muscles through their full range of motion.
For more information on the spine and back exercises see Biomechanics and Kinesiology of Exercise.