This means that the natural lumbar curve must preserved no matter what your legs and arms are doing. (fitnessandpower.com)
However in a standing forward fold, if you can not maintain a neutral lumbar curve (tight hamstrings for one), then you are putting excessive force on the discs. (sequencewiz.org)
If you do have discomfort in one of those areas, then flat lumbar curve could a factor and should be addressed as part of your treatment plan. (sequencewiz.org)