What does a backrest actually do to the lumbar spine?

It is generally believed that a backrest facilitates lumbar lordosis. To test this, the spontaneously adopted postures of 12 healthy subjects were measured by a statometric method during 2-h sitting periods on three types of chairs in a stratified sequence. The only difference between the three workstations regarded backrest: 'A' had no backrest; 'B' had a vertical lumbar backrest; and 'C' had an anteriorly curved backrest. In general, the most lordotic postures were assumed with backrest C, whereas backrest B rather facilitated kyphosis as compared with sitting without a backrest. However, when specifically considering passive sitting, i.e. reading, both types of backrest facilitated kyphosis. Moreover, spinal shrinkage was evaluated by measuring exact height before and after each 2-h sitting period. This was done to assess spinal load. From this perspective, backrest C induced the greatest load on the spine. In conclusion, the traditional conception that a backrest facilitates lordosis is apparently not true. It seems rather that backrests actually facilitate the opportunity for the user to stabilize their lumbar spines by providing their lower backs with support, resulting in relative kyphotic increases. The practical ergonomic applications from this study are unclear. However, traditional concepts in backrest ergonomy should be re-considered.