ABSTRACT
The lumbar curve could be actively maintained by contraction of the muscles in the back (e.g., latissimus dorsi and the sacrospinalis) but this is very tiring. So, when sitting on a seat with a backrest, the pelvis will rotate backwards until the person’s back comes into contact with the support. In a well designed seat, the weight of the trunk is taken by the backrest, the muscles are then relaxed and the curve of the lumbar spine is supported. Conversely, in a poorly designed seat, the lumbar curve is °attened (a loss of lordosis) increasing pressure within the discs (Figure 8.2), straining the spinal ligaments and gluteal muscles and increasing the thoracic c-shaped curve in the upper spine (increase in kyphosis). In a car, this slouched posture could be exacerbated by design elements, such as low headroom space or a seat cushion length, which is too long. So, although this slouching reduces the need for muscular effort in the trunk, it increases disc loading.
