Drivers Beware:
Lumbar Support Can Be Harmful To Your Health!



By Dennis Zacharkow, PT

(Note: This article is a slightly modified version of the original article, published in Physical Therapy Forum, Volume 9, No. 35, September 10, 1990.)

Most health professionals and ergonomists, when asked what are the major faults with sitting posture and chair design, would probably cite the loss of the normal standing lumbar lordosis when sitting, and the lack of proper lumbar support in chair design.

McKenzie (1981) has probably had the greatest impact on the widespread use of lumbar support (lumbar rolls) when sitting, in order to assure proper posture. However, the basic assumptions behind this philosophy of lumbar support have never been thoroughly analyzed.

According to McKenzie (1981), "A good sitting posture maintains the spinal curves normally present in the erect standing position." However, this statement is in opposition to the essential biomechanical action of the spine and pelvis when sitting, which is a posterior rotation of the pelvis and a flattening of the lumbar lordosis (Akerblom, 1948; Anderson et al., 1979). The lumbar lordosis will be less in all sitting postures compared to standing, regardless of the degree of hip flexion mobility present (Bridger et al., 1989).

Porter and Norris (1987) found that sitting upright resulted in a large decrease in the lumbar lordosis compared to standing erect. They found that the maximum displacement of the lumbar spine from the backrest changed from a mean of 6 cm when standing to 2 cm when sitting upright. The preferred location of the lumbar support when sitting for the 20 healthy young adults studied (10 males and 10 females) was only 2 cm forward of the backrest.

The primary reason for the observed posterior pelvic rotation and decrease in lumbar lordosis from erect standing to erect sitting is as follows: In erect standing, the line of gravity of the trunk passes through the hip joints (slightly posterior to the center of the hip joints) (Woodhull et al., 1985). In erect upright sitting, the weight of the upper body is transferred to the seat through the ischial tuberosities, with the center of gravity of the trunk being directly above the ischial tuberosities (Schoberth, 1962; Bridger et al., 1989). As the ischial tuberosities are located posterior to the hip joints in erect standing, the pelvis must rotate posteriorly in order to position the ischial tuberosities vertically below the center of the gravity of the trunk in erect, upright sitting (Bridger et al., 1989).

Regarding proper placement of a lumbar roll as a sitting support, McKenzie (1981) advocates its placement "at or just above the belt line, affecting approximately the area of the third and fourth lumbar vertebrae." However, back support in this region does not assure proper pelvic stabilization when sitting, which is critical for postural stability (Branton, 1969; Zacharkow, 1988).

Stabilization of the pelvis is critical, as the hip joints are in a mid-position when sitting, and therefore stabilization is not possible through ligamentous restraint (Meyer, 1873). Branton (1966, 1969) mentioned that even if an individual appears to sit still, his body is continuously moving. The freedom of the pelvis to move, which will be present in all sitting postures when the upper sacrum is not supported by a backrest, will result in "continuous hunting" or relatively fast oscillatory movements of the pelvis rocking over the ischial tuberosities (Branton, 1969).

As the shape of the lumbar spine when sitting depends directly on the position of the sacrum, support should be given to the upper sacrum and not the lumbar spine (Cohn, 1886; Schoberth, 1969; Branton, 1969). Support across the upper sacrum when sitting will also help stabilize the sacroiliac joints.

Regarding the stresses in the lumbar spine from vibration, recent work supports the view that the bending in the lower lumbar spine due to vibration actually arises from a rocking motion of the pelvis over the ischial tuberosities (Sandover and Dupuis, 1987). Therefore, this study also reinforces the need for sacral-pelvic support rather than lumbar support.

Health professionals should check whether their clients using lumbar rolls actually keep these supports positioned in the L3-L4 region, as advocated by McKenzie (1981). They will probably find that most individuals position the lumbar rolls below the lumbar spine, in order to achieve sacral support and thereby increase their sitting comfort and stability.

Other problems with the use of lumbar supports are as follows (Zacharkow, 1988):

  1. Lumbar supports will distort the proper axial relationship of the thorax and pelvis, with the upper trunk being displaced posterior to the hips (Figure 1).  This posterior displacement of the upper trunk with lumbar support will move the individual further away from the work surface. Trying to maintain this posture when working or driving will often cause increased stress to the neck and upper back.
  2. No lumbar support is possible in a forward-leaning posture, which is a very common work position. Once the individual leans forward from the backrest, lumbar support will be lost. (Unlike lumbar support, sacral support can provide proper pelvic and spinal stabilization in a forward-leaning posture. When leaning forward, there is a slight rocking over the ischial tuberosities, and a tendency for the pelvis to slide backward on the seat (Cotton, 1904). The combination of proper sacral support, along with an open space below the sacral support for the buttocks, will allow the individual to still obtain sacral support in a forward-leaning posture.)
  3. The use of a lumbar support or lumbar roll will result in relaxation and overstretching of the deep lower abdominal muscles, which are critical postural muscles for both sitting and standing posture.
  4. A lumbar roll or support will not correct the main postural fault of a kyphotic sitting posture - the resulting "postural depression" or approximation of the sternum to the pelvis.

As opposed to the segmental view of the body when sitting which considers lumbar support the "cure-all," a more holistic view of the body involves support to both the sacrum and the lower thoracic spine (Zacharkow, 1988). The philosophy behind sacral-lower thoracic support when sitting is that the proper axial relationship of the thorax and pelvis must be restored, with the upper trunk being brought over the hips (Figure 2).

The sacral support will stabilize the pelvis in the proper neutral position, and the lower thoracic support will apply an extension force to the lower thoracic spine, resulting in elevation of the sternum. The ribs, chest, and diaphragm will then be raised to their most optimal positions.

Proper sitting posture should facilitate activation of three key muscle groups: the deep lower abdominals, the lower thoracic erector spinae, and the diaphragm.

With proper activation of the deep lower abdominals when sitting, there will be a beneficial increase in the resting intra-abdominal pressure, resulting in a lengthened spine. Proper activation of the deep lower abdominals will be facilitated by gently pressing the sacrum against the sacral support in a posterior-superior direction (Haynes, 1928). This slight movement will help activate the extensor reflex of the trunk, which involves a co-contraction of the deep lower abdominals and lower thoracic erector spinae (Haynes, 1928; Rathbone, 1934).

Lower thoracic support is also critical, as the approximation of the sternum to the pelvis in slumped kyphotic sitting postures will localize stress to the lower thoracic spine and thoracolumbar junction. This region is where the extensor mechanism of the spine is the weakest (Rathbone, 1934; Wiles, 1937). Lower thoracic support can also play an important role in counteracting the increase in thoracic kyphosis and the decrease in thoracic spinal extension that have been found to occur with advancing age (O'Gorman and Jull, 1987).

Finally, the pressure from the thoracic support against the lower thoracic region of the back can also promote proper diaphragmatic breathing when sitting. Phrenic motoneurons have been demonstrated to be under the control of reflexes elicited by afferent stimulation of the lower intercostal nerves and the dorsal rami of the lower thoracic spinal nerves. Mechanical stimuli applied to the lower thoracic region, such as pressing on the back muscles or on the lower part of the rib cage, have been shown to have an excitatory effect on the diaphragm from an increase in phrenic motoneuron activity (von Euler, 1968; Decima et al., 1969).

In order to have a significant impact on the epidemic back pain associated with motor vehicle and computer seating, health professionals and ergonomists must first increase their knowledge of the dynamics of sitting. This includes questioning the assumptions behind the prevailing recommendation of lumbar support for improving sitting posture.


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