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


The Instability of Unsupported Sitting: Pelvic and Respiratory Factors

by Dennis Zacharkow, PT
© 2014

Pelvic Instability

The cross-legged sitting position on the floor is the predominant sitting posture in non-industrialized cultures. With the flexion, abduction, and external rotation of the hips in the cross-legged sitting position, the sartorius muscles are activated to prevent the backward rotation of the pelvis (Coe, 1987). (The long strap-like sartorius muscles connect from the tibia bones, just below the knees, to the front of the pelvis.)

However, in the conventional raised sitting posture in a chair, with approximately 90-degree angles at the hips and knees, the sartorius muscles remain slack, and their pelvic stabilizing function is lost. With the sartorius muscles relaxed and the hip and knee joints near the midpoint of their range of motion, and therefore in a state of maximum mobility, there is a much greater potential for pelvic instability with conventional chair seating.

This freedom of the pelvis to move will result in relatively fast oscillatory movements of the pelvis rocking over the ischial tuberosities, unless the pelvis is stabilized with a sacral support (Branton, 1969). Without sacral support, the rocking motion of the pelvis over the ischial tuberosities results in the greatest bending motion occurring at the level of the lowest lumbar vertebrae: L5 and L4 (Sandover and Dupuis, 1987; Sandover, 1988). This correlates with the most frequent level of disc herniation being at L5-S1, the second most frequent at L4-L5.

Respiratory Instability

In unsupported sitting on a stool, with experiments involving both quiet breathing and deep breathing, the sway path of the center of pressure was significantly greater for unsupported sitting compared to standing (Bouisset and Duchene, 1994).

The sway path was also significantly increased during deep breathing compared to quiet breathing.

According to the authors, the subjects seated on a stool without back support "clearly show greater respiration-linked instantaneous instability. Therefore, in sitting posture, autonomic movements such as respiratory movements, as well as voluntary movements, can perturb body balance."

Branton's experiments (1966) in unsupported sitting also showed that part of the continuous movements of the Center of Gravity is related to corrections to body balance due to the Center of Gravity's displacement by breathing. When subjects held their breath, the movements of the Center of Gravity became less pronounced.

Due to the greater postural instability with deep breathing in unsupported sitting, the tendency will be for unsupported sitters to favor shallow, upper chest breathing. This shallow inspiration will minimize the displacement of the Center of Gravity and the resulting postural instability, but it will lead to a chronic state of fatigue.

These challenges to optimal diaphragmatic breathing with unsupported sitting can be solved with the addition of lower thoracic support.

Lower thoracic support has the following advantages for optimizing respiration when sitting, without destabilizing the sitting posture:

  1. Lower thoracic support prevents the shifting forward of the Center of Gravity with deep diaphragmatic inspirations.
  2. The pressure stimulation from the lower thoracic support against the lower thoracic region of the back immediately facilitates proper diaphragmatic breathing by eliciting the intercostal-to-phrenic reflexes (Decima et al., 1969; Leanderson et al., 1987). In other words, the nerve connected to the diaphragm muscle (the phrenic nerve) is under the control of reflexes that are activated by sensory stimulation of other nerves in the region of the lower thoracic spine.

    Pressure from the lower thoracic support against this specific region of the back is one way to elicit these reflexes.

  3. Mechanical support to the lower thoracic spine (T10-T12) elevates the rib cage and prevents its downward movement.

    This rib cage stabilization improves the diaphragm's ability to lift the lower ribs, and hence to expand the lower rib cage. In addition, this mechanical support immediately reduces some of the static load on the scalene muscles.


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