- ‘Correct’ upright sitting posture requires considerable effort at odds with the body energy saving demands.
- So, not only is it almost impossible to maintain but also incorporates factors that have and produce negative bio-medical effects with the potential to result in musculo-skeletal breakdown of skeletal components.
- These consist of the intervertebral discs and the ligament and muscular structures that lie posteriorly and occurs mainly at the lumbo-sacral junction and cervical joints.
- It counters how the spine has adapted to accomadate these effects that occurred with hominid bipedalism, orthograde posture and enlarged cranialisation shown by paleo-anthropology. Lumbar vulnerability and Paleoanthropology →.
- On adopting an upright seated posture there are a number of changes from the balanced standing position that can affect the vulnerable lumbo-sacral junction.
The adverse effects on the upright sitting posture.
1. Backward pelvic tilt leading to loss of lumbar lordosis and reduction of the protective wedge angle.
- When sitting there is backward tilting (anat; forward rotation) of the pelvis, which reduces or reverses the protective wedge angle of the lower lumbar joints The backward pelvic tilt with the loss of low lumbar lordosis reduces or even reverses the protective wedge angle of the lower lumbar joint.
- These effects are augmented by: →
- Misplaced lumbar support .
- (support should be ‘pelvic’. directed to the iliac crest. (Gorman J.)) ☛(See Lumbar & spinal support→).
- Hips at 90° due to seat being parallel to floor.
- With the pull of the hamstring a gluteal muscles/
- (Ideally hip at 110-135°which is achieved by a FTS or in a reclined mode)
- Axial loading. . With the currently advocated upright sitting posture there is an increase of spinal loading 500% above that of lying supine which is relevant to the 2T reclined mode. Determined by Wilke (Wilke1999) as 0.10 MPa for standing and sitting 0.55 MPa. See ☛Loading→
- Axial compression + flexion. As shown above the axial loading force comes to lie posterior to the pivot point at the Ischial Tuberosities. A backward turning movement results in backward tilt of the pelvis with flexion of the vulnerable lower joints . This all results in the main factors in retro-pulsion of the IV disc contents.
3. Stretching of the posterior elements
Including ligaments which can become permanent and allow instability.
Effect of flexion on the IV disc
In the flexed position, the anterior portion of the annulus fibrosis undergoes compression while the posterior portion is under tensile stress. This augments the intra-discal pressure largely due to tension of the posterior ligaments. In full flexion this can be as much as 100%. The pressure gradient, increased anteriorly, tends to retropulsion of the nucleus pulposus (NP).
The pMRI evidence
The above effect has been shown incontrovertibly to occur by pMRI scans (Smith FE 2006). From which the following pictures are derived (arrows, etc, are added).
pMRI scan in reclined, relaxed, sitting mode shows the NP in a safe mid-position.
The hip angle is at 135°.
pMRI scan in an upright sitting mode shows the NP has translated posteriorly which can culminate in protrusion.
Hips are at an angle of 90° with the seat-pan horizontal.
This confirms the bio-mechanical evidence.
The axial compression, whether raised or not, which occurs while sitting in the upright position and usually compounded by a near extreme range of spinal flexion, is postulated as a likely to be an important cause of compromise of a degenerate disc.
If the annulus is unable to contain the increased hydrostatic pressure of the nucleus, while being stressed vertically and simultaneously stretched posteriorly by forward pelvic tilting, retropulsion of the nucleus pulposus becomes more likely. Anterior bulging also occurs and in time becomes contained by osteophytic (‘Parrot’s beaks’ in French) outgrowths from the adjoining vertebral margins. These are evident on x-rays and are silent clinically. This is in contradistinction to disc bulging with a backward or a postero-lateral protrusion where impingement is likely on a number of pain sensitive structures. Herniation or rupture of the annulus can then occur with posterior nuclear or, more commonly, a postero-lateral protrusion which can threaten the emerging nerve root. A large midline extrusion can rupture the posterior longitudinal ligament (PLL) resulting in the surgical emergency of a Corda Equina syndrome.
Office workers seated positions
The Cambridge 2T project study. In an ad hoc observation of sitting positions in a nearby office, students came up with the following results in relation to the postures shown above. In the absence of reclined chairs 52% + 30% avoided the upright posture.
Hermann Miller also show research on body/chair interface pressures in the upright positions. Pressure maps are shown but none of a semi-reclined mode are shown as in the Okamura research.
Effect of sitting for prolonged periods.
Sitting for prolonged periods is obvious to most patients and clinicians who treat them as an association with LBP with and results in a high reporting of LBP by sedentary workers and was recognised in the early work as a risk factor (Bendix 1994).
Sitting in the ordinary way is not associated with spinal pathology unless prolonged for about half a workday (Lis AM, 2007) and a higher prevalence rate has also been reported in those occupations that require the worker to sit for the majority of a working day and is significantly higher than the prevalence rate of the general population (Papageorgiou 1975) particularly among those aged 35 years and older who have had sedentary jobs for several years (Kelsey JL. 1971) and if combined with awkward postures or Whole Body Vibration (Lings S, 2000) which occurs with driving a car (Kelsey 1975), tractor or, worst, a helicopter. This association was refuted in a later study with 45 pairs of identical twins (Battie et al., 2002).
It is not a general view but long-term, low-level chronic stressors that occur with prolonged sitting (Wood and McLeich, 1974.) may be as important as acute impact forces such as falls and lifting strains of heavy manual work. Sitting for more than 6 hours daily increases mortality by 37% for females and 17% for men. The gender difference is unexplained (Patel.2013).
There is a strong presumption that prolonged sitting in adverse ergonomic circumstances, especially in childhood, is fundamental to the explosive rise in the prevalence and incidence of spinal pathology and LBP.
For full remediation see: The 2T concept⟶