Biomechanics research for safe sitting.

22 Ott Biomechanics research for safe sitting.

Ligaments

Adams et al. (1980) showed that the supraspinous-interspinous ligaments segments are the first ligamentous tissues to become stressed with forward bending of the lumbar spine.

Inter-vertebral disc

Thus, the disc is capable of withstanding the large compressive forces that result from muscular recruitment. Hutton and Adams (1982) found that cadaver discs from males between the ages of 22 and 46 could, on average, withstand single loads of over 10,000 N before failure occurred. In most cases, the failure was in the thin bony membrane that forms the boundary between the disc and the vertebral body (vertebral endplate) rather than through nuclear prolapse. Since the disc is an avascular structure, the health of the endplate is critical for nutrient exchange, and even small failures may hasten the degenerative process.

Researchers have found that prolapsed discs occurred more frequently when the vertebral segments were wedged to simulate extreme forward bending of the spine (Adams and Hutton, 1982). In this position, the anterior portion of the annulus fibrosis undergoes compression while the posterior portion is under tensile stress. Over 40 percent of the cadaver discs tested by Adams and Hutton (1982) prolapsed when tested in this hyperflex posture, and with an average of only 5,400 N of compression force applied. This finding shows that the disc is particularly susceptible to bending stresses. In a later study in which Adams and Hutton (1985) simulated repetitive loading of the disc, previously healthy discs failed at 3,800 N, again mostly through trabecular fractures of the vertebral bodies. Taken together, these studies show that the disc, especially the vertebral endplate, is susceptible to damage when loading is repetitive or when exposed to large compressive forces while in a severely flexed posture.

Muscle action

From a biomechanical perspective, co-contraction is a way in which joints can be stiffened, stabilized, and moved in a well-controlled manner. Cocontraction, however, also has the potential to substantially increase the mechanical loads (compression, shear, or torsion) or change the nature of the loads placed on the body’s articulations during an exertion or motion. This is because any co-contraction of fully or partially antagonistic muscles requires increased activation of the agonistic muscles responsible for generating or resisting the desired external load. Thus, the co-contraction increases the joint loading first by the antagonistic force, and second by the additional agonist force required to overcome this antagonistic force. Therefore, work activities in which co-contraction is more common impose greater loads on the tissues of the musculoskeletal system.

Neurological pathways

Investigations have identified pain pathways for joint pain, pain of disc origin, longitudinal ligaments, and mechanisms for sciatica. In the case of facet pain, several mechanisms were identified including an extensive distribution of small nerve fibers and endings in the lumbar facet joint, nerves containing substance P, high-threshold mechanoreceptors in the facet joint capsule, and sensitization and excitation of nerves in the facet joint and surrounding muscle when the nerves were exposed to inflammatory or algesic chemicals (Dwyer, Aprill, and Bogduk, 1990; Ozaktay et al., 1995; Yamashita et al., 1996). Evidence for disc pain was also identified via an extensive distribution of small nerve fibers and free nerve endings in the superficial annulus of the disc and small fibers and free nerve endings in the adjacent longitudinal ligaments (Bogduk, 1991, 1995; Cavanaugh, Kallakuri, and Ozaktay, 1995; Kallakuri, Cavanaugh, and Blagoev, 1998).

Several studies have also shown how sciatic pain can be associated with mechanical stimulation of spine structures. Moderate pressure on the dorsal root ganglia resulted in vigorous and long-lasting excitatory discharges that would explain sciatica. In addition, sciatica could be explained by excitation of dorsal root fibers when the ganglia were exposed to the nucleus pulposus. Excitation and loss of nerve function in nerve roots exposed to phospholipase A₂ could also explain sciatica (Cavanaugh et al., 1997; Chen et al., 1997; Ozaktay, Kallakuri, and Cavanaugh, 1998). Finally, the sacroiliac joint has also been shown to be a significant, yet poorly understood source of low back pain (Schwarzer, Aprill, and Bogduk, 1995). Hence, these studies clearly show that there is a logical and well demonstrated rationale to expect that mechanical stimulation of the spinal structures can lead to low back pain perception and reporting. How these relate operationally to clinical syndromes is less certain.

There also appears to be a strong temporal component to ligament status recovery. Ligaments appear to require long periods of time to regain structural integrity, and compensatory muscle activities are recruited (Solomonow et al., 1998; Stubbs et al., 1998; Gedalia et al., 1999; Solomonow et al., 2000; Wang et al., 2000). The time needed for recovery can easily exceed the typical work-rest cycles observed in industry.

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.

 The   pMRI evidence

An investigation using Whole-body Positional MRI (pMRI), by FW. Smith, Bashir W (2007) who found that the upright position, at 90°, caused disc contents to move the most, while the relaxed position (135°/45° reclined) caused disc contents to move the least. This confirms that the upright position is the worst for the back, while the relaxed position is the best.

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).  This confirms the bio-mechanical evidence.

pMRI scan in reclined, relaxed, sitting mode shows the NP in a safe mid-position.  The hip angle is at 135°.    the NP is in the safe  mid-position.   This is practical and preferable and is advocated for the 2Tilt principle in the reclined mode.

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 is visual confirmation of the bio-mechanical evidence.

  For full  remediation see: The 2T concept⟶

Next see  ☛ Loading→

This entry was posted in BIOMECHANICS on November 22, 2015.

BIOMECHANICS that determine safe sitting

Bio-mechanical factors and their effects that may determine spinal pathology and lead to backache (Low Back Pain. LBP & further pathology).

The final common pathway to IV Disc pathology at the joints of the lumbar-sacral junction (L3/4, L4/5, L5/S1) is reduction of the L/S joint angle resulting in posterior movement of the intervertebral disc (IVD) contents and can be seen, on pMRI scan (Smith F 2006).    This retropulsion of disc contents can progress to protrusion and extrusion (see ☛The intervertebral disc→).

Prolonged stretching of the ligaments can lead to laxity, is irreversible and can result in ‘Cumulative Trauma Disorder (CTD). ☛ Ligaments & CTD→

Backward pelvic tilt. Effect of upright sitting.

• 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  ☛(See Lumbar & spinal support→).
  • ←Flexion  The axial loading force comes to lie posterior to the pivot point at the Ischial Tuberosities. A backward turning movement results in backward tilt in of the pelvis.
  • The  backward pelvic  tilt flattens the lumbar lordosis and reduces the protective wedge angle of the lower intervertebral discs.  Resulting in  Posterior translation of the disc contents (NP),, see below→
• The loss of low lumbar lordosis reduces or even reverses the protective wedge angle of the lower lumbar joints
• 

2. Axial loading.

With  the currently advocated upright sitting posture there is an increase of compression on the L3 disc is x2.5 above that when lying supine (Nachemson).   This has been questioned by later work and determined by  Wilke (Wilke1999) as 0.10 MPa for standing and  sitting  0.55 MPa.  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.   For more detail see ☛ Loading→

The intra-discal pressure is augmented In the flexed position, largely due to tension of the posterior ligaments,  while the anterior portion of the annulus fibrosis (AF) undergoes compression   In full flexion this can be as much as 100%.   The pressure gradient increases anteriorly,   tending to retropulsion of the nucleus pulposus (NP).

• Axial compression + flexion. 
•  Disc prolapse  occurred more frequently when the vertebral segments were wedged to simulate extreme forward bending of the spine (Adams and Hutton, 1982). In this position, the anterior portion of the annulus fibrosis undergoes compression while the posterior portion is under tensile stress. Over 40% of the cadaver discs tested by Adams and Hutton (1982) prolapsed when tested in this hyper flexed posture, and with an average of only 5,400 N of compression force applied. This finding shows that the disc is particularly susceptible to bending stresses. In a later study in which Adams and Hutton (1985) simulated repetitive loading of the disc, previously healthy discs failed at 3,800 N, again mostly through trabecular fractures of the vertebral bodies. Taken together, these studies show that the disc, especially the vertebral endplate, is susceptible to damage when loading is repetitive or when exposed to large compressive forces while in a severely flexed posture.
• Observed effects :-

Disc narrowing and anterior annular bulge.

End-plate bowing or rupture.

Increased intra-discal pressure

Posterior or poster-lateral prolapse of disc contents   

For remediation to avoid LBP these need to be addressed……    See ☛ Remediation→ 

pMRI. (Positional MRI)

Confirms and validates assumptions derived from the bio-mechanical studies (above) and clinical observation.  These pMRI scans (Smith FW,2007) show that the reclined mode ensures migration  of IVD nuclear material away from dangerous position near the back of the inter-vertebral joint which occurs with upright (90°) sitting.  It is difficult to refute this evidence.

The mid-upright mode is the worst possible position. How did it happen?   ☛ Why? Mandal explains→

And is still recommended….  Some of these diagrams suggest that iliac back support is incorporated.  This is a partial solution providing it is correctly configured. See ☛ Lumbar support→

Posterior elements

• Stretching of the posterior elements (including ligaments) which can become permanent and allow instability. (☛Ligaments & CTD→) & (see ☛ ‘Liagament integrity & creep’)→ 
• The likely forces that must be resisted by the ilio-lumbar and the supraspinous ligament (shown in blue) when sitting in the usual lumbar support seat, which allows backward tilting of the pelvis, with a bodyweight 40kg (excluding the legs) can be calculated.  Simple moments about the centre of the L5 disk suggests a ligament tension of about 70 kg (700 Newtons).  This is probably a worst-case estimate.   (JD Gorman)

Constrained upright sitting

The adverse effects have been long and variously described    ☛Importance of MOVEMENT→.  There is a recognition that prolonged constrained static postures are uncomfortable and deleterious for both spinal and general health.   Recently there has been interest in continuous small amplitude movement for upright chairs, the chair re-aligning with the users centre of gravity,  and termed  ‘Dynamic Seating’.     Exercise is required to maintain this position. This provides proprioceptive feedback and frequent small amplitude pressure changes which may be comforting for short periods and helps multifidus muscle action. Rani  Lueder  gives a review account (Lueder R 2002) and the referenced evidence→is considerable.

The pMRI evidence

An investigation using Whole-body Positional MRI (pMRI), by FW. Smith, Bashir W (2007) who found that the upright position, at 90°, caused disc contents to move the most, while the relaxed position (135°/45° reclined) caused disc contents to move the least. This confirms that the upright position is the worst for the back, while the relaxed position is the best.

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).  This confirms the bio-mechanical evidence.

pMRI scan in reclined, relaxed, sitting mode shows the NP in a safe mid-position.  The hip angle is at 135°.    the NP is in the safe  mid-position.   This is practical and preferable and is advocated for the 2Tilt principle in the reclined mode.

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 is visual confirmation of the bio-mechanical evidence.

Activation of   nociceptors (pain receptors) in the posterior layers of the disc (annulus fibrosis, AF) and adjoining structures (ligaments, dura mater and emerging nerve roots) produce back pain (LBP) and sciatica.

It can be said that as bipedalism and upright (orthograde) posture evolved, the hominin spine was developed in an active orthograde group with a lifespan of about 30 years.   ☛ ‘Paleo-anthropology’→.    After adopting a sedentary lifestyle a wrongly shaped seating artefacts were adopted in historically recent times.   A paradigm change  is now required if these adverse effects are to be corrected.  ☛Remediation→    ☛A Full Solution→

Bio-mechanical factors only have been considered.  The aetiology of LBP is much more complicated.

LBP. Cause (Aetiology).

The causes of LBP are multifactorial  and ill understood.    It is suggested (Adam et al.2002)  that  there is a need to integrate the evidence from genetics, biomechanics, biochemistry, cell biology and psychology to construct a comprehensive model.

Twin studies suggest that only 70% of the UK population is liable to LBP (Spector 1999). Other twin studies strongly emphasise the genetic factor in the aetiology of LBP  (Battie et al., 2002).   Genetics are not a cause of LBP but a risk factor acting through, possibly, spinal configuration or components of the disc composition.

Psycho-social factors, regarded as an important in the large increase in the last 20 years (Waddell 1996), is  more a response rather than a cause, resulting increases reporting.     Anecdotal evidence from patients with chronic LBP should not be ignored.  Although usually unaware of any causative incident, they are fully aware of what makes their symptoms better or worse.

The prevalence and incidence of LBP in populations that do not use Western style upright chairs but other ‘natural’ styles of sitting. which are culturally determined, is very low or nil, suggesting an adverse effect of the use of Western style upright chairs.    In Japan, this increases as elements of the population adopt Western chairs (Schlemper 1987).

Nutrition, environment and lifestyle may all play a part in the aetiology of LBP, but the only new factors that might account for the present epidemic is lack of exercise and a sedentary Western lifestyle.    Driving, office work, computers and TV,  the modern worker spends an increasing amount of time in a chair.

Epidemiological studies of  backache prevalence.

The literature on epidemiological evidence is extensive but, in relation to aetiology, confusing due to uncertainties and imprecision in describing reliably the condition that is under review.  Low back ‘trouble’ extends from LBP of psycho-genic origin to a midline IVD protrusion causing a corda equina syndrome, which is a surgical emergency.  Meta-analysis of systematic reviews, useful for evidence based therapeutics, have to be viewed with caution in extrapolating to clinical conditions. (Furfan et al 2001).  I have often seen wrong conclusions in my own field.

A study by the US Department of Health and Human Resources (NIOSH 1997) reviewed a number of factors and the general conclusions seem to suggest that the evidence was contradictory and confusing. There was an emphasis on non-physical psycho-social factors and heavy load handling.   Much of the research into the causation of spinal breakdown has concentrated on violent or inappropriate spinal usage afflicting manual workers today and hominins in prehistory (See Origins of lumbar vulnerability→).

In a Swedish review (Linton, van Tulder.2001) of controlled trials of preventive interventions for back pain problems only exercise gave evidence of relatively moderate significance perhaps suggesting that sitting has no influence on LBP.   The divergence of the  clinical and other evidence from that of some epidemiological studies suggests that the methodology of the latter should be reassessed rather than an immediate conclusion that seating plays no part in the symptomatology of LBP.

Invisibility is possibly due to  the universality of the upright sitting posture on chairs becoming  a part of the human condition in Western orientated populations, so that it fails to become apparent to recognition in systematic reviews.  This impacts on research, where musculo-skeletal funding is minuscule, in spite of the economic, which is about £13billion annually in the UK, and personal cost .

Next ☛Effects on the upright seated posture.→

This entry was posted in BIOMECHANICS on May 13, 2014.

EXERCISE & movement

Exercise, spinal movement & change of position are important for a number of reasons including IV disc nutrition, for general health and for the avoidance of LBP.    

Intervertebral Disc Nutrition starvation plays a part in nuclear degeneration.  Nutrients are only barely supplied to the central part of the Intervertebral disc by diffusion from surrounding blood vessels in the circumference and the endplates .  Fluid flow also plays a part, especially for the transport of the larger molecular metabolites, and is dependent on the pumping action of pressure changes of the compression/decompression that occur with movement, such as changing from a supine to upright posture (Wilke H-J, 1999.).    Some disc narrowing occurs rapidly but is then followed further, as water is forced out over a longer timescale, and is near maximal in about 6 hours.  This is evident in diurnal variation when the torso is 15-25 mm longer (and stiffer) in the morning when intra-discal fluid pressure is at its highest. Fluid is expelled during the day, while standing, when the compressive force exceeds the osmotic pressure force (Adams MA, Hutton WC.).     During recumbency the disc regains 71% of it’s height in the initial 3.75 hrs but with 3 hrs of upright posture 80% of height loss has occurred.  This accounts for some of the early morning stiffness experienced by those with minor degenerative disc changes and also for the change in height of astronauts on their return from a weightless environment.

General wellbeing, for example, in aiding blood flow and venous return, with other health risks avoided (Aaras, 200045).  Rani Lueder, a respected American ergonomist and researcher, reviews the subject of spinal movement and comes down firmly in support of movement based chairs(200245}.  Gorman (201051), on the other hand, remarked “However the history of chairs with lots of movement is that people don’t like them and they die out pretty quickly…… Also if, as I suggest, pumping and movement is not the central point of low back pain then lots of the justification for such a chair is gone.  I have always argued that it is the flexion of the lowest two discs beyond the natural limit that is the central problem. (”48 & see his URL → )

The negative consequences of constrained sitting has been described by a number of authorities (Linton 2001) (Adams and Hutton, 1983; Duncan and Ferguson, 1974; Eklund, 1967) (Grandjean, 1987). (Kilbom, 1986) (Graf et al. 1993 and 1995) (McGill and Hughson, 2000). Hunting et al. (1981) Kumar (2001) (Bendix 1994)  (Winkel and Oxenburgh, 1990).  for Leg oedema (Winkel 1981; Winkel and Jorgensen, 1986). (van Deursen et al, 2000)   It is “reasonable to assume a causal relationship between constrained postures at terminals and other office machines and impairments and symptoms.” (Hunting, et al, 1981).  Movement reduces these risks ( Aaras et al, 1997). More recent work shows  that high sedentary levels increase heart failure risk regardless of physical activity levels. (DR Young et al.  2014). This was confirmed in another study that also found that  patients with coronary artery disease spend an average of 8 hours each day sitting” (University of Ottawa Heart Institute.)      It has to be emphasised hat no one should sit in a chair all day.  Office managers should arrange for staff to be standing or exercising for 2 hours during a work day.

‘Dynamic Seating’

There is a recognition that prolonged constrained static postures are uncomfortable and deleterious for both spinal and general health.   Recently there has been interest in continuous small amplitude movement for upright chairs, the chair re-aligning with the users centre of gravity,  and termed  ‘Dynamic Seating’.     Exercise is required to maintain this position This provides proprioceptive feedback and frequent small amplitude pressure changes which may be comforting for short periods and helps multifidus muscle action. Rani  Lueder  gives a review account (Lueder R 2002) and the referenced evidence→ is considerable.

A few chairs are considered in this respect → various chairs. How do they measure up? .

Other ‘movement’  systems

The Dondola hinge system allows a controlled all-round instability and a trial when incorporated in a Wagner chair showed reduction of morbidity when compared to the chair without the system.  However, there have been versions of floating, movable seats over the years with springs, rubber doughnuts, wobbly balls etc.  Not hugely commercially successful, they lacked stability in sitting and so were tiring and uncomfortable particularly for the legs which had to be used to maintain the stability that was not provided by the seat. This may be the same with the Dondola free float but the website is uninformative on the mechanism.   They say   “Nowadays we spend up to 14 hours in a seated position. 50% of all Germans suffer at least once a year from back pain, 25% already suffer chronically. For many this

means a permanent psychological strain. The top priority for Wagner is to go against this and increase the performance by caring about the people’s well-being. The most important criteria when buying an office chair, is not if it moves, but how it moves!”

A recently advertised product from Steelcase has taken this to extremes.   Will it succeed?  Comfort, movement, freedom and lack of constraint are good.  Will this make up for potential lack of correct biomechanics?

Who wants to sit on a wobble ball in the office?

The Sit Stand concept 

This has slowed following work showing less difference in sitting and standing intra-discal pressures than was first thought.  A comeback is occurring as the  importance of keeping staff exercised is recognised.   The Sit Stand concept is bio-mechanic superior to the use of existing upright chairs and can be an  adjunct to a 3M system creating a 4M concept (and a ‘full’ solution).. ☛4M workstation→.

Most people prefer sitting, because it is energy efficient with less action by the Erector Spinae muscles than when standing.   However there are situations where the concept is applicable and as the  importance of keeping staff exercised is increasingly recognised.   The negative consequences of constrained sitting has been described by a number of authorities.

A ‘stand’ mode can  be incorporated into the 2T concept with a 3M ‘desk-less’ workstation.  This extra mode results in the 4M, the first change since the inception of the concept in 1998.   (See ☛workstations→). 

Effect of movement on the IV disc

It has been shown that disc nutrition depends on the pumping action of pressure changes due to changes of position and is probably important in avoiding later degenerative changes.  In the context of chair design, movement is comforting and avoids the adverse effects of a prolonged constrained static upright mode on general health (See (Lueder R 2002).

Effect of axial loading  on sitting

Originally Nachemson and others showed that the standing position had a lower intradiscal pressure than upright sitting.  This suggested that incorporating sitting and standing, with a sit/stand desk, could be beneficial if designed into an office environment.

This concept  has slowed following later work (Wilke 1999) showing less difference in sitting and standing intra-discal pressures and the general recent ergonomic improvement of office chairs.     A number of bio-mechanically efficient stools have been produced which usually incorporate a FTS, contouring and sometimes iliac support.

☜ The Pesko ERGO FLEX M 1_1 also incorporates a ‘wobble”.

A recent, more complex example is the Freedman chair in which the 2 halves of the seat can move independently, intending to accommodate spinal irregularity (scoliosis) and pelvic side tilting.  The emphasis is on the FTS concept and movement.

Therapeutic exercise

The effort of balancing to maintain this position exercises the small deep (Multifidus) muscles of the back and helps them to regain their reflex supply following an episode of LBP by what is known as proprioceptive neuromuscular feed back. (☛Muscles→) Spinal instability occurs quickly after any spinal disorder and does not easily recover (Hides 1996) so physiotherapist have developed a wobbly ball for therapeutic exercise when the pelvis is in the correct position. Wilke (1999) gives an intradiscal pressure of MPa 0.5 for sitting on an ergonomic sitting ball with straight back compared to MPa 0.27 when sitting slouched.  The 3M chair has a similar effect in the unstable mode with the uncertain advantage that the lateral component, which may strain the facet joints if used continuously, is avoided. So a patient can exercise discreetly and safely following an episode of backache while using a 2T chair. Who wants to sit on a wobbly ball in an office?

 STOOLS

The exact converse system is designed to get the user off the stool to make way for another user.   A good example is an expensive designer stool with 4 legs and painted black as used in Mac shops.
Being an enthusiastic  Mac user myself, I am only too familiar with these.  They have a round, horizontal seat without conturing which allows point pressure between the ischial tuberosities (ITs) and the hard surface.

Problems may arise for staff & users who have to sit on them for longer periods. The view is of myself but It can be seen repeated in some random shots of staff
users.   In the unavoidable, high loaded, upright position, the pelvis tilts backwards due to  the horizontal surface of the stool seat.  This is augmented by leaning forward, but some slight positive effect may occur by taking weight on the arms.

A light hearted account of 19c office conditions is relevant here.

The 2T inherent exercise system

In the context of the 2Tilt concept, movement is at the users command in the unstable intermediate mode.   The 2T concept is simple and requires two STABLE positions. (‘stable’ is used in the ergonomic and not the engineering use of the term) and an unstable intermediate transition mode.  With physiological benefits it can be considered as a mode the 2T becomes a Triple-Mode (3M) concept.

Exercise in the transition mode.

•  Effort, and abdominal muscle activity is required to bring the chair up from the reclining to the upright sitting mode.  The frequency of this maneuver depends on the number of activities performed in this position   As already described, position changes are important for the nutrition of the intervertebral disc which occurs with spinal movement and in particular by the pumping action of the compression/decompression when changing from a supine to upright posture.
• A further exercise system emerges when the user moves back a few degrees from the forward upright mode and enters the near upright unstable intermediate mode.   Here small amplitude movement is required providing a choice equivalent to  ‘Dynamic Seating’.
• Additionally 2T chair has to be pulled or swung towards the work-top or pushed away from it, depending on which mode is suitable for the task in hand.
• All-round wobble would be preferable to simple anterior/posterior  instability.

Additional systems

These can include a sprung footrest to provide comforting exercise for the calf muscles. These muscles have an important pumping action to aid venous return to the heart and to prevent venous thrombosis.

A standing mode is easily included in a 3M workstation and would be an advantage in working with a colleague.  This extra mode results in a workstation.   See ☛ 4M concept.→

 Exercise in the office

Although a 3M chair provides more exercise than a constrained upright chair this is insufficient, as is for any chair, for normal full requirements. This and the implications should be recognised by the individual and also by an employer who has a ‘duty of care’ for the employee.   Applications, such as “Workpace”, have been developed to interrupt the computer and supervise the operator in simple exercises. A worthy idea but not very exciting.

Sleep.

A reverse concept!  Short periods of sleep (napping) has benefits resulting in increased productivity   See ☛ SLEEP→

Next ☛SLEEP→

This entry was posted in BIOMECHANICS, THE 2 TILT CONCEPT on October 7, 2013.

Intervertebral disc angles and the lumbar-sacral junction.

 Spinal configuration of the intervertebral disc angles at the lumbar-sacral junction is important for preventing IV Disc pathology  and developed as a result of hominins adopting an upright (orthograde) stance for efficient bipedalism.  Lordosis at the vulnerable lumbar-sacral junction increased and this also protected the joints.

Lordosis occurs, as already shown, at two levels of the human spine, cervical and lumbar. Both these spinal levels are where mobile segments meet a solid mass, the skull and the pelvis.  This is where mechanical spinal pathology mostly occurs (Harrison DD 1998) and Cyriax wrote in 1946 that “the spinal joints subject to internal derangement are the 4th, 6th  & 7th cervical and the 4th & 5th lumbar”.   Differences are found when comparing LBP patients with healthy patients (Jackson,1994⁴⁹).

This configuration of the intervertebral disc angles occurred as a result of humans adopting an upright (orthograde) stance for efficient bipedalism.

The importance of IV Disc wedging

The lordotic wedging of the Inter Vertebral Discs (IVD) have an important function in protecting the discs ((Cyriax JH. 1946 Harrison DD 1998 ) and is compromised by some sitting positions.   These views were based on the preservation of the wedge shape of the disc to prevent retropulsion of the disc contents.

•  In moderate extension the position of the disc is stable. The  disc contents lie centrally and the back of the joint is closed which prevents their backward displacement.
• In flexion the back of the joint is opened up so that the wedge shape is lost and the joint surfaces become more parallel or the wedge is even reversed.  The anterior portion of the annulus fibrosis undergoes compression while the posterior portion is under tensile stress increasing the intra-discal pressure in addition to existing axial compression.  This is an undesirable position as the  altered forces acting on the disc contents, tends to force them posteriorly with a liability for prolapse to occur.
• The posterior extent of the AF is weaker than the anterior, with fewer lamellae.
• The effect has already been illustrated with the following diagram.
• This effect is clearly shown on pMRI scanning (below).The wedge angles of IV discsare shown outlined in white on the pMRI scan  (left). (Francis W. Smith et al.)   They are measured more accurately by the shape system used by Meakin et al, shown (right) with dotted  outlines.

The wedge angles of IV discs

 These angles are shown outlined in white on the pMRI scan  (left). (Francis W. Smith et al.)   They are measured more accurately by the shape system used by Meakin et al, shown (right) with dotted  outlines.

The wedge angles of IV discs are the simplest to understand.   It is this angle at the vulnerable L4/5 & L5/S1  ( known as Pre Sacral, PS, 1 & 2 in paleoanthropology) discs that  must be preserved if retropulsion and protrusion are to be avoided.   (Pictures are derived from Francis W. Smith et al.)

In lumbar flexion (left) shows posterior nuclear shift of the nuclear disc contents.

In lordotic extension (right) the nucleus is in the safe mid position.

The Lumbo-sacral (L5/S1) & L4/5 IV disc angles.

The  joints at risk of breakdown are the lower lumbar (L4/5, L5/S1 & to a lesser extent L3/4).   Approximately two-thirds of total lumbar lordosis occurs at the inferior two segments (L4-L5-S1). The total and segmental lumbar lordosis at L4-L5 significantly decreases with age.  As explained these mobile joints are under greater mechanical stress as they adjoin the fused sacral joints which form the posterior wall of the mass of the pelvis.   Having a greater wedge angle than the upper joints, it is likely that this has a protective function which would be lessened if this angle be reduced so that the disc surfaces become parallel or if the wedge is reversed.

The Lumbo-sacral disc angles in sedentary groups liable & not liable to LBP.

Gorman JD reviewing  the x-rays from work by (Pearcy et al.) noted that in the cohort of 11 non- LBP sufferers, a  sedentary group who had not experienced backache in the previous twelve months the standing mean wedge angles were 18° at L5/S1 increasing to 23° with extension.  At L4/5 the mean wedge angle was 16°.

Note that in this group the wedge angles (Left , in extension) in full flexion (Right)  was preserved although reduced.   Loss of this effect is what is likely to result in joint breakdown, including IV Disc prolapse.   This effect is shown on pMRI scan

. This study suggests that bipedalism requires a high wedge angle for protection of the lower lumbar discs (IVDs).  Individuals who do not have a tendency for LBP preserve the low lumbar disc wedge angles even when the spine is flexed.  It has been suggested that they may not have used upright chairs when young.   People who are liable to LBP, some 70% of the UK population, do not have this ability to preserve the wedge angle on spinal flexion and the angle may even become 0° or reversed, resulting in retropulsion of the nucleus of the disk towards sensitive structures.

The wedge angles depend on the configuration  of the lower lumbar joints which has been extensively studied and are relevant to chair design.

Note, in the diagram above, that the upper surface of S1 forms part of both the Sacral horizontal angle and the wedge angle of L5/S1.  The tilt of the pelvis therefore modifies L5/S1 angle.  Upright sitting effects the configuration and reduces the wedge angles.

Lumbar Lordosis (LA)

A new method of calculating  LA from skeletal remains, based on vertebral body wedge angle and the articular process angles, has enabled a study of LA of extinct pre-human and hominin individuals (Been et al., 2010).  Although complicated and with a lack of numbers of extinct forms, it gives a good account of the development of the LA related to bipedalism.

The morphological trend showed the LA jumping from the primitive condition seen in modern apes (22.1°+/- 3.4) to the more human-like LA of early hominoids such as the australopithecines (41° +/_  5)  and hominins such as H. erectus (45°) and then slowly to fossil  H. sapiens (54°8 +/- 14) and modern humans  (51.1° +/-11.0) .  The reduction of the LA in modern humans, at 51.1° from 54°8, who probably sat in chairs since childhood, compared to their extinct Cro-Magnon ancestors, who probably didn’t, might suggest that sitting in chairs has an effect.   The odd ones out are the Neanderthals who show a reversal  with a decreased lordosis (29° +/- 5) when compared with other hominins. with the possible exception of hominins from Sima de los Huesos, who are potentially Neanderthal ancestors. The reduced lordosis of Neanderthals suggests a slightly different gait.  Although having greater body mass and strength, being slower, the reduced LA might imply a reduced LSA or  lumbo-sacral wedge angle with a possibly  increased  liability to LBP and disc protrusion. which might have hastened their demise in the confrontation with H. sap.      The centre of gravity lies further forward later in pregnancy In humans, so that a more pronounced lordosis is required.

 Lumbo-sacral angle (LSA)

More specifically localised to the lumbar-sacral junction which accounts for most of the LA is the Lumbo-sacral angle (LSA) which was used by Abitbol  (Abitbol 1987)  in the study of the phylogenic & ontogenic development of lordosis.

The angle is almost nil in non-erect mammals and only slight in monkeys becoming greater in primates which assume an occasional upright stance and bipedal behaviour. It is much greater, about 30˙, in the early hominids.   The angle includes slight wedging of the lower discs  and, to a lesser extent, the vertebral bodies by 5-10°.  This ‘wedge angle’ has an important protective mechanical effect in preventing posterior prolapse of disc components.   In modern humans the LSA is between 70° to  90°.   At birth the angle is about 20° increasing, as the infant assumes the erect posture and  begins to walk, to about 70° at the age of 5 years.

Sacral-horizontal angle

Next consider the Sacral-horizontal angle which indicates the tilt of the pelvis.

This was used by Pearcy in Xray measurements of a cohort of 11 with no, or low,  incidence of LBP.  This showed a variation between 22-70° and determines the wedge angles of the lower two discs.  The angles being greater than those usually shown in textbooks & journals (Pearcy et al.).  Measurements from this study were further researched by JD Gorman (51).  who, based also on the work of Lovejoy (Tague,  Lovejoy 198652, 53,)  postulated that as the large brained later hominins emerged a larger pelvic birth canal was required.   This was achieved by development of a greater  sacral-horizontal angle which had the effect of increasing the wedge angles of the lowest 2 IV discs and lordosis in additional to the lordosis of early bipedalism.          To see Gorman’s account→

 Paleo-anthropology

Lumbar vulnerability to breakdown at the lumbo-sacral junction originates in the prehistory of the modern and earlier hominins.  Recent changes in human behaviour patterns may have submitted the spine to strains for which it was not developed resulting in increase of the incidence of back pain (LBP) 

The mammalian spine, in it’s various forms, by natural selection has become a beautifully efficient structure.  This has been effected by compromise for often incompatible functions.

•  The spine has to provide midline central support for the whole body.
• Give firm anchorage for the weight bearing limb girdles.
• At the same time it has to allow mobility and respond cybernetically to limb movement and position changes.
• It has to protect the spinal cord and allow the spinal nerve roots egress.
• It may also act as a shock absorber to protect the brain.
• The upright (orthograde) stance adds additional,  sometimes conflicting, requirements.
• These included partial solution to the lumbar vulnerability at the lumbar-sacral junction of the mobile lumbar vertebrae and the mass of the pelvis and a configuration to limit breakdown.
• It has resulted in a number of midline spinal curves, backward kyphosis and forward, lordosis and lumbar vulnerability origins may arise from even minor variations.

Orthograde advantages

The adoption of the inherently unstable upright orthograde posture allowed :-

• Greater height, to see further.
• Improvement in thermo-regulation required for the savannah environment.
• Efficient locomotion to escape predators, hunt prey and outrace scavenging rivals.
• The freeing of the arms from their locomotor function resulted in an increase in manual dexterity allowing new skills and increasing intelligence.
• Rapid effective response and social organisation were further developed by, and were necessary for, hunting skills.
• These, in turn, have determined important human social attitudes and behaviour.

Bipedalism began about 7-5 million years ago (MYA) with the advent of the  Hominidae family, which included ancient forms of  modern orangutans, gorillas and chimpanzees and their extinct relatives such as Ardipithecus  and  Australopithecus and far down the line, towards modernity, to include humans.  Early hominids were originally arboreal, tree living, creatures that began to walk upright along branches and occasionally on land.  The basic adaption of bipedalism was well advanced in Africa about 3-4 million years ago (MYA). Climate change had replaced the jungle habitat to arid grassland and small shrub Savannah and the Australopethecines were advantaged by  exchanging an arboreal for a  mainly terrestrial lifestyle and development of an upright stance and bipedal gait similar  to that of humans.   The morphological changes are well illustrated by the Australopithecine group, typified by A afarensis and one individual, “Lucy” (or officially Al 288-1), who lived in North East Africa.

Bipedalism and the lumbo-sacral junction  

Anthropoid apes have a straight spine and the torso weight lies anterior to the centre of gravity,   Loading can be brought further back by flexion of the hips and knees, described as   ‘Bent Hips/Bent Knees’  (BHBK, gait)  is required For an upright stance.  This incurs higher energy requirements and a slower gait. Simulation of BHBK walking by humans increases energy consumption by 50%. This is because 80% of energy is conserved by the exchange of potential for kinetic energy by the rising and falling of the centre of gravity.

Bipedalism requires anatomical changes so that the torso can remain balanced upright for most activities and there is an ability to stride forward with the swinging gait which is characteristically human.  This requires the lumbar and cervical spine to be extended in a lordotic configuration so that the axial load of the body is directed down to the ground in a near straight line when standing.   The head, specifically the foramen magnum, is balanced vertically over the plane of the hip joints, in males, and the point of contact of the foot with the ground. This has been achieved by pelvic rotation to enable the hips and knees to straighten.  These lordotic angles and the pelvic rotation are  measurable.

Anatomically, an upright (orthograde) posture and Bipedalism results in :-

• Lordotic changes to the lumbar spine to avoid walking with bent hips & knees (BHBK).
• An increase of the IV Disk wedge angle.   This confers a degree of protection from NP retropulsion.
• Rotation of the pelvic iliac blades for muscles to change from being extensors to abductors ensure pelvic stability.
• Shortening of the ilium.
• Relative reduction of the size of the birth canal.

Brain size

The early hominids, such as Homo erectus, had a brain of 900 cc. and its primitive variant, of 1.8 MYA, found at  the Dmanisi (Georgia) site was only 650-780cc. These were probably the earliest hominids outside of Africa (Lordkipanidze, 2005).    H. sapiens, with a volume of about 1300 cc appeared about 130,000 years ago according to the previous ‘out of Africa’ theory’ (Stringer 1970).

Bipedal rats and others 

 Hominids are the only known creatures which are truly bipedal and able to adopt our swinging gait apart possibly wingless birds, such as the ostrich, which are evolved from bipedal dinosaurs. The upright posture occurs in other animals but is usually for short periods and an examination of the skeleton, for example in the penguin, shows a different arrangement with only an analogous appearance of bipedalism.  Performing Japanese monkeys (Macaca fuscata).  can be trained to adopt an upright posture resulting in lumbar lordosis and bipedalism.  Over time some bone remodelling occurs.   However energy expenditure is higher than when plantigrade and they revert to this posture when retired from performing (Nakatsukas 2004).  Slijper, in 1942, gave a detailed account of the changes in the skeleton of a phocomelic goat that had  been born without forelegs. The spine and pelvis had been remodeled with changes suggestive of those found in bipedal animals.   Rats have been shown (Cassidy 1968) to adopt a bipedal stance and gait if their forelegs are amputated at birth. Their ability to function is remarkable. Their posture and locomotion are surprisingly similar to that of humans and provides the nearest animal model to the human bio-mechanical condition at the lumbar spine. The lumbar spine adapts by becoming lordotic and approximates to that of the human spine and there are changes in the muscles acting around the pelvis. It can be shown that there is increased axial loading on the lumbar spine and a high proportion of these rats develop back disorders which are usually, almost uniquely, only found in humans. These include degenerative changes, disc protrusion, facet joint degeneration and spinal stenosis.

For an associated account  of spinal loading, see ☛ IntraDiscal Pressure→

Next for the obstetric pelvis  and it’s evolutionary importance ☛ see next post → Obstetric pelvis of A afarensis

This entry was posted in ANATOMY, ANGLES OF THE IVD, BIOMECHANICS on October 2, 2013.

Positional MRI

Positional MRI (pMRI)                                                                                                                     The development of upright positional MRI (pMRI) scanners, enables imaging of the spine in both upright and functional weight-bearing positions as well as the recumbent position. This allows imaging of the spine in the load-bearing postures which are relevant to sitting.

A pMRI study 2006                                                                                                                             The first pMRI study of backward migration of the Nucleus Pulposus (NP) in functional positions was done at the Positional MRI Centre, Woodend Hospital, Aberdeen, U.K by Professor Francis Smith in 2006.   For this study 11 symptom-free volunteers were recruited, aged between 18 and 60 years. .

Results                                                                                                                                            

The authors concluded “These results support for the first time the validity of clinical assumptions about disc behavior in functional positions: sitting postures may increase risk of posterior derangement, and prone and supine may be therapeutic for symptoms caused by posterior disc displacement.”   These results confirmed the views of JH Cyriax which he had published as far back as 1945² and subsequently by others^3, ,4,5,6.   It also validates the 2Tilt concept as the most effective, possibly the only, way to ensure ‘Safe Sitting’.

The relevant weight bearing positions included sitting upright unsupported, with lumbar support (shown here) and slumped (also shown here). A standing position, which includes lumbar support is also shown here.   Intradiscal pressure in this position was originally measured by Nachemson (1964) showed that intradiscal pressure to be 500-800 N  for a 75 Kg man. See .

Upright sitting, slumped.The usual position and was represented as 30% by students at Cambridge in a quick survey. Intradiscal pressure /load  measured by    Wilke (2001) was 0.48MP  and by Sato  1127 kPa , 800N.  This was less than sitting upright and can be explained by the pressure relieving effect of the abdominal cavity.

Upright sitting with lumbar (not iliac) support.  As can be seen, the support is applied above the Iliac crest and can be expected to have  an opposite effect at the lower 2 joints (Gorman).   This is shown to be so.

In the flexed sitting view (left), It can be seen that there is an adverse posterior position of the Nucleus Pulposus (NP) of the vulnerable L4/5 & L5/S1 discs. When compared to non-loaded, uncompressed disc scans (right), the NP has migrated, or in clinical terms there is retropulsion, to the position that is potentially dangerous.

This is precisely what Cyriax had postulated in 1945.

Unexpectedly confirming the view postulated by JD Gorman the scan shows that support at the upper lumbar joints has a reverse, adverse, effect at the important lower joints.   Lumbar v. pelvic support→See Gorman’s view

For a review from Working Ergonomics→

This entry was posted in BIOMECHANICS on September 25, 2013.

Loading

There is some confusion on the extent and role of spinal compression.

Axial loading & intra-discal pressure (IDP, IAP).

Early in vitro loading studies (Jayson, 1975) showed little or no effect on intact discs in the young.  In most cases, failure occurred in the vertebral endplate rather than by nuclear (NP) prolapse.  Cadaveric discs from males between the ages of 22 and 46 could, on average, withstand single loads of over 10,000 N before failure occurred (Adams AC and Hutton 1982).     When hyper flexed in vitro discs prolapsed at an average of only 5,400 N instead of 10,000 N  (Adam et al.2002).

Rather than through peripheral nuclear prolapse, in most cases, the failure was in the vertebral endplate, resulting in Schmorl’s nodes.   Since the disc is a largely avascular structure, the health of the endplate is critical for nutrient exchange, and even small failures may hasten the degenerative process.

In vivo studies

Loading studies, in vivo by direct measurement, in addition to degeneration and joint flexion,  is complicated and fraught with variations in the state of the disc, neuromuscular activity, facet joint load, ligamentous laxity or  tension.  However all show Intra Discal Pressure (IDP) to be lower in the reclined posture than with mid-upright sitting. IDP studies were pioneered  by Nachemson  (Nachemson A. 1964) who   showed that intradiscal pressure (IDP) to be 500-800 N  for a 75 Kg man, while standing,  Sitting  IDP was higher.

• Standing  100  ( In standard units based on %.)
• Lying supine  (face upwards)  25
• Forward flexion (bending forwards) 150
• Sitting in flexion 185,     sitting and lifting     280,
• Lifting  220,
• Cough   140,   Laugh  150
•  Later modified to lying, 150-250 N,      sitting 700-1000 N.

Later work by others contradicted this and reported near identical pressures for both sitting or standing, being approximately 0.30 MPa (Schultz , 1982).

Modifying factors occur with :-

•  Degree of disc degeneration (Sato et al., 1999)., which is eventually nearly universal.
• Joint flexion (Sato et al., 1999).which occurs with unmodified upright sitting.
• prolapsed discs occurred more frequently when the vertebral segments were wedged to simulate extreme forward bending of the spine (Adams and Hutton, 1982). In this position, the anterior portion of the annulus fibrosis undergoes compression while the posterior portion is under tensile stress. Over 40 percent of the cadaver discs tested by Adams and Hutton (1982) prolapsed when tested in this hyperflex posture, and with an average of only 5,400 N of compression force applied. This finding shows that the disc is particularly susceptible to bending stresses.
• In a later study in which Adams and Hutton (1985) simulated repetitive loading of the disc, previously healthy discs failed at 3,800 N, mostly through trabecular fractures of the vertebral bodies.
• Taken together, these studies indicate that the disc, especially the vertebral endplate, is susceptible to damage when loading is repetitive or when exposed to large compressive forces while in a severely flexed posture.
• When combined with flexion IDP loading can produce regions of higher stress in the posterior annulus strongly associated with disc prolapse (McNally et al., 1993).
• The state of the disc,  it’s volume, surface area,  previous activity and hydration.
• In this respect it should be noted that during 7 hours of a night’s sleep,  intradiscal pressure may change as much as from 0.1 MPa to an average of 0.24 MPa in the morning, an increase of over 100% (Wilke 1995) (Zander T, 2010).
•  The zygapohyseal (facet) joint function and pathology becomes more pronounced with the disc narrowing that occurs with OA..
• Reduction of IDP occurs with extreme spinal extension (lordosis) when the neural arches become compressed together (Adams 1994).   This transfers load to posterior elements, the posterior extent of the annulus, ligaments and facet joints.   These are supplied with nociceptors (Kuslich 1991).
• Compression (IDP) is not simply due to the weight of the body that lies above the level of the specific disc.  The addition is due to co-contraction of the muscles that preserve the upright posture  of the trunk and allow the spinal joints to be stiffened, stabilized, and moved in a well-controlled manner.   This increases in work activities, which includes the position that occurs when sitting upright, and impose greater loads on the discs and other tissues of the musculo-skeletal system (Wilke & Wolf 1995)  and in some spinal flexion deformities.
• Relevant to disc protrusion are torsion and shear forces when applied to a vulnerable disc.

Considerations on the later studies on intradiscal pressures.

Studies, from Japan, Sato  (Sato et al., 1999), using a piezoresistive sensor in the transducer needle detecting the pressure laterally at a window and not at the needle tip, confirmed the pioneering findings of Nachemson and examining the changes in the more relevant L4/5 disc rather than the L3/4.

• This later work showed the load of the spine changes in a curvilinear fashion, when moved from a flexed to an extended position.  In healthy subjects with an average body weight of 73 kg and an average L4–L5 disc cross- sectional area of 16 cm² the compression pressure for prone lying was found to be 144 N, and for Standing  800 N & Sitting upright  996 N, confirming Nachemson’s findings.  Also  intradiscal pressure fluctuate with breathing, which may help disc nutrition when sitting in a cramped position.  Reduction in hydration which occurs with degeneration of the  N. Pulposus significantly reduced the pressure as degeneration progressed.    Sato also radiographically evaluated the disc wedge angle for each measurement and concluded that the spinal load was highly dependent on this angulation.

More recent works differ from that of Nachemson and Sato in that pressure on sitting upright is similar or only slightly more than when standing.  As subjects without LBP or radiological evidence of degeneration were used, this has little relevance to the effect of IDP on discogenic effects.

• Wilke (Wilke et al. 1999) in a study on a single subject using telemetry to record from an implanted transducer  transmitting data, during a variety of daily activities, over 24 hours.  Recorded pressures included :-

1. Standing intradiscal pressure of  0.48 MPa,
2. Relaxed erect sitting without, or not using the  backrest, slightly slumped, as is usual was 0.44 MPa.
3. With lumbar support 0.33 MPa Sitting with actively straightened back with no support was 0.55 MPa.
4. Later work (Wilke et al. 2003) showed loading to be thee times higher during unsupported flexion and almost three times greater than with relaxed sitting.

• Stadiometry, radiological measurement of spinal height used to infer the effect of disc compression, showed sitting postures to have lower compression than upright standing (Althoff et al., 1992, McGill et al., 1996, Lievseth and Drerup, 1997; van Deursen et al., 2005) contrary to needle-transducer results.
• Data from load-cell fixators indicated similar results, consistent with implanted-transducer disc pressure.

Disc protrusion. Pathogenesis

To assess the biomechanical factors involved in the protection of the vulnerable lumbo-sacral junction, knowledge is required of :-

1. Intradiscal compression (IDP). The intra-discal measurements are those of the NP which being aneural is not a pain source..
2. Disc wedge angle of the disc (Sato 1999).  Extension transfers load to posterior elements, the posterior extent of the annulus, ligaments and facet joints which have nociceptors (Kuslich 1991).
3. Position of disc contents (NP)  as shown by pMRI (Smith 2006).

For research studies, in both intradiscal pressure and pMRI, standardised positions are needed for evaluating sitting positions.  My recommendation (ex forward with

arms supported) are shown below :

(My apologies!  The upright forward sitting position with arms resting on desk was forgotten (52% incidence, above).  The photographs were taken hurriedly in the Cambridge MfI department using a passing student.  It was hot in 2012, the London Olympics were in progress and that seemed to be the student ‘dress code’.  HAS)

Other views on discogenic pathogenesis           

Garun Garg at NIOSH  concluded that intradiscal pressure alone  as a risk factor is minimal at best. Protrusion can occur at low IDPs (Kumar, S. (1998, 2001).   The subject was reviewed in Australia (Clause Hides 2008), and concluded “If sitting is a greater threat for development of low back pain than standing, the mechanism is unlikely to be raised IDP.”  but mainly quoting sources confined to studies on intact discs.

Next ☛Intervertebral disc angles and the lumbar-sacral junction.→

This entry was posted in BIOMECHANICS on July 5, 2013.

Lumbar vulnerability and Paleoanthropology.

Lumbar vulnerability to breakdown at the lumbo-sacral junction originates in the prehistory of the modern and earlier hominins.  Recent changes in human behaviour patterns may have submitted the spine to strains for which it was not developed resulting in increase of the incidence of back pain (LBP) 

Development by natural selection has resulted in the mammalian spine, in it’s various forms, becoming a beautifully efficient structure.  This has been effected by compromise for often incompatible functions.  Lumbar vulnerability origins may arise from even minor variations.

• The spine has to provide midline central support for the whole body.
• Give firm anchorage for the weight bearing limb girdles.
• At the same time it has to allow mobility and respond cybernetically to limb movement and position changes.
• It has to protect the spinal cord and allow the spinal nerve roots egress.
• It may also act as a shock absorber to protect the brain.
• The upright (orthograde) stance adds additional,  sometimes conflicting, requirements.
• These included partial solution to the lumbar vulnerability at the lumbar-sacral junction of the mobile lumbar vertebrae and the mass of the pelvis and a configuration to limit breakdown.
• It has resulted in a number of midline spinal curves, backward kyphosis and forward, lordosis.

Orthograde advantages

The adoption of the inherently unstable upright orthograde posture allowed :-

• Greater height, to see further.
• Improvement in thermo-regulation required for the savannah environment.
• Efficient locomotion to escape predators, hunt prey and outrace scavenging rivals.
• The freeing of the arms from their locomotor function resulted in an increase in manual dexterity allowing new skills and increasing intelligence.
• Rapid effective response and social organisation were further developed by, and were necessary for, hunting skills.
• These, in turn, have determined important human social attitudes and behaviour.

The development of Agriculture occurred in the Middle East only about 10,000 years ago, probably as a result of climatic changes (Rowley-Conway, 2007) and allowed a more settled lifestyle and may have involved more non-erect, sedentary activities, such as food preparation and repetitive use of groundstone tools for grinding grain.  This was mainly a female’s activity while the men were away happily hunting.  Archeological evidence shows an increase of spinal degeneration at this time which affected females more than males (Molleson 1994).

Spinal curvature.   DEVELOPMENT

Bipedalism began about 7-5 million years ago (MYA) with the advent of the  Hominidae family, which included ancient forms of  modern orangutans, gorillas and chimpanzees and their extinct relatives, such as Ardipithecus  and  Australopithecus and far down the line, towards modernity, to include humans.  Early hominids were originally arboreal, tree living, creatures that began to walk upright along branches and occasionally on land.  The basic adaption of bipedalism was well advanced in Africa about 3-4 million years ago (MYA). Climate change had replaced the jungle habitat to arid grassland and small shrub Savannah and the Australopethecines were advantaged by  exchanging an arboreal for a  mainly terrestrial lifestyle and development of an upright stance and bipedal gait similar  to that of humans.   The morphological changes are well illustrated by the Australopithecine group, typified by A afarensis and one individual, “Lucy” (or officially Al 288-1), who lived in North East Africa.

Bipedalism and the lumbo-sacral junction  

Bipedalism requires anatomical changes so that the torso can remain balanced upright for most activities and there is an ability to stride forward with the swinging gait which is characteristically human.  This requires the lumbar and cervical spine to be extended in a lordotic configuration so that the axial load of the body is directed down to the ground in a near straight line when standing.   The head, specifically the foramen magnum, is balanced vertically over the plane of the hip joints, in males, and the point of contact of the foot with the ground. This has been achieved by pelvic rotation to enable the hips and knees to straighten.

Anthrapoid apes have a straight spine and the torso weight lies anterior to the centre of gravity,   Loading can be brought further back by flexion of the hips and knees, described as   ‘Bent Hips/Bent Knees’  (BHBK, gait)  is required For an upright stance.

This incurs higher energy requirements and a slower gait. Simulation of BHBK walking by humans increases energy consumption by 50%. This is because 80% of energy is conserved by the exchange of potential for kinetic energy by the rising and falling of the centre of gravity.

The orthograde upright spinal configuration was achieved, through natural selection, by the lordotic and kyphotic curves (below).

Lumbar vulnerability origins become included when the wedge angle of the IV disc is reduced..

The angles that determine lordosis have subsequently been extensively studied (See “Lumbar Lordosis→”).   Note, in the diagram above, that the upper surface of S1 forms part of both the Sacral horizontal angle and the wedge angle of L5/S1.  The tilt of the pelvis therefore modifies L5/S1 angle.  Upright sitting effects the configuration

and reduces the wedge angles.

Already mentioned, the origins of lumbar vulnerability show that lordosis developed at two levels of the human spine, cervical and lumbar. Both these spinal levels are where  mobile segments meet a solid mass, the skull and the pelvis, and where mechanical spinal pathology mostly occurs and differences are found when comparing LBP patients with healthy patients (Jackson, 1994).   Cyriax wrote in 1946 that “the spinal joints subject to internal derangement are the 4^th, 6^th  & 7^th cervical and the 4^th & 5^th lumbar”.       Cyriax also recognised that the lordotic wedging of the Inter Vertebral Discs (IVD) have an important function in protecting the discs (Harrison DD 1998) and is compromised by some sitting positions.

Anatomically, an upright (orthograde) posture and Bipedalism results in :-

• Lordotic changes to the lumbar spine to avoid walking with bent hips & knees (BHBK).   See below⟶
• An increase of the IV Disk wedge angle.   This confers a degree of protection from NP retropulsion.
• Rotation of the pelvic iliac blades for muscles to change from being extensors to abductors ensure pelvic stability.
• Shortening of the ilium.
• Relative reduction of the size of the birth canal.

Brain size

The early hominids, such as Homo erectus, had a brain of 900 cc. and its primitive variant, of 1.8 MYA, found at  the Dmanisi (Georgia) site was only 650-780cc. These were probably the earliest hominids outside of Africa (Lordkipanidze, 2005).    H. sapiens, with a volume of about 1300 cc appeared about 130,000 years ago according to the previous ‘out of Africa’ theory’ (Stringer 1970).

Bipedal rats and others 

 Hominids are the only known creatures which are truly bipedal and able to adopt our swinging gait apart possibly wingless birds, such as the ostrich, which are evolved from bipedal dinosaurs. The upright posture occurs in other animals but is usually for short periods and an examination of the skeleton, for example in the penguin, shows a different arrangement with only an analogous appearance of bipedalism.  Performing Japanese monkeys (Macaca fuscata).  can be trained to adopt an upright posture resulting in lumbar lordosis and bipedalism.  Over time some bone remodelling occurs.   However energy expenditure is higher than when plantigrade and they revert to this posture when retired from performing (Nakatsukas 2004).  Slijper, in 1942, gave a detailed account of the changes in the skeleton of a phocomelic goat that had  been born without forelegs. The spine and pelvis had been remodeled with changes suggestive of those found in bipedal animals.

Rats have been shown (Cassidy 1968) to adopt a bipedal stance and gait if their forelegs are amputated at birth. Their ability to function is remarkable. Their posture and locomotion are surprisingly similar to that of humans and provides the nearest animal model to the human bio-mechanical condition at the lumbar spine. The lumbar spine adapts by becoming lordotic and approximates to that of the human spine and there are changes in the muscles acting around the pelvis. It can be shown that there is increased axial loading on the lumbar spine and a high proportion of these rats develop back disorders which are usually, almost uniquely, only found in humans. These include degenerative changes, disc protrusion, facet joint degeneration and spinal stenosis.

Next for the obstetric pelvis  and it’s evolutionary importance ☛ see next post →

This entry was posted in BIOMECHANICS, PALEO-ANTHROPOLOGY on July 5, 2013.

Effects on the upright seated posture.

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.    There is an opinion that sitting is irrelevant to backache.  Really?  

The adverse effects on the upright sitting posture.  

1. Backward pelvic tilt leading to loss of lumbar lordosis.

• 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→00

• 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.

•  Failure to maintain the wedge angle of the lower two lumbar joints
•  2. Lack of variation in posture and loading.
• See ☛Importance of movement→
• 3. Stretching of the posterior elements (including ligaments) which can become permanent and allow instability.
• ☛Ligaments & CTD→
• 4. The effects of an upright backrest

A typical ‘correct’ lumbar support maintains extension at the upper lumbar joints but allows adverse backward tilting of the pelvis which flexes the vulnerable lower lumbar joints.

Remediation of the rounded back of the slouched position should be by exerting pressure only at the Iliac crests.

☛ Mandal.  The Seated Man→

See ☛Lumbar v pelvic support→

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.

Clinical effects

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.