Wednesday, February 23, 2011

Analysis of sitting posture

Introduction: Occupations & occupational demands have changed in the recent era. Modernization & Industrialization has changed the face of occupational activities & need of work related physical performances. We have slowly crawled in to mostly a sedentary era. On the other side passive leisure time pursuits are taking over active leisure time pursuits. The situation is such that the average office going adult hardly moves his or her axial & apendicular joints in it’s full range of motion.
Mean temporal classification of ADL (activity of daily living) closely equates 7-8 hours of sleeping; 8-10 hours of working and rest of hours are spent in house hold activities like washing, watching TV, purchasing grocery, rarely gardening or a sports etc. Office hour activities span more than one third of the day. Except in blue collar jobs (manual labor class) office hours consists of at least 2-4 hours sitting to full office hour invested in sitting like in banking, IT sector jobs, and similar office going professionals etc. Invention of computers has lead to increased sitting hours in the office. However occupational sitting differs in western & eastern populations. Urban & rural occupations are different and thus different occupational sitting postures are seen in the people dwelling in these areas. Mostly in Asian rural areas cross legged sitting or it’s variant is used in occupation.
With change in demands on sitting posture at working & home environment there is parallel increase in low back pain in the working population (both office bound & non-office bound) & population in general to epidemic proportions. However, till the last medical review published in 2010, causal relationship of LBA (low back ache) to that of occupational sitting has not been established.

Hence we have chosen to highlight the impact of sitting of lumbar spine ergonomics, possible ways to explain pain production in sitting form dynamic Mckenzie disc model. In the due course we would try to explain the Lumbar spine flexion relaxation phenomenon & stress transfer from contractile to inert structure, spine asymmetry & asymmetric sitting, acquiring awkward posture, ideal spine- neutral zone weight transfer & ideal sitting posture, simple ergonomic correction advices (postural correction, lumbar roll, orthopedic back rest).

Implication of occupational LBA form epidemiological studies:

Low back pain is a major cause of morbidity in high-, middle- and low-income countries, yet to date it has been relatively under-prioritised and under-funded (Hoy D).
LBA in world population: According to a review article by Schochat T et al on epidemiological studies of LBA in the general population two different set of information are available. First comes from the general health surveys and the other from surveys with specific reference to back pain. Data from general health surveys depict point prevalences of LBA between 0.8% and 41% and 1-year prevalences between 15% and 56%. LBA data specially designed to evaluate the prevalence of back pain shows prevalence ranges between 14% and 42% and the lifetime period prevalence between 51% and 84% with highest prevalence is found at age 50 to 64. Either there is no difference between men and women or only a slightly higher prevalence in women.
LBA prevalence rates in high & low income countries & it’s implication: A literature review by Volinn E on LBA epidemiology in high & low income countries is interesting in our context of discussion. According to this study high-income countries comprise less than 15% of the world's population. There is an intra population variation in both high & low income countries depending on where they live. Urban dwelling population in each case had more prevalence rates then their rural counter parts. A 2-4 time higher prevalence is noted among the general populations of high-income countries. This type of consistent behavior indicates a certain factor common in urban population in both high & low income countries noted among workers in particular worksites, referred to as "enclosed workshops." Higher rates among workers in enclosed workshops of low-income countries suggest a disturbing trend: low back pain prevalence may be on the rise among vast numbers of workers as urbanization and rapid industrialization proceed. Considerably lower rates among populations of low-income farmers compared with rates of the affluent populations is contrary to the hypothesis that hard physical labor itself is not necessarily related to low back pain.

Different varieties sitting postures:

A. Common sitting postures:

1. Chair sitting
2. Crossed sitting
3. Crossed sitting with arms wrapped around both knees & locked in front
4. Half crossed sitting
5. Crook sitting
6. Inclined sitting (to back)
7. Inclined sitting (to sides)
8. Inclined long sitting
9. Side sitting
10. Stoop sitting
11. Fall out sitting
12. Ride sitting
13. Kneel sitting
14. Crouch sitting

B. Activities in sitting:

1. Twisting in sitting
2. Bending & reaching in sitting (sidewise- office works & in front- driving)
3. Hitching & Hiking (to relieve pressure on buttocks in prolonged sitting)

C. Co-existing unavoidable stress factors in sitting:

1. Whole body vibration (driving)
2. Noise stress
3. Visual stress
4. Psychological stress

Analysis of muscle work in ideal quiet sitting posture:

Ideal sitting posture: The following discussion is in the context of a quiet ideal chair sitting posture without any upper limb activity. The position is taken on a flat base chair or stool, the height & width of the sitting area allow the thighs to supported & hips and knees is flexed to 900 . In ideal sitting femora are parallel to each other & feet rest on the floor with ankle at 900 where as hells are vertically below the knees.

Muscle work in ideal sitting posture:

a. Joints of lower extremity have no muscle work except at hip. Flexors of hip work in reverse origin insertion fashion to prevent slumping of the lumbar spine.
b. Joints of spine:
i. Global extensor muscles of the spine (Ex-Multifidus): these postural muscles keep the trunk upright. Action of these muscles may be counterproductive at lumbar & cervical spine where it’s action produces a bow string effect & increases the lordotic curvature leading to reduction in the over all height of the spine at these places. Therefore at the lumbar & cervical spine this action must be counteracted by the local flexors (lumbar & cervical spine flexors) to ensure local spine lengthening and maintain the correct & ideal local spine posture.
ii. Flexors of the lumbar spine (Abdominals): In sitting they must work to prevent the bow string effect produced by the global extensor muscle. Scientific literature indicates they contract in an in to out fashion. Hence transverse abdominis is of prime importance in maintaining the core stability & correct spinal alignment. Where as the straight abdominals (rectus abdominis) maintain the correct pelvic tilt matching the spine alignment so that correct contact points are maintained at the chair base- body interface.
iii. Flexors of the cervical spine (pre-vertebral neck muscles) act to prevent the bow string effect produced by the global extensor muscle.
iv. Posture of the head on the cervical spine is finely controlled at the CVJ & at atlanto-axial joint by own set of flexors-extensors to maintain this sagittal posture.

c. Other joints in sitting:
i. TMJ: elevators of the mandible close the mouth against the pull of the gravity.
ii. Thoraco-scapular junction: Thoraco scapular muscle (rhomboids) retracts the scapula so that the glenoid cavity faces laterally. Cervico scapular muscles (levator scapulae) work to elevate a depress scapula due to the pull of the gravity.

d. Joints of upper extremity: No muscle work is required for quite sitting but sitting with occupational arm demands may leads to more activation of the external rotators of the arm, abductors, elbow flexors, forearm pronators, wrist extensors & finger flexors.

Search words:
Low back pain, Occupational low back pain, occupational low back pain in sitting, Low back pain epidemiology, Ergonomics, Work ergonomics, dynamic spine model, Mechanical diagnosis & therapy, Mckenzie therapy etc

Can occupational sitting cause LBA?


Sociocultural, economic, and manufacturing factors are 3 different major factors that influence sitting. "Correct" seated posture is associated with spine health was known to people possibly from Hippocratic era. But the debate has not dampened yet whether the sitting is directly a cause of LBA or it predisposes or precipitates LBA. From 1980s much of research has been put into ergonomically correct sitting & providing a work environment for correct sitting posture. However there is a question; is there any medically correct sitting posture? The correct sitting posture refers to maintenance of a correct lumbar spine lordosis actively (muscle contraction) or passively (chair or sitting back support) (described below elaborately in neutral spine position). Passive maintenance is sought because it is noticed that within minutes of sitting lordosis is reversed or lost. According to Ernst even the correct sitting is not out of danger because of the repetitive nature of compressive loads on vital spine tissues having multiple avenues of pain production. But certainly correct sitting posture & correct work ergonomics may lead to minimize injury.

Importance of Lordosis in seating ergonomics:

Sagittal balanced spinal posture is a goal for spinal surgery and conservative ergonomics. The configuration of the spine, postural position, and weight transfer is different in the 3 types of sitting: anterior, middle, and posterior. Among these 3 different; flat, long lordosis, short lordosis seating postures, flat posture show the least muscle activity similar to the slump posture.

Ergonomic advices for jobs with sitting: Lumbar lordosis is affected by trunk-thigh angle (THA) and the knee angle (KA). THA 110 to 130 degrees with arm rest, back rest & concomitant lumbar support (or a pneumatic support) has the lowest disc pressures and lowest muscle activity from spinal muscles. A seat-bottom posterior inclination of 5 degrees and armrests can further reduce lumbar disc pressures and muscle activity while seated. To reduce forward translated head postures, a seat-back inclination of 110 degrees is preferable over higher inclinations.

Ergonomic advices for jobs with sitting & whole body vibration: Adjustable seat back incline of 110 degrees from horizontal, a changeable depth of seat back to front edge of seat bottom, adjustable height, an adjustable seat bottom incline, firm (dense) foam in the seat bottom cushion, horizontally and vertically adjustable lumbar support, adjustable bilateral arm rests, adjustable head restraint with lordosis pad, seat shock absorbers to dampen frequencies in the 1 to 20 Hz range, and linear front-back travel of the seat enabling drivers of all sizes to reach the pedals. The lumbar support should be pulsating in depth to reduce static load. The seat back should be damped to reduce rebounding of the torso in rear-end impacts.

This is a part of one of my international seminar on “Occupational health & hygiene” held at bhubaneswar, Orissa, India 23-25 feb 2011. I was invited as a speaker on the first day of the congress. My topic was “Can sitting cause occupational LBA?”

* With due apology we want to mention that the references here are not published.