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Ergonomics of postures and movements
Yes, we need to move but all movements are not the same.
A review of the research literature.
Courtesy of Allsteel
Abstracts of studies cited in our review of the research,
listed in alphabetical order (still incomplete).
Over the last two decades, our focus has gradually shifted from identifying the best single sitting posture towards a more dynamic view of sitting and movement.
While this emphasis on movement has helped us avoid ergonomic risk factors, it also confuses the issues. Movement is critical, but it is not the only consideration. Taken to extremes, a strict emphasis on movement may even introduce new risk factors.
Movements are not the same; we ought to avoid some movements. This research review aims to revisit what we know about sitting postures and movements and the associated implications for sitting and seating.
1. Aaras, A.; Ro, O., (2001) Work with the Visual Display Unit (VDU). Health Consequences. Proceedings of the 35th Annual Conference of the Ergonomics Society of Australia. Oct. 1999, Edited by A. Summers, S. Chew, I. Gibson, J. Miller and W. Pietrocola. 77, 33-49.
This paper is a review of the international literature regarding health consequences for VDU workers. The two main problems reported by VDU workers, visual discomfort and musculoskeletal pain are particularly focused. Important factors for designing the lighting system and workplace are given. A procedure for optometric corrections is discussed. The paper emphasizes the importance of multidisciplinary cooperation in order to reduce visual discomfort and musculoskeletal pain.
2. Adams, M. A.; Dolan, P. (1995) Recent advances in lumbar spinal mechanics and their clinical significance. Clin Biomech (Bristol, Avon). 10, 1, 3-19.
Of the many problems associated with low back pain, those which are most amenable to biomechanical investigation are identified. Recent advances in lumbar spinal mechanics are then reviewed in five sections dealing with mechanical function, mechanisms of failure, movements in vivo, loading in vivo, and the biological consequences of mechanical loading. The discussion suggests that mechanical fatigue damage may frequently be the underlying cause of low back pain, even when degenerative changes are evident in the tissues, and the review ends by suggesting some priority areas for future research.
3. Adams, M. A.; Green, T. P.; Dolan, P. (1994) The strength in anterior bending of lumbar intervertebral discs. Spine. 19, 19, 2197-2203.
This was a motion segment experiment. OBJECTIVES. To determine the strength in bending of lumbar intervertebral discs.
METHODS. Eighteen "motion segments" consisting of two vertebrae and the intervening disc and ligaments were loaded to simulate forward bending movements in life. The range of flexion and the resistance to bending of each specimen was compared before and after removal of the posterior elements. Nine of the discs were tested after the intradiscal pressure had been reduced by damaging the vertebral end plate in compression.
RESULTS. The posterior elements restricted the disc to 80% of its full range of flexion. The strength in anterior bending of isolated discs decreased with age and was 33 Nm on average. Reducing intradiscal pressure did not affect the discs’ strength in bending.
CONCLUSIONS. Disc failure in bending occurs through overstretching of the outer anulus in the vertical direction. In life, the posterior elements may not adequately protect the posterior anulus from fatigue damage.
4. Adams, M. A.; Hutton, W. C. (1980) The effect of posture on the role of the apophysial joints in resisting intervertebral compressive forces. J Bone Joint Surg Br. 62, 3, 358-362.
Cadaveric lumbar intervertebral joints were loaded to simulate the erect standing posture (lordosis), and the erect sitting posture (slightly flexed). The results show that, after the intervertebral disc has been reduced in height by a period of sustained loading, the apophysial joints resist about 16 per cent of the intervertebral compressive forces in the erect standing posture, whereas in the erect sitting posture they resist none. The implications of this in relationship to degenerative changes and to low backache are discussed.
5. Adams, M. A.; Hutton, W. C. (1983) The effect of posture on the fluid content of lumbar intervertebral discs. Spine. 8, 6, 665-671.
Cadaveric lumbar motion segments were creep loaded for 4 hours, and the fluid content of the intervertebral discs was measured and compared with that of unloaded discs. Half of the specimens were wedged to simulate sitting with the lumbar spine flexed, and the other half were wedged to simulate sitting or standing erect. The results showed that "flexed" discs lost more fluid, especially from the nucleus pulposus, than did the "erect" discs. It is concluded that fluid flow in flexed postures can aid the nutrition of the lumbar discs.
6. Adams, M. A.; McMillan, D. W.; Green, T. P.; Dolan, P. (1996) Sustained loading generates stress concentrations in lumbar intervertebral discs. Spine. 21, 4, 434-438.
Cadaveric motion segment experiment. Measurements on each specimen were compared before and after creep loading. OBJECTIVES. To show how sustained "creep" loading affects stress distributions inside intervertebral discs.
SUMMARY OF BACKGROUND DATA. The central region of an intervertebral disc acts like a hydrostatic "cushion" between adjacent vertebrae. However, this property depends on the water content of the tissues and may be lost or diminished after creep.
METHODS. Twenty-seven lumbar motion segments consisting of two vertebrae and the intervening disc and ligaments were loaded to simulate erect standing postures in life. The distribution of compressive stress in the disc matrix was measured by pulling a miniature pressure transducer through the disc in the midsagittal plane. Profiles of vertical and horizontal compressive stress were repeated after each specimen had been creep loaded in compression for 2-6 hours.
RESULTS. Creep reduced the hydrostatic pressure in the nucleus by 13-36%. Compressive stresses in the anulus were little affected when the profiles were measured at 1 kN, but at 2 kN, localized peaks of compressive stress appeared (or grew in size) in the posterior anulus after creep.
CONCLUSIONS. Increased loading of the apophysial joints causes an overall reduction in intradiscal stresses after creep. In addition, water loss from the nucleus causes a transfer of load from nucleus to anulus. Stress concentrations may lead to pain, structural disruption and alterations in chondrocyte metabolism. Disc mechanics depend on loading history as well as applied load.
7. Adams, M. A.; McNally, D. S.; Dolan, P. (1996) ’Stress’ distributions inside intervertebral discs. The effects of age and degeneration. J Bone Joint Surg Br. 78, 6, 965-972.
We investigated the distribution of compressive ’stress’ within cadaver intervertebral discs, using a pressure transducer mounted in a 1.3 mm diameter needle. The needle was pulled along the midsagittal diameter of a lumbar disc with the face of the transducer either vertical or horizontal while the disc was subjected to a constant compressive force. The resulting ’stress profiles’ were analyzed in order to characterize the distribution of vertical and horizontal compressive stress within each disc.
A total of 87 discs from subjects aged between 16 and 87 years was examined. Our results showed that age-related degenerative changes reduced the diameter of the central hydrostatic region of each disc (the ’functional nucleus’) by approximately 50%, and the pressure within this region fell by 30%. The width of the functional annulus increased by 80% and the height of compressive ’stress peaks’ within it by 160%.
The effects of age and degeneration were greater at L4/L5 than at L2/L3, and the posterior annulus was affected more than the anterior. Age and degeneration were themselves closely related, but the stage of degeneration had the greater effect on stress distributions. We suggest that structural changes within the annulus and endplate lead to a transfer of load from the nucleus to the posterior annulus. High ’stress’ concentrations within the annulus may cause pain, and lead to further disruption.
8. Akerblom, B. (1954) Chairs and sitting. Floyd, W.F. and Welford, A.T., Symposium on Human Factors in Equipment Design. Proceedings of the Ergonomics Research Society., 2, 29-35
9. Althoff, I.; Brinckmann, P.; Frobin, W.; Sandover, J.; Burton, K. (1992) An improved method of stature measurement for quantitative determination of spinal loading.
Application to sitting postures and whole body vibration. Spine. 17, 6, 682-693. Abstract: A refined procedure for measuring stature is described; this provides a reproducibility error of 0.4 mm. The procedure accommodates the natural diurnal change in stature and permits estimation of the net stature change caused by a change in spinal loading. A series of measurements done with a cohort of 20 young and middle-aged persons showed that stature decrease was related linearly to the quasi-static load on the spine.
The coefficient of proportionality between load and height loss was inversely proportional to the cross-sectional area of the lumbar discs. This method was used to investigate sitting postures and whole-body vibration to demonstrate the applicability of the procedure to quantify spinal strain (and, therefore, estimate comparative loading) in applied ergonomics. Sitting invariably led to an increase in stature, regardless of the type of chair used or the posture maintained. Whole-body vibration did not induce any loss of stature. Thus, this novel approach was able to enhance understanding of spinal behavior under different loading conditions.
10. Andersson, B. J.; Ortengren, R. (1974) Lumbar disc pressure and myoelectric back muscle activity during sitting. II. Studies on an office chair. Scand J Rehabil Med. 6, 3, 115-121.
11. Andersson, B. J.; Ortengren, R.; Nachemson, A.; Elfstrom, G. (1974) Lumbar disc pressure and myoelectric back muscle activity during sitting. I. Studies on an experimental chair. Scand J Rehabil Med. 6, 3, 104-114.
12. Andersson, B. J.; Ortengren, R.; Nachemson, A. L.; Elfstrom, G.; Broman, H. (1975) The sitting posture: an electromyographic and discometric study. Orthop Clin North Am. 6, 1, 105-120.
The disc pressure of the third lumbar disc and the myoelectric activity of several muscles of the back were measured. Three standing and nine unsupported sitting positions were studied as well as eight support parameters and six sedentary tasks. 1. Myoelectric activity is about the same in standing and in relaxed unsupported sitting. In the unsupported sitting positions the highest level of activity is found in anterior sitting and the lowest in posterior sitting. In the muscles of the cervical and lumbar regions the activity is always lower than in the muscles of the thoracic region.
The disc pressure is considerably higher in unsupported sitting than in standing. In the unsupported sitting positions the highest disc pressure is found in anterior sitting and the lowest in sitting straight. 2. Both the myoelectric activity and the disc pressure decrease when the back is supported. Of the support parameters, the backrest inclination is the most important, myoelectric activity and disc pressure both decreasing with an increase in inclination. The disc pressure is considerably reduced also when the lumbar support is increased and when armrests are used. 3. Myoelectric activity and disc pressure are both comparatively low in writing, higher in typing, and still higher in lifting. In the car driver’s seat the disc pressure increases both when the gear is shifted and when the clutch pedal is depressed. When the gear is shifted, there is also an increase in myoelectric activity.
13. Andersson, G.B.J. (1980) The load on the lumbar spine in sitting postures. Levis, D.J. Oborne and J.A.: Human Factors in Transport Research., New York, Academic Press. 231-239.
14. Au, G.; Cook, J.; McGill, S. M. (2001) Spinal shrinkage during repetitive controlled torsional, flexion and lateral bend motion exertions. Ergonomics. 44, 4, 373-381.
This experiment analyzed the spinal shrinkage due to repetitive exertions confined to each of three separate axes (twist, lateral bend, flexion). While the experiment was performed twice with small technique modifications in the twisting task (and thus two data collections were performed), the essential components were as follows. A total of 20 subjects were loaded with an equal moment of 20 Nm in each of the three axes, on 3 separate days (one axis per day). Subjects performed each task for 20 min at 10 repetitions min(-1), where stadiometer measurements of standing height were taken prior to and immediately following the 20 min exertion. The twisting task demonstrated significant spinal shrinkage (1.81 and 3.2 mm in the two experiments) between the pre- and post-stature measurements while no clear effect emerged for the other two tasks. These data suggest that repetitive torsional motions impose a larger cumulative loading on the spine when compared with controlled lateral or flexion motion tasks of a similar moment.
15. Beasley, R.; Raymond, N.; Hill, S.; Nowitz, M.; Hughes, R. (2003) eThrombosis: the 21st century variant of venous thromboembolism associated with immobility. Eur Respir J. 21, 2, 374-376.
The association between immobility with prolonged sitting and venous thromboembolism has been recognized for › 60 yrs, most recently with long distance air travel. The case of a 32-yr-old male, in whom immobility associated with sitting for long periods at a computer represented the major provoking risk factor for his life-threatening venous thromboembolism, is presented. The authors propose the term "eThrombosis" to describe this 21st Century variant of venous thromboembolism associated with immobility from prolonged sitting. In view of the widespread use of computers in relation to work, recreation and personal communication, the potential burden of eThrombosis may be considerable.
16. Bendix, T (1984) Seated trunk posture at various seat inclinations, seat heights, and table heights. Human Factors. 26, 6, 695-703.
Rani Lueder’s comments: Danish surgeon (Mandal, 1982, 1984) emphasizes that the seat pan should adjust forwards in order to increase the thigh-to-torso angle, as well as reduce leg edema, enhance leg mobility, and facilitate rising from a seat. Related suggestions were earlier also provided by Staffel and Schlegel. Forward slopes do increase lumbar lordosis, although findings by Bendix (1984) have been less dramatic than those of Mandal (1984). Mandal, who has worked extensively with these seats, consistently maintains that once adapted to the seat, users come to prefer it and will use it correctly. However, at least during the initial phases, many people typically dislike slopes greater than 5 or 6°.
17. Bendix, T.; Krohn, L.; Jessen, F.; Aaras, A. (1985) Trunk posture and trapezius muscle load while working in standing, supported-standing, and sitting positions. Spine. 10, 5, 433-439.
A study of standing, supported-standing ("riding" on a rounded seat), and sitting postures was carried out on persons simulating assembly work in places with poor leg space. These postures and the upper trapezius muscle load were examined using statometric and electromyographic methods, respectively. While supported-standing or sitting, the lumbar spine moved toward kyphosis, even where there was no backward rotation of the pelvis. In adopting the position for anteriorly placed work, the upper arms were raised 30° forward or more; then, if a greater reach was necessary, the trunk was flexed as well. It is concluded that if leg space is poor, variation between supported-standing and standing should be encouraged, and an ordinary office chair avoided. Working level should be arranged so that it is lower than 5 cm above elbow level if no arm/wrist support is possible.
18. Bendix, T. (1987) Adjustment of the seated workplace – with special reference to heights and inclinations of seat and table. Dan Med Bull. 34, 3, 125-139.
The influence of varying workplace adjustment on posture, muscular load, and subjective acceptability has been investigated in eight studies. Workplace variables were: seat height, seat inclination, backrest position, table height, and desk inclination. Moreover, some of the adjustments were investigated in relation to the tasks: desk – work, typing, large-scale drawing, and simulated assembly work. Concerning posture, the static aspect was investigated by statometry, while the dynamic aspect was estimated by analyzing the movements transferred to a tiltable seat. Muscular load was assessed by means of amplitude distribution functions derived from electromyography. A five-point scale was used for subjective rating.
It is recommended that the seat should be height adjusted approximately 3-5 cm above popliteal level, and the table top 3-5 cm above elbow level. A slanted desk, placed on a horizontal table, is recommended, as it had greater positive influence on the posture than varying seat adjustments. These recommendations are in accordance to subjective preferences, and to a compromise between optimizing the posture and using the backrest. In addition, the recommended seat adjustment seemed to induce more body movements than the traditionally recommended height just below popliteal level. With the high chair adjustment, the seat should be able to incline forward.
A tiltable seat seemed preferable, but it is still a question whether or not such equipment facilitates movements. The backrest has been demonstrated to be a much lesser determinator for shifting the lumbar spine towards lordosis, than a slanting desk or an increased seat height. The backrest should be pushed against the back after the subject’s selection of the position on the seat. This usually corresponds to an ischial-tuberosity position 6-7 cm behind the axis for seat-tilting. During touch-typing the keyboard should be at the level of the elbows, whereas the significance of a forearm/wrist support is unclear.
19. Bendix,T. (1994) Low Back Pain and Seating. Hard Facts about Soft Machines: The Ergonomics of Seating, Edited by R. Lueder and K. Noro. Taylor & Francis, London. 25, 147-155.
Effects on the musculo-skeletal system; Effector system; Posture; Seating.
20. Bendix, T.; Biering-Sorensen, F. (1983) Posture of the trunk when sitting on forward inclining seats. Scand J Rehabil Med. 15, 4, 197-203.
Changes in posture during one hour of sitting were measured by a statometric method on ten subjects. Four seats were used, one horizontal and three with forward inclinations respectively of 5°, 10° and 15°. With increasing forward inclination of the seat, the spine moved toward lumber lordosis. A supplementary sample showed that 1/3 of the body’s adaptation to the seat inclination took place in the spine and 2/3 in the hip joints. A tendency to a more vertical position of the trunk as a whole was observed on the 5° chair, but the posture of the cervical spine was not influenced by the seat inclination. During the hour of sitting a slight tendency was observed for the subjects to sit with the trunk in the most vertical position at the first measurements. The back curves did not change in any systematic way during the hour. A comfort evaluation showed the 5° forward inclination and the horizontal seats to be preferred.
21. Bendix, T.; Poulsen, V.; Klausen, K.; Jensen, C. V. (1996) What does a backrest actually do to the lumbar spine? Ergonomics. 39, 4, 533-542.
It is generally believed that a backrest facilitates lumbar lordosis. To test this, the spontaneously adopted postures of 12 healthy subjects were measured by a statometric method during 2-h sitting periods on three types of chairs in a stratified sequence. The only difference between the three workstations regarded backrest: ’A’ had no backrest; ’B’ had a vertical lumbar backrest; and ’C’ had an anteriorly curved backrest. In general, the most lordotic postures were assumed with backrest C, whereas backrest B rather facilitated kyphosis as compared with sitting without a backrest.
However, when specifically considering passive sitting, i.e. reading, both types of backrest facilitated kyphosis. Moreover, spinal shrinkage was evaluated by measuring exact height before and after each 2-h sitting period. This was done to assess spinal load.
From this perspective, backrest C induced the greatest load on the spine. In conclusion, the traditional conception that a backrest facilitates lordosis is apparently not true. It seems rather that backrests actually facilitate the opportunity for the user to stabilize their lumbar spines by providing their lower backs with support, resulting in relative kyphotic increases. The practical ergonomic applications from this study are unclear. However, traditional concepts in backrest ergonomy should be re-considered.
22. Bhatnager, V.; Drury, C. G.; Schiro, S. G. (1985) Posture, postural discomfort, and performance. Hum Factors. 27, 2, 189-199.
Key words: Adult, Fatigue/physiopathology, Human Engineering, Male, Posture, Stress/physiopathology, Task Performance and Analysis
23. Boudrifa, H.; Davies, B. T. (1984) The effect of backrest inclination, lumbar support and thoracic support on the intra-abdominal pressure while lifting. Ergonomics. 27, 4, 379-387.
The effect of backrest inclination, lumbar support and thoracic support on the erector spinae muscle when lifting while sitting has been investigated. It was found that the lowest values of the iEMG of the back muscle were obtained when: (1) the lumbar support was positioned +4 cm forward, the thoracic support inclined to +10° and the backrest inclination at 110°; (2) the lumbar support was positioned +4 cm forward, the thoracic support inclined to +10° and the backrest inclined to 100°; (3) the lumbar support was positioned +1 cm forward, the thoracic support inclined to +10° and the backrest inclination 110°; (4) the backrest was inclined to 100° the thoracic support inclined to 10° and the lumbar support 1 cm forward; (5) the backrest inclination and lumbar support were both increased; (6) the thoracic support and the backrest inclination were both increased.
24. Branton, P. (1969) Behaviour, body mechanics and discomfort. Ergonomics. 12, 2, 316-327.
With technological advances and the continuing increase in the time for which people sit, the problem (between seats and body malfunctions) is unlikely to diminish in importance and, even if some may harbor the suspicion that there are no ideal solutions, the search must continue. Perhaps this is the moment to pause and reconsider the conceptual framework of research on sitting on seats.
This paper will try to show that behavioral study reveals some gaps to exist in this framework and will suggest ways of bridging them. In our view the gaps result from the fact that the problems are of an interdisciplinary nature and there are three areas for further researching in which our understanding could be considerably advanced by joint effort. The first is the area between body mechanics and behavior, insofar as it lies between the biological and behavioral approaches to human action. The second is the area between behavior and subjective feelings, where we might look for a theoretical basis for research into comfort and its measurement. In the third area, the technological, demands raised in the other two are to be translated into hardware and then tested systematically to secure validity as a precondition for acceptance by the public at large.
25. Branton, P.; Grayson, G. (1967) An evaluation of train seats by observation of sitting behaviour. Ergonomics. 10, 1, 35-51.
Rani Lueder’s comments: People move continuously (Branton, 1969), and have been found to go through postural cycles that may occur in ten to twenty minute sequences throughout the day (Branton and Grayson, 1967). Various factors affect how people sit. One of these is gender; females consistently sit more upright (e.g., Branton and Grayson, 1967; Floyd and Ward, 1964; Le Carpentier, 1969; Ridder, 1953). Another is the kind of seat (e.g., Floyd and Ward, 1964), although in this case male office workers were more influenced by "better" office furniture than were females.
26. Bridger, R. S.; Von Eisenhart-Rothe, C.; Henneberg, M. (1989) Effects of seat slope and hip flexion on spinal angles in sitting. Hum Factors. 31, 6, 679-688.
Lumbar and thoracic spinal angles of 25 male and 25 female subjects were measured in four sitting postures, with standing angles used as reference. Subjects sat with either 90° or 65° of hip flexion on either flat or forward-sloping seats. Lumbar kyphosis was greatest when the flat seat/90° posture was adopted and least when the sloping seat/65° posture was adopted. The opposite was observed for the thoracic angles, and intermediate results were observed for the other two sitting postures. No statistically significant interactions were observed among seat slope, hip flexion, and subject sex. The findings are discussed with reference to the anatomy of sitting and factors influencing pelvic tilt and the implications for the ergonomic design of chairs.
27. Brodeur, R.R. and Reynolds, H.M. (1990) Passive mechanics of the lumbo-pelvic spine for erect and slumped seated postures. Proceedings of the May 1990 International Conference on Spinal Manipulation. FCER: Arlington, Virginia. 190-193.
28. Brunswic, M. (1984) Seat design in unsupported sitting. Proceedings of the International Conference on Occupational Ergonomics., Toronto, 294-298.
Rani Lueder comments: Every 10° of decreased knee angle is equivalent to increasing the thigh-torso angle by 5° (Brunswic, 1984). Small knee angle good -> relaxes hamstrings. 2:1 ratio knee angles relative to torso angle Brunswick -> lumbar lordosis in lab, not in field (likely due to lower work surface height)
29. Callaghan, J. P.; McGill, S. M. (2001) Low back joint loading and kinematics during standing and unsupported sitting. Ergonomics. 44, 3, 280-294.
The aim was to examine lumbar spine kinematics, spinal joint loads and trunk muscle activation patterns during a prolonged (2 h) period of sitting. This information is necessary to assist the ergonomist in designing work where posture variation is possible – particularly between standing and various styles of sitting. Joint loads were predicted with a highly detailed anatomical biomechanical model (that incorporated 104 muscles, passive ligaments and intervertebral discs), which utilized biological signals of spine posture and muscle electromyograms (EMG) from each trial of each subject.
Sitting resulted in significantly higher (p < 0.001) low back compressive loads (mean ±SD 1698 ±467 N) than those experienced by the lumbar spine during standing (1076 ± 243 N). Subjects were equally divided into adopting one of two sitting strategies: a single ’static’ or a ’dynamic’ multiple posture approach. Within each individual, standing produced a distinctly different spine posture compared with sitting, and standing spine postures did not overlap with flexion postures adopted in sitting when spine postures were averaged across all eight subjects. A rest component (as noted in an amplitude probability distribution function from the EMG) was present for all muscles monitored in both sitting and standing tasks. The upper and lower erector spinae muscle groups exhibited a shifting to higher levels of activation during sitting.
There were no clear muscle activation level differences in the individuals who adopted different sitting strategies. Standing appears to be a good rest from sitting given the reduction in passive tissue forces. However, the constant loading with little dynamic movement which characterizes both standing and sitting would provide little rest/change for muscular activation levels or low back loading.
30. Congleton, J. J.; Ayoub, M. M.; Smith, J. L. (1985) The design and evaluation of the neutral posture chair for surgeons. Hum Factors. 27, 5, 589-600.
A neutral posture chair was designed and evaluated to determine if it was a possible means for reducing or eliminating fatigue experienced by surgeons because of seated body posture during microsurgical procedures. The neutral body posture is defined as the posture found in weightlessness, where the muscle, tendon, and ligament systems acting over the joints are in total balance. The neutral posture chair is a unique combination of a forward-sloping cultivator seat and an English saddle, with wraparound leg trough support. On subjective questionnaires, surgeons rated the neutral posture chair as being generally superior to a currently utilized surgical chair for general comfort, body-part comfort, and chair features.
31. Coppes, M. H.; Marani, E.; Thomeer, R. T.; Groen, G. J. (1997) Innervation of "painful" lumbar discs. Spine. 22, 20, 2342-2349; discussion 2349-2350.
The authors investigated the innervation of discographically confirmed degenerated and "painful" human intervertebral discs.
OBJECTIVE: To determine the type and distribution patterns of nerve fibers present in degenerated human intervertebral discs.
SUMMARY OF BACKGROUND DATA: The innervation of intervertebral discs has previously been extensively described in fetal and adult animals as well as humans. However, little is yet known about the innervation of severely degenerated human lumbar discs. The question may be posed whether a disc that has been removed for low back pain possesses an increased innervation compared with normal discs.
METHODS: The presence of nerve fibers was investigated using acetylcholinesterase enzyme histochemistry, as well as neurofilament and substance P immunocytochemistry. From 10 degenerated and 2 control discs, the anterior segments were excised and their nerve distribution studied by examining sequential sections.
RESULTS: In all specimens, nerve fibers of different diameters were found in the anterior longitudinal ligament and in the outer region of the disc. In 8 of 10 degenerated discs, fibers were also found in the inner parts of the disc. Substance P-immunoreactive nerve fibers were sporadically observed in the anterior longitudinal ligament and the outer zone of the anulus fibrosus.
CONCLUSIONS: Findings indicate a more extensive disc innervation in the severely degenerated human lumbar disc compared with normal discs. The nociceptive properties of at least some of these nerves are highly suggested by their substance P immunoreactivity, which provides further evidence for the existence of a morphologic substrate of discogenic pain.
32. Corlett, E.N., (1999) Are You Sitting Comfortably? International Journal of Industrial Ergonomics 24 10, 1, 7-12.
Historical constraints on seat design are discussed, and some of the proposed solutions presented, with their limitations. A new design of a seat is shown which overcomes many of these limitations, whilst giving the sitter a greater freedom of choice for seat height. It is also argued that the design provides greater protection against back injuries by putting the spine under less initial stress.
33. Corlett, E.N. and Eklund, J.A.E. (1984) How does a backrest work? Applied Ergonomics. 15 (2), 111 – 114.
34. Dimberg, L.; Ullman, J.; Joseffson, L.G.; Ericson, M.; Ericsson, B., (1994) A Comparison between the Effect on Low-Back Pain and Disorders of Sitting in Non-Traditional Chairs with Forward-Sloping Seats and Sitting in Traditional Chairs with Horizontal Seats. The Implementation of Change: NES ’94 – Proceedings of the Nordic Ergonomics Society Annual Conference, Stenungsund, Sweden, 14-16 September 1994, Edited by G. Palmerud. Ergonomics Society of Sweden, Goteborg, Sweden. 1, 144-147.
A randomized, prospective study with a cross-over design of two office chairs, the Ullman and Nomi 3, was carried out over a period of one year (2 x 6 months) among 100 workers with a history of back complaints and with predominantly sedentary jobs (more than 4 hrs/day) in a Swedish engineering company. Back mobility, pain index, analgesic medication and sickness absenteeism were monitored during the periods and at the end of each sitting period the volunteers were asked for a subjective evaluation of the chair. Ninety-four individuals (94%) completed the study.
The number of sick leave periods for back complaints was reduced by about half during the Ullman periods (29) compared with the Nomi periods (53), and about equally for both groups (p=0.03). The total number of days of sick leave for the two groups together during the Ullman chair period was 780 days (8.3 days/individual) as against 843 days (9.0 days/individual) for the Nomi period and of those, 353 days (3.8 days/individual) and 387 days (4.1 days/individual) were due to reported back complaints. No difference between the groups was seen with regard to observed back mobility, reported pain index or pain killers during the period. Thirty-seven individuals gave the Ullman chair an overall rating of 8-10 on a rating scale between 0 (extremely bad) and 10 (excellent) compared with 29 for the Nomi chair. On the other hand, 10 individuals rated the Ullman chair 0-3 compared to 5 individuals for the Nomi chair. No rating parameters gave significant differences. Although not proven, there is some support that the difference observed in reported low-back illness is caused by the difference between the chairs.
35. Dolan, P.; Adams, M. A. (2001) Recent advances in lumbar spinal mechanics and their significance for modeling. Clin Biomech (Bristol, Avon). 16 Suppl 1, S8-S16.
Mathematical models are often used to quantify the overall forces and moments acting on the lumbar spine. However, if the purpose of the research is to explain how spinal tissues can be injured, it is necessary to distribute the overall forces and moments between (and within) different spinal structures, because it is the concentration of force which causes injury, and elicits pain. This paper reviews recent experimental evidence concerning the distribution of forces and moments acting on the lumbar spine. Lordotic postures increase loading of the posterior annulus and apophysial joints, whereas moderately flexed postures tend to equalize compressive stress across the disc, and unload the apophysial joints. Sustained compression reduces the volume and pressure of the nucleus pulposus, while increasing compressive stresses in the annulus and neural arch. Sustained compression also reduces disc height, giving some slack to collagen fibres in the intervertebral disc and ligaments, and causing them to resist bending less. Disc degeneration has a similar effect on disc height, and stress distributions.
On the other hand, discs and ligaments can be subjected to greater bending moments following a period of sustained or repetitive bending, because sustained bending impairs the normal protective reflex from the back muscles, and repetitive bending fatigues the back muscles, reducing their ability to protect the spine. Incorporating this information into mathematical models will make them better able to identify which activities are most likely to injure the lumbar spine in life.
36. Dowell, W.R. (1995) An estimation of lumbar height and depth for the design of seating. Proceedings of the Human Factors & Ergonomics Society 39th Annual Meeting 1995. Santa Monica: Human Factors & Ergonomics Society. v. I, 409-411.
The horizontal and vertical position of the apex of the lumbar curve of 773 seated persons were measured. The position of the lumbar landmark is given. Three common seated anthropometric measurements were also taken: popliteal height, buttock-popliteal length, and elbow rest height. The three common measurements are compared with data from an existing anthropometric database.
37. Dowell, W.R., Yuan, F. and Green, B.H. (2001) Office seating behaviors: An investigation of posture, task and job type. Proceedings of the Human Factors & Ergonomics Society. 1245-1248.
38. Duncan, J.; Ferguson, D. (1974) Keyboard operating posture and symptoms in operating. Ergonomics. 17, 5, 651-662.
Keywords: Australia, Biomechanics, Evaluation Studies, Fingers, Human Engineering, Laterality, Male, Muscle Cramp/epidemiology, Occupational Diseases, Occupational Medicine, Posture, Shoulder, Ulna, Wrist
39. Dunlop, R. B.; Adams, M. A.; Hutton, W. C. (1984) Disc space narrowing and the lumbar facet joints. J Bone Joint Surg Br. 66, 5, 706-710.
Cadaveric lumbar spine specimens of "motion segments", each including two vertebrae and the linking disc and facet joints, were compressed. The pressure across the facet joints was measured using interposed pressure-recording paper. This was repeated for 12 pairs of facet joints at four angles of posture and with three different disc heights. The results were that pressure between the facets increased significantly with narrowing of the disc space and with increasing angles of extension. Extra-articular impingement was found to be caused, or worsened, by disc space narrowing. Increased pressure or impingement may be a source of pain in patients with reduced disc spaces.
The above abstracts are listed alphabetically through Dunlop. This list is still incomplete.
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