Sitting with Adjustable Ischial and Back Supports: Biomechanical Changes

Study Design. The seat and back contact force, pressure distribution, lumbar lordosis, and low back muscle activities associated with a new seat design with adjustable ischial support and backrest were investigated using kinematic, kinetic, electromyographic, and radiographic measurements. Objectives. To investigate the biomechanical effects of adjusting ischial and backrest supports during sitting. Summary of the Background Data. Sitting may induce posterior rotation of the pelvis, reduction of lumbar lordosis, and increases in muscle tension, disc pressure, and pressure on the ischium and coccyx, which may be associated with low back pain. A device that reduces the ischial load and maintains lumbar lordosis may help increase seating comfort and reduce low back pain. Methods. Fifteen office workers with no known low back pain history were tested. Contact pressure distributions, reaction forces between the buttock-thighs and seat and between the back and backrest, load carried by the seat pan and backrest, sacral inclination, lumbar lordosis, intervertebral space of lumbar spine, and muscular activity in stabilizing the trunk were measured for sitting with and without ischial support and with adjustable back support. Results. When the ischial support was relieved, the center of the force on the seat and on the legs of the chair, and the peak center of pressure on the seat, were significantly (P < 0.002) shifted forward toward the thighs. The total contact area on the seat pan and on the backrest was significantly decreased and increased, respectively (P < 0.001). The sacral inclination, total and segmental lumbar lordosis, and lumbar spine disc height were significantly increased for sitting upright with backrest, with the lumbar curve close to that during standing. Conclusions. Sitting with reduced ischial support and fitted backrest to the lower spine altered the contact area, reduced peak pressure under the ischia, reduced muscular activity, maintained total and segmental lumbar lordosis, rotated the sacrum forward, and increased lumbar intervertebral disc heights, which could potentially reduce low back pain.

[1]  J. McIntosh,et al.  Lumbar disc disease. , 1982, Archives of physical medicine and rehabilitation.

[2]  J. Kostuik,et al.  Spinal Fusions to the Sacrum in Adults with Scoliosis , 1983, Spine.

[3]  J. J. Keegan,et al.  Alterations of the lumbar curve related to posture and seating. , 1953, The Journal of bone and joint surgery. American volume.

[4]  W. Marras,et al.  Spine Loading Characteristics of Patients With Low Back Pain Compared With Asymptomatic Individuals , 2001, Spine.

[5]  R. Narechania,et al.  Relationship of Spine Deformity and Pelvic Obliquity on Sitting Pressure Distributions and Decubitus Ulceration , 1985, Journal of pediatric orthopedics.

[6]  Brandstater Me,et al.  Efficacy of three measures to relieve pressure in seated persons with spinal cord injury , 1994 .

[7]  D. Anton.  Occupational biomechanics , 1986 .

[8]  R. Ortengren,et al.  Lumbar disc pressure and myoelectric back muscle activity during sitting. II. Studies on an office chair. , 1976, Scandinavian journal of rehabilitation medicine.

[9]  E. Itoi.  Roentgenographic Analysis of Posture in Spinal Osteoporotics , 1991, Spine.

[10]  R Aissaoui,et al.  Evaluation of the new flexible contour backrest for wheelchairs. , 2000, Journal of rehabilitation research and development.

[11]  A. Tokuhiro,et al.  Evaluation of wheelchair cushions by means of pressure distribution mapping. , 1998, Acta Medica Okayama.

[12]  E. Bleck,et al.  Radiographic Determination of Lordosis and Kyphosis in Normal and Scoliotic Children , 1983, Journal of pediatric orthopedics.

[13]  D. Hobson.  Comparative effects of posture on pressure and shear at the body-seat interface. , 1992, Journal of rehabilitation research and development.

[14]  A Pedotti,et al.  Comparative biomechanical evaluation of different wheelchair seat cushions. , 2000, Journal of rehabilitation research and development.

[15]  Kevin W. McEnery,et al.  An Analysis of Sagittal Spinal Alignment in 100 Asymptomatic Middle and Older Aged Volunteers , 1995, Spine.

[16]  G R Fernie,et al.  Mechanical Response of the Lumbar Spine to Seated Postural Loads , 1997, Spine.

[17]  W C Hutton,et al.  The effect of posture on the role of the apophysial joints in resisting intervertebral compressive forces. , 1980, The Journal of bone and joint surgery. British volume.

[18]  G. Elfström,et al.  The sitting posture: an electromyographic and discometric study. , 1975, The Orthopedic clinics of North America.

[19]  A F Mak,et al.  Posture effect on seating interface biomechanics: comparison between two seating cushions. , 1996, Archives of physical medicine and rehabilitation.

[20]  W C Hutton,et al.  The effect of posture on the lumbar spine. , 1985, The Journal of bone and joint surgery. British volume.

[21]  R. Daffner,et al.  Lumbar Lordosis Measurement: A New Method Versus Cobb Technique , 1998, Spine.

[22]  R. Jackson,et al.  Radiographic Analysis of Sagittal Plane Alignment and Balance in Standing Volunteers and Patients with Low Back Pain Matched for Age, Sex, and Size: A Prospective Controlled Clinical Study , 1994, Spine.

[23]  M. Brandstater,et al.  Efficacy of three measures to relieve pressure in seated persons with spinal cord injury. , 1994, Archives of physical medicine and rehabilitation.

[24]  G. Waddell.  Low Back Pain: A Twentieth Century Health Care Enigma , 1996, Spine.

[25]  R. Cailliet,et al.  Elliptical modeling of the sagittal lumbar lordosis and segmental rotation angles as a method to discriminate between normal and low back pain subjects. , 1998, Journal of spinal disorders.

[26]  H. Saraste,et al.  Radiographic Measurement of the Lumbar Spine: A Clinical and Experimental Study in Man , 1985, Spine.

[27]  D W Hukins,et al.  Comparison of Structure, Mechanical Properties, and Functions of Lumbar Spinal Ligaments , 1990, Spine.

[28]  Manohar M. Panjabi,et al.  Clinical Biomechanics of the Spine , 1978 .

[29]  DANIEL E. FOX,et al.  Evaluation of Lumbar Lordosis: A Prospective and Retrospective Study , 1985, Spine.

[30]  G. Andersson,et al.  The Influence of Backrest Inclination and Lumbar Support on Lumbar Lordosis , 1979, Spine.

[31]  T. Beeker,et al.  Toward Standards for Posture: Postural Characteristics of the Lower Back System in Normal and Pathologic Conditions , 1985, Spine.

[32]  A F Mak,et al.  Evaluation of an active seating system for pressure relief. , 1995, Assistive technology.

[33]  Von U. Burandt.  Röntgenuntersuchung † über die Stellung von Becken und Wirbelsäule beim Sitzen auf vorgeneigten Flächen , 1969 .

[34]  D G Wilder,et al.  The biomechanics of lumbar disc herniation and the effect of overload and instability. , 1988, Journal of spinal disorders.

[35]  R. Patterson,et al.  Sitting forces and wheelchair mechanics. , 1990, Journal of rehabilitation research and development.

[36]  Michael A. Adams,et al.  Posture and the compressive strength of the lumbar spine , 1994 .

[37]  P. Dolan,et al.  The clinical biomechanics award paper 1993 Posture and the compressive strength of the lumbar spine. , 1994, Clinical biomechanics.

[38]  J A Hawley,et al.  A Comparison of the Effects of Two Sitting Postures on Back and Referred Pain , 1991, Spine.

[39]  D G Wilder,et al.  Spine radiographs in patients with low-back pain. An epidemiological study in men. , 1984, The Journal of bone and joint surgery. American volume.

[40]  Cobb,et al.  Outlines for the study of scoliosis , 1948 .

[41]  Y L Chen,et al.  Vertebral centroid measurement of lumbar lordosis compared with the Cobb technique. , 1999, Spine.

[42]  J L Saumet,et al.  Influence of external pressure on transcutaneous oxygen tension and laser Doppler flowmetry on sacral skin. , 1996, Clinical physiology.

[43]  D L Bader,et al.  The recovery characteristics of soft tissues following repeated loading. , 1990, Journal of rehabilitation research and development.

[44]  M. Boninger,et al.  Displacement between the seating surface and hybrid test dummy during transitions with a variable configuration wheelchair: a technical note. , 2000, Journal of rehabilitation research and development.

[45]  G. Costanzo,et al.  Reciprocal Angulation of Vertebral Bodies in a Sagittal Plane: Approach to References for the Evaluation of Kyphosis and Lordosis , 1982, Spine.

[46]  M. Schendel,et al.  Effect of patient position on the sagittal-plane profile of the thoracolumbar spine. , 1996, Journal of spinal disorders.

[47]  G. Andersson.  On myoelectric back muscle activity and lumbar disc pressure in sitting postures , 1974 .

[48]  T M Cook,et al.  Effect of seat angle and lumbar support on seated buttock pressure. , 1988, Physical therapy.

[49]  H. Farfan,et al.  Lumbar intervertebral disc degeneration: the influence of geometrical features on the pattern of disc degeneration--a post mortem study. , 1972, The Journal of bone and joint surgery. American volume.

[50]  E Johanning,et al.  Evaluation and management of occupational low back disorders. , 2000, American journal of industrial medicine.

[51]  A. Hargens,et al.  Lumbar Spine Disc Height and Curvature Responses to an Axial Load Generated by a Compression Device Compatible with Magnetic Resonance Imaging , 2001, Spine.

[52]  M Bernhardt,et al.  Segmental Analysis of the Sagittal Plane Alignment of the Normal Thoracic and Lumbar Spines and Thoracolumbar Junction , 1989, Spine.

[53]  M. J. Lord,et al.  Lumbar Lordosis: Effects of Sitting and Standing , 1997, Spine.

[54]  Bader Dl,et al.  The recovery characteristics of soft tissues following repeated loading. , 1990 .

[55]  V. Dabbs,et al.  Correlation between disc height narrowing and low-back pain. , 1990, Spine.

[56]  G. Vanhorn,et al.  Lumbar disc disease. , 1985 .