Conference Paper

The Way You Sit Will Never Be the Same! Alterations of Lumbosacral Curvature and Intervertebral Disc Morphology in Normal Subjects in Variable Sitting Positions Using Whole-body Positional MRI

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PURPOSE To define the optimal sitting posture by investigating variable seated positions using positional MRI. Normal lumbar spine position during sitting has long been debated. In 1953, Keegan hypothesised a relationship between lordosis and trunk-thigh angle using plain radiography of only 4 individuals. No one has ever replicated his original study. METHOD AND MATERIALS Measurements of lumbar lordosis angles, intervertebral disc (IVD) heights and translation of the nucleus pulposus (NP) were made on 22 normal subjects with no history of back pain or surgery using a 0.6 Tesla whole-body positional MRI scanner. We then determined which sitting position showed the least biomechanical stress on the lumbar spine. This study would not have been possible on a regular fixed ‘tunnel’ recumbent scanner. RESULTS We found IVD height showed a tendency to decrease as lumbar lordosis decreased in variable sitting positions from reclining to forward flexion. The NP showed movement within a limited range in normal IVDs without change of its overall area. The optimal sitting position was with a trunk-thigh angle of 135 degrees. This position was shown to cause least 'strain' on the lumbar spine, most significantly when compared with an upright 90 degree sitting posture. CONCLUSION We have adapted a small plain radiography study from 53 years ago and shown the best biomechanically and anatomically 'neutral' sitting postion using positional MRI. It is well known that a relationship exists between seating posture and back pain and therefore this study has provided data that will help to reduce the incidence of chronic back problems from bad sitting positions. CLINICAL RELEVANCE/APPLICATION We have shown that positional MRI is of value in the evaluation of future seating design and as a result help to reduce chronic back problems from bad seating posture.

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... In addition to support and spinal articulation, the reclined working posture has advantages. Benefits such as reduced loading on the intervertebral disks and decreased muscle activity are associated with recline (Andersson & Ortengren, 1974a, 1974b, 1974cAndersson et al., 1974;Bashir, Torio, Smith, Takahashi, & Pope, 2006). This finding seems intuitive because during recline, the seated occupant is decreasing the loading on the buttocks and thighs by increasing the loading on the thorax (Bush & Hubbard, 2007), and thus the load through the disks is reduced. ...
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The authors sought to use biomechanical measures, including motion and pressure, to compare four office chairs. The fit of a person to a chair is related to the geometric and kinematic compatibility between the two. This geometric compatibility influences the motions that are allowed or prohibited and the support pressures at the body-chair interface. Thus, during evaluation, it is necessary to treat the chair and user as a system. Four dynamic test conditions were evaluated with 14 participants of varying anthropometries. Test conditions were selected to compare the ability to accommodate primary and secondary motions (recline and spinal articulation) of seated occupants. The ability of a chair to allow recline, yet maintain head and hand positions, was compared across chairs. Also, the ability of each chair to allow and support spinal articulation was evaluated. Motion data for the chair, head, thorax, pelvis, and extremities were collected along with chair back pressures. Upon completion of testing, subjective assessments were also conducted. Statistically significant differences were found between chairs relative to head and hand motions. Also, significant differences were noted for the chairs' ability to move with the body during spinal articulation and the ability to provide support. Subjective assessments also yielded differences. Biomechanical analyses using motions and pressures can be conducted on office chairs with significant differences detected in their performance. Biomechanical assessments can be used to compare and contrast office chairs in terms that are relatable to fatigue reduction as well as operator performance.
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