Sitting with adjustable ischial and back supports: biomechanical changes
From: Spine. 2003 Jun 1;28(11):1113-21
Low back pain is acute or chronic pain involving the lumbosacral, buttock, and/or thigh. Discogenic low back pain is aggravated by the sitting position, which is necessary in many occupations and daily activities. About 100 million workdays are lost annually in the United States due to low back pain. Despite improved knowledge and health care resources for spinal pathology, chronic disability resulting from nonspecific low back pain is rising exponentially. Although the causes of discogenic low back pain are multifactorial and complex, sitting postures could increase stresses within the disc and contribute to disc degeneration and pain. Two major occupational risk factors are static muscle load and flexed curvature of the lumbar spine; both are involved in seated work tasks.
During sitting, the head, arm and trunk weight is carried mainly by the ischial tuberosities and surrounding tissues. High pressure at the tuberosities is closely associated with high load to the spine. A significant mechanical spine loading is associated with low back pain resulting from trunk muscle coactivation. Ischial and lower back interface pressure vary with different sitting postures and body positioning. Repositioning of the lumbar support to redistribute the interface pressure and load is essential in preventing low back pain associated with inappropriate sitting in a working environment. Therefore, a device that decreases the sitting pressure and load carried by the ischial tuberosity may decrease forces within the disc and associated degeneration and pain.
Physiologic lumbar lordosis in the standing position ranges from 40° to 60°, with the lordosis occurring mainly at S1-L5 and L4-L5, and with the sacral inclination ranging from 30° to 40°. Compared to standing or lying supine, sitting could cause the pelvis to rotate posteriorly, resulting in decreased sacral inclination and lumbar lordosis and increased forces at the discs. A number of investigators have reported interaction between low back pain and biomechanical changes such as decreased lumbar lordosis, malalignment of lumbar curvature, and narrowing of disc spaces. Williams et al reported that use of a lumbar roll that increased lumbar lordosis reduced low back pain, and the chair backrest also helps increase the lumbar lordosis and decrease intradiscal pressure.
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Consciously postural sway and cervical vertigo after whiplash injury
From: Spine. 2008 Jul 15;33(16):E539-42.
Cross-sectional study of whiplash injury patients with vertigo and healthy volunteers consciously pretending to have postural sway as in malingering. The aim of this study was to evaluate the postural sway in malingerers by posturography. Malingering is not a problem in the majority of cases with whiplash injury and diagnosis should be made carefully. However, some patients with whiplash injury might exaggerate their symptoms or be malingerers because of the potential gain associated with insurance claims. We designed a diagnostic study to screen putative malingerers.
Subjects were 20 healthy volunteers who were tested under standing condition (normal group), consciously swaying the body under standing condition like malingerers (pseudomalingering group) and 32 patients who complained of neck pain with vertigo or dizziness after whiplash injury with whiplash associated disorders grade I and II . The movement of the center of pressure (COP) was measured using a force platform to quantify postural sway. Static posturography was performed under open and closed eyes. We analyzed (1) total envelop area per unit of time, (2) shifting length per second, (3) sway pattern, and (4) Romberg rate representing total shifting length under eyes-closed/eyes-open.
In open eyes condition, the values of envelop area and length per second were significant higher under pseudomalingering than both of the control and whiplash associated disorders groups. The Romberg rate was 1.30 +/- 0.17, 1.13 +/- 0.19, and 1.83 +/- 0.94 in control, pseudomalingering, and whiplash associated disorders group, respectively, and was less than 1.0 in 45% of pseudomalingering. There were significant differences in the envelop area, length per second, and Romberg rate between pseudomalingering and the other 2 groups.
Results suggest that compared with normal subjects and whiplash associated disorder patients, malingerers are more likely to exhibit a wide envelop area, a long sway length per second, and a low Romberg rate. Malingering must be diagnosed carefully and posturography could be a helpful supplementary tool for differentiating whiplash associated vertigo from malingering.
Effects of slouching and muscle contraction on the strain of the iliolumbar ligament
From: Man Ther. Volume 13, Issue 4, Pages 325-333 (August 2008)
The cause of lower back pain is often attributed to intolerable high intradiscal pressure. Use of the spinal compression model is often referred to for workload standards and is the starting point for spinal buckling instability models for lifting in stooped postures. The precise mechanism producing back sprain is, however, still under discussion. Therefore, we decided to explore a novel approach. In contrast to established biomechanical research we do not relate injury risk to forward trunk inclination, but take the unconstrained erect posture as a starting point.
In a previous study we developed a biomechanical model on sitting with hyperkyphosis while leaning against a high backrest. For verification of the model we measured in vitro stepwise backward tilt of the pelvis combined with forward flexion of the spine. We found that during forward flexion of the L5 vertebra the sacrum moved in the opposite direction (counternutation). During the same test we measured (indirectly) elongation of the iliolumbar ligaments. The increase of strain on the iliolumbar ligament by forward flexion of L5 was similar to that reported earlier.
Because patients suffering from acute lower back pain often present with pain at the site of the iliolumbar ligament we decided to develop a model on sudden slouching. Starting point was the absence or delay of protective muscle force. A higher incidence of lower back pain was found in athletes showing delayed muscle reflex response on a quick force release in trunk flexion, extension and lateral bending. In continuation of our earlier biomechanical model on sitting we decided to model dynamic slouching of the upright trunk. The aim of the present study was to assess failure risk of the iliolumbar ligament by means of explorative calculations and to measure in vitro if such risk could be prevented by back muscles. The following hypothesis was postulated: tension in the iliolumbar ligament increases with forward flexion of L5 and decreases by multifidus and erector spinae muscle contraction.
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Association between sitting and occupational Lower Back Pain
From: European Spine Journal. 2007 February; 16(2): 283–298.
Lower back pain has been identified as one of the most costly disorders among the worldwide working population. Sitting has been associated with risk of developing lower back pain. The purpose of this literature review is to assemble and describe evidence of research on the association between sitting and the presence of lower back pain. The systematic literature review was restricted to those occupations that require sitting for more than half of working time and where workers have physical co-exposure factors such as whole body vibration and/or awkward postures. Twenty-five studies were carefully selected and critically reviewed, and a model was developed to describe the relationships between these factors. Sitting alone was not associated with the risk of developing lower back pain. However, when the co-exposure factors of whole body vibration and awkward postures were added to the analysis, the risk of lower back pain increased fourfold. The occupational group that showed the strongest association with lower back pain was Helicopter Pilots.
For all studied occupations, the odds ratio increased when whole body vibration and/or awkward postures were analyzed as co-exposure factors. whole body vibration while sitting was also independently associated with non-specific lower back pain and sciatica. Vibration dose, as well as vibration magnitude and duration of exposure, were associated with lower back pain in all occupations. Exposure duration was associated with lower back pain to a greater extent than vibration magnitude. However, for the presence of sciatica, this difference was not found. Awkward posture was also independently associated with the presence of lower back pain and/or sciatica. The risk effect of prolonged sitting increased significantly when the factors of whole body vibration and awkward postures were combined. Sitting by itself does not increase the risk of lower back pain. However, sitting for more than half a workday, in combination with whole body vibration and/or awkward postures, does increase the likelihood of having lower back pain and/or sciatica, and it is the combination of those risk factors, which leads to the greatest increase in lower back pain.
Lower back pain is an important public health problem in all industrialized countries. It remains the leading cause of disability in persons younger than 45 years old and comprises approximately 40% of all compensation claims in the United States. More than one-quarter of the working population is affected by lower back pain each year, with a lifetime prevalence of 60–80% and a large percentage of lower back pain claims for long durations (more than 90 workdays lost.
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