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.
Numerous chairs or cushions have been developed to reduce or redistribute the sitting pressure on the ischial tuberosities using custom-fit seat pans. Others chairs are designed to maintain lumbar lordosis by adjusting back support or using a forward tilted seat. However, few chairs use adjustable mechanisms for both ischial release and lumbar support.
The purpose of this study was to investigate the biomechanical effects of tilting down the back part of seat and adjusting the backrest. The hypotheses were the following: 1) when the back part of seat is tilted down, load on the ischial tubercles will be reduced and shifted to the thighs, and low back muscle activity will be reduced; and 2) an increase in lumbar lordosis, forward rotation of the sacrum, and larger disc height will be observed when the back part of seat is tilted downward in combination with a properly adjusted back support.
This study investigated quantitatively the biomechanical effects induced by adjusting ischial and back supports, including the contact pressure distributions, reactive forces between the buttock-thighs and seat and between the back and backrest, muscular activity in back muscles, sacral inclination, lumbar lordosis, and intervertebral space of the lumbar spine. It was found that sitting with this lowered back part of seat and adjusted backrest distributed contact pressure more evenly, significantly reduced peak pressure under ischia, reduced muscular activity, rotated the sacrum forward, increased total and segmental lumbar lordosis, and increased lumbar intervertebral disc height.
When the back part of seat was tilted downward by θ = 18° and the backrest was utilized, Ftot exerted on the backrest was significantly increased. The peak contact pressure on the backrest was significantly increased, and it was significantly decreased on the seat; the largest change was found for the upright with backrest condition. The center of pressure and center of force were significantly shifted anteriorly to the thighs for all sitting conditions. All of these measurements for load and pressure redistribution on the seat and backrest indicated that load on the ischial tubercles was significantly decreased and shifted to the thighs, while load on the back support was increased, mainly with the load component in the posteroanterior direction to maintain lumbar lordosis. Using this new sitting concept made the thighs take up more load over a larger surface area, with less load on the ischial tubercles.
Measurement of load and contact pressure redistribution are important in assessing tissue viability, as prolonged sitting can lead to pressure sore development, increased disc degeneration, and low back pain. Repositioning of the lumbar support to redistribute the interface pressure and load is essential to prevent low back pain. Furthermore, for patients who have limited mobility, body repositioning remains the only way to change their pressure distribution at the body-seat interface.
Muscle activity was decreased in most of the sitting conditions when the back part of seat was tilted down, especially in the lumbar region. Relieving the ischial support may have made the pelvis rotate forward and relaxed muscles in the lumbar region more than in the thoracic regions. Another reason may be different amounts of pressure on the lumbar and thoracic regions during EMG recording for sitting conditions using backrest, especially for the relaxed condition. Under the relaxed sitting condition, the body did not show any significant change in muscle activity at the thoracic regions.
The upright with backrest sitting condition was found to be more efficient to change the pelvis and lumbar structures when the back part of seat was tilted down. The results from load and contact pressure redistribution on the seat and backrest indicated that this sitting condition gave the best results to reduce load on the ischial tubercles and lower spine. Thus, this sitting condition upright with backrest was used to evaluate changes in the lumbar spine and pelvis structures with the backrest fitted partially or fully to the lower spine using radiographs.
Tilting down the back part of seat maintained sacral inclination approaching that of the standing posture. The total and segmental lumbar lordosis were also increased and resulted in an appearance in which the abdomen was particularly prominent and resembled closely that of the standing position. The sacral inclination and lumbar lordosis results from other studies for sitting and standing are given. A comparison between these results and the results from the present study confirmed the similarity.
The benefit of lumbar lordosis was suggested in a postmortem study with an association between decreased lumbar lordosis and increased disc degeneration at L5-S1, suggesting a protective effect for increased lumbar lordosis on the lumbosacral junction. Andersson et al found lordosis to be inversely proportional to intradiscal pressure. Lumbar curvature affects disc pressure by changing the distribution of load between disc and apophyseal joints and also by changing tension in the intervertebral ligaments. The increased intradiscal pressure may also be the underlying factor for the association of decreased lordosis and low back pain. In a study of osteoporotic patients by Itoi, decreased lordosis was associated with increased low back pain. Keegan, in a study of the relation between lordosis and sitting, found the most important factor in low back pain with prolonged sitting to be a decreased trunk-thigh angle with consequent flattening of the lumbar curve. Using a lumbar roll that increases lordosis has been found to decrease low back pain. With decreased lordosis, sitting pressure increases over the ischium and coccyx with resultant pain. Others have also found distinct differences in lumbar lordosis when comparing low back pain patients with healthy patients.
All disc heights were significantly increased by lowering the back part of seat. Extensive and consequent segmental lordosis may decrease intradiscal pressure. The effect of disc height on mechanical properties caused by compressive forces was investigated and found that biomechanical axial stress for the intervertebral disc increased most at L4-L5 due to the decreased disc height in upright posture. The highest incidence of lumbar disc diseases was generally found at L4-L5. 52 The reason for the relatively high frequency of lumbar disc disease was connected to a relatively wide range of motion and high loads at L4-L5 and L5-S1 with lumbar flexion and extension.
It has been reported that a backrest with a protruded part to support the lumbar spine would result in an increase of the lumbar lordosis and the load on the back. The present study supports such an observation. Furthermore, a much greater load reduction was observed in this study for proper lumbar support combined with the ischial tubercles load relief. However, the benefits of unloading the ischia were investigated during a short sitting time. The outcome needs to be evaluated for longer period of sitting with unsupported ischia with the concern that load shifted to the thighs may cause hip pain. Tilting the back part of seat down and up alternately is needed during prolonged sitting. Future study is needed to find the optimal tilting angle of the back part of seat and optimal period for tilting the back part of seat down and up during long periods of sitting.
