From: Spine. 2008 Jul 1;33(15):1643-9

Injury to the cervical facet capsular ligaments is a potential mechanism for chronic neck pain after acute whiplash injury. Distending the facet capsule by injecting contrast media has produced whiplash like pain patterns in normal individuals, and anesthetic blocks have isolated the cervical facet joints as the source of pain in about half of a chronic whiplash population. More recently, in vivo animal models of facet capsule loading have shown that group III and IV afferents (thought to mediate pain) from the facet capsule have a graded electrical response to mechanical loading of the facet joint in the goat and have suggested that a capsular ligament strain threshold exists above which allodynia pain in response to a normally nonnoxious stimulus is produced. These data support a facet capsule based mechanism for whiplash injury, but do not establish whether human capsular ligaments are injured in the low speed rear end collisions to which many whiplash injuries are attributed.

Whiplash patients who had their head turned at impact have more severe and persistent symptoms than patients who were facing forward. These findings have prompted biomechanical studies using human cadaveric necks to investigate why a head turned posture increases injury potential. Dynamic rear impact tests of prerotated ligamentous spines (occiput-T1) produce increased neck flexibility (interpreted as injury) in extension, lateral bending and axial rotation. Though concentrated in the lower cervical spine, these injuries were not isolated to particular spinal ligaments. Detailed measurements of the strain field in the facet capsule have also shown that a head-turned posture generates higher capsular strains than a neutral head posture, but the quasi static loads applied during those tests were limited to pure neck flexion/extension moments and did not include the axial compression or posterior shear present during whiplash loading. Thus the question of how a head turned posture combined with multiaxial whiplash loads affects facet capsular ligament strain has yet to be answered.

The goals were to use human cadaveric motion segments to: (1) quantify the intervertebral kinematics and facet capsule strains under whiplash like loads in the presence of an initial axial rotation, and (2) compare the capsule strains generated by these combined loads to the previously published strains needed to injure these ligaments in isolated shear failure. The overall hypothesis was that capsular strains during this simulated whiplash exposure are similar to those needed to injure the capsular ligament.

Axial pretorque and the resulting axial rotation of the intervertebral joint have a large effect on the maximum principal strain in the cervical facet joint capsule when combined with compression, shear, and extension loads simulating a low speed rear end automobile impact. Peak strains in the capsule with an ipsilateral pretorque were double the previously reported peak strains without a pretorque but similar to the previously reported strains to cause partial failures in these specimens. These findings potentially explain the increased severity and persistence of whiplash neck symptoms in patients who had their head turned at impact.

Previous findings suggest that the facet capsules located on the side of the neck towards which a vehicle occupant’s head is turned are most likely to be injured in a rear end crash, although they could find no clinical or epidemiological data to support or refute this proposition.

The quasi static loading rates used in the current flexibility tests and previously published failure tests were similar, but nonetheless lower than those present during actual whiplash exposures. Quasi static loading rates have been shown to affect the magnitude of the load at failure, but maximum principal capsular strain and displacement to failure are not significantly affected by loading rate. Thus aside from the unwinding effect, the capsular strains reported here are expected to be similar to those present during dynamic whiplash events.

During the multiaxial tests, 2 of 13 specimens exceeded the strain needed to cause partial failure of the capsule. Although they could not discount the possibility that other specimens experienced a partial failure during the whiplash like exposures, the potential for 15% of specimens to exceed a threshold for partial failure is consistent with earlier quasi static work and more recent dynamic work. Similar levels of capsule strains have produced behavioral and electrophysiological evidence of short and long term pain in animals, although both animal experiments strained the dorsal aspect of the capsule rather than the lateral aspect studied here. This 15% risk of partial failure in the capsule is similar to the 12% risk of whiplash exposed individuals suffering chronic symptoms ( over 6 months), though considerable work remains to determine whether these similar risk values are related or coincidental.

Two other specimens exceeded the strain needed to cause gross failure of the capsule. There was no evidence of gross failure during their tests and thus this finding likely highlights limitations in their technique. They previously assumed that failures occurred in the element with the highest maximum principal strain, yet in this study they compare whiplash and failure strains quadrant-by-quadrant rather than element-by-element. Regional differences in the ligament could also result in different mechanical tolerances at different locations within a quadrant or element. Moreover, the failure tests were conducted along the anteroposterior axis of the facet joint, whereas the whiplash tests exposed the joint to compound three-dimensional displacements. This means that different ligament fibers may have borne the loads during the whiplash and failure tests. Thus even though their technique provides more detailed strain field information than other recently published techniques, even finer techniques perhaps looking at region specific or fiber specific strains are needed to capture regional differences and properly characterize the capsular ligament’s full three dimensional behavior during whiplash.

The high strain caused by pretorque alone raises the question of why facet capsular ligaments in these joints are not injured when rotating one’s head maximally to the side. Aside from the large rotations taken up at the atlantoaxial joint, one reason may lie in the regional differences described above. The facet capsule likely develops the necessary shape, slack, and tolerance to accommodate voluntary head rotations. The superposition of vertebral retraction during whiplash loading may then shift peak strain to fibers in the capsule that are normally not highly strained during voluntary rotation or combined loading scenarios. Alternatively, the small increase in strain produced by the whiplash loads may be sufficient to injure ligament fibers that are near their limit as a result of a prerotation. Further exploration of this phenomenon will require a more detailed characterization of the dynamic, full field strains in the facet capsule, and definition of the overall and regional tolerances of the facet capsular ligament and its microstructural components.

In summary, they examined the intervertebral kinematics and facet capsule strains under whiplash like loads in the presence of an initial axial rotation. We found that an axial rotation doubles the maximum principal strain in the capsular ligament compared to the neutral posture. We also found that capsular strains during the simulated whiplash exposure with the head turned were not significantly different from maximum principal strain associated with partial failure of the capsule. Thus these findings support the overall hypothesis that excessive capsular strains experienced by some individuals during some whiplash conditions may be responsible for painful capsular whiplash neck injury.

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.

From: J Manipulative Physiol Ther. 2008 Jun;31(5):348-54

The purpose of this study was to determine the reliability of 3 isometric muscle endurance tests in subjects with postural neck pain. Twenty-one subjects with chronic postural neck pain performed 3 submaximal muscle tests twice on the first occasion and once at the second session 3 days later. The tests examined isometric neck flexion, neck extension, and scapular muscle endurance. Reliability was excellent for the neck flexor test, moderate for the scapular test, and good for the neck extensor test. The standard error of measure for the tests was 6.4, 10.9, and 25.9 seconds, respectively. The minimum change required to represent real change in muscle endurance was 17.8 seconds for the neck flexor test, 30.1 seconds for the scapular test, and 71.3 seconds for the neck extensor test. This study showed the reliability of 3 cervical spine and shoulder girdle submaximal muscle endurance tests in patients with postural neck pain.

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.

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.

The unconstrained upright standing posture was the starting point of this study on slouching. In this position back muscle activity is minimal or absent because the upper body weight is centred above the spine and above or just behind the hip joints. Therefore, during sudden and fast slouching, the response of back muscles to protect the change from lumbar lordosis into lumbar hyperkyphosis may be delayed. Muscle delay can exist after a period of prolonged stretch of dorsal ligaments and muscles and muscle fatigue. The explorative calculations show that failure load of the spine and the iliolumbar ligament could be reached during sudden slouching. This change of form in the upright position is called the click clack movement and is characterized by backward tilt of the pelvis combined with forward flexion of the spine.

The present study on the strain of the iliolumbar ligament by spinal flexion may provide an explanation for acute lower back pain in situations with neglectable axial spinal load. Newman states that slouching resembles the primitive reflex mechanism including limb movement to the centre of the body; he refers to this type of movement in patients who have had an acute attack of lumbago while shaving and he attributes the acute attack of pain to bending forward in a position of flexion of the spine with the erector spinae muscles relaxed. This flexion relaxation mechanism was, however, revised by Andersson et al., who used measurements with fine wire electrodes to demonstrate that quadratus lumborum and deep lateral erector spinae were activated when the flexion relaxation phenomenon was present for the superficial medial erector spinae muscles. Therefore, we do not refer to forward trunk inclination but rather to the absence of protective dorsal muscle activity while slouching in the upright posture. We suppose that this only occurs in humans. A human, when in a static standing posture, has his centre of gravity positioned just behind the hip joints, requiring low or absent dorsal muscle activity. This enables the erect slouched posture to be adopted by humans, but not by apes. The bent legged bipedal posture of the ape is the consequence of limited hip extension. Therefore, the centre of gravity is anterior to the hip joints requiring stabilizing dorsal muscle activity.

Strain of the iliolumbar ligaments may occur in dynamic and in static situations. Deursen van et al. reported that lower back pain patients experience significantly more pain provocation in slouched standing postures (e.g. vacuuming, brushing teeth and washing dishes) than in stooped postures.

Sims and Moorman suggest that stress at the ligamento-osseous junction of the iliolumbar ligament at the ilium, and stress in the innervated iliolumbar ligament, can trigger pain from both tissues, which is the premise behind local injections of anaesthetics. Although controversial, evidence exists for the significant role of the iliolumbar ligament in lower back pain. Of interest to our study is that, of the 19 clinical parameters addressed during medical examination of a group of children reporting lower back pain, only one was significantly more prevalent: i.e. pain on palpation at the insertion site of the iliolumbar ligament on the iliac crest.

Further support for our model on slouching can be found in the loss of thoracolumbar curvature in pregnant women, in astronauts during flight and in individuals suffering from psychosocial distress. Our study on the slouched sitting was reported in a previous article, which included measurements on fresh human specimen. For the present study we used a small sample size of embalmed specimen which, however, resulted in similar load deformation patterns. This is in agreement with expectation because the orientation of the iliolumbar ligament is, although individual differences are reported in literature, in all cases in the direction of loading.

The present study provides preliminary information on the effects of muscle contraction on the strain of the iliolumbar ligament. It shows that the lateral erector spinae muscles and the sacral part of the multifidus muscles are capable of protecting the L5-S1 disc against hyperflexion and the iliolumbar ligament against excessive tension, i.e. by hollowing of the lower back. This followed from load tests with the use of embalmed specimen, which implies stiffer material than in vivo. We expect, however, that the relation between the applied load on L5 and the iliolumbar ligament elongation will show the same trend in the in vivo situation. The rotation between the sacrum and ilium remained smaller than the error of measurement, which is attributed to stiffness of the embalmed specimens.

For the effect of back muscle force on the sacroiliac joints we made a distinction between the effect of erector spinae and multifidus muscle. This is of interest because the sacral part of the multifidus muscle has the unique ability to produce the isolated action of extension of the L5-S1 intervertebral disc together with forward rotation of the sacrum with respect to the ilium (nutation). This suggests co-activation with pelvic floor muscles according to the agonist antagonist action about the sacroiliac joints. Therefore, it may be expected that lower back pain due to strain on the iliolumbar ligament may be related to pelvic floor problems. Pool-Goudzwaard et al. demonstrated a significant increase of pelvic floor activity in a population of postpartum low back and pelvic pain patients accompanied by frequency, urgency and stress incontinence.

This study lends credibility to the idea that back sprain can be the result of slouching in the upright posture. The iliolumbar ligament is designated as a possible source of pain, because this ligament was strained by flexion of L5. In line with these measurements were the explorative calculations pointing to the possibility of injury to the iliolumbar ligament and spine by slouching. Ease of the iliolumbar ligament was obtained by simulated back muscle force. Conditional for the conceptual model on back sprain is the delay or absence of muscle recruitment which has to prevent the click clack movement, i.e. backward tilt of the pelvis combined with forward flexion of the spine. This suggests that the attention for lifting in a stooped posture may be diverted to dynamic or static slouching with the trunk in the upright position as the main cause of lower back pain in daily life activities.

From: J Manipulative Physiol Ther. 2008 Jun;31(5):348-54

The purpose of this study was to determine the reliability of 3 isometric muscle endurance tests in subjects with postural neck pain. Twenty-one subjects with chronic postural neck pain performed 3 submaximal muscle tests twice on the first occasion and once at the second session 3 days later. The tests examined isometric neck flexion, neck extension, and scapular muscle endurance. Reliability was excellent for the neck flexor test (intraclass correlation coefficient [ICC] = 0 93), moderate for the scapular test (ICC = 0.67), and good for the neck extensor test (ICC = 0 88). The standard error of measure for the tests was 6.4, 10.9, and 25.9 seconds, respectively. The minimum change required to represent real change in muscle endurance was 17.8 seconds for the neck flexor test, 30.1 seconds for the scapular test, and 71.3 seconds for the neck extensor test. This study showed the reliability of 3 cervical spine and shoulder girdle submaximal muscle endurance tests in patients with postural neck pain.

From: Gait Posture. 2008 Jun 7; [Epub ahead of print]

Insight into the mechanisms of altered postural control in persons with low back pain could lead to better interventions for patients with low back pain. This study investigated (1) whether persons with recurrent low back pain have an altered body inclination, and (2) whether anticipation of postural instability further alters body inclination. Thirty-three young healthy individuals and 56 young persons with recurrent low back pain participated in this study. The upright standing posture was evaluated by means of two piezo-resistive electrogoniometers and a force platform for the conditions as follows: (1) quiet stance with and without vision, and (2) in anticipation of postural instability due to a ballistic arm movement or ankle muscle vibration. No differences in body inclination were observed when visual information was available between the two groups. However, significant more forward inclination was seen in the persons with recurrent low back pain when vision was occluded (+7.4%) and in anticipation of postural instability (+19%) compared to the healthy individuals.

The results suggest that young persons with recurrent low back pain have an altered body inclination that might be caused by anticipation of postural instability. The adopted forward inclined posture may potentially be a factor in the recurrence of low 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.

Approximately one-third of American workers are in jobs that contribute to an increased risk of developing back disorders, although the causes of lumbar disorders are complex and difficult to identify. This situation is similar in Great Britain, where the average estimated year-prevalence derived from British population surveys is 38%.

Because of the potential economic and social benefits to be gained from reducing the magnitude of lower back pain in industry, many investigations have focused their attention on the factors that lead to injury, particularly on those activities and events associated with the onset of symptoms. The major thrust of research about lower back pain has been to identify occupational risk factors associated with its presence and occurrence.

With the rapid development of modern technology, sitting has now become the most common posture in today’s workplace. Reinecke et al. reports that three-quarters of all workers in industrialized countries have sedentary jobs that require sitting for long periods. For many activities, however, sitting can be a less straining posture than standing. Both postures are commonly used, and have advantages and disadvantages depending on the task to be performed. In this review, sitting is discussed in the context of Western standards. Sitting is described as an erect posture in which the head and trunk are vertical, the lower legs are bent at about 90° at the hips and knees, and the feet are firmly placed on the floor. The concept of sitting is very different in non-Western societies. In India and Southeast Asia, for example, it is still common to see workers sitting cross-legged on the ground, squatting (knees bent severely, the thighs close to the trunk, and weight concentrated on the heels), or kneeling. Although common, these variations of sitting are not discussed in this review.

The existence of the relationship between workplace factors and the occurrence of lower back pain has long been recognized. Among the occupational exposures identified, sitting is commonly cited as a risk factor in the literature, in addition to heavy physical work, heavy or frequent lifting, non-neutral postures (i.e., trunk rotation, forward bending), pushing and pulling, and exposure to whole body vibration (whole body vibration) (i.e., motor vehicle driving).

Laboratory studies have focused on biomechanical hypotheses to explain the association between sitting and lower back pain. The literature on this subject is extensive, but the evidence is somewhat contradictory. It has been shown that intradiscal pressure is increased in the sitting posture. In a more recent investigation, Wilke et al. reevaluated the intradiscal pressure in sitting and found that, in fact, it can be lower in sitting than in erect standing. Another hypothesis is that prolonged static sitting postures may have a negative effect on the nutrition of the intervertebral disc. Thus, a constantly changing position may promote the flow of fluid (nutrition) to the disc.

The purpose of this paper is to review the more recent literature that examines the association between sitting and the report of lower back pain among working populations. This review also intends to identify if there is an interaction between occupational lower back pain/sciatica and sitting in combination with whole body vibration and/or awkward postures. The importance of this association cannot be underestimated. Understanding the risk factors for lower back pain and sciatica can lead to prevention programs that have the potential to mitigate the high prevalence rates of occupational lower back pain. Although other factors (i.e., physiological and psychosocial) may also be important, they are not the focus of this paper.

For this review, sitting is defined as a sustained upright trunk posture with limited possibilities to change posture or position. lower back pain is represented here by reported or examined ache, pain, stiffness, or discomfort in the lumbar spine. Sciatic pain is considered a reported or examined radiating pain extending below the knee in one or both legs. Workers in occupations who performed tasks while sitting may also be exposed to whole body vibration and/or awkward posture, and these exposures are independently analyzed in the literature. Awkward posture represents a non-neutral trunk posture (i.e., bending forward and/or twisting of the trunk). Postural load and unfavorable posture are terms used in the literature that refer to awkward posture. whole body vibration is present when the body receives continuous vibration transmitted from the seat over a period of time.

This systematic review seeks to examine the association between the presence and/or occurrence of lower back pain and sciatica among occupational groups in which the seated posture is the major physical requirement for the work. The following questions were considered: (1) Is there evidence in the recent epidemiological literature for an association between occupational groups exposed to sitting and reported lower back pain?, (2) Is there evidence in the recent epidemiological literature of an association between exposure to whole body vibration while sitting and reported lower back pain?, (3) Is there evidence in the recent epidemiological literature for an association between awkward posture while sitting and reported lower back pain?, and (4) Is there evidence in the recent epidemiological literature for an association between the combined effect (both whole body vibration and awkward posture) while sitting and reported lower back pain?

The prevalence rates of lower back pain have been affecting the economy of industrialized countries in many ways. The prevalence rate of reported lower back pain in those occupations that require the worker to sit for the majority of a working day is significantly higher than the prevalence rate of the general population. Physical factors, psychosocial factors, and individual characteristics contributed to these findings, but only the physical factors will be discussed here.

Sitting

Sitting has been a complex topic for researchers of lower back pain. For many years, the sitting position has been identified as one of the major risk factors for developing lower back pain. Nachemson and Elfström found that body position affects the magnitude of the loads on the lumbar spine, and that the magnitude of the loads increases markedly when sitting is compared with upright standing and well-supported reclining. This finding created much controversy, and while some subsequent lab studies supported this finding, others have found different results. For instance, the conclusions of Wilke et al. reevaluate the assumption that there is an increased presence of higher intradiscal pressure while sitting as opposed to erect standing. In fact, Rohlmann et al. reports that intradiscal pressure is up to 10% less when standing. Some studies have reported odds ratio’s as low as 0.7 for occupations in which the major physical requirement is sitting, indicating that people in these occupations may actually have a lower risk of lower back pain than in other occupations. However, although the rate of lower back pain among occupations requiring extended periods of sitting may not be as high as the rate of lower back pain among more strenuous occupations, Lee et al. reported that this group had the highest hospitalization rate for lower back pain, indicating that when low back injuries occur among persons in these occupations, these injuries tend to be more severe. Hence, the risk of prolonged sitting in the workplace should not be overlooked.

A definitive experimental conclusion about the effect of sitting on lower back pain has yet to be confirmed. However, the majority of the literature reviewed for this paper has shown that sitting by itself does not imply a markedly increased association with the presence and/or occurrence of reported or examined lower back pain. In fact, a considerably stronger association was found only when the occupational groups studied were exposed to whole body vibration and/or awkward postures while sitting. It is still unknown if other confounders as psychosocial factors may affect the strength of the association between sitting alone and lower back pain.

Sitting and exposure to whole body vibration

One of the major co-exposure factors for all the occupations analyzed was whole body vibration. There is a clear indication of an increased risk of reported lower back pain and/or sciatica in occupations with exposure to whole body vibration while sitting, mostly in professional driving occupations. Many authors have carefully reviewed the risk effect expressed as increased associations from exposure to whole body vibration and lower back pain, and have concluded that there is indeed an association between whole body vibration and lower back pain. However, there is limited evidence regarding a dose–response relationship. There are many confounding factors that interfere with the relationship between lower back pain and whole body vibration exposure. A worker’s age, duration of exposure, history of lower back pain, previous exposure, and even posture while being exposed to whole body vibration seems to affect that relationship. Chen et al. found that daily increments of the vibration dose by each m2/s4-hour resulted in an increase in the odds ratio of lower back pain by 3.7 even after controlling for age and professional seniority, which is almost analogous to duration of exposure.

Even though the number of studies that use control and exposure groups to analyze the association between lower back pain and whole body vibration is limited, there is strong evidence of a major association between whole body vibration while sitting and lower back pain/sciatica. For instance, occupational groups such as helicopter pilots have shown a very high odds ratio for a vibration-dose exposure greater than 800 m2 h/s4 after adjusting for awkward postures. Only one study did not find an association between sitting plus whole body vibration and lower back pain; however, the low response rate in that study compromised its quality. Therefore, this does not affect the conclusion that there is a strong relationship between sitting plus whole body vibration and lower back pain.

In addition to dose, the magnitude and duration of exposure have also been found to be important exposure factors for the presence of occupational lower back pain and sciatica. The articles reviewed showed that the duration of the exposure had a slightly stronger association with the presence of lower back pain than did magnitude of vibration. This finding suggests a cumulative effect. As the duration of exposure increases, the risk of lower back pain increases. Bovenzi and Betta found that prolonged tractor driving and exposure to whole body vibration were the factors most associated with chronic lower back pain and sick leave. They suggested an excessive accumulated vibration dose effect as the main reason, which is in agreement with the Boshuizen findings. However, in the case of sciatica the pattern is not the same. The duration of the exposure was constantly related to sciatica to a lesser extent than vibration magnitude for all the occupations. The reasons for such results are not clear. Moreover, it is important to understand that in both cases either duration or magnitude were positively associated with the presence of reported lower back pain or sciatica symptoms. It is also important to take into account that the recommended ISO standards did not reveal any preventive health effect. Even with following ISO recommendations, the levels of vibration magnitude to which most drivers are exposed seems to be higher than the recommended maximum level of exposure, especially when exposed for long periods of time. Many authors argue that these recommendations should be reviewed.

Sitting and awkward posture

It has been postulated that sustained awkward seating posture (lordosed or kyphosed, overly arched, or slouched) can result in higher intradiscal pressure and may be injurious to spinal postural health. Therefore, awkward postures while sitting have been described as possible risk factors for the presence of lower back pain. Many experimental studies have demonstrated that postural changes affect spinal loads. Keyserling et al. used a computer-aided system to investigate trunk posture during work by measuring the time spent in neutral and non-neutral postures. Their results suggested that by controlling non-neutral trunk posture, the risk of developing back pain on industrial jobs can be reduced. Non-industrial occupations such as professional drivers, dentists, and helicopter pilots are also potentially exposed to awkward postures. However, only four epidemiological studies showed the associated risk of being exposed to awkward postures and having lower back pain and/or sciatica. The results of these studies confirms that in the case of bus drivers, tractor drivers, road washing vehicle drivers, and helicopter pilots, the risk of having lower back pain increases due to awkward posture while driving. The Road Washing Vehicle drivers have the highest odds ratio of all the occupations It may be important to note that these drivers are exposed to trunk flexion, bending, and twisting, whereas the other occupational groups are exposed to only one of these factors. Hence, flexion, bending, and twisting may be more detrimental than only flexion. The question to be asked in this case is if the observed adverse effects of driving in these studies should be attributed to the exposure to awkward posture alone or to a combination of prolonged sitting, twisting, and bending postures, once sitting per se was not investigated.

A proposed model

The literature reviewed has indicated an increased risk of lower back pain and sciatica for individuals in those occupations that require prolonged sitting (defined as sitting for more than half of a working day). However, the risk increases after combining factors such as whole body vibration and awkward postures. In fact, for all the occupations studied, these co-exposure factors were the variables that led to a significantly increased risk. The fact of being seated for an extended period does not significantly demonstrate an impressive risk of having lower back pain and/or sciatica. Bovenzi and Betta compared a group of agricultural tractor drivers with a group of office workers. Both groups were exposed to static load due to prolonged sitting. However, only the tractor drivers group was exposed to the combined factors of whole body vibration and awkward posture. They found that tractor drivers were 2.39 times more likely to report lower back pain than office workers. The association was similar when they looked at whole body vibration while sitting (adjusted for awkward posture). The association increased when postural load was analyzed adjusting for vibration dose.

A model was constructed based on whole body vibration and awkward postures. The model aims to describe how the risk effect of prolonged sitting increases significantly as other co-exposure factors (in this case whole body vibration and awkward postures) play a role. As such, just the fact of sitting probably does not present a risk until the worker is exposed to a certain level of whole body vibration and/or an awkward posture. When these co-exposure factors are combined, the risk for reporting lower back pain and/or sciatica increases. Bovenzi and Betta tried to demonstrate this finding after analyzing the combined effect of postural load and total vibration dose. They found that the combination of vibration dose and postural load increases the risk of reporting lower back pain. In fact, there was a linear trend of increasing prevalence of lower back pain as the combined effect increases, perhaps demonstrating a dose–response relationship.

It could be argued that this model has many limitations due to the fact that lower back pain is multi-factorial in its origin. Through this literature review, some physical (mechanical) factors (whole body vibration, awkward posture, and a combination of both) were identified that are associated with the occupations performed while sitting. However studies have shown that there are other risk factors for lower back pain to which workers are exposed during the time period when they are sitting or not sitting, such as manual material handling or lifting activities, or psychosocial factors.

Summary

No previous research has been reported that investigated the sitting posture with respect to lower back pain and sciatica in a manner similar to this literature review, so no direct comparisons can be made. However, it could be argued that the conclusions of this paper, as well as of this model, have some limitations due to the fact that lower back pain is multi-factorial in its origin. Through this literature review, an association was made between some work-related factors while sitting (whole body vibration, awkward posture, and the combination of both) and reported lower back pain and sciatica. A quantitative determination of how much exposure to these risk factors (whole body vibration and awkward posture) would alter the risk of occupational-related lower back pain was not conducted (dose–response relationship). Just being seated on the job is not a risk factor; sitting becomes risky when combined with other risk factors. There is a lack of research on how much the risk increases for all populations. Other individual or psychosocial risk factors that have been found to be predictive of lower back pain were not studied in this literature review. Thus, no conclusions can be made about the interactions between these factors while sitting and the presence of lower back pain/sciatica.

More epidemiological studies are needed to provide clear evidence of the association between sitting and occupational lower back pain. However, our review suggests the following conclusions:

• Sitting by itself does not show an increased association with the presence of reported lower back pain.
  
• Sitting in combination with other co-exposures such as whole body vibration and awkward posture does increase the association with the presence of lower back pain.
  
• Sitting in combination with whole body vibration and awkward posture seems to have the strongest association with the presence of lower back pain. However, this conclusion is based on only one study whose results have not since been replicated. Hence more studies are needed to confirm this hypothesis.
  
• Occupational groups exposed to whole body vibration while sitting are at an increased risk of having lower back pain. Also, the influence of the duration of the exposure seems more important than the magnitude of the exposure, suggesting a cumulative effect.
  
• Although awkward posture while sitting has not been as well investigated as whole body vibration, the results of the preliminary research reveal a strong association with the presence of lower back pain. Thus, occupational groups exposed to awkward postures while sitting have an increased risk of having lower back pain. However, further research is needed on this topic due to the lack of valid and reliable instruments for its measurement.
  
• The interaction of factors such as sitting, whole body vibration, and awkward posture should also be carefully analyzed in terms not only of their association with lower back pain, but also of their dose–response.
  
• Awkward posture and whole body vibration have been previously associated with lower back pain, even without sitting. Further research should be conducted to probe if sitting adds to the risk of lower back pain.
  

From: Journal of Electromyography and Kinesiology. 2008 May 30; [Epub ahead of print]

It has been suggested that increased fatigue of neck muscles could be related to neck pain. However, studies on the matter present contradicting results which could be explained by the different test positions used. The purpose of this study was to investigate the influence of test position on muscle fatigue of neck flexor and extensor muscles in healthy controls.

Twenty-five women without neck pain sustained neck flexion and neck extension isometric contractions at 25% and 75% of their maximal voluntary contraction (MVC) in two test positions: sitting and supine lying. Using surface electromyography, the change over time of the median frequency of the power spectrum (MDF slope) of the myoelectric signal of the sternocleidomastoid and splenius capitis muscles was measured and compared between both positions.

At 75% MVC, splenius capitis muscles presented higher fatigue in lying compared to sitting, while sternocleidomastoid demonstrated no difference between positions. No statistically significant effect of test position was found at 25% MVC for both sternocleidomastoid and splenius capitis muscles as they generally did not present myoelectric manifestations of fatigue. These results underline the need to standardise the test position when investigating neck muscle fatigue, especially for neck extensors at high loads.

From: Surgical and Radiological Anatomy. 1994;16(4):367-71

To determine the postural role of longus colli and dorsal neck muscles, we have studied the relationship between their cross sectional areas related to their force of contraction and the lordosis and the length of the neck. This study was carried out in 36 healthy subjects. Muscle cross-sectional areas were measured by computerized tomography. The index of lordosis and the length of the neck were measured on an X-ray profile. The cross sectional area of longus colli was correlated to the lordosis index whereas all the other parameters were not correlated. The authors conclude that longus colli counteracts the lordosis increment related to the weight of the head and to the contraction of the dorsal neck muscles. Postural functions of longus colli and postcervical muscles are complementary. They form a sleeve which encloses and stabilizes the neck in all positions of the head.