Epidemiological and aetiological aspects of low back pain in vibration environments - an update

Reducing whole-body vibration of vehicle drivers with a new sitting concept

A new car seat design, which allows the back part of the seat (BPS) to lower down while a protruded cushion supports the lumbar spine, was quantitatively tested to determine its effectiveness in reducing whole-body vibration (WBV) in automobile drivers. Results on 12 drivers show that, by reducing contact between the seat and the ischial tuberosities (TTs), the new seating design reduced both contact pressure and amplitude of harmful vibrations transmitted through the body. Significant reduction of WBV, in terms of RMS and VDV, was found as large as 30% by this seating design (P < 0.05), especially at lumbar spine region. This reduction in WBV allows more sustained driving than permitted by conventional seating devices, by around 2 hours daily, before reaching harmful WBV levels. The new seating design also promotes improved posture by restoring normal spinal curvature. Such seating devices, implemented in cars, buses, large trucks, and other high-vibration vehicles, may effectively reduce the risk of musculoskeletal disorders among long term drivers.

Frequency response of pig intervertebral disc cells subjected to dynamic hydrostatic pressure

The pathogenesis of vibration-induced disorders of intervertebral disc at the cellular level is largely unknown. Dynamic loads with frequencies close to that of the in vivo human spine resonant frequency (4-6 Hz) have a destructive effect, which may induce extracellular disc matrix (ECM) degradation. To investigate this issue, three-dimensional (3D) alginate cultures of normal pig intervertebral disc nucleus and inner annulus cells were tested under dynamic hydrostatic loading. Alginate cultures of each region were divided into six groups; five groups were exposed to cyclic hydrostatic pressures of frequencies 1, 3, 5, 8, and 10 Hz with the same amplitude (1 MPa), and group 6 was the control group (no loading). Cultures of different groups were loaded for 3 days (30 min daily) in a hydraulic chamber. Effects of loading frequency on disc collagen and protein metabolism were investigated by measuring 3H-proline-labeled proteins associated with the cells in the extracellular matrix and release of 3H-proline-labeled molecules into culture medium. The results indicated a poor synthesis rate and more degradation near the 5 Hz frequency. The repeatability of experiments was verified by performing two experiments with the same protocol. Both experiments indicated that a threshold frequency of around 5 Hz disrupted protein metabolism.

Spatially varying material properties of the rat caudal intervertebral disc

STUDY DESIGN

The use of a microscopy based material testing technique to assess the local material properties of rat caudal intervertebral discs under uniaxial compression.

OBJECTIVES

To better understand the cell environment of rat caudal intervertebral discs during mechanical loading and elucidate better the role of the nucleus pulposus to the overall disc material properties.

SUMMARY OF BACKGROUND DATA

Rat tail models of disc degeneration have been widely used for their similarity with the degeneration phenomena in human beings. Degenerative patterns in the disc are often inhomogeneous, however, only average material properties of rodent discs have been studied. Knowledge of the spatially varying properties within the disc is necessary to understand the disc cell milieu during tissue loading.

METHODS

Rat caudal motion segments were tested intact, sectioned, and with alterations of nucleus pulposus using microscopy based techniques. Local displacements and strains were obtained using digital image correlation. Strains and load measurements were used to get the average apparent Young's modulus, peak stress, local Young's modulus, and local Poisson's ratio.

RESULTS

There was no difference observed in the average apparent Young's modulus among experimental groups. Peak stresses decreased significantly when the nucleus pulposus was replaced with extremely fluid-like materials. The axial displacement field showed 3 distinct linear distributions in samples which were sectioned. The center region in all groups had significantly smaller axial strain and showed a higher local Young's modulus.

CONCLUSIONS

The average equilibrium Young's modulus may be dependent on short-range ultrastructural organization. Spatially varying material properties within the intervertebral disc may be caused by orientation of fiber bundles in the different regions of the anulus fibrosus. The fiber bundles are better able to resist compressive loads when oriented parallel rather than perpendicular to the loading direction. At equilibrium, the anulus fibrosus also appears to have a shielding effect independent of the material filling up the nucleus pulposus space.

Evaluation of an occupational health intervention programme on whole-body vibration in forklift truck drivers: a controlled trial

OBJECTIVES

To evaluate process and outcome of a multifaceted occupational health intervention programme on whole-body vibration (WBV) in forklift truck drivers.

METHODS

An experimental pretest/post-test control group study design. The authors trained occupational health services (OHS) in the experimental group in the use of the programme. OHS in the control group were asked to deliver care as usual. In total, 15 OHS, 32 OHS professionals, 26 companies, and 260 forklift drivers were involved. Post-test measurements were carried out one year after the start of the programme.

RESULTS

Baseline data before the start of the programme showed no difference between experimental and control group. Results of the outcome evaluation indicate a slight, although not statistically significant, reduction of WBV exposure in the experimental group (p = 0.06). Process evaluation revealed a positive influence on company policy toward WBV, attitude and intended behaviour of forklift drivers, and a trend towards an increase in knowledge of OHS professionals and company managers. The number of observed control measures with a major impact (levelling of surface and reduction of speed) was rather low. In those cases where control measures had been taken, there was a significant reduction in WBV exposure. This limited effect of the programme might be caused by the short period of follow up and the dropout of participants. The feasibility and the usefulness of the programme within the OHS setting were rated good by the participants.

CONCLUSIONS

This programme to decrease WBV exposure was partially effective. Significant effects on intermediate objectives were observed. More research on the effectiveness of intervention in the field of WBV is needed.

The effect of weight-bearing exercise with low frequency, whole body vibration on lumbosacral proprioception: a pilot study on normal subjects

Patients with low back pain (LBP) often present with impaired proprioception of the lumbopelvic region. For this reason, proprioception training usually forms part of the rehabilitation protocols. New exercise equipment that produces whole body, low frequency vibration (WBV) has been developed to improve muscle function, and reportedly improves proprioception. The aim of this pilot study was to investigate whether weightbearing exercise given in conjunction with WBV would affect lumbosacral position sense in healthy individuals. For this purpose, twenty-five young individuals with no LBP were assigned randomly to an experimental or control group. The experimental group received WBV for five minutes while holding a static, semi-squat position. The control group adopted the same weightbearing position for equal time but received no vibration. A two-dimensional motion analysis system measured the repositioning accuracy of pelvic tilting in standing. The experimental (WBV) group demonstrated a significant improvement in repositioning accuracy over time (mean 0.78 degrees) representing 39% improvement. It was concluded that WBV may induce improvements in lumbosacral repositioning accuracy when combined with a weightbearing exercise. Future studies with WBV should focus on evaluating its effects with different types of exercise, the exercise time needed for optimal outcomes, and the effects on proprioception deficits in LBP patients.

Gender-based differences in postural responses to seated exposures

BACKGROUND

Individuals may respond differently to various chair designs and the factors that influence these sitting behaviours are not well understood. There is very little information in the scientific literature regarding the observation and documentation of gender differences in seated postures. In particular, anecdotal observations of potential gender-specific sitting behaviours led us to test the influence of gender on the postural responses to different seated conditions.

METHODS

Sixteen healthy university students (8 males and 8 females) were tested on four different chair configurations. Upper body kinematics (spine angles and centre of mass) and seat pressure profiles (centre of pressure, peak pressure) were obtained during each testing session.

FINDINGS

Regardless of the chair used or the task performed, average lumbar and trunk angles were significantly more flexed for males than for females (P=0.047 and P=0.0026, respectively). Males exhibited average lumbar spine and trunk angles of 65.4 degrees (SD 16.2 degrees ) and 29.8 degrees (SD 28.3 degrees ), respectively, while female lumbar spine and trunk angles were 49.6 degrees (SD 23.1 degrees ) and -3.3 degrees (SD 20.4 degrees ), respectively. The pelvis was posteriorly rotated for males (7.6 degrees (SD 8.2 degrees )) and anteriorly rotated for females (-5.5 degrees (SD 9.3 degrees )) (P=0.0008). Significant gender *chair interactions of the location of the individual on the chair seat were most marked for the pivoting chair with a back rest. Females positioned their centre of mass and hip joints anterior to the chair pivot point while males' centre of mass (P=0.0003) and hip joints (P=0.0039) were located posterior to the pivot point. Females also sat with their centre of mass closer to the seat pan centre of pressure than males when a back rest was present (P=0.0012).

INTERPRETATION

Males and females may be exposed to different loading patterns during prolonged sitting and may experience different pain generating pathways. Therefore, gender-dependent treatment modalities and/or coaching should be implemented when considering methods of reducing the risk of injury or aggravation of an existing injury.

Reducing whole-body vibration and musculoskeletal injury with a new car seat design

A new car seat design, which allows the back part of the seat (BPS) to lower down while a protruded cushion supports the lumbar spine, was quantitatively tested to determine its effectiveness and potentials in reducing whole-body vibration (WBV) and musculoskeletal disorders in automobile drivers. Nine subjects were tested to drive with the seat in: 1) the conventional seating arrangement (Normal posture); and 2) the new seating design (without BPS (WO-BPS) posture). By reducing contact between the seat and the ischial tuberosities (ITs), the new seating design reduced both contact pressure and amplitude of vibrations transmitted through the body. Root-mean-squared values for acceleration along the z-axis at the lumbar spine and ITs significantly decreased 31.6% (p < 0.01) and 19.8% (p < 0.05), respectively, by using the WO-BPS posture. At the same time, vibration dose values significantly decreased along the z-axis of the lumbar spine and ITs by 43.0% (p < 0.05) and 34.5% (p < 0.01). This reduction in WBV allows more sustained driving than permitted by conventional seating devices, by several hours, before sustaining unacceptable WBV levels. Such seating devices, implemented in large trucks and other high-vibration vehicles, may reduce the risk of WBV-related musculoskeletal disorders among drivers.

On the relationship between whole-body vibration exposure and spinal health risk

A conceptual framework provides the possibility to identify factors determining the effects of whole-body vibration (WBV) on the spine and the internal stress-strain relationships. Epidemiological studies were critically evaluated with respect to their significance for the derivation of quantitative exposure-effect relationships. The approach of deriving such relationships from a comparison with self-generated accelerations during daily activities was considered as unsuited. Trunk muscle activity and control with apparently identical accelerations of body parts during self-generated and forced motions differ widely. Simple biodynamic models coupled with experimental in vivo and in vitro data permitted a preliminary deduction of quantitative relationships between WBV and spinal health with the consideration of individual factors and exposure conditions. Examples of anatomy-based verified finite element models and their application are provided. Such models are considered as a very promising instrument. They can be used to assess quantitatively preventive measures and design. Future research needs concern the examination of (1) the nonlinearity of biodynamics, (2) the effects of WBV in x- and y-axes, (3) the strength of the spine for shear, (4) the contact parameters between the seat and man, (5) the significance of postures and muscle activity, and (6) material properties of spinal structures.

Nutrition of the intervertebral disc

STUDY DESIGN

A review of the literature on disc nutrition.

OBJECTIVES

To summarize the information on disc nutrition in relation to disc degeneration.

SUMMARY OF THE BACKGROUND DATA

The disc is avascular, and the disc cells depend on diffusion from blood vessels at the disc's margins to supply the nutrients essential for cellular activity and viability and to remove metabolic wastes such as lactic acid. The nutrient supply can fail due to changes in blood supply, sclerosis of the subchondral bone or endplate calcification, all of which can block transport from blood supply to the disc or due to changes in cellular demand.

METHODS

A review of the studies on disc blood supply, solute transport, studies of solute transport in animal and human disc in vitro, and of theoretical modeling studies that have examined factors affecting disc nutrition.

RESULTS

Small nutrients such as oxygen and glucose are supplied to the disc's cells virtually entirely by diffusion; convective transport, arising from load-induced fluid movement in and out of the disc, has virtually no direct influence on transport of these nutrients. Consequently, there are steep concentration gradients of oxygen, glucose, and lactic acid across the disc; oxygen and glucose concentrations are lowest in the center of the nucleus where lactic acid concentrations are greatest. The actual levels of concentration depend on the balance between diffusive transport and cellular demand and can fall to critical levels if the endplate calcifies or nutritional demand increases.

CONCLUSIONS

Loss of nutrient supply can lead to cell death, loss of matrix production, and increase in matrix degradation and hence to disc degeneration.

Transmissibility of 15-hertz to 35-hertz vibrations to the human hip and lumbar spine: determining the physiologic feasibility of delivering low-level anabolic mechanical stimuli to skeletal regions at greatest risk of fracture because of osteoporosis

STUDY DESIGN

Experiments were undertaken to determine the degree to which high-frequency (15-35 Hz) ground-based, whole-body vibration are transmitted to the proximal femur and lumbar vertebrae of the standing human.

OBJECTIVES

To establish if extremely low-level (<1 g, where 1 g = earth's gravitational field, or 9.8 ms-2) mechanical stimuli can be efficiently delivered to the axial skeleton of a human.

SUMMARY OF BACKGROUND DATA

Vibration is most often considered an etiologic factor in low back pain as well as several other musculoskeletal and neurovestibular complications, but recent in vivo experiments in animals indicates that extremely low-level mechanical signals delivered to bone in the frequency range of 15 to 60 Hz can be strongly anabolic. If these mechanical signals can be effectively and noninvasively transmitted in the standing human to reach those sites of the skeleton at greatest risk of osteoporosis, such as the hip and lumbar spine, then vibration could be used as a unique, nonpharmacologic intervention to prevent or reverse bone loss.

MATERIALS AND METHODS

Under sterile conditions and local anesthesia, transcutaneous pins were placed in the spinous process of L4 and the greater trochanter of the femur of six volunteers. Each subject stood on an oscillating platform and data were collected from accelerometers fixed to the pins while a vibration platform provided sinusoidal loading at discrete frequencies from 15 to 35 Hz, with accelerations ranging up to 1 g(peak-peak).

RESULTS

With the subjects standing erect, transmissibility at the hip exceeded 100% for loading frequencies less than 20 Hz, indicating a resonance. However, at frequencies more than 25 Hz, transmissibility decreased to approximately 80% at the hip and spine. In relaxed stance, transmissibility decreased to 60%. With 20-degree knee flexion, transmissibility was reduced even further to approximately 30%. A phase-lag reached as high as 70 degrees in the hip and spine signals.

CONCLUSIONS

These data indicate that extremely low-level, high-frequency mechanical accelerations are readily transmitted into the lower appendicular and axial skeleton of the standing individual. Considering the anabolic potential of exceedingly low-level mechanical signals in this frequency range, this study represents a key step in the development of a biomechanically based treatment for osteoporosis.

Shock absorption in lumbar disc prosthesis: a preliminary mechanical study

Lumbar disc prostheses have been used in treating symptomatic degenerative disc diseases. A few prostheses of the ball-socket design are currently available for clinical use, the joint mechanism being materialized either with a hard polymer core or a metal-to-metal couple. Other prostheses of "shock absorber" design were not available at the time of the study. The objective of this work was to establish whether there was a difference in the shock absorption capacity between a device having an ultra-high-molecular-weight polyethylene center core and a device having a metal-on-metal bearing. Vibration and shock loading were applied to two lumbar total disc prostheses: PRODISC, manufactured by Spine Solutions, and MAVERICK Total Disc Replacement, manufactured by Medtronic Sofamor Danek. The shock absorption capacity of the device was evaluated by comparing the input and the output force measurements. The disc prosthesis was mounted onto a test apparatus. Each side of the device was equipped with a force sensor. The input shock load and the output resulting forces were simultaneously measured and recorded. The loading force pattern included 1). a static preload of 350 N plus an oscillating vibration of 100 N with frequency sweeping from 0 to 100 Hz and 2). a sudden shock load of 250 N applied over a 0.1-second interval. Both input and output signal data were processed and were transformed into their frequency spectrums. The vibration and shock transmissibility of the device, defined as the ratio of the output spectrum over the input spectrum, were calculated in sweeping the frequency from 0 to 100 Hz. The phase deviation was calculated to characterize the shock absorber effects. For both tested devices under vibration and shock loading, the phase angle displacement between the input and the output signals was 10 degrees. Under oscillating vibration loading, both tested devices had a transmission ratio higher than 99.8%. Over the frequency interval 1-100 Hz, the difference in transmission ratio between the two devices was <0.3%. Under sudden shock loading, both tested devices had a transmission ratio higher than 98%. The difference between the two devices was <0.8%. Both tested devices have identical vibration and shock transmissibility.

Working hours spent on repeated activities and prevalence of back pain

BACKGROUND AND AIMS

Back pain is the most common reason for filing workers' compensation claims in the United States and affects large numbers of workers in many other countries. To evaluate the associations between working hours spent on repeated activities and back pain, data gathered through the 1988 National Health Interview Survey were analysed. The data were also used to identify high risk occupations.

METHODS

A total of 30 074 workers participated in the survey. They were asked to provide information on their job, including the time spent on repeated strenuous physical activities (RSPA) and the time spent on repeated bending, twisting, or reaching (RBTR) on a typical job. A case of back pain was defined as a worker who had back pain every day for a week or more during the past 12 months. Each case was asked to report the cause of back pain. Those who attributed their back pain to repeated activities (RA) or a single accident or injury (AI) were asked to recall whether they performed RA or had the AI at work.

RESULTS

Whereas the prevalence of back pain increased as the number of working hours spent on RSPA or RBTR increased, the dose-response relations were not linear for either factor, suggesting the involvement of other unmeasured factors. The estimated overall prevalence of RA back pain was 8.9% among male workers and 5.9% among female workers. "Carpenters" had the highest prevalence (19.2%) and most cases (338 000) among the major occupations of men, and "nursing aides, orderlies, and attendants" had the highest prevalence (15.2%) and most cases (217 000) among the major occupations of women.

CONCLUSIONS

The number of hours spent on repeated activities at work was associated with the prevalence of back pain. This study identified high risk occupations for future research and intervention.

Examination of the flexion relaxation phenomenon in erector spinae muscles during short duration slumped sitting

OBJECTIVE

The purpose of this study was to examine the myoelectric activity of the erector spinae muscles of the back in order to determine if the flexion relaxation phenomenon occurs in seated forward flexion or slumped postures.

BACKGROUND

The flexion relaxation phenomenon during standing forward flexion is well documented. However, flexion relaxation in seated forward flexion has not been studied. It is possible that flexion relaxation could be linked with low back pain that some individuals experience during seated work.

METHODS

Twenty-two healthy subjects (11 males, 11 females) participated in the study. Surface electromyography was used to measure the level of muscle activity at the thoracic and lumbar levels of the erector spinae muscles. An electromagnetic tracking device measured the three-dimensional movement of the lumbar spine. Five trials each of standing and seated forward flexion were performed.

RESULTS

A slumped sitting posture yielded flexion relaxation of the thoracic erector spinae muscles, whereas the lumbar erector spinae muscle group remained at relatively constant activation levels regardless of seated posture. Thoracic erector spinae silence occurred at a smaller angle of lumbar flexion during sitting than the flexion relaxation angle observed during standing flexion relaxation.

CONCLUSIONS

Since the myoelectric activity of the lumbar erector spinae did not increase, it is likely that the passive tissues of the vertebral column were loaded to support the moment at L4/L5. Ligaments contain a large number of free nerve endings which act as pain receptors and therefore could be a potential source of low back pain during seated work.

RELEVANCE

Examination of flexion relaxation during seated postures may provide insight into the association between low back pain and seated work.

The dynamic response of L(2)/L(3) motion segment in cyclic axial compressive loading

OBJECTIVE: The dynamic response and load sharing amongst passive elements of an L2-L3 motion segment during axial compressive cyclic loading was investigated. DESIGN: A validated viscoelastic nonlinear finite element model of L2-L3 was used for a detailed stress/strain analysis during axial cyclic loading. BACKGROUND: The repetitive loading of the spine has been implicated as a risk factor in developing low back disorders. However, the quantitative description of injury mechanisms and the internal load sharing have been lacking. METHODS: The applied cyclic axial compressive loading was controlled, peak to peak, from 600 to 1000 N at 0.5 Hz for 15 cycles. The stress/strain and strain energy density of various elements were quantified and the effects of cyclic loading on these parameters were investigated. RESULTS: The axial stiffness of the motion segment decreased, while intradiscal pressure (IDP) and the strain in anulus fibers of the outermost lamella increased. The axial stresses of outer lamellae in the anulus matrix reduced, in contrast to the increased strain at the endplate. CONCLUSIONS: The load sharing amongst the passive elements of the motion segment changed. The response of the motion segment to the same external axial load depends on the history of loading. The anulus fibers in the innermost layer were slack due to compression, hence not at risk of failure. The loss of disc height and increased disc bulge led to higher strain in anulus fibers of outermost layer. In future, more complex loading conditions with a longer duration should be considered.

Low back joint loading and kinematics during standing and unsupported sitting

The aim was to examine lumbar spine kinematics, spinal joint loads and trunk muscle activation patterns during a prolonged (2 h) period of sitting. This information is necessary to assist the ergonomist in designing work where posture variation is possible -- particularly between standing and various styles of sitting. Joint loads were predicted with a highly detailed anatomical biomechanical model (that incorporated 104 muscles, passive ligaments and intervertebral discs), which utilized biological signals of spine posture and muscle electromyograms (EMG) from each trial of each subject. Sitting resulted in significantly higher (p<0.001) low back compressive loads (mean +/- SD 1698 +/- 467 N) than those experienced by the lumbar spine during standing (1076 +/- 243 N). Subjects were equally divided into adopting one of two sitting strategies: a single 'static' or a 'dynamic' multiple posture approach. Within each individual, standing produced a distinctly different spine posture compared with sitting, and standing spine postures did not overlap with flexion postures adopted in sitting when spine postures were averaged across all eight subjects. A rest component (as noted in an amplitude probability distribution function from the EMG) was present for all muscles monitored in both sitting and standing tasks. The upper and lower erector spinae muscle groups exhibited a shifting to higher levels of activation during sitting. There were no clear muscle activation level differences in the individuals who adopted different sitting strategies. Standing appears to be a good rest from sitting given the reduction in passive tissue forces. However, the constant loading with little dynamic movement which characterizes both standing and sitting would provide little rest/change for muscular activation levels or low back loading.

To what extent do current and past physical and psychosocial occupational factors explain care-seeking for low back pain in a working population? Results from the Musculoskeletal Intervention Center-Norrtälje Study

STUDY DESIGN

A population-based case referent study.

OBJECTIVES

To determine whether current and past physical and psychosocial occupational factors are associated with care-seeking for low back pain in working men and women.

SUMMARY OF BACKGROUND DATA

The importance of physical and psychosocial workloads as causal factors of low back pain has mostly been investigated in special occupational groups and with a cross-sectional design, which makes generalizability and interpretations more difficult.

METHODS

The study comprised 2118 working men and women 20 to 59 years old (695 cases, and 1423 referents). Cases were defined as persons seeking care by any caregiver for low back pain. The exposure assessments were made through questionnaires and interviews about current and past physical and psychosocial loads during work and leisure time.

RESULTS

In a logistic regression analysis, physical load from forward bending in men (RR = 1.8) and high physical load, in general, in women (RR = 2.0) showed increased relative risks. Psychosocial factors alone seemed to be of less importance in women, but "poor job satisfaction" and "mostly routine work without possibilities of learning" increased the risk in men. Combined current and past exposures further increased the risks. A combination of high physical and psychosocial loads increased the risk substantially, but few were exposed to such loads. Adjustment for lifestyle and other loads outside work did not change the results.

CONCLUSION

Current and past physical and psychosocial occupational factors, both separately and combined, seem to be gender-specific, and to have a moderate impact on care-seeking for low back pain in a general working population.

The long-term effects of rally driving on spinal pathology

OBJECTIVES

To investigate the consequences of rally driving on lumbar degenerative changes.

BACKGROUND

Vehicular driving is suspected to accelerate disc degeneration through whole-body vibration, leading to back problems. However, in an earlier well-controlled study of lumbar MRI findings in monozygotic twins, significant effects of lifetime driving on disc degeneration were not demonstrated. Another study of machine operators found only long-term exposure to vibration on unsprung seats led to a reduction in disc height.

DESIGN

Case-control study comparing rally drivers with population sample.

METHODS

Eighteen top rally drivers and co-drivers, mean age 43 yrs (SD, 10), volunteered for the study. The subjects were interviewed and imaged with a MR scanning and lumbar images were analyzed for degenerative findings using a standard scoring protocol previously published. The reference group was composed of 14 men, mean age 55 yrs (SD, 10), selected from a population sample.

RESULTS

Overall results showed no significant differences in lumbar degenerative findings as assessed from MR images between the rally drivers and the reference group; age-adjusted differences were not statistically significant for disc heights, bulges, herniations, end-plate irregularities, or osteophytes.

CONCLUSION

Even extreme vehicular vibration as experienced in rally driving does not appear to have significant effects on disc generation.

RELEVANCE

The study results do not support driving, and its associated whole body vibration, as a significant cause of disc degeneration and question the theory that the higher incidence of back pain among drivers is due to accelerated disc degeneration. Other driving-related factors, such as postural stress, may deserve more attention.

A review of studies on seated whole body vibration and low back pain

The paper reviews the epidemiologic evidence linking low back pain (LBP) and exposure to whole body vibration (WBV). Particular emphasis is placed on studies where the exposure is quantified. Biomechanical studies show a resonance at 4-5 Hz. At the resonance the transmissibility exceeds 1.0, and is dependent on seat attenuation, posture and seat back inclination. Increased spinal loading is evidenced by increased muscle activity, muscle fatigue, and disc pressure, and by decreased stature. Physiologic changes also occur with WBV. The risk of LBP can be reduced by vibration damping, good ergonomic design, reducing exposure, and reducing other risks such as lifting.

Variation in spinal load and trunk dynamics during repeated lifting exertions

OBJECTIVES

To quantify the variability in lifting motions, trunk moments, and spinal loads associated with repeated lifting exertions and to identify workplace factors that influence the biomechanical variability.

DESIGN

Measurement of trunk dynamics, moments and muscle activities were used as inputs into EMG assisted model of spinal loading.

BACKGROUND

Traditional biomechanical models assume repeated performance of a lifting task produces little variability in spinal load because the assessments overlook variability in lifting dynamics and muscle coactivity.

METHODS

Five experienced and seven inexperienced manual materials handlers performed 10 repeated lifts at each combination of load weight, task asymmetry and lifting velocity.

RESULTS

Box weight, task asymmetry and job experience influenced the magnitude and variability of spinal load during repeated lifting exertions. Surprisingly, experienced subjects demonstrated significantly greater spinal loads and within-subject variability in spinal load than inexperienced subjects. Trial-to-trial variability accounted for 14% of the total variation in compression overall and 32% in lateral shear load. Although the mean spinal load was safely below the NIOSH recommended limit; due to variability about the mean, more than 20% of the lifts exceeded the recommended limit.

CONCLUSION

Spinal load changed markedly from one exertion to the next despite identical task requirements. Trial-to-trial variability in kinematics, kinetics, and spinal load were influenced by workplace factors, and may play a role in the risk of low-back pain.

RELEVANCE

Ergonomic assessments considering only the mean value of spinal load overlook the fact that a large fraction of the lifts may exceed recommended levels.

Three-dimensional biomechanical model for simulating the response of the human body to vibration stress

Several investigations have revealed that long-term exposure to whole-body vibrations can induce low back pain. In analogy to materials handling, the health risk can be assessed if the forces transmitted in the spine during vibration are known. To estimate the forces a biomechanical model has been developed in which the human trunk, neck, head and arms are represented by 16 rigid bodies. An additional body simulates the vibrating seat. The bodies are connected by visco-elastic joint elements, and 56 force elements imitate the trunk and neck muscles. The motion equations are derived by means of the dynamics of systems of rigid bodies, and the motions are simulated in three directions. The frequency-response functions between the accelerations of the seat and the head satisfactorily correspond to data reported in the literature. The spine forces are composed of a static part, due to body posture, and a vibration-induced part. The relation between the oscillating parts of the forces transmitted from seat to pelvis and the spine forces are also described by frequency-response functions. To assess the health risk the simulated spine forces must be compared with the strength of the spine, bearing in mind that this is dependent on the number of load cycles.

Muscular response to sudden load. A tool to evaluate fatigue and rehabilitation

STUDY DESIGN

Subjects were exposed to fatiguing and restorative interventions to assess their response to sudden loads.

OBJECTIVES

To investigate the erector spinae and rectus abdominis response characteristics to "sudden load" and the effect of fatigue and rehabilitation.

SUMMARY OF BACKGROUND DATA

Unexpected loads which people often experience, can lead to high forces in the spine and may be a cause of low back injury.

METHODS

Muscle responses to sudden load were mediated by fatigue, walking, expectation, method of load application, exposure to vibration, and cognitive-behavioral rehabilitation in patients with chronic low back pain. A novel technique, perfected in this work, called wavelet analysis, was used to analyze these data.

RESULTS

Reaction time was affected by fatigue and expectation. Vibration exposure significantly increased the muscle response time. Walking was able to ameliorate that effect. Back muscles responded differently, depending on whether loads were applied to the back through the hands or through the trunk. Electromyographic reaction time and magnitude decreased in patients after a 2-week rehabilitation program.

CONCLUSIONS

Sudden loads can exacerbate fatigue effects. Walking after driving reduces the risk to the back caused by handling unpredictable loads. Vibration exposure guidelines should be more conservative. Patients have longer response times than healthy subjects, but patients can improve their response to sudden loads via rehabilitation. Patients exhibit a flexion-extension oscillation at 5 Hz in response to a sudden load, suggesting that the 5-Hz, seated, natural frequency observed during whole-body vibration may result from neurophysiologic control limits.

Mechanical stress reduction during seated jolt/vibration exposure

The risk of experiencing low back pain is associated with mechanical factors. Anatomic factors, such as advancing pregnancy, can also place extra mechanical stress on the lower back. Mechanical factors, such as those related to the workplace, can be minimized by ergonomic interventions. A constrained, seated posture, in combination with exposure to whole-body, jolt/vibration can impose significant stresses on the posterior intervertebral disc and can lead to back muscle fatigue. Interventions that reduce the jolt/vibration magnitude and duration of exposure will decrease the mechanical work performed on the intervertebral disc. Such interventions range from jolt/vibration isolating seats and vehicle cabs, to decreasing exposure time and maintaining simple supported postures during ingress and egress. Improvements in seat configuration can reduce the intervertebral disc pressure and the strain on the posterior disc.