A comparison of peak vs cumulative physical work exposure risk factors for the reporting of low back pain in the automotive industry

The dose-response relationship between cumulative lifting load and lumbar disk degeneration based on magnetic resonance imaging findings

BACKGROUND

Lumbar disk degeneration (LDD) has been related to heavy physical loading. However, the quantification of the exposure has been controversial, and the dose-response relationship with the LDD has not been established.

OBJECTIVE

The purpose of this study was to investigate the dose-response relationship between lifetime cumulative lifting load and LDD.

DESIGN

This was a cross-sectional study.

METHODS

Every participant received assessments with a questionnaire, magnetic resonance imaging (MRI) of the lumbar spine, and estimation of lumbar disk compression load. The MRI assessments included assessment of disk dehydration, annulus tear, disk height narrowing, bulging, protrusion, extrusion, sequestration, degenerative and spondylolytic spondylolisthesis, foramina narrowing, and nerve root compression on each lumbar disk level. The compression load was predicted using a biomechanical software system.

RESULTS

A total of 553 participants were recruited in this study and categorized into tertiles by cumulative lifting load (ie, <4.0 × 10(5), 4.0 × 10(5) to 8.9 × 10(6), and ≥8.9 × 10(6) Nh). The risk of LDD increased with cumulative lifting load. The best dose-response relationships were found at the L5-S1 disk level, in which high cumulative lifting load was associated with elevated odds ratios of 2.5 (95% confidence interval [95% CI]=1.5, 4.1) for dehydration and 4.1 (95% CI=1.9, 10.1) for disk height narrowing compared with low lifting load. Participants exposed to intermediate lifting load had an increased odds ratio of 2.1 (95% CI=1.3, 3.3) for bulging compared with low lifting load. The tests for trend were significant.

LIMITATIONS

There is no "gold standard" assessment tool for measuring the lumbar compression load.

CONCLUSIONS

The results suggest a dose-response relationship between cumulative lifting load and LDD.

Why don't most runners get knee osteoarthritis? A case for per-unit-distance loads

Peak knee joint contact forces ("loads") in running are much higher than they are in walking, where the peak load has been associated with the initiation and progression of knee osteoarthritis. However, runners do not have an especially high risk of osteoarthritis compared with nonrunners. This paradox suggests that running somehow blunts the effect of very high peak joint contact forces, perhaps to provide a load per unit distance (PUD) traveled that is relatively low.

PURPOSE

This study aimed to compare peak and PUD knee joint loads between human walking and running.

METHODS

Fourteen healthy adults walked and ran at self-selected speeds. Ground reaction force and motion capture data were measured and combined with inverse dynamics and musculoskeletal modeling to estimate the peak knee joint loads, PUD knee joint loads, and the impulse of the knee joint contact force for each gait with a matched-pair (within-subject) design.

RESULTS

The peak load was three times higher in running (8.02 vs 2.72 body weight, P < 0.001), but the PUD load did not differ between running and walking (0.80 vs 0.75 body weight per meter, P = 0.098). The impulse of the joint contact force was greater for running than for walking (1.30 vs 1.04 body weight per second, P < 0.001). The peak load increased with increasing running speed, whereas the PUD load decreased with increasing speed.

CONCLUSIONS

Compared with walking, the relatively short duration of ground contact and relatively long length of strides in running seem to blunt the effect of high peak joint loads, such that the PUD loads are no higher than that in walking. Waveform features other than or in addition to the peak value should be considered when studying joint loading and injuries.

Weight knowledge and weight magnitude: impact on lumbosacral loading

Several factors can impact lumbosacral loads during lifting, including weight knowledge and weight magnitude. However, interaction between them has never been tested. This study investigated the interaction effect of these variables on lumbosacral forces and moments. Participants performed symmetrical lifts using three different weights. Weight knowledge involved known and unknown weight conditions. A biologically assisted dynamic model was used to calculate spinal loading parameters. Weight impacted all variables, while knowledge impacted only compression, by a moderate amount (5%), and spinal moments. Lifting a lightweight resulted in a difference of 16% and 7.2% between knowledge conditions for compression and anterior-posterior shear forces, respectively, compared with a negligible difference of < 1% when lifting a heavy weight. Increased spinal loading with light unknown weight can be attributed to increased muscular co-contraction. Weight knowledge is important to consider at low weight levels as it can increase tissue loading to values equivalent to lifting a heavier weight.

The assessment of material-handling strategies in dealing with sudden loading: the effect of uneven ground surface on trunk biomechanical responses

As a major risk factor of low back injury, sudden loading often occurs when performing manual material-handling tasks on uneven ground surfaces. Therefore, the purpose of the current study was to investigate the effects of a laterally slanted ground on trunk biomechanical responses during sudden loading events. Thirteen male subjects were subjected to suddenly released loads of 3.4 and 6.8 kg, while standing on a laterally slanted ground of 0°, 15° and 30°. The results showed that 8.3% and 5.6% larger peak L5/S1 joint compression forces were generated in the 30° condition compared with the 0° and 15° conditions, respectively. The increase of L5/S1 joint moment in the 30° condition was 8.5% and 5.0% greater than the 0° and 15° conditions, respectively. Findings of this study suggest that standing on a laterally slanted ground could increase mechanical loading on the spine when experiencing sudden loading. Practitioner Summary: Sudden loading is closely related to occupational low back injuries. The results of this study showed that the increase of slanted ground angle and magnitude of load significantly increase the mechanical loading on the spine during sudden loading. Therefore, both of these two components should be controlled in task design.

L4-L5 compression and anterior/posterior joint shear forces in cabin attendants during the initial push/pull actions of airplane meal carts

The aim of the present study was to assess the acute low back load of cabin attendants during cart handling and to identify working situations which present the highest strain on the worker. In a setup, 17 cabin attendants (ten females and seven males) pushed, pulled and turned a 20 kg standard meal cart (L: 0.5m × W: 0.3 m × H: 0.92 m) loaded with extra 20 kg and 40 kg, respectively on two different surfaces (carpet and linoleum) and at three floor inclinations (-2°, 0° and +2°). Two force transducers were mounted as handles. Two-dimensional movement analysis was performed and a 4D WATBAK modelling tool was used to calculate the acute L4-L5 load. No working situations created loads greater than the accepted values for single exertions, however compression and anterior/posterior shear forces during pulling and turning were much higher when compared with pushing. There were significant effects of handling the cart on different floor types, at the varying inclinations and with different cart weights. Additionally, when external forces were reduced, the cabin attendants did not decrease push/pull force proportionally and thus the L4-L5 load did not decrease as much as expected.

Prevalence of occupation-related pain among baristas and an examination of low back and shoulder demand during the preparation of espresso-based beverages

Many baristas complain of low back pain (LBP) and upper extremity discomfort while at work. This study documented the prevalence of LBP and shoulder pain, via questionnaire, among a population of baristas to determine whether cumulative low back loads and shoulder moments are associated with pain reporting. Fifty-nine baristas completed the questionnaire; ten were also video-recorded for biomechanical analysis while making espresso beverages and cumulative and peak low back loads and shoulder moments were calculated. Seventy-three percent of those who completed the questionnaire reported having experienced LBP, and half attributed this pain to their job as a barista. Furthermore, 68% reported having experienced shoulder pain and half also attributed this pain to their job. Those who suffered from LBP had higher peak low back compression and those with shoulder pain had, in general, higher moments about their dominant shoulder.

FMS™ scores and low-back loading during lifting--whole-body movement screening as an ergonomic tool?

BACKGROUND

Previous research suggests that a general whole-body movement screen could be used to identify personal movement attributes that promote potentially injurious low-back loading patterns at work. The purpose of this study was to examine the relationship between Functional Movement Screen™ (FMS) composite scores and the low-back loading response to lifting.

METHODS

Fifteen men who scored greater than 14 on the FMS (high-scorers) and 15 height- and weight-matched low-scorers (FMS < 14) performed sagittally symmetric and asymmetric laboratory-based lifting tasks. A three-dimensional dynamic biomechanical model was used to calculate peak low-back loading levels, and the angle of the lumbar spine was captured at the instant when the peak compressive force was applied.

RESULTS

Regardless of the lifting task performed, there were no differences in peak low-back compression (p ≥ 0.4157), anterior/posterior reaction shear (p ≥ 0.5645), or medial/lateral reaction shear (p ≥ 0.2581) forces between the high- and low-scorers. At the instant when peak compressive forces were applied, differences in the lumbar spine angle between high- and low-scores were not statistically significant about the lateral bend (p ≥ 0.4215), axial twist (p ≥ 0.2734), or flexion/extension (p ≥ 0.1354) axes, but there was a tendency for the lumbar spine to be more deviated in the low-scorers.

CONCLUSIONS

Using the previously established injury prediction threshold value of 14, the composite FMS score was not related to the peak low-back loading magnitudes in lifting. Though not statistically significant, the tendency for the lumbar spines of low-scorers to be more deviated when the peak low-back compression force was imposed could be biomechanically meaningful because spinal load tolerance varies with posture. Future attempts to modify or reinterpret FMS scoring are warranted given that several previous studies have revealed links between composite FMS scores and musculoskeletal complaints.

Cumulative mechanical low-back load at work is a determinant of low-back pain

OBJECTIVES

Reported associations of physical exposures during work (eg, lifting, trunk flexion or rotation) and low-back pain (LBP) are rather inconsistent. Mechanical back loads (eg, moments on the low back) as a result of exposure to abovementioned risk factors have been suggested to be important as such loads provide a more direct relationship with tissue failure and thus LBP. Since information on the effect of such load metrics with LBP is lacking yet, we aimed to assess this effect in a prospective study.

METHODS

Of 1131 workers, categorised into 19 groups, LBP was prospectively assessed over 3 years. Video and hand force recordings of 4-5 workers per group (93 in total) were used to estimate mechanical low-back loads (peak load and three cumulative load metrics, ie, linear weighted load, squared weighted load and load weighted to the tenth power) during manual materials handling (MMH) tasks using a video analysis method. These data were combined with static mechanical load estimates based on structured observation of non-MMH tasks. Associations of mechanical loads and LBP were tested using generalised estimating equations.

RESULTS

Significant effects on LBP were found for cumulative low-back moments (linear and squared weighted; both p<0.01 and ORs of 3.01 and 3.50, respectively) but not for peak and cumulative moments weighted to the tenth power.

CONCLUSIONS

Results of this first prospective study on the effect of mechanical low-back load on LBP support a LBP aetiology model of cumulative loads, potentially due to accumulation of microdamage or fatigue. Therefore, prevention of LBP should focus on reducing cumulative low-back loads, especially in highly exposed occupational groups, for example, by reducing handling of heavy loads and working in awkward body postures.

Effects of age and its interaction with task parameters on lifting biomechanics

This study investigated the age-related differences in lifting biomechanics. Eleven younger and 12 older participants were instructed to perform symmetric lifting tasks defined by different combinations of destination heights and load magnitudes. Lifting biomechanics was assessed. It was found that the trunk flexion in the starting posture was 32% lower and the peak trunk extension velocity was 46% lower in older participants compared with those in younger ones, indicating that older adults tended to use safer lifting strategies than did younger adults. Based on these findings, we recommend that physical exercise programmes may be a more effective ergonomic intervention for reducing the risks of low back pain (LBP) in lifting among older workers, compared with instructions of safe lifting strategies. As for younger workers, instructions of safe lifting strategies would be effective in LBP risk reduction.

Continuous assessment of back and upper arm postures by long-term inclinometry in carpet weavers

Awkward back and shoulder postures have been suggested to be a cause of back and shoulder discomfort in carpet weavers. This study aimed at continuous assessment of the upper arm and back postures and estimation of biomechanical load subtasks using inclinometers during 4 h. Median of trunk flexion angle in weavers was 18° and 13° during knotting and compacting subtasks, respectively. The weavers worked with arms elevated greater than 45° for %4.5 of the work time. The average cumulative compression load for males and females were estimated at 22 MN-S and 13 MN-S, respectively. In addition to poor workstation design, constrained posture of the trunk and low elevation and velocity for both arms may be the main risk factors for developing fatigue and disorders in the back and shoulder regions among carpet weavers. Therefore, any ergonomic interventions should be focused on reducing trunk flexion and the constrained postures of weavers.

Cumulative spine loading and clinically meaningful declines in low-back function

OBJECTIVE

The objective was to assess the role of cumulative spine loading measures in the development of a clinically meaningful decline in low-back function.

BACKGROUND

Cumulative spine loading has been a suspected risk factor for low-back pain for many years, yet the measures that characterize risk have not been well delineated.

METHODS

A total of 56 cumulative exposure measures were collected in a prospective field study of distribution center workers. An individual's risk for a clinically meaningful decline in low-back function (true cases) was explored with daily, weekly, and job tenure cumulative exposure measures using univariate and multivariate statistical modeling techniques. True noncases were individuals with no decline in low-back function.

RESULTS

An individual's risk for a clinically meaningful decline in low-back function (true cases) was predicted well versus true noncases (sensitivity/specificity = 72%/73%) using initial low-back function (p(n)), cumulative rest time, cumulative load exposure, job satisfaction, and worker age.

CONCLUSIONS

Cumulative rest time was identified as an important component for predicting an individual's risk for a clinically meaningful decline in low-back function.

APPLICATION

This information can be used to assess cumulative spine loading risk and may help establish guidelines to minimize the risk of a clinically meaningful decline in low-back function.

The NIOSH lifting equation and low-back pain, Part 1: Association with low-back pain in the backworks prospective cohort study

OBJECTIVE

The aim of this study was to evaluate relationships between the revised NIOSH lifting equation (RNLE) and risk of low-back pain (LBP).

BACKGROUND

The RNLE is commonly used to quantify job physical stressors to the low back from lifting and/or lowering of loads. There is no prospective study on the relationship between RNLE and LBP that includes accounting for relevant covariates.

METHOD

A cohort of 258 incident-eligible workers from 30 diverse facilities was followed for up to 4.5 years. Job physical exposures were individually measured. Worker demographics, medical history, psychosocial factors, hobbies, and current LBP were obtained at baseline. The cohort was followed monthly to ascertain development of LBP and quarterly to determine changes in job physical exposure. The relationship between LBP and peak lifting index (PLI) and peak composite lifting index (PCLI) were tested in multivariate models using proportional hazards regression.

RESULTS

Point and lifetime prevalences of LBP at baseline were 7.1% and 75.1%, respectively. During follow-up, there were 123 incident LBP cases. Factors predicting development of LBP included job physical exposure (PLI and PCLI), history of LBP, psychosocial factors, and housework. In adjusted models, risk (hazard ratio [HR]) increased per-unit increase in PLI and PCLI (p = .05 and .02; maximum HR = 4.3 and 4.2, respectively). PLI suggested a continuous increase in risk with an increase in PLI, whereas the PCLI showed elevated, but somewhat reduced, risk at higher exposures.

CONCLUSION

Job physical stressors are associated with increased risk of LBP. Data suggest that the PLI and PCLI are useful metrics for estimating exposure to job physical stressors.

Efficacy of the revised NIOSH lifting equation to predict risk of low-back pain associated with manual lifting: a one-year prospective study

OBJECTIVE

The objective was to evaluate the efficacy of the Revised National Institute for Occupational Safety and Health (NIOSH) lifting equation (RNLE) to predict risk of low-back pain (LBP).

BACKGROUND

In 1993, NIOSH published the RNLE as a risk assessment method for LBP associated with manual lifting. To date, there has been little research evaluating the RNLE as a predictor of the risk of LBP using a prospective design.

METHODS

A total of 78 healthy industrial workers' baseline LBP risk exposures and self-reported LBP at one-year follow-up were investigated. The composite lifting index (CLI), the outcome measure of the RNLE for analyzing multiple lifting tasks, was used as the main risk predictor. The risk was estimated using the mean and maximum CLI variables at baseline and self-reported LBP during the follow-up. Odds ratios (ORs) were calculated using a logistic regression analysis adjusted for covariates that included personal factors, physical activities outside of work, job factors, and work-related psychosocial characteristics.

RESULTS

The one-year self-reported LBP incidence was 32.1%. After controlling for history of prior LBP, supervisory support, and job strain, the categorical mean and maximum CLI above 2 had a significant relationship (OR = 5.1-6.5) with self-reported LBP, as compared with the CLI below or equal to I. The correlation between the continuous CLI variables and LBP was unclear.

CONCLUSIONS

The CLI > 2 threshold may be useful for predicting self-reported LBP. Research with a larger sample size is needed to clarify the exposure-response relationship between the CLI and LBP.

Lumbar facet joint and intervertebral disc loading during simulated pelvic obliquity

BACKGROUND CONTEXT

Intervertebral disc and facet joints are the two primary load-bearing structures of the lumbar spine, and altered loading to these structures may be associated with frontal plane spinal deviations.

PURPOSE

To determine the load on the lumbar facet joint and intervertebral disc under simulated frontal plane pelvic obliquity combined loading, an in vitro biomechanical study was conducted.

STUDY DESIGN/SETTING

An in vitro biomechanical study using a repeated-measures design was used to compare L4-L5 facet joint and intervertebral disc loading across pure moment and combined loading conditions.

METHODS

Eight fresh-frozen lumbosacral specimens were tested under five loading conditions: flexion/extension, lateral bending, axial rotation using pure moment bending (±10 Nm), and two additional tests investigating frontal plane pelvic obliquity and axial rotation (sacrum tilted left 5° and at 10° followed by a ±10-Nm rotation moment). Three-dimensional kinematics, facet load, and intradiscal pressures were recorded from the L4-L5 functional spinal unit.

RESULTS

Sagittal and frontal plane loading resulted in significantly smaller facet joint forces compared with conditions implementing a rotation moment (p<.05). The facet joint had the highest peak load during the 10° combined loading condition (124.0±30.2 N) and the lowest peak load in flexion (26.8±16.1 N). Intradiscal pressure was high in lateral flexion (495.6±280.9 kPa) and flexion (429.0±212.9 kPa), whereas intradiscal pressures measured in rotation (253.2±135.0 kPa) and 5° and 10° combined loading conditions were low (255.5±132.7 and 267.1±127.1 kPa, respectively).

CONCLUSIONS

Facet loading increased during simulated pelvic obliquity in frontal and transverse planes, whereas intradiscal pressures were decreased compared with sagittal and frontal plane motions alone. Altered spinopelvic alignment may increase the loads experienced by spinal tissue, especially the facet joints.

Inter-rater reliability of a video-analysis method measuring low-back load in a field situation

Valid and reliable low-back load assessment tools that can be used in field situations are needed for epidemiologic studies and for ergonomic practice. The aim of this study was to assess the inter-rater reliability of a low-back load video-analysis method in a field setting. Five raters analyzed 50 work site manual material handling tasks of 14 workers. Peak and mean moments at the level of L5S1, and segment angles were obtained using the video-analysis method. Intra-class correlation coefficients (ICCs) and median standard deviations across raters were calculated. ICCs revealed excellent inter-rater reliability (>0.9) for peak and mean moments, ICCs of segment angles were variable. Median standard deviations showed relatively small inter-rater variance for moments (standard deviation <10 Nm) and segment angle variation ranging from 0° to 20°. The proposed video-analysis method, provides a reliable tool for obtaining low-back loads from occupational field tasks.

Region specific response of intervertebral disc cells to complex dynamic loading: an organ culture study using a dynamic torsion-compression bioreactor

The spine is routinely subjected to repetitive complex loading consisting of axial compression, torsion, flexion and extension. Mechanical loading is one of the important causes of spinal diseases, including disc herniation and disc degeneration. It is known that static and dynamic compression can lead to progressive disc degeneration, but little is known about the mechanobiology of the disc subjected to combined dynamic compression and torsion. Therefore, the purpose of this study was to compare the mechanobiology of the intervertebral disc when subjected to combined dynamic compression and axial torsion or pure dynamic compression or axial torsion using organ culture. We applied four different loading modalities [1. control: no loading (NL), 2. cyclic compression (CC), 3. cyclic torsion (CT), and 4. combined cyclic compression and torsion (CCT)] on bovine caudal disc explants using our custom made dynamic loading bioreactor for disc organ culture. Loads were applied for 8 h/day and continued for 14 days, all at a physiological magnitude and frequency. Our results provided strong evidence that complex loading induced a stronger degree of disc degeneration compared to one degree of freedom loading. In the CCT group, less than 10% nucleus pulposus (NP) cells survived the 14 days of loading, while cell viabilities were maintained above 70% in the NP of all the other three groups and in the annulus fibrosus (AF) of all the groups. Gene expression analysis revealed a strong up-regulation in matrix genes and matrix remodeling genes in the AF of the CCT group. Cell apoptotic activity and glycosaminoglycan content were also quantified but there were no statistically significant differences found. Cell morphology in the NP of the CCT was changed, as shown by histological evaluation. Our results stress the importance of complex loading on the initiation and progression of disc degeneration.

Loads worn by soldiers predict episodes of low back pain during deployment to Afghanistan

STUDY DESIGN

Prospective cohort study.

OBJECTIVE

To describe low back pain (LBP) sustained by soldiers deployed to Afghanistan and identify demographic, fitness, and occupational risk factors of LBP.

SUMMARY OF BACKGROUND DATA

LBP is the most common injury in deployed soldiers. It results in lost duty days, medical evacuations, and permanent disability. Risk factors for LBP have not been investigated in this population.

METHODS

This study investigated a US Army Brigade Combat Team deployed to Afghanistan for 1 year. Demographic, fitness, and occupational variables were recorded before and after deployment. Episodes of LBP were assessed using self-report data. Logistic regression was used to investigate predictors of (1) moderate or worse LBP developed during deployment and (2) all LBP developed during deployment.

RESULTS

The incidence of moderate or worse LBP was 22% in the 805 soldiers studied. Soldiers with elevated risk of LBP were male, in Cavalry or Infantry units, wore body armor for more than 6 hr/d, worked at a desk for 1 to 4 hours a day, spent 5 to 8 hr/d lifting, spent more than 1 hr/wk on walking patrol, spent 21 to 40 hr/wk in a tactical vehicle, or wore equipment. The most parsimonious set of significant predictors for moderate or worse LBP included age, OR = 1.04 (1.01-1.08); fitness score, OR = 0.99 (0.989-0.999); the amount of time spent wearing body armor, OR = 1.16 (1.10-1.23); the amount of time spent on walking patrol, OR = 1.01 (1.003-1.02); and weight of the equipment worn, OR = 1.01 (1.002-1.013). The incidence of all LBP (minor or worse) was 77%. The most parsimonious set of significant predictors for all LBP included age, OR = 1.06 (1.02-1.09); fitness score, OR = 0.99 (0.985-0.997); history of LBP, OR = 2.94, (1.87-4.62); equipment weight, OR = 1.008 (1.003-1.01); and time spent wearing body armor, OR = 1.13 (1.07-1.19).

CONCLUSION

This study found that being older, having lower fitness scores, wearing armor longer, and wearing heavier loads increased the risk of LBP in deployed soldiers. Units more likely to wear loads such as the Infantry and Cavalry and tasks commonly involving wearing loads such as patrolling all increased the risk of LBP as well.

Differences in physical workload between military helicopter pilots and cabin crew

PURPOSE

The 1-year prevalence of regular or continuous neck pain in military helicopter pilots of the Dutch Defense Helicopter Command (DHC) is 20%, and physical work exposures have been suggested as risk factors. Pilots and cabin crew perform different tasks when flying helicopters. The aims of the current study were to compare the exposures to physical work factors between these occupations and to estimate the 1-year prevalence of neck pain in military helicopter cabin crew members.

METHODS

A survey was completed by almost all available helicopter pilots (n = 113) and cabin crew members (n = 61) of the DHC. The outcome measures were self-reported neck pain and exposures to nine physical work factors. Differences in the proportions of helicopter pilots and cabin crew members reporting being often exposed to the particular physical factor were assessed with the χ(2) test.

RESULTS

The 1-year prevalence of regular or continuous neck pain among cabin crew was 28%. Significantly more cabin crew members than pilots reported being often exposed to manual material handling, performing dynamic movements with their torsos, working in prolonged bent or twisted postures with their torsos and their necks, working with their arms raised and working in awkward postures. Often exposure to prolonged sitting and dynamic movements with the neck were equally reported by almost all the pilots and cabin crew members.

CONCLUSION

Flight-related neck pain is prevalent in both military helicopter pilots and cabin crew members. The exposures to neck pain-related physical work factors differ between occupations, with the cabin crew members subjected to more factors. These results have implications for preventative strategies for flight-related neck pain.

Study protocol title: a prospective cohort study of low back pain

BACKGROUND

Few prospective cohort studies of workplace low back pain (LBP) with quantified job physical exposure have been performed. There are few prospective epidemiological studies for LBP occupational risk factors and reported data generally have few adjustments for many personal and psychosocial factors.

METHODS/DESIGN

A multi-center prospective cohort study has been incepted to quantify risk factors for LBP and potentially develop improved methods for designing and analyzing jobs. Due to the subjectivity of LBP, six measures of LBP are captured: 1) any LBP, 2) LBP ≥ 5/10 pain rating, 3) LBP with medication use, 4) LBP with healthcare provider visits, 5) LBP necessitating modified work duties and 6) LBP with lost work time. Workers have thus far been enrolled from 30 different employment settings in 4 diverse US states and performed widely varying work. At baseline, workers undergo laptop-administered questionnaires, structured interviews, and two standardized physical examinations to ascertain demographics, medical history, psychosocial factors, hobbies and physical activities, and current musculoskeletal disorders. All workers' jobs are individually measured for physical factors and are videotaped. Workers are followed monthly for the development of low back pain. Changes in jobs necessitate re-measure and re-videotaping of job physical factors. The lifetime cumulative incidence of low back pain will also include those with a past history of low back pain. Incident cases will exclude prevalent cases at baseline. Statistical methods planned include survival analyses and logistic regression.

DISCUSSION

Data analysis of a prospective cohort study of low back pain is underway and has successfully enrolled over 800 workers to date.

Towards establishing an occupational threshold for cumulative shear force in the vertebral joint - an in vitro evaluation of a risk factor for spondylolytic fractures using porcine specimens

BACKGROUND

Injury models for spondylolytic fracture of the pars interarticularis have long considered repetitive shear loading as a risk factor without quantifying the relationship between shear force magnitude and fatigue life. This investigation sought to quantify the relationship using a basic in vitro approach.

METHODS

Thirty-two (16 C3-C4, 16 C5-C6) porcine cervical specimens were exposed to repetitive shear loading to 20%, 40%, 60%, or 80% of their calculated ultimate anterior shear failure tolerance. Shear force was cyclically applied at 1Hz for 21,600cycles or until bone failure was detected. Cumulative shear force and the number of cycles sustained until failure were calculated. Failure patterns were also documented.

FINDINGS

Cumulative shear and the number of cycles sustained prior to failure demonstrated a strong non-linearly decreasing relationship with increased force magnitude. In particular, sustained cumulative shear by the 40% group was 2.52 and 2.63MN∗s higher than for the 60% and 80% groups (P<0.0001). Despite undergoing an average of 230 more loading cycles, cumulative shear force sustained by the 60% group was not statistically different from the 80% group. Bilateral fractures of the cranial vertebra's pars interarticularis were most common, but less consistent at higher force magnitudes.

INTERPRETATION

Our investigation suggested that pars interarticularis damage may begin non-linearly accumulating with shear forces between 20% and 40% of failure tolerance (approximately 430 to 860N). Models of pars interarticularis injury and estimates of cumulative shear exposure may be enhanced from a tissue-based weighting method for low-back shear.

Cumulative knee adductor load distinguishes between healthy and osteoarthritic knees--a proof of principle study

Cumulative knee adductor load reflects repetitive exposures to medial knee loading that are encountered during daily activity. The purpose of this proof of principle study was to investigate whether cumulative knee adductor load distinguished between adults with and without knee osteoarthritis (OA). Thirty-one adults with radiographic knee OA (53.2±6.1 years old) and 30 healthy adults (33.5±8.0 years old) participated. A non-normalized knee adduction moment waveform was calculated from gait data collected using a motion analysis system with synchronized force plate. The peak and impulse from knee adduction moment was calculated. Cumulative knee adductor load was the product of the knee adduction moment impulse during stance and the mean number of steps taken per day, measured with a uni-dimensional accelerometer. One thousand bootstrap t-tests determined whether cumulative knee adductor load was at least as good as the peak knee adduction moment in discriminating between the healthy and OA groups. Cumulative knee adductor load was nearly two times larger in the knee OA compared to the healthy group (p=0.001). Cumulative knee adductor load was better than the peak knee adduction moment at discriminating between groups (p=0.04). This work provides evidence of validity for cumulative knee adductor load. In the calculation of cumulative loads, non-normalization of the adduction moment impulse to magnitude or time emphasized the importance of the total loads borne through the medial knee compartment during each step.

Cumulative low back load at work as a risk factor of low back pain: a prospective cohort study

PURPOSE

Much research has been performed on physical exposures during work (e.g. lifting, trunk flexion or body vibrations) as risk factors for low back pain (LBP), however results are inconsistent. Information on the effect of doses (e.g. spinal force or low back moments) on LBP may be more reliable but is lacking yet. The aim of the present study was to investigate the prospective relationship of cumulative low back loads (CLBL) with LBP and to compare the association of this mechanical load measure to exposure measures used previously.

METHODS

The current study was part of the Study on Musculoskeletal disorders, Absenteeism and Health (SMASH) study in which 1,745 workers completed questionnaires. Physical load at the workplace was assessed by video-observations and force measurements. These measures were used to calculate CLBL. Furthermore, a 3-year follow-up was conducted to assess the occurrence of LBP. Logistic regressions were performed to assess associations of CLBL and physical risk factors established earlier (i.e. lifting and working in a flexed posture) with LBP. Furthermore, CLBL and the risk factors combined were assessed as predictors in logistic regression analyses to assess the association with LBP.

RESULTS

Results showed that CLBL is a significant risk factor for LBP (OR: 2.06 (1.32-3.20)). Furthermore, CLBL had a more consistent association with LBP than two of the three risk factors reported earlier.

CONCLUSIONS

From these results it can be concluded that CLBL is a risk factor for the occurrence of LBP, having a more consistent association with LBP compared to most risk factors reported earlier.

Description and analysis of hand forces in medicine cart pushing tasks

The primary objectives of this study were to describe and analyze the hand force exertion patterns of experienced nursing home nurses and nursing students during dynamic medicine cart pushing tasks in Initial, Sustained, Turning, and Stopping motion phases. A 2 × 2 × 2 factorial experiment was conducted with 22 participants to estimate the effects of lane congestion, precision cart control, and floor surface on horizontal hand forces. Root mean squared (RMS) lane deviation patterns were also described to provide an indicator of cart handling difficulty across the different study conditions. Descriptive statistics revealed that nurses exerted greater mean hand force (10%) and made more (12%) lane deviation than students and that the highest two-hand forces of 147N were measured in the Turning phase on carpet. Strong correlations between work experience group, body mass, and BMI required that force data for nurses and students be collapsed in analytical models where no group differences existed. Predicted pushing forces on carpeted floor surface were significantly greater than on tile in Initial (14N), Sustained (14N) and Turning (18N), except in stopping where pulling forces were 37N lower. High lane congestion predicted significant peak force increases of 4N and 7N in Sustained and Turning, respectively, but decreased by 20N in Initial. High precision control led to significant decreases in two-hand forces that ranged from 4 to 20N across motion phases. Complex interactions among the experimental factors suggest that work environment (lane congestion and floor surface) and work demands (precision control) should be included in the evaluation of pushing tasks and considered prior to making renovations to nursing home environments.

Muscle Contributions to L4-5 Joint Rotational Stiffness following Sudden Trunk Flexion and Extension Perturbations

The purpose of this study was to investigate the contribution of individual muscles (MJRS_m) to total joint rotational stiffness (MJRS_T) about the lumbar spine's L_4-5 joint prior to, and following, sudden dynamic flexion or extension perturbations to the trunk. We collected kinematic and surface electromyography (sEMG) data while subjects maintained a kneeling posture on a parallel robotic platform, with their pelvis constrained by a harness. The parallel robotic platform caused sudden inertial trunk flexion or extension perturbations, with and without the subjects being aware of the timing and direction. Prevoluntary muscle forces incorporating both short and medium latency neuromuscular responses contributed significantly to joint rotational stiffness, following both sudden trunk flexion and extension motions. MJRS_T did not change with perturbation direction awareness. The lumbar erector spinae were always the greatest contributor to MJRS_T. This indicates that the neuromuscular feedback system significantly contributed to MJRS_T, and this behaviour likely enhances joint stability following sudden trunk flexion and extension perturbations.

Pooling job physical exposure data from multiple independent studies in a consortium study of carpal tunnel syndrome

Pooling data from different epidemiological studies of musculoskeletal disorders (MSDs) is necessary to improve statistical power and to more precisely quantify exposure-response relationships for MSDs. The pooling process is difficult and time-consuming, and small methodological differences could lead to different exposure-response relationships. A sub-committee of a six-study research consortium studying carpal tunnel syndrome: (i) visited each study site, (ii) documented methods used to collect physical exposure data and (iii) determined compatibility of exposure variables across studies. Certain measures of force, frequency of exertion and duty cycle were collected by all studies and were largely compatible. A portion of studies had detailed data to investigate simultaneous combinations of force, frequency and duration of exertions. Limited compatibility was found for hand/wrist posture. Only two studies could calculate compatible Strain Index scores, but Threshold Limit Value for Hand Activity Level could be determined for all studies. Challenges of pooling data, resources required and recommendations for future researchers are discussed.

PRACTITIONER SUMMARY

There is a need for standardised measures and measurement protocols of physical exposure for the upper extremity. This study may provide guidance for those planning to conduct an epidemiological study on quantified job physical exposures, or planning to merge physical exposure data from similar studies with some methodologic differences.

Performance evaluation of a wearable inertial motion capture system for capturing physical exposures during manual material handling tasks

With a long-term goal of improving quantification of physical exposures in the workplace, this study examined the ability of a commercially available inertial motion capture (IMC) system in quantifying exposures during five different simulated manual material handling tasks. Fourteen participants repeated all these tasks in three 20 min sequential time blocks. Performance of the IMC system was compared against an optical motion capture (OMC) system ('gold standard') in terms of joint angles, angular velocities and moments at selected body parts. Though several significant changes in performance over time were found, the magnitudes of these were relatively small and may have limited practical relevance. The IMC system yielded peak kinematic values that differed by up to 28% from the OMC system. The IMC system, in some cases, incorrectly reflected the actual extremity positions of a participant, and which can cause relatively large errors in joint moment estimation. Given the potential limitations, practical recommendations are offered and discussed.

PRACTITIONER SUMMARY

Use of an inertial motion capture system can advance the quantification of physical exposures in situ. Results indicate a good potential capacity for capturing physical exposure data in the field for an extended period, while highlighting potential limitations. Future system application can help provide better understandings of dose-exposure relationships.

The contribution of load magnitude and number of load cycles to cumulative low-back load estimations: a study based on in-vitro compression data

BACKGROUND

Cumulative low-back load is suggested to be associated with low back pain, possibly due to (micro-)fractures of spinal segments. Based on available in vitro data it can be assumed that, in order to predict spine segment failure from cumulative compressive loading, load magnitude should be weighted with an exponent higher than one, whereas the number of cycles should be weighted with an exponent lower than 1. The aim of the present study was to assess both exponents based on available in-vitro data.

METHODS

Data on loading to fatigue fracture of spinal segments under cyclic compression in-vitro were used and converted to survival probability for 5 load levels and 5 levels of number of cycles. Three optimization procedures were used to estimate the exponent of load magnitude and load cycles separately, and load magnitude and load cycles combined. Goodness of fit was assessed by comparing the Akaike's Information Criterion (AIC) between models.

FINDINGS

The best fit, based on AIC and average error per data point was obtained with weighting of load magnitude and number of load cycles with exponents of approximately 2.0 and 0.2, respectively.

INTERPRETATION

The results show that a combination of load magnitude and number of load cycles weighted with exponents of approximately 2 and 0.2 respectively provides a suitable measure of cumulative spinal compression loading. This finding may be of relevance for assessing cumulative low-back loads in studies on the etiology of low-back pain.

Tolerance of the lumbar spine to shear: a review and recommended exposure limits

BACKGROUND

The lumbar spine may experience significant shear forces during occupational tasks due to the force of gravity acting on the upper body when bending the trunk forward, or when performing tasks involving pushing or pulling. Shear force limits of 1000 N and 500 N have been recommended by previous authors for maximum permissible limit and action limit, respectively.

METHODS

The present paper reviews literature in terms of shear tolerance (ultimate shear stress and fatigue life in shear stress) of the lumbar spine and develops recommended limits based on results of studies examining shear loading of human motion segments. Weibull analysis was used to assess fatigue failure data to estimate distributions of failure at different percentages of ultimate shear stress.

FINDINGS

Based on Weibull analysis of fatigue failure data from the best available data, a 1000 N shear limit would appear acceptable for occasional exposure to shear loading (≤ 100 loadings/day); however, a 700 N limit would appear appropriate for repetitive shear loading (100-1000 loadings/day) for most workers.

INTERPRETATION

Results of the current analysis support the 1000 N limit for shear stress, but for a rather limited number of cycles (<100 per day). Due to the logarithmic nature of the fatigue failure curve, a 700 N shear limit would appear to be acceptable for frequent shear loadings (100-1000 per day). This value is slightly higher than the action limit of 500 N previously recommended.

The effect of posture category salience on decision times and errors when using observation-based posture assessment methods

Observation-based posture assessment methods (e.g. RULA, 3DMatch) require classification of body postures into categories. This study investigated the effect of improving posture category salience (adding borders, shading and colour to the posture categories) on posture selection error rates and decision times of novice analysts. Ninety university students with normal or corrected normal visual acuity and who were not colourblind, were instructed to select posture categories as quickly and accurately as possible, in five salience conditions (Plain (no border, no shading, no colour); Grey Border; Red Border; Grey Shading (GS) and Red Shading (RS)) for images presented in randomised blocks (240 classifications made by each participant) on a computer interface. Participants responded quickest in the Border conditions, classifying postures about 5% faster than in the Plain condition. Coloured diagrams significantly reduced posture classification errors by approximately 1.5%. Overall, the best performance, based on both error rate and decision time combined, resulted from incorporating a Grey Border to the posture category diagrams; a simple enhancement that could be made to most current observation-based posture assessment tools.

PRACTITIONER SUMMARY

The salience of posture diagrams used in observation-based posture assessment tools was evaluated with respect to analyst error rates and decision times. The best performance resulted from incorporating a grey border to the posture diagrams; a simple enhancement that can be made to most current observation-based posture assessment tools.

Can the Revised NIOSH Lifting Equation Predict Low Back Pain Incidence in a ‘90-day-pain-free-cohort’?

LBP is a major public health concern with enormous human and economic burden. The validity of the Revised NIOSH Lifting Equation (RNLE), a widely used job assessment tool, has not been studied previously in individuals with past LBP. The primary aim of this research was to study the relationship between RNLE measures and incident LBP episode risk after a 90-day pain free period using a prospective study design. One hundred and thirty industrial workers were identified as eligible to become an incident case for LBP. Univariate relationships between RNLE measures and incidence of LBP episode were studied using the Cox proportional hazards model. Significant associations between RNLE measures and incident LBP risk were found. It is concluded that the RNLE is predictive of incident LBP episode risk in individuals with past pain.

The benefits of an additional worker are task-dependent: assessing low-back injury risks during prefabricated (panelized) wall construction

Team manual material handling is a common practice in residential construction where prefabricated building components (e.g., wall panels) are increasingly used. As part of a larger effort to enable proactive control of ergonomic exposures among workers handling panels, this study explored the effects of additional workers on injury risks during team-based panel erection tasks, specifically by quantifying how injury risks are affected by increasing the number of workers (by one, above the nominal or most common number). Twenty-four participants completed panel erection tasks with and without an additional worker under different panel mass and size conditions. Four risk assessment methods were employed that emphasized the low back. Though including an additional worker generally reduced injury risk across several panel masses and sizes, the magnitude of these benefits varied depending on the specific task and exhibited somewhat high variability within a given task. These results suggest that a simple, generalizable recommendation regarding team-based panel erection tasks is not warranted. Rather, a more systems-level approach accounting for both injury risk and productivity (a strength of panelized wall systems) should be undertaken.

Work-site musculoskeletal pain risk estimates by trained observers--a prospective cohort study

Work-related musculoskeletal pain (MSP) risk assessments by trained observers are often used in ergonomic practice; however, the validity may be questionable. We investigated the predictive value of work-site MSP risk estimates in a prospective cohort study of 1745 workers. Trained observers estimated the risk of MSP (neck, shoulder or low-back pain) using a three-point scale (high, moderate and low risk) after observing a video of randomly selected workers representing a task group. Associations of the estimated risk of pain and reported pain during a three-year follow-up were assessed using logistic regression. Estimated risk of neck and shoulder pain did (odds ratio, OR: 1.45 (95% confidence interval, CI: 1.01-2.08); 1.64 (95% CI: 1.05-2.55)), however, estimated risk of low-back pain did not significantly predict pain (OR: 1.27 (95% CI: 0.91-1.79)). The results show that observers were able to estimate the risk of shoulder and neck pain, whereas they found it difficult to estimate the risk of low-back pain. Practitioner Summary: Work-related musculoskeletal pain risk assessments by observers are often used in ergonomic practice. We showed that observers were able to estimate shoulder and neck pain risk, but had difficulties to estimate the risk of low-back pain. Therefore, observers' risk estimates might provide a useful method for musculoskeletal pain risk assessments.

Lumbar bone mass predicts low back pain in males

STUDY DESIGN

Longitudinal study of lumbar bone mass as predictor of low back pain (LBP).

OBJECTIVE

To investigate whether low bone mineral content (BMC) and bone mineral density (BMD) values at the age of 36 years are associated with the prevalence of LBP at the age of 42 years among the study population of the Amsterdam Growth and Health Longitudinal Study.

SUMMARY OF BACKGROUND DATA

Results of epidemiological, clinical, and in vitro studies indicate that spinal injuries, caused by mechanical loading, might be a cause of LBP. BMC and BMD are determinants of spinal strength. We therefore hypothesized that BMC and BMD are associated with LBP.

METHODS

At the age of 36 years, the lumbar BMC and BMD were determined by dual-energy x-ray absorptiometry in 140 men and 152 women. At the age of 42 years, the participants were asked whether they had experienced LBP in the previous 12 months. Logistic regression analyses were performed to determine odds ratios (ORs)-adjusted for stature, body weight, physical activity, and smoking-for the relationship of BMC and BMD with LBP.

RESULTS

BMC and BMD at the age of 36 years were significantly associated with the reported 12-month prevalence of LBP at the age of 42 years. This association, however, was observed only for men and not for women. Men within the quartile with the lowest BMC or BMD values had higher odds for LBP with ORs of 4.78 (95% confidence interval, 1.52-15.00) and 3.48 (95% confidence interval, 1.23-9.85), respectively.

CONCLUSION

For a male population that is not characterized by osteoporosis or old age, lower lumbar BMC and BMD values at the age of 36 years are associated with an increased risk of reporting to have had LBP in the previous 12 months at the age of 42 years.

Trunk postural demands of occupational activities of some merchant pregnant women in Benin, West Africa

Strenuous physical work puts expectant mothers at risk of experiencing back pain during the gestational months. Pregnant women in Benin perform physically demanding occupational tasks that include the lifting and carriage of heavy loads on their heads for commercial activities. A large percentage of pregnant subjects (58%) reported having back pain episodes since the start of their pregnancy. However, the mean Oswestry score of the affected participants was relatively low with a mean score of 0.2 (SD: 0.12), on a scale form 0 to 1. An evaluation of the postural demands of the occupational activities of these women revealed that they performed on average 328 trunk flexions at angles exceeding 60°, with 66 of these flexions sustained for more than 4 s, during the average 7.9 h where trunk postures were recorded. They also spent on average 36% of the recording time at trunk flexion angles larger than 20°. These results show that the merchant pregnant women in the Porto-Novo area in Benin are at great risk for developing back disorders during pregnancy.

PRACTITIONER SUMMARY

Results will make a first contribution to the literature by identifying the stressful postures adopted during a typical day. The findings of this study can help in the development of preventative concepts and postural modification techniques to decrease the occurrence of back pain during pregnancy for women in Benin.

Shear strength of the human lumbar spine

BACKGROUND

Shear loading is recognised as a risk factor for lower back pain. Previous studies of shear loading have either not addressed the influence of age, bone mineral density, axial height loss due to creep or were performed on animal specimens.

METHODS

Intact human lumbar motion segments (L2-3) were tested in shear using a modified materials testing machine, while immersed in a Ringer bath at 37°C. Vertebrae were rigidly embedded in neutral posture (0° flexion) and subjected to a constant axial compression load of 500 N. Shear was applied to three groups: 'Young-No-Creep' (20-42 years), 'Young-Creep' (22-38 years, creep 1000 N for 1h) and 'Old-No-Creep' (44-64 years). Failure was induced by up to 15 mm of anterior shear displacement at a rate of 0.5mm/s. The trabecular and apophyseal joint bone mineral densities were evaluated from computed tomography images of the intact lumbar spines.

FINDINGS

Peak shear force correlated positively with trabecular bone mineral density for specimens tested without axial creep. No significant differences were observed with respect to age. During shear overload specimens increased in height in the axial direction.

INTERPRETATION

Trabecular bone mineral density can be used to predict the peak force of lumbar spine in shear in neutral posture.

Two linear regression models predicting cumulative dynamic L5/S1 joint moment during a range of lifting tasks based on static postures

Previous studies have indicated that cumulative L5/S1 joint load is a potential risk factor for low back pain. The assessment of cumulative L5/S1 joint load during a field study is challenging due to the difficulty of continuously monitoring the dynamic joint load. This study proposes two regression models predicting cumulative dynamic L5/S1 joint moment based on the static L5/S1 joint moment of a lifting task at lift-off and set-down and the lift duration. Twelve men performed lifting tasks at varying lifting ranges and asymmetric angles in a laboratory environment. The cumulative L5/S1 joint moment was calculated from continuous dynamic L5/S1 moments as the reference for comparison. The static L5/S1 joint moments at lift-off and set-down were measured for the two regression models. The prediction error of the cumulative L5/S1 joint moment was 21 ± 14 Nm × s (12% of the measured cumulative L5/S1 joint moment) and 14 ± 9 Nm × s (8%) for the first and the second models, respectively. Practitioner Summary: The proposed regression models may provide a practical approach for predicting the cumulative dynamic L5/S1 joint loading of a lifting task for field studies since it requires only the lifting duration and the static moments at the lift-off and/or set-down instants of the lift.

Constrained Bayesian streak artifact reduction approach for contrast enhanced computed tomography imaging of the intervertebral disc

A promising approach for the study of progressive herniation damage of the intervertebral discs under flexion/extension motions as well as compressive loads is the use of contrast-enhanced computed tomography (CECT). One of the biggest limitations of using CECT is the presence of significant streak artifacts in the acquired tomograms, due primarily to the contrast agent injected into the intervertebral disc. To address this issue, a novel constrained Bayesian approach to streak artifact reduction in CECT imagery is introduced in this paper. The problem of artifact reduction is formulated as a constrained Bayesian estimation problem in projection space, and a non-parametric Parzen window estimation approach is employed to estimate the underlying posterior distributions. Experimental results show that the proposed approach provides significant artifact reduction while preserving the intervertebral disc regions to allow for clear visualization of progressive intervertebral disc damage.

Maximum acceptable weight of lift reflects peak lumbosacral extension moments in a functional capacity evaluation test using free style, stoop and squat lifting

It is unclear whether the maximum acceptable weight of lift (MAWL), a common psychophysical method, reflects joint kinetics when different lifting techniques are employed. In a within-participants study (n = 12), participants performed three lifting techniques--free style, stoop and squat lifting from knee to waist level--using the same dynamic functional capacity evaluation lifting test to assess MAWL and to calculate low back and knee kinetics. We assessed which knee and back kinetic parameters increased with the load mass lifted, and whether the magnitudes of the kinetic parameters were consistent across techniques when lifting MAWL. MAWL was significantly different between techniques (p = 0.03). The peak lumbosacral extension moment met both criteria: it had the highest association with the load masses lifted (r > 0.9) and was most consistent between the three techniques when lifting MAWL (ICC = 0.87). In conclusion, MAWL reflects the lumbosacral extension moment across free style, stoop and squat lifting in healthy young males, but the relation between the load mass lifted and lumbosacral extension moment is different between techniques.

PRACTITIONER SUMMARY

Tests of maximum acceptable weight of lift (MAWL) from knee to waist height are used to assess work capacity of individuals with low-back disorders. This article shows that the MAWL reflects the lumbosacral extension moment across free style, stoop and squat lifting in healthy young males, but the relation between the load mass lifted and lumbosacral extension moment is different between techniques. This suggests that standardisation of lifting technique used in tests of the MAWL would be indicated if the aim is to assess the capacity of the low back.