Steven A. Lavender

Biomechanical risk factors for occupationally related low back disorders

A continuing challenge for ergonomists has been to determine quantitatively the types of trunk motion and how much trunk motion contributes to the risk of occupationally-related low back disorder (LBD). It has been difficult to include this motion information in workplace assessments since the speed at which trunk motion becomes dangerous has not been determined. An in vivo study was performed to assess the contribution of three-dimensional dynamic trunk motions to the risk of LBD during occupational lifting in industry. Over 400 industrial lifting jobs were studied in 48 varied industries. The medical records in these industries were examined so that specific jobs historically categorized as either low, medium, or high risk for occupationally-related LBD could be identified. A tri-axial electrogoniometer was worn by workers and documented the three-dimensional angular position, velocity, and acceleration characteristics of the lumbar spine while workers worked at these low, medium, or high risk jobs. Workplace and individual characteristics were also documented for each of the repetitive lifting tasks. A multiple logistic regression model indicated that a combination of five trunk motion and workplace factors predicted well both medium risk and high risk occupational-related LBD. These factors included lifting frequency, load moment, trunk lateral velocity, trunk twisting velocity, and trunk sagittal angle. Increases in the magnitude of these factors significantly increased the risk of LBD. The analyses have enabled us to determine the LBD risk associated with combined changes in the magnitudes of the five factors. The results indicate that by suitably varying these five factors observed during the lift collectively, the odds of high risk group membership may decrease by over ten times. These results were related to the biomechanical, ergonomic, and epidemiologic literature. The five trunk motion and workplace factors could be used as quantitative, objective measures to redesign the workplace so that the risk of occupationally-related LBD is minimized.

The Effects of Preview and Task Symmetry on Trunk Muscle Response to Sudden Loading

The effect of warning time (preview) and task symmetry on the trunk muscular response to sudden loading conditions was investigated. Eleven subjects were asked to catch falling weights with four levels of preview (0, 100, 200, and 400 ms) in sagittally symmetric posture and asymmetric posture. For each of the eight muscles sampled with surface electrodes, the integrated electromyographic (EMG) signal was interpreted in terms of its peak value, mean value, onset rate, and lead/lag time with reference to the weight drop. Results show linear relationships between preview times and peak EMG, preview times and mean EMG, and preview times and lead times. The results show significant change when going from symmetric to asymmetric conditions across most dependent measures. Analysis of peak changes in compression were performed across all conditions but yielded unexpected results.

The effects of initial lifting height, load magnitude, and lifting speed on the peak dynamic L5/S1 moments

The purpose of this study was to quantify the peak dynamic bending moments on the spine during sagittal plane lifting as a function of the loads initial height above the floor, the loads magnitude, and the lifting speed. Ten male subjects participated in a repeated measures experiment in which 24 lifts were performed. The boxes lifted by the subjects contained loads that were 20, 100, 200, and 300 N. The lifts originated from three vertical locations (7.5 cm or “floor” level, knee level, and knuckle level), and were lifted at two qualitatively defined lifting speeds (“normal” and “fast”). All lifts were symmetric about the bodys mid-sagittal plane and the boxes were gripped with both hands using the handles. Kinetic and kinematic data were used in a “bottom-up” linked segment dynamic model to determine the peak sagittal bending moment experienced by the subjects during each lift. The peak moments were significantly greater when lifting from lower lift heights (p<0.001), at faster lifting speeds (p<0.001), and with heavier loads (p<0.001). Moreover, the peak moments were dependent upon the combination of these three factors. Based on these data, a regression model was developed to predict the peak dynamic moment given the load magnitude, the starting height, and a qualitative estimate of lifting speed. This model was then used to assess whether the current National Institute for Occupational Safety and Healths (NIOSHs) model can control the peak bending moments acting on the spine as the limits for safe lifting are adjusted according to changes in the initial lifting height. It was found that the current NIOSH guidelines under-represent the increased bimechanical load experienced during low-level lifting.

Biomechanical analyses of paramedics simulating frequently performed strenuous work tasks

Firefighters performing emergency rescue functions are at an elevated risk of musculoskeletal injuries. The objective of the current study was to analyze the biomechanical stresses placed on the body based on simulations of the following strenuous and frequently performed emergency rescue tasks: (1) transferring a patient from a bed to a stretcher using bedsheets, (2) transferring a patient from the ambulance stretcher to a hospital gurney, (3) carrying a victim down a set of stairs and through a landing using a stairchair, (4) carrying a victim down a set of stairs and through a landing using a backboard, and (5) carrying a victim down a straight set of stairs using a stretcher. Postural data were analyzed using the University of Michigans Three-Dimensional Static Strength Prediction Program and the relative risk of low back disorder (LBD) was quantified using the trunk motion model published by Marras et al. (1993, spine 18, 617-628). Peak compression values and the probabilities from the Marras et al. (1993) model indicated that the most hazardous tasks performed as part of this simulation included pulling a victim from a bed to a stretcher, the initial descent of a set of stairs when using the stretcher, and lifting a victim on a backboard from the floor. Overall, the two models were well correlated in their assessment of the task components modelled (r = 0.78). These data indicate where engineering changes to equipment regularly used by emergency rescue personnel would have the greatest impact in reducing the risk of musculoskeletal injury.

Coactivation of the trunk muscles during asymmetric loading of the torso

Materials handling tasks in industry are rarely performed in the midsagittal plane. Often these tasks, labeled nonsagittally symmetric or asymmetric lifting tasks, can be expected to lead to an unequal distribution of forces between the left and right sides of the body. Because of the large number of muscles capable of resisting loads in the torso, researchers are forced to make simplifications when using biomechanical models to estimate mechanical loading of the spine during such tasks. Simplifications and assumptions regarding the coactivation of antagonistic muscles are frequently used because sufficient experimental data do not exist. The present study was designed to quantify coactivation of the trunk musculature in response to applied asymmetric loads. This load was varied in direction from an anterior midsagittal plane orientation to a posterior midsagittal plane orientation in 15-deg increments. The results showed little coactivation when the applied load directions were anterior and within 45 deg of the midsagittal orientation. When load directions were greater than 45 deg, coactivation was quantifiable in ipsilateral and posterior muscle groups.

Postural analysis of paramedics simulating frequently performed strenuous work tasks.

Paramedics who perform emergency rescue functions are highly susceptible to musculoskeletal injuries. Through an interview and survey process firefighters, many of whom are cross-trained paramedics in a consortium of 14 suburban fire departments, identified and rated tasks that were perceived to be both strenuous and frequently performed. The objective of the current study was to describe the working postures and the forces applied as firefighter/paramedics (FF/Ps) simulated specific roles within the following tasks identified by the survey: (1) transferring a patient from a bed to a stretcher using bedsheets, (2) transferring a patient from the ambulance stretcher to a hospital gurney, (3) carrying a victim down a set of stairs and around a landing using a stairchair, (4) carrying a victim down a set of stairs and around a landing using a backboard, and (5) carrying a victim down a set of stairs using a stretcher. Ten two-person teams of FF/Ps participated and were videotaped to obtain postural data for the upper and lower extremities as they performed each role in the simulated two-person tasks. Trunk postures were obtained using lumbar motion monitors. Static hand forces were estimated using a hand-held dynamometer at the most physically demanding points for each role within each task. The postural and force data were averaged across subjects performing identical roles to quantify the postures assumed by the FF/Ps at the most strenuous moments during task performance. Based on these analyses we concluded that: (1) when transferring victims from a bed to a stretcher the FF/P on the bed was able to maintain an upright and more stable posture by standing as opposed to kneeling, (2) an interface board should be used to reduce the frictional forces when transferring victims from a bed to a stretcher or from a stretcher to a gurney, thereby reducing the need to lift the victim with flexed torsos and/or shoulders, and (3) equipment and training that encourages the FF/P in the leader role to walk facing forward during victim transport, especially when descending stairs, potentially results in safer transit.

Quantitative Dynamic Measures of Physical Exposure Predict Low Back Functional Impairment

Study Design. Prospective field study of work exposure and changes in back function. Objective. Quantify dynamic physical exposures in the workplace and their association with decreases in kinematic back function (indicative of low back pain [LBP]). Summary of Background Data. Previous epidemiologic studies of work have measured gross categories of exposure and found moderate relationships with LBP. More precise quantitative measures of exposure and spine function were hypothesized to increase the chances of identifying any significant associations. Methods. Three hundred and ninety real-time physical exposure measures were collected from distribution center workers performing repetitive manual materials handling tasks. Low back health effect measures were quantitatively measured prospectively for workers performing each of the jobs using a kinematic measure of function. Results. Significant decreases in spine function were observed in workers associated with 40% of the jobs sampled. Numerous significant univariate odds ratios were identified that indicated an association between physical exposure and decreased function. A multivariate model including right lateral trunk velocity, timing of the maximum dynamic asymmetric load moment exposure, and the magnitude of the dynamic sagittal bending moment predicted reduced spine function well. The model resulted in excellent sensitivity (85%) and specificity (87.5%) as well as excellent positive predictive value (89.5%) and negative predictive value (82.4%). Conclusion. This study suggests that with proper quantification of job exposure and spine function, it is possible to identify which dynamic physical exposures are associated with reduced spine function and increases in LBP.

Variance in the measurement of sagittal lumbar spine range of motion among examiners, subjects, and instruments.

Study Design Repeated measurements were made of lumbar sagittal range of motion by 14 examiners using three different measurement instruments. Objectives To determine the reliability of lumbar range of motion measurements among examiners, subjects, and instruments, and to determine whether variance is due to subject inconsistency, examiner inconsistency, differences between examiners, or differences between instruments. Summary of Background Data Measurements of lumbar spine range of motion are widely used in research and clinical applications as well as in disability rating systems for patients with low back pain. Methods Fourteen examiners measured the sagittal range of motion. Using three instruments, 18 healthy subjects were measured twice in a randomized sequence with blinded readings when performing full flexion, and pertial flexion to a defined midpoint. None of the examiners routinely used the particular instruments in their practices. Results The mean test-retest reliability was 4.90. The intraexaminer reliability did not differ significantly among the examiners. Furthermore, there was no systematic difference resulting from instruments or posture condition. However, there was a statistically significant variance among examiners—i.e., a poor interexaminer reliability. Conclusion The most likely explanation for these findings is the variability among examiners in locating bony landmarks. The results indicate that range of motion measurements must be interpreted with caution in clinical, research, and disability applications. Test administrator training may improve results, but this could not be determined from this study.

A reliable and accurate method for measuring orthosis wearing time.

Study Design. Compliance monitor measurement of orthosis wearing time during laboratory climate tests and normal volunteer subject tests were compared to normal diaries. Objective. To develop and test the accuracy and reliability of a device designed to measure spinal orthosis wearing time. Summary of Background Data. Orthosis wearing time is an important factor in orthotic treatment for spine disorders. A reliable and objective method for measuring orthosis wearing time currently is lacking. Methods. Four pressure switches and a data logger embedded in each thoracolumbosacral orthosis recorded orthosis wearing time. Orthoses were assumed to be worn when at least two switches were “on.” Laboratory climate tests and normal volunteer tests were conducted to assess the ability of the compliance monitor to measure orthosis wearing time. A manual wearing-event diary was kept during all the tests. The length of each wearing-time interval, the daily wearing time, and the cumulative wearing time were calculated from data recorded by the compliance monitor and the manual diaries. Results. A linear regression was performed on all orthosis wearing-time intervals as recorded by the compliance monitor and by the manual diaries. Climate chamber tests yielded 121 sensor trigger-event intervals (R2 = 0.998; slope = 1.003;P < 0.0001). Normal subject testing yielded 72 orthosis wearing-time intervals (R2 = 0.998; slope = 0.998;P < 0.0001). Conclusion. As indicated by the regression analyses, the compliance monitor accurately quantified the orthosis wearing-time intervals during the laboratory climate tests and the tests with normal volunteers.

Comparison of five methods used to determine low back disorder risk in a manufacturing environment.

STUDY DESIGN Five methods for quantifying work-related low back disorder (LBD) risk were used to assess 178 autoworkers from 93 randomly selected production jobs. OBJECTIVE To determine if five occupational LBD risk evaluation methods yielded similar assessments of manual material handling tasks. SUMMARY OF BACKGROUND DATA Several techniques are available for quantifying LBD risk in the workplace and are used in industry for job evaluation and redesign. It is unknown whether the methods yield similar results. METHODS The five job evaluation methods were the 1993 National Institute for Occupational Safety and Health model, the Static Strength Prediction Program, the Lumbar Motion Monitor model, and two variations of the United Auto Workers (UAW)-General Motors Ergonomic Risk Factor Checklist. These methods were selected because they represent common practice within the automotive industry, the result of governmental efforts to protect the workforce, or models thought to be the most scientifically advanced. RESULTS Intercorrelations between methods ranged between 0.21 and 0.80. Pairwise analysis of risk group classifications identified biases on the part of the National Institute for Occupational Safety and Health equation, which considered jobs to be of higher risk relative to other methods, and on the part of the Static Strength Prediction Program, which considered nearly all the jobs sampled to be low risk. CONCLUSIONS There is little agreement among the five quantitative ergonomic analysis methods used. In part, this may be because of their differential focus on acute versus cumulative trauma, thereby suggesting that greater consideration needs to be given to the underlying causes of LBD within a facility before selecting an ergonomic evaluation method.