Shoulder disorders and postural stress in automobile assembly work.

OBJECTIVES: A case-referent study was conducted in an automobile assembly plant to evaluate the risk of shoulder disorders associated with nonneutral postures. METHODS: The cases were workers who reported shoulder pain to the plant clinic during a 10-month period and met symptom criteria (pain frequency or duration in the past year) in an interview; more than one-half also had positive findings in a physical examination. The referents were randomly selected workers who were free of shoulder disorders according to the clinic records, the interview, and the physical examination. For each of the 79 cases and 124 referents, 1 job was analyzed for postural and biomechanical demands by an analyst blinded to the case-referent status. RESULTS: Forty-one percent of the subjects flexed or abducted the right arm "severely" (above 90 degrees) during the job cycle, and 35% did so with the left arm. The peak torques at the shoulder were rather low. Shoulder disorders were associated with severe flexion or abduction of the left [odds ratio (OR) 3.2, 95% confidence interval (95% CI) 1.5-6.5] and the right (OR 2.3, 95% CI 1.2-4.8) shoulder. The risk increased as the proportion of the work cycle exposed increased. The relationships were similar for the cases with and without physical findings. Use of hand-held tools increased the risk and also modified the association with postural stress, although the joint exposure distributions limited full analysis of this finding. CONCLUSIONS: The findings support the conclusion that severe shoulder flexion or abduction, especially for 10% or more of the work cycle, is predictive of chronic or recurrent shoulder disorders.

Development and evaluation of an observational method for assessing repetition in hand tasks.

Several physical stressors, including repetitive, sustained, and forceful exertions, awkward postures, localized mechanical stress, highly dynamic movements, exposures to low temperatures, and vibration have been linked to increased risk of work-related musculoskeletal disorders. Repetitive exertions have been among the most widely studied of these stressors, but there is no single metric for assessing exposure to repetitive work. A new methodology enables repetitive hand activity to be rated based on observable characteristics of manual work. This method uses a series of 10-cm visual-analog scales with verbal anchors and benchmark examples. Ratings for repetition reflect both the dynamic aspect of hand movements and the amount of recovery or idle hand time. Trained job analysis experts rate the jobs individually and then agree on ratings. For a group of 33 jobs, repetition ratings using this system were compared to measurements of recovery time within the cycle, exertion counts, and cycle time. Amount of recovery time within the job cycle was found to be significantly correlated with the analysis ratings (r2 = 0.58), as were the number of exertions per second (r2 = 0.53). Cycle time was not related to the analyst ratings. Repeated analyses using the new method were performed 1 1/2 to 2 years apart on the same jobs with the same group of raters. Ratings for repetition differed less than 1 point (on the 10-cm scale), on average, among the different sessions. These results indicate that the method is sensitive to exertion level and recovery time, and that the decision criteria and benchmark examples allow for a consistent application of these methods over a period of time. This method of rating repetition can be combined with similar scales for other physical stressors.

A computerized method for assessment of musculoskeletal discomfort in the workforce: a tool for surveillance.

A musculoskeletal discomfort survey was conducted to assess musculoskeletal discomforts among rural mail carriers in two post offices. Perceived musculoskeletal discomfort was collected directly from the workforce by means of a computerized discomfort assessment system (DAS). This investigation aimed at: (1) assessing the rural mail carrier's perception of DAS; and (2) assessing the rural mail carrier's musculoskeletal discomforts resulting from work. Most participants in the study found the computer tool easy to learn and easy to use. The information collected by DAS was used: (1) to determine the number of participating employees who were experiencing some kind of musculoskeletal problem; (2) to determine subtasks associated with discomfort; (3) to determine the body areas most affected by different subtasks; and (4) to investigate the patterns of discomfort that occurred with time.

Back disorders and nonneutral trunk postures of automobile assembly workers.

A case-referent study was conducted in an automobile assembly plant to evaluate the health effect of trunk postures, such as bending and twisting, that deviate from anatomically neutral. Cases of back disorders were all those of workers who reported back pain to the medical department in a ten-month period and met the severity criteria of an interview. The referents were randomly selected workers free of back pain according to medical department records, an interview, and an examination. For each of the final 95 cases and 124 referents, the job was analyzed for postural and lifting requirements with a video recording and software analysis system by analysts blinded to the case/referent status. Back disorders were associated with mild trunk flexion [odds ratio (OR) 4.9, 95% confidence interval (95% CI) 1.4-17.4], severe trunk flexion (OR 5.7, 95% CI 1.6-20.4), and trunk twist or lateral bend (OR 5.9, 95% CI 1.6-21.4). The risk increased with exposure to multiple postures and increasing duration of exposure.

Effect of handle height on lower-back loading in cart pushing and pulling.

This paper presents results of a study conducted to estimate lower back loadings in cart pushing and pulling. Experiments were conducted in the laboratory using a cart. Six subjects with different weights (ranging from 50 to 80 kg) were tested for three different pushing and pulling forces (98, 196 and 294 newtons), three different heights of exertion (660, 1090 and 1520 mm high) and two different moving speeds (1.8 and 3.6 km/h). It was found that, in general, pushing a cart results in lesser lower-back loading than pulling. Subject body weight affected the lower-back loadings more significantly in pulling (50% increase as body weight increased from 50 kg to 80 kg) than in pushing (25% increase). Handle height of 1090 mm was found to be better than other handle heights in pushing while 1520 mm handle height was better for pulling in reducing lower-back loadings.

Monitoring industrial injuries: a case study.

This paper presents a new statistical approach for actively using information commonly available in medical surveillance systems to monitor the incidence and severity of industrial injuries on particular operations. The procedures require the practitioner to track the exposure time between injuries. An "out of control" situation is detected when the test statistic exceeds an optimal decision limit, which is determined by minimizing the total injury costs per exposure hour on each operation. The use of this test is presented in the context of a case example from an industrial assembly operation.

Prediction of overexertion injuries using biomechanical and psychophysical models.

This paper summarizes a three year epidemiological study conducted in five large industrial plants in order to evaluate the validity of two alternative modeling approaches to overexertion injury prediction. Detailed biomechanical and psychophysical job evaluations were performed for 55 industrial jobs comprised of 2934 potentially stressful manual materials handling tasks. The medical experiences of 6912 incumbent workers were monitored retrospectively for two years and prospectively for one year to establish a data base for comparison of the different models. The results show that each of the models can be used to predict both the incidence and severity of certain overexertion types of injuries such as contact, musculoskeletal and back injuries. The application of these models to identify or design administrative and engineering controls, however, may be limited as a result of the inherent correlation between the available indices and multifaceted jobs.

A study of lumbosacral orientation under varied static loads.

This paper describes a study of lumbar and sacrum orientations in select standing postures with the patient holding different loads in the hands. Vertebral rotation data were gathered from two young, healthy men and two young, healthy women while they performed static lifts at two load levels and in postures ranging from erect standing to deep squatting. The results disclosed a predictable reorientation of the sacrum and lumbar spine for both men and women as a function of torso and knee angles. Nonlinear second-order regression models of torso angle and knee angle versus reorientation angle were fit to the data with resulting r2 values of about 0.89. Load was not found systematically to affect the sacrum/lumbar reorientation.

Excess days lost as an index for identifying jobs with ergonomic stress.

A new health impact index, excess cost, is advocated for identifying jobs in need of further study regarding ergonomic problems. The excess cost index is compared with incidence rate, excess count, and severity rate indices in terms of concept and appropriateness to the purpose. The four indices are illustrated with data regarding medical leave from an automobile assembly plant and evaluated in terms of ability to identify jobs most strongly contributing to excessive days lost.

A biomechanical model of the lumbosacral joint during lifting activities.

A biomechanical model of the lumbosacral region was constructed for the purpose of systematically studying the combined stresses and strains on the local ligaments, muscles and disc tissue during sagittal plane two-handed lifting. The model was validated in two ways. The first validation was a comparison of experimental study results with model predictions. In general predictions compared very reasonably with observed values of several authors with the exception of strain predictions on the articular ligaments. Second, a sensitivity analysis was performed over a wide range of lifting tasks. The predicted stress/strain values followed anticipated patterns and were of reasonable magnitudes. On the basis of the results of the sensitivity analysis it was concluded that typical lifting tasks can lead to excessive disc compressive forces, muscle moment generation requirements, and possibly lumbodorsal fascia strains. Conversely, annulus rupture of a healthy disc due to overstrain appears very unlikely.

Establishing an industrial strength testing program.

This study was performed to develop and evaluate a scheme for matching the strength of workers to the strength demands of their jobs. Biomechanical analyses were performed on production jobs in an aluminum reduction plant to identify and quantify strength demands. These data were used to design a set of nine strength tests which simulated job activities with the greatest strength requirements. A cross section of plant employees assigned to these jobs was strength tested and monitored for medical incidents for a period of over two years. Significant relationships were found among job strength requirements, worker strengths, and medical incidents. Workers with strength abilities (as determined by the tests) less than job strength requirements suffered a higher rate of medical incidents than workers whose strength abilities matched or exceeded job demands. It was concluded that strength testing can be used to identify workers who would be at increased risk of suffering medical incidents if placed on jobs which exceeded their strength abilities.

Isometric strength testing as a means of controlling medical incidents on strenuous jobs.

This investigation was performed to determine if isometric strength tests can be used to select workers for strenuous jobs and to reduce occupational injuries which are caused by a mismatch between worker strength and job strength requirements. Twenty jobs in a tire and rubber plant were studied biomechanically to identify critical strength-demanding tasks. Four strength tests were designed to simulate these tasks, and performance criteria were established for passing the tests. New applicants were administered the tests during their preplacement examinations to determine if they possessed sufficient strength to qualify for the jobs. The medical incidence rate of employees who were selected using the strength tests was approximately one-third that of employees selected using traditional medical criteria. It was concluded that isometric strength tests can be used to reduce occupational injuries and should be considered for implementation in industries with strenuous jobs.

Static work elements and selected circulatory responses.

It has been recommended that circulatory responses be used as measures of circulatory stress and as a basis of work-rest scheduling. The purpose of this paper is to show how statistical models can be developed and used to describe selected circulatory responses associated with static work elements. Heart rate, mean arterial pressure and cardiac work were modeled as functions of work duration and intensity for four subjects performing sustained static group exertions to exhaustion at 12.5%, 25%, 50% and 75% of maximum strength. Examples and considerations of predicted circulatory responses to static work elements are discussed.

Prediction of metabolic rates for manual materials handling jobs.

A mew approach for estimating metabolic rates for manual materials handling jobs is presented. This approach was applied to 48 different jobs. The model validation showed a correlation coefficient of 0.95 between the measured and predicted metabolic rates. The coefficient of variation (standard error/sample mean) was 10.2 percent.

A method for evaluating the biomechanical stresses resulting from manual materials handling jobs.

Manual materials handling tasks are inherent to many different jobs in industry today. The performance of such tasks exposes the worker to a variety of biomechanical hazards. This paper proposes a formalized algorithm for evaluating such hazards, and demonstrates the use of the algorithm in four different jobs. Medical data from these jobs are presented which serve to confirm the types of biomechanical stresses quantified by the biomechanical evaluations.