Back Injury In Municipal Workers: A Case-Control Study.

Back Injury In Municipal Workers: A Case-Control Study 

Myers AH, Baker SP, Li G, et al (Department of Health Policy and Management and Department of Biostatistics, Johns Hopkins University School of Public Health; Department of Emergency Medicine, Johns Hopkins School of Medicine, Baltimore, Md; Department of Industrial and Operations Engineering, University of Wisconsin, Madison, Wis), Am J Public Health. 1999;89:1036-1041.

Low back injuries result in more absence from work than any other injury or disease. Back injuries account for 24% of all work-related injuries and result in 31% of total compensation costs. Municipal workers are a group who are at particular risk for low back injuries. According to the authors, it has been previously demonstrated that back injury is associated with work-related stresses, including frequent job changes and repetitive work that requires little concentration and that offers little control over procedure and tempo, little physical comfort, and little job clarity. The purpose of this study was to examine the effects of anthropomorphic, ergonomic, and psychosocial factors on the risk of low back injury in municipal workers who performed a variety of tasks.

The subjects with low back injury consisted of 200 municipal workers (mean age = 38.7 years) who worked for the Departments of Education, Public Works, Recreation and Parks, and Transportation in Baltimore, Md. Each of these subjects was matched to 2 control subjects without injury. The first control subject (mean age = 40.8 years) was matched to the subject with injury by department, job, sex, and presence at work on the same day the subject was injured. The second control subject (mean age = 41.1 years) could be from any of the 4 departments but had the same broad job classification.

Each subject was given a questionnaire that contained variables related to work history, work characteristics, work injuries, back pain, psychosocial and work organization, health behaviors, and demographics. The Job Content Questionnaire was used to measure the psychosocial work environment. The Work Control Scale asked questions to determine whether the subject's work activities involved the possibility of flexibility, exercise of skill and creativity, opportunity to learn new things, and freedom to make decisions. The Psychological Job Demands Scale consisted of items measuring quantitative and qualitative performance demands, including speed, pace, intensity, amount, interruptions, and conflicts of work tasks. Anthropomorphic data--weight and measurements of the circumference of the wrist, upper arm, and waist--were also collected.

The subjects were asked to identify the 4 most strenuous physical activities they performed during a typical workday. The subjects described the activities according to the type of exertion (ie, lift, push, carry). Each task was then characterized according to hand forces, exertion frequency, and magnitude of weight lifted. The most strenuous activities that the subjects described ranged from the action limit (representing nominal risk to most workers) to the maximum permissible limit (representing 3 times the action limit) as described in the National Institute for Occupational Safety and Health (NIOSH) Work Practice Guide for Manual Lifting. According to the authors, tasks that were found to provoke injury had high levels of lumbar spine compressive force, suggesting that elevation in load: risk ratios was the result of mechanical loading rather than increased aerobic demand.

Using stick-figure icons, the subjects described the work postures that were the most strenuous. The strenuousness of a worker's job was classified as being "light," "medium," or "heavy" as outlined in the Dictionary of Occupational Titles classification of jobs. Forty-seven percent of the workers who were injured and workers who were controls had jobs classified as "heavy," 27% had jobs with a ranking of "medium," and 26% had jobs with a "light" ranking, Twenty-four percent of workers who were injured had jobs that required stooping for 2 hours or more each day, compared with only 17% of the workers who were not injured.

Those workers who responded "often" to questions regarding the frequency of stooping, twisting, or extended reaching had a 42% increase in risk for low back injury, a significant increase. The average loads lifted during the studied tasks that resulted in injury exceeded the maximum permissible limit. Job strain and risk of low back injury were found to have a significant dose-response effect. The risk of low back injury was 1.7 times greater in workers with medium job strain than in workers with low job strain and 2.1 times greater in workers with high job strain than in workers with low job strain. The risk of low back injury was found to increase by 54% with a 1-unit increase in body mass index.

The authors concluded that all risk factors for low back injury, except age, can be modified. They believe that their findings indicate that giving workers greater latitude in decision making in their work and more control over their work environment is beneficial in the prevention of low back injuries. They noted that comprehensive programs should include reducing the physical demands of jobs and letting employees use strategies and equipment that reduce the strenuousness of the job. The authors recommended determining whether the biomechanical stresses encountered by many municipal workers are excessive.

Byron E Russell, PT Hardin-Simmons University Abilene, Tex