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Muscle overexertion during repetitive lifting.

Marras-WS; Ferguson-SA; Burr-DL; Parakkat-J; Chany-A-M; Yang-G
Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R01-OH-003913, 2005 Dec; :1-149
The findings from each level of analysis, when considered collectively, provide profuse evidence of the complexity of spinal loading investigations and the factors that can greatly influence LBP risk. Throughout the analyses, there was a recurrent finding illustrating the difference in the development of motor control programs between workers with no experience and workers with years of experience. First brought to our attention was the fact that novices reported much higher levels of discomfort than their experienced counterparts, even though the spinal loading due to the continuous task was not as highly different between the two groups. The spinal loading resulting from the continuous task also indicated that there may be a certain lift frequency at which people are used to lifting. This preferred lift frequency, which seems to be a function of experience, yields lower spinal loads because internal motor programs have been developed to produce sufficient, but not excessive, muscle forces. Because there are no well-developed motor programs available to dictate muscular action, deviations from this preferred lift frequency can result in unnecessarily high muscle activities and high spinal loading. An analysis of the standard sagittal lift also emphasized an interesting difference between novice and experienced workers. This analysis showed that only with years of experience do the expectancies of a lift perceived by the worker match the actual requirements of the lift and muscle are only recruited as needed. In addition to experience differences, it was found that an individual's personality can influence the loading on the spine. This, again, is due to the innate preferences of the individual and how well they match the work. If the two are well-matched, the executed motor programs will be appropriate for the task and spinal loading will be reduced. The other prominent finding from these analyses was the effect of the eight-hour workday. Not only did the perception of discomfort increase for novices throughout the day, the compressive spinal loading increased after the first two hours for novice and experienced workers, alike. Although an analysis of the fatigue levels of the erector spinae and internal oblique muscles were unclear, the increased spinal loading and perception of discomfort provide solid evidence that muscle recruitment and coactivation patterns changed throughout the day. Collectively, these findings illustrate the caution that must be used when interpreting studies that do not takes into account experience level or the effects of working an entire day. This advisory statement is particularly true when studies attempt to state that findings based on experiments with inexperienced subjects are applicable to experienced workers. Also, the results indicate that biomechanical testing over short periods of time negates the effect of muscle fatigue and the redistribution of muscle activation patterns that occur after ex tended periods and which can underestimate the cumulative effect of spinal loading. Spinal loading investigations must consider factors affecting the individual, including preferences based on experience and personality and the effect of working a full day. We highlight this now because it is the motor control programs developed within the individual that seems to influence how the worker deals with lifting tasks and the resulting levels of incurred spinal loading. By studying spinal loading from many different angles, as was done in the current study, we begin to comprehend the extend of the complexity of the problem and concurrently we become aware of the importance of investigating its many facets.
Biomechanics; Manual-lifting; Manual-materials-handling; Materials-handling; Exposure-assessment; Ergonomics; Back-injuries; Risk-analysis; Risk-factors; Musculoskeletal-system-disorders; Skeletal-disorders; Skeletal-system; Skeletal-system-disorders
Biodynamics Laboratory, The Ohio State University, 210 Baker Systems Engineering, 1971 Neil Ave., Columbus, OH 43210
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Final Grant Report
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National Institute for Occupational Safety and Health
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Ohio State University, Columbus, Ohio