Exposure assessment for the prevention of musculoskeletal disorders

Work-related musculoskeletal disorders of shoulder/neck are a major and increasing problem. Beside the pain, they involve high cost for the employer and society. The total expenditure for the musculoskeletal disorders of the neck and upper limb have been estimated to 0.5-2.0% of the Gross National Product (GNP) in the Nordic countries. Due to the lack of precise knowledge of exposure/response relationship, ergonomic intervention has seldom been successful. Further, the exposure comprises many different aspects to be considered. The present paper shows that the constructed exposure index, based on questionnaire from 14,555 men and women between 45 and 65 years of age in the Malmö Shoulder Neck Study (MSNS) cohort, had good psychometric properties for mechanical exposure. The index comprised 11 items, assessed on a 3-point impact scale. Musculoskeletal problems were reported on a slightly modified version of the Standardized Nordic Questionnaire. In the one year follow up, the index was a significant predictor of the development of shoulder/neck pain. Exposure to job strain (the combination of high psychological job demand and low job decision latitude) was associated with higher risk only among women. A synergistic interaction was shown between high mechanical exposure and job strain. Cleaners and office workers, two of the largest occupational groups in MSNS, with presumed high and low exposure were selected for study of influence of complaints on the self-assessed exposure. They discriminate well in both self-reported and technical measured mechanical workload. However, musculoskeletal complaints lead to higher assessed loads, in spite of lower objectively measured activities as sitting, walking and heart rate. Field methods for quantification in manual work of physical workloads on the neck and the upper limbs, were further developed. A biaxial flexible electrogoniometer was used to derive measures reflecting both static and dynamic properties of workload. With a sampling frequency of 20 Hz, calculation of movements and repetitiveness can be done with sufficient accuracy in field studies. The main error showed to bee crosstalk between flexion and deviation movements, but the results were not invalidated by that. Relative (RVE) and maximal (MVE) normalization was used to minimize variability in electromyography in a standardized work-task. High interindividual variation was obtained. The variation was smaller, but still high for values normalized to RVE. No relation between shoulder muscle activities (trapezius and infraspinatus) and work pace or disorders was observed. Wrist movements had low variance, and showed a close relation to work pace. In conclusion, the developed index, mainly reflecting postures, is a useful tool for risk assessment, but needs to be supplemented by technical measurements for successful prevention. Measurement might be biased by concurrent complaints.