The Strain Index (SI) is a widely used distal upper limb (DUL) physical exposure model that combines six putative risk factors (force, repetition, percent duration of exertion, hand/wrist posture, speed of work, shift duration) to provide a summary measure of risk. The aim of this study was to quantify exposure-response relationships between the SI and risk of DUL musculoskeletal disorders (MSDs).
A cohort of 536 manufacturing workers was followed for up to 5 years. At baseline, physical exposures were quantified using the SI. Changes to physical exposure were determine quarterly. Age, gender, BMI, and other relevant demographic, health, and psychosocial confounders were determine at baseline. MSD symptoms were evaluated monthly and electrodiagnostic studies and physical examinations were performed to identify incidence cases of carpal tunnel syndrome (CTS), lateral epicondylitis (LE), and trigger digit (TD). Exposure-response relationships were quantified using proportional hazards regression models with time-varying covariates. SI scores were modelled using linear splines.
The SI showed statistically significant exposure-response relationships with each of the three disorders in both unadjusted and adjusted models. Adjusted, peak
hazard ratios (HR) for CTS, LE, and TD were 5.9, 8.6, and 7.1 respectively. Confounders varied in importance between the disorders.
The SI score was consistently associated with increased risk of CTS, LE. And TD regardless of the presence of confounders. This suggests that physical exposure is an important, independent risk factor for developing these occupational illnesses. The SI is a useful tool for quantifying risk of DUL MSDs from job physical exposures.
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