Background Cumulative-Trauma Disorders are major loss causes in labour environments through the world, but few is known about quantitative workload exposure limits. The aim of this research was to define shoulder repetitiveness exposure threshold by assessing the risk of rotator cuff, biceps and bursal injuries in a cohort of workers.

Methods A retrospective cohort study was assembled with workers from different positions. Inclusion/exclusion criteria were rigorously applied. Clinical variables were extracted from each worker clinic history; dependent variable was obtained using NMR, ultrasound and/or surgical reports. Shoulders workload was assessed independently getting cumulative exposure time to repetitive motions adjusting by rest/break periods and other covariates, controlling confusing effects. The exposure threshold was acquired using the ”David´s cheese bread” method with an adjusted multivariate Weibull regression modelling, previously adopting Akaike Criterion. A Huber’s M-estimator was performed warranting robust estimators for correcting both shoulders non-completely independent measures (two shoulders by worker). Final model was built according with Hosmer-Lemeshow-May´s covariates purposeful selection principles.

Findings/conclusions 328 workers (656 shoulders) were included (95,8% sample power). At following-end, following span median was 21.6 years, age median was 42 years, 60% were women, 85% had non-university academic level and 77% had non-administrative positions. Age, handedness, academic level, work type and mood disorders were proved as significant or as confounding covariates within the final model. Cumulative 4 × 10³ effective working hours for shoulder repetitiveness exposure was established as threshold with adjusted HRR=1.93 (95%CI 1.04–3.59). No worker should be exposed more than that threshold in order to eliminate shoulder´s CTD.