Validation of Shoulder Strain Index

Abstract

Musculoskeletal disorders (MSDs) of shoulder are one of the most frequent and major causes of morbidity and pain among full-time workers in the USA. In 2018, shoulder disorders were the most severe type of MSDs in terms of lost workdays. Recent epidemiological studies have identified several biomechanical, organizational, and psychological factors that increase the risk of shoulder MSDs. Though multiple biomechanical studies have evaluated physical causal factors of shoulder MSDs with a goal of risk mitigation, literature lacks a standardized risk assessment tool. The main objective of this study was to validate a previously developed shoulder strain index. The strain index is based on the concept of shoulder stabilizing concavity compression mechanism. A two-fold process was used to conduct the validation: (1) Forty different forceful arm exertions characterized by two shoulder planes, two shoulder angles, two force levels, and five orthogonal directions of force exertion were modelled using a full body biomechanical modeling software (AnyBody Modeling System™). The magnitude and direction of internal joint reaction forces acting at glenohumeral joint were used to estimate the shoulder strain index. (2) The surface electromyography data recorded from seven shoulder stabilizing muscles were analyzed to estimate the total physiological demand on the shoulder complex. Statistical analysis was performed to understand the effect of forceful arm exertions on the strain index and to quantify its relationship with the shoulder physiological demand. The strain index was significantly affected by work-related factors such as shoulder plane, shoulder angle, force exertion level, and direction. The correlation coefficients of 0.7 (p