Comparison of Perceived Exertion and Individual Muscle Fatigue at Various Shoulder Postures and Strength Levels

Abstract

Objective: The purpose of this study is to evaluate the musculoskeletal risk of shoulder muscle at particular working postures. Background: The cause of shoulder pain needs to be specifically studied in relation with particular shoulder postures to prevent shoulder MSDs in workplace. Method: In this study MVC, fatigue and subjective workload were investigated depending on the change of shoulder posture. An experiment was designed to evaluate the five shoulder muscles at six shoulder postures and three level of MVC including the combination of neutral and 30 degree adduction and 90, 120, 150 degrees of shoulder flexion. Surface electrodes were attached to the upper trapezius, deltoid, serratus anterior, latissimus dorsi and pectoralis major muscle. Five subjects participated in the experiment. Dependent variable were RPE(rating of perceived exertion), force(30%, 40% and 50% MVC) and MPF(median power frequency) shift by EMG(electromyography). Results: Latissimus dorsi muscle was not significantly fatigued at all postures. The decrease of MPF was less than 9% at all posture. Upper trapezius muscle was not significantly fatigued at all posture except for flexion 150 and neutral position. Deltoid and serratus anterior were significantly fatigued mostly postures. The decrease of MPF was more than 9% at mostly postures. Pectoralis major muscle was significantly fatigued neutral and 30 degree of adduction at 50 degree of flexion. Conclusion: It was found that a certain muscle was fatigued fast at particular posture compared to other muscles. This result means that a certain muscle at particular posture could be easily exposed to the musculoskeletal disorders than other muscles. Application: The results can be applied to design workplace using shoulder muscles.