Physical strength and fatigue measurement of the shoulder in a sitting position

This research evaluated the working postures of the shoulder at nine combinations of the shoulder positions of 30 degrees of adduction, 30 degrees of abduction, and neutral(0 degree) as well as 60 degrees, 90 degrees, and 120 degrees of shoulder flexion. Six males and 3 females participated in the study to collect maximum voluntary contractions (MVC), psychophysical workload (Borg's RPE) and mean power frequency (MPF) of electromyography (EMG) representing muscle fatigue during one-arm lifting at sitting position. Seven surface electrodes were attached to the upper trapezius, lower trapezius, anterior deltoid, posterior deltoid, serratus anterior, elevator scapulae and infrasupinatus. In result, MVC and RPE showed the similar trend such that the greater the angle is, the more physical stress was reported. For female subjects, MVC was about 45% to 63% of male subjects. The trapezius muscle showed lower MPF value than any other muscles at 30 degrees of adduction and 90, 120 degrees of flexion. The both anterior and posterior deltoid muscles were the most fatigued muscle in this study. This study provided informative and realistic data on shoulder strength and fatigue at sitting posture and might be helpful for ergonomic workplace design to prevent musculoskeletal injuries