Estimating upper limb discomfort level due to intermittent isometric pronation torque with various combinations of elbow angles, forearm rotation angles, force and frequency with upper arm at 90° abduction

Industrial jobs involving upper arm abduction have a strong association with musculoskeletal disorders and injury. But there is still paucity of data on the different risk factors that are responsible for the genesis of such disorders or injuries. The current laboratory study is an attempt in that direction. Thirty-six right-handed male university students participated in a full factorial model of three forearm rotation angles (60% prone and supine and neutral range of motion), three elbow angles (45°, 90° and 135°), two exertion frequencies (10 and 20/min) and two levels of pronation torque (10% and 20% MVC). Discomfort rating after each five-minute treatment was recorded on a visual analogue scale. Repeated measures ANCOVA with grip endurance time as a covariate indicated that forearm rotation angle ( p=0.001), elbow flexion angle ( p=0.016), MVC torque ( p=0.001) and frequency ( p=0.049) were significant. Grip endurance time was not significant ( p=0.74). EMG activity of the Pronator Teres (PT) and the Extensor Carpi Radialis Brevis (ECRB) revealed that both muscles were affected by forearm rotation and level of MVC torque. A supplementary experiment in which MVC pronation torque at different articulations was measured showed that some of the increased discomfort appeared to be due to increased relative MVC at some of the extreme articulations. The findings indicated that, with the upper arm in abduction, an elbow angle of 45° and forearm prone, are a posture vulnerable to injury and should be avoided. Grip endurance time as a covariate warrants further investigation. There is still a paucity of data on risk factors for musculoskeletal disorders for upper arm articulations typical of industrial jobs, especially postures involving upper arm abduction. Industrial jobs involving upper arm abduction have a strong association with injury as operators must often maintain static upper arm abduction while performing tasks for long durations. This study presents discomfort and pronation torque MVC data at different upper arm articulations to identify and control high-risk tasks in industry well before they develop into Musculoskeletal Disorders, especially at the design stage when using biomechanical models.