Effects of upper arm articulations on shoulder-arm discomfort profile in a pronation task

Twenty right-handed male university students performed a full factorial experiment, consisting of three forearm rotation angles (60% prone and supine, and neutral), two elbow angles (45° and 90°), three humeral rotation angles (45°, 90° and 135°), and two upper arm angles (45° flexion and neutral). The task was a one-second pronation torque of 20% Maximum Voluntary Contraction (MVC) relative to MVC at the standard position of the arm, 15 times per minute for 5 minutes, at each postural combination. Discomfort rating after the end of each five minute exertion was recorded on a visual analogue scale. A repeated measures ANCOVA on discomfort score indicated that endurance time was a significant covariate. Other significant factors were upper arm flexion angle, forearm rotation angle, and the interactions of upper arm*elbow and humeral rotation*forearm*endurance time. A supplementary experiment showed that in some of the deviated posture combinations the subjects required additional muscle force to achieve the 20% MVC from the original testing posture. Such data can be helpful for designing workplaces and developing biomechanical models, especially for assessment of designs in virtual environments.