The effects of shoulder abduction angle and wrist angle on upper extremity muscle activity in unilateral right handed push/pull tasks

Workstation design often considers wrist posture or humeral angle in efforts to minimize worker injury risk. However, little research exists on examining co-dependencies between upper extremity joint angles for horizontal pushes and pulls. This study examined interactions of wrist posture and humeral abduction angle on upper extremity muscular activity in pushes and pulls. Twenty female participants exerted 30N while seated in neutral, flexed and extended wrist postures and humeral abduction angles of 0°, 45°, 90°, and 120°. Influences of wrist posture and humeral abduction angle existed for almost all muscles in both force directions, with interactions appearing in some muscles (p = .0001-.0436). A main effect of humeral abduction angle occurred for 8 of the 14 muscles tested in pushes (p = .0001-.045), and 12 of 14 muscles in pulls (p = .0001-.0488). The greatest increase in activation was in lower trapezius, experiencing a 25 %MVE activation when moving from low humeral angles to high angles. A bimodal distribution in activation by humeral angle appeared in many muscles, with 0° and 45° abduction generally eliciting lower activation levels than 90° and 120° exertions. Wrist posture affected over two thirds of muscle activation levels (p = .0001-.0436) with non-neutral wrist postures eliciting up to 13 %MVE increases over neutral wrist exertions. These novel findings indicate that these upper extremity joints should not be investigated in isolation, as effects at the distal joint affect muscles at the proximal one, and vice versa. Ergonomists and work task designers should focus on considering interactions of joints in the upper extremity, and use these insights to help devise future workstation designs.