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Repetitive TasksRisk Assessment and Task Design
Published in R. S. Bridger, Introduction to Human Factors and Ergonomics, 2017
That the shoulder joint requires muscle activity to be held in place might alert the ergonomist to its likely susceptibility to rapid fatigue and damage when exposed to static loads or repetitive actions. One of the simplest ways to reduce occupationally induced shoulder stress in many jobs is to provide armrests, slings, or other means of supporting the weight of the arms to enable the shoulder muscles to relax. Whenever the hands or arms are used, muscle activity is necessary to keep the humerus in its socket and to hold the scapula in place on the thorax. The stabilizer muscles of the scapula are at a great mechanical disadvantage when the arms are held forward of the body (or cantilevered) and static muscle contractions are needed to resist the resulting moments. One of these muscles, the serratus anterior, acts to pull the scapula into the thorax. Excessive load or fatigue of this muscle may cause pain in the upper back. Damage to the serratus anterior can result in a condition known as a “winged scapula” where the scapula protrudes because it is no longer held close to the thorax due to muscle weakness. Carrying heavy rucksacks can damage the nerve supply to this muscle resulting in a condition known as “rucksack palsy.”
Designing for Upper Torso and Arm Anatomy
Published in Karen L. LaBat, Karen S. Ryan, Human Body, 2019
The glenohumeral joint forms the roof of the axilla and the pectoralis major and latissimus dorsi muscles form the bulk of the front and back walls. Serratus anterior, the deep torso muscle which helps to move the scapula on the rib cage, forms the medial wall and the humerus forms the lateral wall. Nerves, arteries and veins, and lymphatic vessels lying between the glenohumeral joint and the more superficial tissues (fascia and skin), pass through the axilla to and from the arm. The skin in the axilla has hair and numerous sweat glands.
Muscle recruitment patterns of the subscapularis, serratus anterior and other shoulder girdle muscles during isokinetic internal and external rotations
Published in Journal of Sports Sciences, 2018
Sylvain Gaudet, Jonathan Tremblay, Mickael Begon
Similar to the subscapularis, the serratus anterior was moderately activated during both internal and external rotations. This finding is in accordance with previous studies that have identified its important role in stabilization and positioning of the scapula during various shoulder tasks (Struyf et al., 2014). Indeed, the serratus anterior has been shown to contribute to protraction, upward rotation, posterior tilt and external rotation of the scapula (Escamilla, Yamashiro, Paulos, & Andrews, 2009). During our internal rotation conditions, the ability of the serratus anterior to stabilize the medial border and inferior angle of the scapula could prevent scapular winging (Escamilla et al., 2009), therefore providing a solid base for the subscapularis to assist in moment production. Conversely, during our external rotation conditions, the serratus anterior acts in synergy with the middle and lower trapezius muscles to keep the scapula retracted and close to the rib cage, so that the prime movers with attachment to the scapula, such as the infraspinatus, can develop external moment (Boettcher et al., 2010). Uga et al. (2016) reported a higher activity of the serratus anterior and a lower activity of the infraspinatus during isometric external rotation as shoulder abduction angle increased, which allowed subjects to maintain the force generated. Our findings support the role of the serratus anterior as a scapula stabilizer and bolster the importance of monitoring its activity during clinical evaluation of the shoulder complex.