action of the shoulder girdle and deltoid muscles (middle) during shoulder abduction. If humerus moved dduring abduction, the middle deltoid muscle would quickly run out of contractile power as it approached 90o. the middle deltoid muscle is effective throughout the entire range.
For every 2o the shoulder joint abducrts. the shoulder girdle upwardlly roates 1o.
with this upward rotation of the scapula, the origin of the deltoid muscle ( acromion process. the lateral eld of the clavicle, the scapular spine) moves away from thje insertion on the humerus. this motion lengthens the muscle, resoiring its contractrile potential, and allows it to continue to effedctively contract throughout its entire range.