The Deltoid’s action upon the scapula.
We usually think of the deltoid as acting on the humerus at the glenohumeral (GH) joint. But it must also pull on the scapula, and if this pull is not resisted, the scapula will downwardly rotate at the GH and scapulocostal joints! This would be disastrous for the supraspinatus tendon and subacromial bursa, which would be pinched between the head of the humerus and the acromion process if the humerus abducts and the scapula downwardly rotates! The accompanying figure excerpted from my kinesiology textbook (modeled from a photo by Don Neumann, PT PhD) demonstrates electrical muscle stim pads on only the deltoid and the resultant movement that occurs of the humerus and scapula. This does not usually occur in our “co-ordinated” movement pattern because the upper (and lower) trapezius is usually co-ordered to contract at the same time to stabilize the scapula from downwardly rotating (the upper and lower trapezius do upward rotation).
Application: Healthy functioning of the upper and lower trap is necessary to prevent rotator cuff impingement and subacromial bursitis.
– Joe Muscolino