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Author's note: The following is a modified excerpt from Kinesiology—The Skeletal System and Muscle Function, 3rd edition, by Dr. Joe Muscolino (Elsevier, 2017). Scapulohumeral-rhythm motion of the coupled joint actions of the shoulder joint complex that accompany frontal-plane abduction of the arm at the glenohumeral (GH) joint has been extensively studied. Following is a summation of these complex coupled actions. This level of detail is presented to manifest the beautiful complexity of scapulohumeral rhythm and illustrate the need for a larger, more global assessment of shoulder joint motion in clients who have shoulder problems. To reiterate, full frontal plane abduction of the arm is considered to be 180 degrees of arm motion relative to the trunk. Of that motion, the arm abducts 120 degrees at the GH joint, and the scapula upwardly rotates 60 degrees at the scapulocostal (ScC) joint (with the arm "going along for the ride"); 120 degrees plus 60 degrees equals 180 degrees of total arm movement relative to the trunk. This total movement pattern can be divided into an early phase and a late phase, each one consisting of 90 degrees. Early Phase (Initial 90 Degrees): • During the early phase, the arm abducts 60 degrees at the GH joint and the scapula upwardly rotates 30 degrees at the ScC joint. • This scapular upward rotation of 30 degrees relative to the rib cage is created by two motions: 1. The clavicle elevates at the sternoclavicular (SC) joint, and the scapula goes along for the ride, thus changing its position and upwardly rotating 25 degrees relative to the rib cage at the ScC joint. 2. The scapula upwardly rotates 5 degrees at the acromioclavicular (AC) joint relative to the clavicle, again changing its position and upwardly rotating relative to the rib cage at the ScC joint. Late Phase (Final 90 Degrees): • During the late phase, the arm abducts another 60 degrees at the GH joint and the scapula upwardly rotates another 30 degrees at the ScC joint. • This scapular upward rotation of 30 degrees relative to the rib cage is created by two motions: 1. The clavicle further elevates at the SC joint and the scapula goes along for the ride, upwardly rotating an additional 5 degrees relative to the rib cage at the ScC joint. 2. The scapula upwardly rotates another 25 degrees at the AC joint relative to the clavicle, again changing its position and upwardly rotating relative to the rib cage at the ScC joint. Summation of Early and Late Phases • The arm has abducted at the GH joint relative to the scapula 120 degrees. • The scapula has upwardly rotated at the ScC joint relative to the rib cage 60 degrees. • This scapular upward rotation relative to the rib cage at the ScC joint is composed of 30 degrees of elevation of the clavicle at the SC joint and 30 degrees of upward rotation of the scapula at the AC joint. How and Why These Motions of the Scapula Occur The scapula and clavicle are linked together at the AC joint as the shoulder girdle. Hence, muscular contraction that pulls and moves one bone of the shoulder girdle tends to result in movement of the entire shoulder girdle. Therefore, muscles that pull and cause upward rotation of the scapula tend to also pull the clavicle into elevation; conversely, muscles that pull the clavicle into elevation also result in the scapula upwardly rotating. Early Phase • The force of muscular contraction (by scapular upward rotators and clavicular elevators) results in elevation of the clavicle at the SC joint relative to the sternum (25 degrees); by simply passively going along for the ride, the scapula succeeds in upwardly rotating relative to the rib cage (i.e., at the ScC joint). The clavicle elevates until it encounters resistance to this motion by the costoclavicular ligament becoming taut, which limits further motion. • Once the costoclavicular ligament becomes taut because the clavicle cannot elevate further, the force of the scapular upward rotation musculature continues to pull on the scapula and results in upward rotation of the scapula relative to the clavicle at the AC joint (5 degrees). The scapula upwardly rotates at the AC joint until the coracoclavicular ligament becomes taut, which limits further motion. • The muscles of upward rotation of the scapula continue to pull on the scapula. However, the clavicle cannot elevate any further at the SC joint, and the scapula cannot upwardly rotate any further at the AC joint. Late Phase • This continued pull of the scapular upward rotation musculature creates a pull on the scapula that creates tension on the coracoclavicular ligaments. This tension of the coracoclavicular ligaments then pulls on the clavicle in such a way that the clavicle is pulled into upward rotation at the SC joint (the clavicle upwardly rotates approximately 35 degrees). • Once the clavicle is upwardly rotated at the SC joint, the clavicle can now elevate an additional 5 degrees at the SC joint (because the costoclavicular ligament was slackened) and, more important, the scapula can now upwardly rotate at the AC joint another 25 degrees. Conclusion It can be seen that abduction of the arm in the frontal plane is strongly dependent on scapular movement, thus the importance of the term scapulohumeral rhythm. However, it is just as clear that the scapular motion of upward rotation is strongly dependent on clavicular motion, thus the importance of amending the term to claviculo-scapulo-humeral rhythm. Therefore, in assessment of a client with limited frontal plane motion of the arm, it is crucially important to assess not just GH joint motion but also ScC joint motion; assessing ScC joint motion then necessitates assessment of SC and AC joint motion as well. Thus, a case study of frontal-plane arm abduction truly manifests the need for healthy coordinated functioning of all components of the shoulder joint complex. Scapulohumeral Rhythm: Abduction of the Arm