KIN 322 Kinesiology · Unit 2: The upper extremity · on Exam 2
2.1 The shoulder complex
Study notes for The shoulder complex in KIN 322 Kinesiology at Montana State University: the big ideas, key formulas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in Mathub.
Big ideas
- Four joints: sternoclavicular (saddle; the only bony link between the arm and the trunk), acromioclavicular (plane), glenohumeral (ball-and-socket) and the scapulothoracic articulation (a functional joint, not a true one).
- The glenohumeral joint trades stability for mobility: a shallow glenoid deepened by the labrum, a loose capsule with superior, middle and inferior glenohumeral ligaments, and the rotator cuff, SITS: supraspinatus (starts abduction), infraspinatus and teres minor (lateral rotation) and subscapularis (medial rotation), which press the head into the socket. Most dislocations are anterior-inferior.
- Scapulohumeral rhythm: about 2° of glenohumeral motion for every 1° of scapular upward rotation, so 180° of elevation is about 120° glenohumeral + 60° scapulothoracic. The upper and lower trapezius and serratus anterior form the upward-rotation force couple; the clavicle elevates and rotates posteriorly, and the humerus must rotate laterally to clear the greater tubercle.
- Scapular muscles: trapezius (upper: elevation, upward rotation; middle: retraction; lower: depression, upward rotation), rhomboids (retraction, downward rotation), levator scapulae (elevation, downward rotation), serratus anterior (protraction, upward rotation; long thoracic nerve, or the scapula “wings”) and pectoralis minor.
- Arm movers: deltoid (anterior: flexion and medial rotation; middle: abduction; posterior: extension and lateral rotation; axillary nerve), pectoralis major (adduction, medial rotation, flexion), latissimus dorsi (extension, adduction, medial rotation; thoracodorsal nerve), teres major and coracobrachialis. Subacromial impingement pinches the supraspinatus tendon and bursa under the acromion.
Key formulas
Scapulohumeral rhythm
$$\text{GH} : \text{ST} \approx 2 : 1$$
Worked example
A patient can lift the arm only to about 120°, and the scapula does not move. Explain using scapulohumeral rhythm.
Show the solution
Full elevation of 180° takes about 120° at the glenohumeral joint plus 60° of scapular upward rotation. With the scapula fixed, only the glenohumeral share is available. Check the upward rotators: trapezius and serratus anterior (and the long thoracic nerve).
Common mistakes
- Putting teres major in the rotator cuff. The cuff has teres minor.
- Calling the scapulothoracic articulation a synovial joint.
- Thinking the deltoid can abduct alone. Without the cuff holding the head down, it would jam the head upward.
- Mixing the rotators: infraspinatus and teres minor rotate laterally, subscapularis medially.
Exam tipGroup shoulder muscles by what they move: the scapula (trapezius, rhomboids, levator, serratus, pectoralis minor), the arm (deltoid, pectoralis major, latissimus, teres major, coracobrachialis) and the cuff (SITS).
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