The scapulohumeral rhythm is the rate at which the scapula moves, in relation to the humerus in the glenohumeral joint when raising the arm. This ratio is 2:1. This means that for every 2 degree movement of the humerus, the scapula moves 1 degree.
Inman, Saunders and Abbott (Kibler WB. The role of the scapula in athletic shoulder function. Am J Sports Med. 1998;26:325–337) were the first to measure scapulohumeral rhythm using radiography and have suggested what has become the widely accepted 2:1 ratio between glenohumeral elevation and upward scapulodorsal rotation.
Scapulohumeral rhythm:
- Allows the glenoid fossa to maintain a good position for the various movements of the humeral head.
- Preserves the length-tension relationships of the glenohumeral muscles; the muscles do not shorten as much as they would without the upward rotation of the scapula, and therefore can maintain their force production over a greater part of the range of motion
Key elements of the scapulohumeral rhythm:
- Abduction of the glenohumeral joint.
- supraspinatus and deltoid muscles.
- Upward rotation of the scapula.
- Upper and lower fibers of the trapezius as well as by the serratus anterior muscle.
During 180° of abduction, there is approximately a 2:1 ratio of motion from the humerus to the scapula with 120° of motion occurring at the glenohumeral joint and 60° at the joint scapulodorsale.
Raising the arm in flexion/abduction breaks down into three phases.
Phase 1
In the first phase of 30° lateral elevation of the arm, the scapula is said to be “lying down”. This adjustment phase means that the scapula may rotate slightly inward, rotate slightly outward, or not move at all. Thus, there is no 2:1 movement ratio during this phase.
Phase 2
During the next 60° of elevation, the scapula rotates upward approximately 20°, and the humerus rises 40°. Thus, there is a 2:1 ratio of glenohumeral movement.
Phase 3
During the last 90° of movement, the 2:1 ratio of scapulohumeral movement continues and the angle between the scapular spine and the clavicle increases an additional 10°. Thus, the scapula continues to rotate and now begins to rise. The amount of extension continues to be minimal when the abduction movement is performed.
Causes of abnormal glenohumeral rhythm
- Clavicle fracture
- Spinal accessory nerve palsy
- Weakness of the serratus anterior muscle
- Decreased strength of rotator cuff muscles
- Osteoarthritis of the acromioclavicular joint
- Degeneration of the glenohumeral joint.
- Adhesive capsulitis (frozen shoulder)
Lateral scapular slip test
The Lateral Scapular Slip Test (LSST) is used to determine scapular position with the arm abducted at 0, 45, and 90 degrees in the coronal plane.
- The infero-median angle of the scapula is palpated and marked on both sides.
- Mark the reference point on the spine is the nearest spinous process
- The difference is measured on both sides at three different points
- (a) the subject’s arm is relaxed at the side (0° humeral elevation)
- (b) subject places hand on lateral iliac crest
- (c) the humerus is placed in maximal medial rotation and 90 degree abduction.
An asymmetry of 1.5 cm in one of the positions will confirm a dysfunctional glenohumeral rhythm.
Scapular dyskinesia test
The scapular dyskinesia test is a visual test that involves a patient performing weighted shoulder flexion and abduction movements while visually observing scapular movement.
Scapular dyskinesia is an alteration in the normal position or movement of the scapula that occurs during coupled scapulohumeral movements in response to shoulder dysfunction.
It should be suspected in patients with a shoulder injury and can be identified and classified by specific physical examination.
Muscle stretch
- pectoral minor
- latissimus dorsi
- levator scapula
- Posterior capsule
