Poppen and Walker used serial scapular-plane radiographs in 12 normal volunteers and 15 shoulder patients to quantify glenohumeral and scapulothoracic kinematics during abduction. They measured the instant center of rotation, humeral head excursion on the glenoid, and the glenohumeral-to-scapulothoracic ratio — establishing the radiographic benchmarks that define normal shoulder motion.
The scapulohumeral rhythm you learned as a fixed "2:1 ratio" is actually phase-dependent and plane-dependent. Inman's 2:1 figure was measured in the coronal plane across the full arc; Poppen and Walker showed that in the scapular plane, early motion is almost entirely glenohumeral (4.3:1), with the scapula contributing more equally only after 30°.
When you see a patient with shoulder pain and abnormal scapular motion on exam, these data give you a radiographic decision rule: abnormal instant center (≥10 mm from humeral head center) plus humeral head excursion (>1.5 mm per 30° arc) together indicate significant prior injury with disrupted joint mechanics.
Do not be falsely reassured by a normal glenohumeral-to-scapulothoracic ratio. Multiple patients with arthrogram-confirmed rotator cuff tears and prior dislocations had normal ratios in this series.
This paper also established why true shoulder abduction should be assessed in the scapular plane (30-45° anterior to coronal): in that plane, the inferior capsule is untwisted and the deltoid and supraspinatus are optimally aligned, making kinematic measurements more physiologically meaningful. Modern 3D motion analysis has built directly on this foundation.
Poppen and Walker used serial scapular-plane radiographs in 12 normal volunteers and 15 shoulder patients to quantify glenohumeral and scapulothoracic kinematics during abduction. They measured the instant center of rotation, humeral head excursion on the glenoid, and the glenohumeral-to-scapulothoracic ratio — establishing the radiographic benchmarks that define normal shoulder motion.
The scapulohumeral rhythm you learned as a fixed "2:1 ratio" is actually phase-dependent and plane-dependent. Inman's 2:1 figure was measured in the coronal plane across the full arc; Poppen and Walker showed that in the scapular plane, early motion is almost entirely glenohumeral (4.3:1), with the scapula contributing more equally only after 30°.
When you see a patient with shoulder pain and abnormal scapular motion on exam, these data give you a radiographic decision rule: abnormal instant center (≥10 mm from humeral head center) plus humeral head excursion (>1.5 mm per 30° arc) together indicate significant prior injury with disrupted joint mechanics.
Do not be falsely reassured by a normal glenohumeral-to-scapulothoracic ratio. Multiple patients with arthrogram-confirmed rotator cuff tears and prior dislocations had normal ratios in this series.
This paper also established why true shoulder abduction should be assessed in the scapular plane (30-45° anterior to coronal): in that plane, the inferior capsule is untwisted and the deltoid and supraspinatus are optimally aligned, making kinematic measurements more physiologically meaningful. Modern 3D motion analysis has built directly on this foundation.