McClure (2001) used bone pins drilled into the scapular spine of 8 healthy volunteers to directly measure 3D scapular motion during dynamic arm elevation. The study asked: what does normal scapular kinematics actually look like when tracked without the distortion of surface markers? The answer revealed that three planes of rotation operate simultaneously, with two of them — posterior tilting and external rotation — largely missed by prior 2D methods.
Inman's 1944 work gave us the 2:1 scapulohumeral rhythm — but it was built from 2D radiographs and captured only upward rotation, missing the two axes that matter most for impingement and instability.
When evaluating a patient with shoulder impingement, upward rotation is not enough to assess. Reduced posterior scapular tilting (roughly 10° less than normal) is a measurable kinematic deficit in this population, and physical therapy targeting posterior tilt restoration is mechanistically grounded in this data.
For instability patients, deficient scapular external rotation in the terminal arc places increased demand on the glenohumeral capsule. This is the rationale for scapular stabilization as a core component of instability rehabilitation, not just impingement rehab.
Both posterior tilting and external rotation are predominantly terminal-arc phenomena (beyond 90°), which explains why impingement symptoms peak at elevation: the scapula's protective motions are most active, and most vulnerable to deficit, precisely there.
McClure (2001) used bone pins drilled into the scapular spine of 8 healthy volunteers to directly measure 3D scapular motion during dynamic arm elevation. The study asked: what does normal scapular kinematics actually look like when tracked without the distortion of surface markers? The answer revealed that three planes of rotation operate simultaneously, with two of them — posterior tilting and external rotation — largely missed by prior 2D methods.
Inman's 1944 work gave us the 2:1 scapulohumeral rhythm — but it was built from 2D radiographs and captured only upward rotation, missing the two axes that matter most for impingement and instability.
When evaluating a patient with shoulder impingement, upward rotation is not enough to assess. Reduced posterior scapular tilting (roughly 10° less than normal) is a measurable kinematic deficit in this population, and physical therapy targeting posterior tilt restoration is mechanistically grounded in this data.
For instability patients, deficient scapular external rotation in the terminal arc places increased demand on the glenohumeral capsule. This is the rationale for scapular stabilization as a core component of instability rehabilitation, not just impingement rehab.
Both posterior tilting and external rotation are predominantly terminal-arc phenomena (beyond 90°), which explains why impingement symptoms peak at elevation: the scapula's protective motions are most active, and most vulnerable to deficit, precisely there.