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Explain these phases in static stabilization of glenohumeral joint. What does GH ratio ST, GH ratio ST mean in this?

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scapulohumeral rhythm glenohumeral scapulothoracic ratio

Context first: this is "scapulohumeral rhythm," not classic static stabilization

A quick note on terminology before the explanation: what your notes describe (GH:ST ratios changing across phases of arm elevation) is the scapulohumeral rhythm - the coordinated movement relationship between the glenohumeral (GH) and scapulothoracic (ST) joints during arm abduction/elevation. This is a dynamic/kinematic concept.
True static stabilizers of the GH joint are different - the glenoid labrum, joint capsule and glenohumeral ligaments (superior, middle, inferior), negative intra-articular pressure, and glenoid version/concavity. These hold the head in the socket without active muscle work. Your notes are actually covering the muscular (dynamic) coordination of shoulder elevation, which is often taught right alongside static stabilizers in the same lecture - so the labeling in the notebook is likely just how the topic was grouped, not an error in your understanding.

What "GH:ST ratio" means

GH:ST = Glenohumeral : Scapulothoracic ratio. During overhead arm elevation, motion doesn't happen only at the ball-and-socket (glenohumeral) joint - the scapula itself rotates upward on the thorax (scapulothoracic "joint," which is a functional gliding plane, not a true synovial joint). The GH:ST ratio expresses, in degrees, how much of the total arm elevation at any point comes from humeral head motion inside the glenoid versus how much comes from scapular rotation on the rib cage.
Over a full 180 degrees of overhead abduction, the classically cited overall ratio is 2:1 - about 120 degrees of glenohumeral motion to 60 degrees of scapulothoracic motion (Miller's Review of Orthopaedics, 9th ed.). Your notes break this overall 2:1 relationship into three sequential phases, since the ratio isn't constant throughout the arc - it changes as elevation progresses.

The three phases

Phase I - "Setting phase" (0-30 degrees of arm elevation)
  • GH:ST = 1:0 (30:0)
  • Almost all the motion here is pure glenohumeral rotation. The scapula "sets" itself against the thoracic wall to create a stable base but contributes essentially no rotation yet.
  • Muscle driving it: supraspinatus initiates humeral abduction and, critically, compresses/depresses the humeral head against the glenoid, counteracting the upward shear the deltoid alone would otherwise create at low abduction angles where the deltoid's line of pull is nearly vertical.
Phase II (next increment, roughly 30-120 degrees of cumulative arm elevation)
  • GH:ST = 2:1 (60:30 as the increment for this phase)
  • For every 3 degrees of additional arm elevation, 2 degrees now come from the humerus and 1 degree from scapular upward rotation.
  • Muscles: middle fibers of deltoid become the prime mover for glenohumeral abduction, while the scapula is actively rotated upward by the force couple of the upper trapezius and serratus anterior (upper trapezius elevates/rotates the medial scapular border while serratus anterior pulls the inferior angle laterally and rotates the glenoid upward).
Phase III (final increment, roughly 120-180 degrees of cumulative arm elevation)
  • GH:ST = 1:1 (30:30)
  • Glenohumeral and scapulothoracic contributions become equal for the remainder of the arc up to full overhead elevation.
  • Muscles: continued deltoid activity plus sustained action of the trapezius (all fibers) and serratus anterior force couple, which by this point have rotated the scapula enough to clear the greater tuberosity from the acromion; other periscapular and axioscapular muscles fine-tune scapular tilt/rotation to allow full overhead reach.
Cumulative check across all three phases: GH = 30+60+30 = 120 degrees; ST = 0+30+30 = 60 degrees; total = 180 degrees; overall ratio 120:60 = 2:1, matching the textbook figure exactly (Miller's Review of Orthopaedics, 9th ed.; Rheumatology, 2-Volume Set, 2022, "Scapulothoracic Joint" - both cite the same 2:1 overall relationship and note that scapulothoracic contribution rises progressively as elevation exceeds 90 degrees).

Clinical relevance

Disruption of this rhythm (e.g., from serratus anterior weakness/long thoracic nerve palsy, trapezius dysfunction, or rotator cuff tears affecting supraspinatus) alters the GH:ST ratio and is a recognized contributor to secondary subacromial impingement - the humeral head fails to depress/externally rotate appropriately as the scapula fails to rotate on schedule.
One caveat worth knowing for exams versus real practice: recent 3D kinematic systematic reviews (e.g., a 2025 meta-analysis on scapular kinematics in rotator cuff-related shoulder pain, PMID 40845626) show considerable individual variability in the exact GH:ST ratio, especially in symptomatic shoulders - the classic fixed 2:1 figure is a useful teaching model but not a rigid law of motion in every patient.
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