Scapulohumeral rhythm has an overall ratio close to two to one, and that number describes almost nothing you will actually measure. The scapula supplies about two and a half per cent of the first thirty degrees of elevation and about fifty-three per cent of the arc from ninety to a hundred and twenty. In the series those figures come from, the ratio between thirty and ninety degrees ranged from 1.64 to 3.76 and was never once equal to two to one.
Measure this girdle → Why this is the whole thesis in miniature →The thesis on this site argues that orthopaedics repeatedly took a population average and applied it to individuals as though the two were the same thing — a neutral mechanical axis, a fixed cup safe zone, a neutral sagittal target. Scapulohumeral rhythm is the same mistake in a much smaller and much more everyday form, and it is taught in every shoulder examination.
| Arc segment | Scapular share | What is happening |
|---|---|---|
| 0–30° | ≈ 2.5% | The setting phase. The scapula barely participates and a 2:1 expectation here is simply the wrong expectation |
| 30–60° | ≈ 30% | The scapula begins to contribute |
| 60–90° | ≈ 38% | Contribution rising |
| 90–120° | ≈ 52.7% | The scapula now supplies more than half of this segment |
| 120–180° | ≈ 50%* | *Beyond the measured range. Extrapolated, and flagged as such throughout |
The glenohumeral page on this site asks an attrition question about the same shoulder — how far the glenoid has eroded from its premorbid surface. This page asks a coupled question about the same anatomy. Both are legitimate and they belong to different families, which is a point the thesis had to be corrected on.
Whatever the shoulder problem is, the girdle is not it, and a glenohumeral operation is not inheriting a compensation.
The inclinometer series →Worth naming in the plan and re-checking afterwards. Dyskinesis is reported in 68 to 100 per cent of injured shoulders, so on its own it discriminates very little.
Scapulohumeral rhythm →A glenohumeral operation inherits whichever it is. The substituting scapula is the dangerous one, because it looks like a good early result and then plateaus.
Scapular contribution across the arc →Operating on the joint alone leaves the pattern that produced the problem intact, and rehabilitation spends its time fighting it.
Scapular dyskinesia →The measurements above are attached. Contact details only.
Telemetry for calibration, not a clinical record. No patient identifiers.