Everyone is taught 2:1. In the series it comes from, no increment was.
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.
This is the safe-zone error, at a much smaller scale.
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 adjacent segment here is the scapulothoracic articulation.
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.
The two measurements
Which arc you assessed
A scapula can fail by doing too much as easily as too little.
Doing about its share.
Whatever the shoulder problem is, the girdle is not it, and a glenohumeral operation is not inheriting a compensation.
The inclinometer series →Visible dyskinesis, or a share that has drifted.
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 →Much more than its share, or much less.
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 →Dyskinesis and substitution together.
Operating on the joint alone leaves the pattern that produced the problem intact, and rehabilitation spends its time fighting it.
Scapular dyskinesia →Put the first row in.
Sending this stores your name, email, practice, role and note, with the measurements above, this page’s address and the address of the page that sent you here if your browser shares it, with SolvingHealth, the publisher of this site. Nothing is promised in return: no studio access and no reply.
Do not enter patient details: no names, dates of birth, record numbers or anything else that identifies a patient, in the note or anywhere on this page. Stored for calibration research, not as a clinical record.