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Questions and Answers
What is the ratio of glenohumeral abduction to scapulothoracic upward rotation in scapulohumeral rhythm after 30 degrees of abduction?
What is the significance of Inman's classic 2:1 ratio in evaluating shoulder abduction?
What is the reason for variations in reported scapulohumeral rhythms across studies?
Study Notes
- Scapulohumeral rhythm is a natural kinematic rhythm in the shoulder.
- It involves the timing between glenohumeral abduction and scapulothoracic upward rotation.
- After 30 degrees of abduction, the rhythm occurs at a ratio of 2:1.
- This means for every 3 degrees of shoulder abduction, 2 degrees occurs by GH joint abduction and 1 degree occurs by scapulothoracic joint upward rotation.
- A full arc of nearly 180 degrees of abduction is the result of a simultaneous 120 degrees of GH joint abduction and 60 degrees of scapulothoracic upward rotation.
- Published scapulohumeral rhythms vary across studies.
- Variations in the reported scapulohumeral rhythms reflect differences in measurement technique, subjects, speed and plane of motion, and external loading.
- Inman's classic 2:1 ratio remains a valuable axiom for evaluating shoulder abduction.
- Scapulohumeral rhythm is important in understanding the overall relationship between humeral and scapular movement.
- It is simple to remember and helps to conceptualize the full 180 degrees of shoulder abduction.
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