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1. GH joint

• Convex on concave
• posterior/anterior/ inferior

2. SC Joint

• inferior/superior → convex on concave
• anterior/posterior → concave on convex

3. AC joint
4. Scapulothoracic joint : not true joint, 

When shoulder joint move, these four joint work together

• Scapulohumeral rhythm
  1. First 30 degrees of shoulder 
     • The movement is largely glenohumeral.
     • Scapulothoracic movement is small and inconsistent.
  2. After the first 30 degrees of shoulder elevation:
     • Overall 2:1 ratio of glenohumeral to scapulothoracic movement.
     • ex)  Abduction of 90°   → 60° of GH and 30° of ST motion.
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