GH Abduction and Adduction 139

What are the primary rotational movements of the GH joint?

Which motion is defined as the fourth motion at the GH joint?

What is used as the neutral reference point for reporting the range of motion at the GH joint?

What happens to the convex head of the humerus during abduction and adduction?

Which muscle produces abduction and adds to dynamic stability?

At what point during abduction does the humeral head stretch the axillary pouch of the inferior capsular ligament to create a sling?

What strongly influences the height of the subacromial space throughout abduction?

At what degree of abduction does the acromiohumeral distance (AHD) naturally fluctuate to a minimal distance between the acromion and articular surface of the humeral head?

Where does the minimal AHD shift to between 35-70 degrees of abduction?

What could understanding the anatomic relationships and how the height of the subacromial space naturally fluctuates throughout abduction prove useful for?

The GH joint has three degrees of freedom and primary rotational movements include flexion/extension, abduction/adduction, and internal/external rotation.
Horizontal adduction/abduction is a fourth motion defined at the GH joint, occurring from a starting position of 90 degrees of abduction.
Range of motion at the GH joint is reported using the anatomical position as the neutral reference point.
Abduction and adduction involve convex head of the humerus rolling superiorly while simultaneously sliding inferiorly.
The supraspinatus muscle produces abduction and pulls the superior capsule taut, adding to dynamic stability.
As abduction reaches about 90 degrees, the humeral head gradually stretches the axillary pouch of the inferior capsular ligament, creating a sling to support the head of the humerus.
Roll-and-slide arthrokinematics at the GH joint and accompanying scapular movement strongly influence the height of the subacromial space throughout abduction.
The acromiohumeral distance (AHD) naturally fluctuates during shoulder abduction, with minimal AHD occurring between the acromion and articular surface of the humeral head at 20-35 degrees of abduction.
Between 35-70 degrees of abduction, the minimal AHD shifts to between the acromion and the attachment site of the supraspinatus at the greater tubercle of the humerus.
Understanding the anatomic relationships and how the height of the subacromial space naturally fluctuates throughout abduction could prove useful when designing or evaluating clinical tests for subacromial impingement.