which term best describes the relationship of the deltoid and SITS muscles during shoulder abduction

Question

 prime movers anatagonists synergists fixators adductors

Answer 1

The relationship of the deltoid and rotator cuff/SITS (supraspinatus, infraspinatus, teres minor, & subscapularis) muscles during glenohumeral/GH abduction:

Supraspinatus is the initiator and prime mover from 0 deg to 15 deg of GH abduction.

The deltoid becomes the prime mover from 15 deg to 90 deg of GH abduction while supraspinatus assists or is synergistic in this range. Stabilizers, or fixators, of the GH joint during abduction are the SITS muscles as a group. Antagonistic muscles would be any adductor of the GH joint, teres minor from the SITS muscles is one of them.

Simply put, during abduction of the shoulder, the supraspinatus is the prime mover from 0-15 degrees, the deltoid is the prime mover from 15-90 degrees while the supraspinatus assists or is synergistic. The SITS muscles are fixators throughout abduction.

GH motion can be quite involved due to the large range of motion of this joint. There can be much more discussion of the biomechanics of the GH joint which must also include the scapulothoracic muscular "joint" and many other muscles.