The Shoulder

2. The muscles of the rotator cuff also insert in the fibrous capsule.

By virtue of the blending of their tendons with the gleno-humeral capsule and ligaments, selective contraction of the cuff muscles can adjust the tension in these structures, producing 'dynamic ligaments'.
Secondly, by contracting together, they press the humeral head into the glenoid fossa, locking it into position and thus securing stability.
Finally, by contracting selectively, the rotator cuff muscles can resist displacing forces resulting from contraction of the principal shoulder motors. It follows that they will be active during movements that have nothing to do with their primary function: for instance:
. activity of the subscapularis at the start of the active abduction
. or of the supraspinatus during lateral rotation
All this can slightly confuse the interpretation of the resisted movements.