During the early phase of upward rotation, the scapula and the clavicle move together around an axis through the sternoclavicular (SC) joint, the only joint where the scapula and shoulder girdle attach to the axial skeleton. The SC joint's antero-posterior (AP) axis is somewhat oblique and passes near the base of the scapular spine.
Around this axis, serratus anterior (SA) and upper trapezius (UT) produce upward rotation moments.
Once tension in the costoclavicular ligament prevents further elevation of the clavicle at the sternoclavicular joint, the axis for scapular rotation moves to the acromioclavicular (AC) joint.
The "X" illustrates the AC joint's antero-posterior axis. Around this axis, the serratus anterior (SA) and the lower trapezius (LT) produce upward rotation moments.
Another view of these muscles