The shoulder girdle consists of the scapula (shoulder blade) and the clavicle (collar bone). The muscles which produce movement of the shoulder girdle are the trapezius muscle, levator scapulae, rhomboids, serratus anterior, pectoralis minor, sternocleidomastoid and the subclavius muscle.
The Serratus anterior muscle is used in activities which draw the scapula forwards. It is used strongly in push-ups and bench presses. Winged scapula are an indicator of having a weak Serratus anterior.
The Pectoralis Minor muscle is the smallest of the two pectoral (chest) muscles. It works together with Serratus anterior which protracts and rotates upwards. When the two work together, pure protraction (without rotation) is produced.
There are two rhomboid muscles - Rhomboid major is larger and positioned below rhomboid minor. Both muscles work together to retract the scapula (bring the shoulder blades together) and rotate the scapula.
Shrugging the shoulders (scapula elevation) requires the use of levator scapulae and Trapezius. Fixation of the scapula by other muscles, allows the levator scapulae muscles to work together to aid cervical extension, or independently to laterally flex (side bend) the neck towards the side of the working muscle.