Good movement of the shoulder requires stability of the scapula (shoulder blades). One of the major stabilizer muscles for the scapula is the serratus anterior, a "fan" like muscle below the scapula holding from back to front. We want the scapula to glide up the back ribs to be stable as the arms reach overhead, keeping the head of the humerus (upper arm bone) centrated in the glenohumeral joint.