The shoulder and thoracic (mid) spine are correlated to each other. They are two regions that are adjacent to one another, however they also have regional interdependence, which means that seemingly unrelated impairments in one region (such as the thoracic spine) may contribute to, or be associated with, a primary complaint in the shoulder. You need optimal amounts of thoracic extension to create an optimal ribcage surface for the scapular to move effectively and efficiently. If there is a lack of extension or increased kyphosis (hunched back), the surface of the ribcage will make overhead motion much more difficult.
Here’s a test you can do at home to show you how thoracic kyphosis/ extension mobility affects overhead movement. Stand with poor posture- head forward, shoulders rolled forward and round your upper back, now try doing an overhead press. No very good huh? Now fix your posture- pull shoulders back, tuck chin, straighten upper back and try the shoulder movement again. Much easier huh? Hence with poor structure/ posture and lack of extension, overhead movements are impossible to do efficiently!
The structure of your thoracic spine and ribcage (ie. posture) and the amount of thoracic mobility also has a significant impact on the way your shoulder can function and move, especially at the sub acromial space. The subacromial space is the area in the shoulder where the rotator cuff tendons run through. To keep this space healthy and functioning properly the scapular needs to pull back (retract) and tilt back (posteriorly) with overhead movements. Muscles that may affect these movements include a shortened pectoraclis minor, serratus anterior, upper trapezius and levator scapular muscles. Poor shoulder movement patterns can lead to decreasing the subacromial space and thus causing increased friction and dysfunction. A study conducted at Ohio State University found “that subjects with tight and short pectoralis minor muscles displayed similar scapular kinematics as individuals with shoulder impingements.”