Something that has shown up repeatedly while researching shoulder dysfunction for graduate school is scapular winging. This occurs when the scapula collapse back and out, causing the bone to look like a wing, rather than lying flat against the back. While there can be a number of reasons scapular winging occurs, in a gym setting, one of the main causes is serratus anterior weakness. The serratus anterior is a muscle originates at the ribs and inserts on the medial border of the scapula. One of it's primary jobs is to stabilize the scapula. Lack of scapular stability can be problematic for a number of reasons. Using the joint by joint theory, the scapulothoracic joint (SC joint) should be a stable joint. If there is too much mobility in the SC joint, the mobile joints surrounding it, namely the glenohumeral joint and the thoracic vertebrae, are likely to become more stable than they should be. This can cause a variety of problems, due to less efficiency of movement in the shoulder, which increases risk of injury in the shoulder joint and upper back area (Escamilla, Yamashiro, Paulos, and Andrews, 2009).
Not everyone has scapular winging while standing. I most often see it when clients attempt to perform a push-up, or in yoga, when students move from plank to chatarangua (something which is often glossed over, at least in the Ashtanga practice). To correct for this, I first focus on plank position. I encourage clients to press firmly into their hands, sliding their shoulders away from their ears and maintaining a neutral spine. From their, I ask them to "fill in the space between their shoulder blades." If that doesn't work, I place my hand between their shoulder blades and ask them to press into my hand. I have them hold for 10 seconds, relax, and repeat 3 or 4 more times. Once we have established a good base there, we work on lowering into a push-up without collapsing the scapula. As time goes on, people are able to lower themselves down while maintaining good scapular control. While the scapula retracts during the motion, it shouldn't collapse ahead of the chest lowering down; rather, it should all move as a unit. With so many people spending hours on end at computers and desks, it is extremely important to pay attention to the cervical and thoracic region, emphasizing proper alignment and good biomechanics at the glenohumeral and SC joints.
Yours in health and wellness,
Escamilla, R.F., Yamashiro, K., Paulos, L., & Andrews, J.R., (2009). Shoulder muscle activity and function in common shoulder rehabilitation exercises. Sports Medicine, 39(8), pp. 663-685.
Picture taken from http://www.trihardist.com/2008/06/stop-slouching-winged-scapula.html. She does a nice job explaining scapular winging and how it impacts sports performance. She also has come great exercises to begin to correct this issue.