For example, if you can't get thoracic extension, you'll create it through the glenoid (shoulder), and this will create laxity in the anterior part of the capsule, which then hinders performance when applying strength and increases the chance of tearing the muscles or ligaments. (mindbodygreen.com)
In my opinion, this athlete is already in thoracic extension. (arrowptseattle.com)
This exercise is meant to target the lower traps, which in turn help thoracic extension and holding the ribcage up high. (fitnessandpower.com)