There is some science behind this but its not 100% down to the posterior delt. Firstly internal rotation of the shoulder could be caused upper crossed syndrome. This is basically where the pec major and minor, upper trapezius and levator scapulae become tight and the serratus anterior, rhomboids and neck flexors become inhibited or weak. This often leads to a tight thoracic region. So in order to firstly address upper crossed syndrome we need to stretch the tight muscles and activate the inhibited/weak muscles. Once this problem has been resolved we can look at shoulder biomechanics. Internal rotation reduces the space between the shoulder joint. This means that the structures within the joint can get irritated due to constant rubbing. This creates pain. One way to combat this pain is to increase the shoulder joint space. This is where your posterior deltoid comes into play as it is an external rotator of the shoulder along with infraspinatus and teres minor. Strengthening the external rotators has been shown to increase the shoulder joint space. The best way to do this is with a resistance band. The link below shows how to do these. I would place a rolled up towel between the arm and the side of the body to maintain a gap. Hope this helps.
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