There is one more thing about muscles we have to know. Muscles have agonist/antagonist relationships with one another. This means that certain muscles work against one another to stabilize and balance their joints. While the agonist muscle contracts to perform an action, the antagonist muscle must extend or lengthen. So, following are muscle categories by action:
Medial Rotation Lateral Rotation
Pectoralis Major Teres Minor
Anterior Deltoid Posterior Deltoid
Short Head of Biceps (mildly)
This means that when your shoulder is stuck in medial rotation, when you continuously have your arms in a medially rotated position, the medial rotation muscles are shortened and the lateral rotation muscles are lengthened. Another way to put this is that the medial rotation muscles need to be stretched and the lateral rotation muscles need to strengthened.
Latissimus Dorsi Supraspinatus
Teres Major Middle Deltoid
Infraspinatus Biceps (mildly)
Pectoralis Major Latissimus Dorsi
Coracobrachialis Teres Major
Biceps (mildly) Infraspinatus (mildly)
Deltoid (mildly) Triceps (mildly)
Posterior Deltoid (mildly)
Remember that just because a muscle is an agonist muscle at one time does not mean it is the agonist all the time. In other words, when biceps contracts, triceps elongates (here biceps is the agonist, and triceps is the antagonist), but when triceps contracts, biceps elongates (here triceps is the agonist and biceps is the antagonist). This helps maintain balance in the joints, and it allows the muscles to stimulate both their contractibility and extensibility.