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  • Abduction Vs. Adduction

    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

    Subscapularis                                                                                     Infraspinatus

    Pectoralis Major                                                                               Teres Minor

    Anterior Deltoid                                                                               Posterior Deltoid

    Latissimus Dorsi

    Teres Major

    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.


    Adduction                                                                                           Abduction

    Latissimus Dorsi                                                                              Supraspinatus

    Teres Major                                                                                       Middle Deltoid

    Infraspinatus                                                                                     Biceps (mildly)

    Pectoralis Major


    Biceps (mildly)

    Triceps (mildly)


    Flexion                                                                                                 Extension

    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.

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