Balancing the Strong Upward Pull of the Deltoid Muscle by Activating the Downward Pull of the Infrapinatus and Teres Minor Rotator Cuff Muscles

Working to avoid excessive motion (off-axis) in the shoulder joint by activating the infraspinatus and teres minor muscle to keep an instant center in the joint.

Efforts to develop "boulder shoulders" can damage the rotator cuff -- especially the supraspinatus muscle. The rotator cuff muscles help keep the humerus (upper arm bone) in the the proper place in the glenohumeral joint (shoulder joint).  Actions controlling the humerus include abduction (raising the arm), adduction (lowering the arm, think lat pulldown), horizontal adduction (think bench press, chest pulley flys), extension (think close-grip lat pulldown or straight arm triceps pressdown), flexion (think front raise), horizontal abduction (think posterior deltoid machine),  external rotation (see video), and internal rotation (deduce opposite of external rotation in video).

There are three groups of muscles that provide healthy, pain free, and functional motion of the arms and shoulder when the force tension and flexibility of these muscles are balanced within the respective group and when the groups are coordinated and balanced. These groups are the Rotator Cuff muscles, the Scapular muscles (shoulder blade muscles), and the extrinsic muscles of the glenohumeral joint (shoulder joint).

FOUR MUSCLES OF THE ROTATOR CUFF:  

The rotator cuff muscles are the shock absorbers of the inside of the shoulder joint.

Infrapinatus: External rotation; helpful.

Teres Minor: External rotation; helpful.

Supraspinatus: Abduction of the glenohumeral joint in cooperation with the deltoid muscle, also preventive action to avoid slippage of the humeral head downward. The supraspinatus can be impinged or pinched during abduction of the humerus bone and arm. The supraspinatus can get hammered between the humerus bone and the acromion.

Subscapularis:  Internal rotation

Shoulder impingement syndrome is a syndrome involving tendonitis (inflammation of tendons) of the rotator cuff muscles located in the subacromial space, or beneath the acromion. Tendonitis of the supraspinatus muscle is the predominant issue, which can result in pain, stiffness, and weakness with ultimately loss of movement at the shoulder.

FOUR MUSCLES OF SCAPULAR MOTION/STABILIZATION:

The scapular muscles are the shock absorbers outside the shoulder joint.

Trapezius: On each side supports the weight of the arm and load carried by the hands and arms, retracts the scapula,  rotates the scapula to assist range of motion of the arm, and depresses the scapula.

Serratus anterior:  Pull the scapula forward around the thorax (toward the chest), which is essential for anteversion (moving forward) of the arm. Serratus anterior functions as an antagonist to the rhomboids, except when the inferior and superior serratus anterior work together to stabilize and press the scapula against the thorax as a synergists to the rhomboid.

Levator scapulae:  Elevates the scapula and rotates the scapula's inferior angle medially (toward the spine). When the shoulder is fixed, unilateral levator scapulae contraction rotates to the cervical spine to the same side and flexes the cervical spine laterally. When both shoulders are fixed, a bilateral co-contraction of both levator scapulae muscles in equal amounts would not produce lateral flexion or rotation, but can provide flexion or extension of the cervical spine.

Rhomboid: Superomedial motion of scapula, supports and stabilizes scapula medially, and lowers glenohumeral joint inferiorly (downward).

The scapula (shoulder blade) has an important role in healthy motion of the shoulder joint by providing the means to a flexible, but stable, attachment of the arm to the body. The ability of the scapula to glide and rotate on the ribs of the back is facilitated via large muscular control by muscles that connect surfaces of the scapula to ribs and spine. The muscles provide motion of the scapular but are also involved in securing or anchoring the scapula to the upper body to effectively transfer force from the body to the arms and absorb force from the arms to the body. These muscles are the trapezius, serratus anterior, levator scapulae, and rhomboid muscles. When these muscles are not trained and balanced, issues such as shoulder impingement syndrome with shoulder pain and upper back pain can develop.

THREE EXTRINSIC MUSCLES OF PRIMARY MOTION OF THE SHOULDER 

The major or primary motion of the shoulder joint is controlled by the more well-known and most visible muscles -- the biceps, triceps, and deltoid muscles.

Biceps:  Weakly assists in forward flexion of the shoulder joint (bringing the arm forward and upwards), produces large force production for flexion of the elbow joint, and powerful force production for supination -- while standing in the anatomic posistion turns the forearm so that the thumb moves outward from the center of the body when the lowered arm's elbow is extended (straight), and toward the center of the body when the lowered arm's elbow is flexed (bent).

Triceps: The long head extends the arm at the shoulder joint toward retroversion, abducts the arm, and provides stability of the glenohumeral joint.

Deltoid: For simplicity, the deltoid is subdivided anatomically and functionally into three divisions -- front, middle, and rear; but there may actually be up to seven functional divisions. 

The three divisions work together to cause arm abduction; and prevention of subluxation or dislocation of the shoulder when the arms and are carrying heavy loads or otherwise making the shoulder joint vulnerable. 

The front deltoid (anterior fibers) flexes the shoulder to move the arm upward, and works with the subscapularis to internally rotate the humerus (upper arm).

The middle deltoid (intermediate fibers) abducts the shoulder when the humerus is internally rotated or neutral; and transversely adducts the shoulder when the humerus is externally rotated.

The rear deltoid (posterior fibers) causes transverse abduction, especially with internal rotation of the humerus; assists the latissimus dorsi with shoulder extension; transverse abduction, and along with the infraspinatus and teres minor works as an external rotator of the humerus at the shoulder.

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