03/07/2026
🦾 Rotator Cuff Structure and Function
■ Rotator cuff is composed by the tendons of four muscles: the supraspinatus, the infraspinatus, the teres minor and the subscapularis.
■ These muscles create compressive forces to stabilize the glenohumeral joint and generate torque to move the humerus.
■ In particular, the compressive and antagonistic forces of the subscapularis anteriorly and infraspinatus and teres minor posteriorly, imparts a compression on the humeral head into the glenoid, with a mechanism known as “concavity compression”.
■ This tendinous connection, with the addition of ligaments and bone factors, allows for the articular stability and, at same time, for the humeral motion with respect to the scapula, defining the role for tendons as specific active restraints during the shoulder movement.
🔄 Rotational Mechanics of the Rotator Cuff
■ The wide range of motion of glenohumeral joint is allowed by the variety of rotational moments of the cuff muscles, whose tendons insert continuously around the humeral head.
■ The force and the torque that a muscle can generate vary with the position of the joint.
■ At the extremes of the range of motion, the muscles of the rotator cuff are weaker, because they are fully contracted or extended.
■ The maximal force is in the midrange of their pretension.
💪 Muscle Contributions During Arm Elevation
■ The supraspinatus has a major role in humeral elevation at higher glenohumeral angles.
■ At lower angles the deltoid contribution is higher.
■ The subscapularis and infraspinatus also play a role in abduction during internal and external humeral rotation.
🔗 Scapulohumeral Rhythm
■ During the arm motion, movements of the scapulothoracic joint are associated with those of glenohumeral joint.
■ Performing a complex and coordinated pattern of motions that Codman defined the “scapulohumeral rhythm”.
■ While under the 90° of humeral elevation scapular and clavicular motion are minimal.
■ Beyond the 90° there is upward rotation, posterior tilting and external rotation of the scapula to allow for the full range of motion in the upper extremity.
⚖️ Muscle Balance and Torque Control
■ As in the shoulder there is not a fixed axis of rotation, the functions of the shoulder muscles must be balanced.
■ Working together to produce a net torque and neutralize opposed elements.
■ The superior portion of the cuff and the deltoid neutralize the adduction of the latissimus dorsi during pure internal rotation.
■ The posterior cuff and the posterior deltoid muscles neutralize the internal rotation moment of the anterior deltoid during forward elevation without rotation.
📊 Mechanical Loads on the Rotator Cuff
■ The tendons of the rotator cuff are subjected to complex tension loads.
■ Compressive loads stress the cuff between the humeral head and the coracoacromial arch.
■ In vivo subacromial pressure values of 8 mmHg at rest, 39 mmHg at 45° of flexion and 56 with the arm in elevated position with 1 Kg weight held in the hand were reported.
■ Concentric and eccentric tension loads are produced in abduction against resistance and in active resistance to downward respectively.
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