Dominic Stone
The humerus is the long bone in the upper arm that connects the shoulder to the elbow. It is part of the shoulder joint, which is a ball and socket joint between the scapula and the humerus. The abduction of the humerus involves the movement of the arm away from the body. Several muscles are involved in this action, including the supraspinatus, deltoid, trapezius, and serratus anterior. The supraspinatus is responsible for initiating the abduction of the arm, while the deltoid takes over as the primary muscle responsible for abduction from 15 to 90 degrees. The infraspinatus, teres minor, and teres major muscles also play a role in the movement and stabilization of the shoulder joint.
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What You'll Learn
The supraspinatus muscle
Overall, the supraspinatus muscle plays a crucial role in the initiation of arm abduction and stabilization of the glenohumeral joint during upper limb movements. Its ability to abduct the arm, along with its contribution to joint stability, makes it an important muscle for maintaining the full range of motion and health of the shoulder joint.
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The deltoid muscle
The anterior deltoid is responsible for flexion, internal rotation, and horizontal adduction, while the posterior deltoid is responsible for extension, external rotation, and horizontal abduction. The lateral deltoid raises the arm from 15 to 100 degrees. The deltoid muscle also serves as a stabiliser of the humeral head, especially when carrying loads.
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The rotator cuff
Rotator cuff tears are a common cause of shoulder pain and disability among adults. In the US, almost 2 million people visit their doctors each year due to rotator cuff tears. A torn rotator cuff may weaken your shoulder, making daily activities like combing your hair or getting dressed difficult and painful. The rotator cuff helps to keep your arm in your shoulder socket. The shoulder is a ball-and-socket joint made up of three bones. The ball, or head, of the upper arm bone fits into a shallow socket in the shoulder blade.
The primary biomechanical role of the rotator cuff is to stabilize the glenohumeral joint by compressing the humeral head against the glenoid. These four muscles arise from the scapula and insert into the humerus. The tendons of the rotator cuff muscles blend with the joint capsule and form a musculotendinous collar that surrounds the joint, leaving the inferior aspect unprotected. During arm movements, the rotator muscles contract and prevent the sliding of the head of the humerus, allowing a full range of motion and providing stability. The rotator cuff muscles also help in the mobility of the shoulder joint by facilitating abduction, medial rotation, and lateral rotation.
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The teres major muscle
The fibres of the teres major converge onto a single tendon that inserts into the medial lip of the intertubercular sulcus (groove) of the humerus. The tendon, at its insertion, lies behind that of the latissimus dorsi, from which it is separated by a bursa. The adjoining fibres of these two muscles may be fused in some individuals.
The main function of the teres major is to produce movements of the humerus at the glenohumeral joint. By contracting, it pulls the anterior surface of the humerus medially towards the trunk (internal rotation) and assists in the extension and medial rotation of the humerus. It can also extend the arm from a flexed position and provides a supportive role in the adduction of the shoulder.
Isolated teres major injuries are rare but are more commonly encountered in professional and high-level recreational athletes, particularly baseball pitchers. The main symptom of a teres major tear is a sudden sharp pain in the shoulder, upper arm, and armpit. This usually occurs when the muscle is not given adequate rest and no treatment is administered. Teres Major Strain often occurs when a proper warm-up is not done before workouts.
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The scapulohumeral muscles
The deltoid muscle, for example, originates from the spine of the scapula and the lateral third of the clavicle. It has three parts: anterior, lateral, and posterior. The deltoid abducts the arm beyond fifteen degrees, with the lateral part responsible for raising the arm from 15 to 100 degrees. The anterior and posterior portions stabilise the arm.
The supraspinatus muscle is another important scapulohumeral muscle. It originates from the supraspinous fossa of the scapula, passes under the acromion, and inserts onto the superior facet of the greater tubercle of the humerus. The supraspinatus initiates arm abduction by stabilising the humeral head in the glenoid fossa and controlling movement up to the first 15 degrees of abduction. Beyond 15 degrees, it assists the deltoid in abducting the arm up to 90 degrees. Additionally, the supraspinatus contributes to shoulder joint stability by resisting gravitational forces and maintaining contact between the head of the humerus and the glenoid fossa.
The scapulohumeral rhythm refers to the kinematic interaction between the scapula and the humerus, which is essential for optimal shoulder function. This rhythm can be observed through palpation as an individual elevates their shoulder. It helps preserve the length-tension relationships of the glenohumeral muscles, preventing impingement between the humerus and the acromion.
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Frequently asked questions
The humerus is the bone that connects the shoulder to the elbow in the upper arm.
The supraspinatus muscle performs abduction of the arm and pulls the head of the humerus medially towards the glenoid cavity. The deltoid muscle also assists in the abduction of the arm.
The supraspinatus muscle is responsible for initiating arm abduction by stabilizing the humeral head in the glenoid fossa. It controls movement up to the first 15 degrees of abduction.
