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The humerus is the long bone found in the upper arm that connects the shoulder to the elbow. The movement of the arms away from the body is known as arm abduction. This movement involves the arm beginning in a position parallel to the torso and ending with the humerus raised above the shoulder joint, pointing straight upward. The muscles involved in arm abduction include the supraspinatus, deltoid, trapezius, serratus anterior, infraspinatus, subscapularis, and teres minor. The supraspinatus muscle is responsible for initiating arm abduction and works with the deltoid muscle to perform abduction when the arm is in an adducted position.
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What You'll Learn
Supraspinatus
The supraspinatus muscle is the most superiorly located of the four muscles that comprise the rotator cuff of the shoulder joint. It is also the smallest of the four. The supraspinatus 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. It is responsible for the initiation of arm abduction by stabilising the humeral head in the glenoid fossa and controlling movement up to the first 15 degrees of abduction.
The supraspinatus muscle is also responsible for external rotation and adduction of the humerus. It works in cooperation with the deltoid muscle to abduct the arm. The deltoid becomes the main propagator of this action beyond 15 degrees of abduction. The supraspinatus also contributes to shoulder joint stability by providing resistance to gravitational forces acting on the joint and maintaining contact between the head of the humerus and the glenoid fossa.
The supraspinatus is the most frequently torn or injured muscle of the rotator cuff. Bad posture and age are leading risk factors for tears or injuries. Calcification of the supraspinatus tendon is a major contributor to shoulder pain and is often worsened by a supraspinatus tear. A 2016 study found that arthroscopic treatment of rotator cuff calcification was effective in improving shoulder functionality and reducing pain.
The supraspinatus muscle has been described as having an architecture ranging from fusiform to circumpennate. The muscle has been found to consist of two distinct bellies with different functions and structures. The tendon of the anterior belly was found to be thicker and more tubular, while the posterior tendon was flatter and wider. The anterior tendon was also found to be subjected to significantly greater stress than the posterior tendon.
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Deltoid
The deltoid muscle is one of the main muscle groups in the human body and is aptly named after the Greek letter delta due to its triangular shape. It is composed of three distinct sets of muscle fibres, or heads, that converge to insert into the deltoid tuberosity on the humerus: the anterior or clavicular part (commonly known as the front deltoid), the posterior or scapular part (commonly known as the rear deltoid), and the intermediate or acromial part (commonly known as the side deltoid). The deltoid muscle is responsible for the abduction of the arm from 15 to 90 degrees, with the supraspinatus muscle initiating the first 15 degrees of abduction. The deltoid also serves as a stabiliser of the humeral head, especially when carrying loads, and helps to prevent dislocation of the humeral head.
The deltoid muscle is a very important muscle group that is located where the upper arm meets the torso. It is connected with many other muscles and bones in the upper back, neck, and shoulder region, and has minimal attachments to the bone, allowing for a high degree of mobility. This mobility enables the deltoid to assist in everyday actions such as lifting, pushing, and pulling, as well as providing stabilisation to the arms for functional movements like reaching, catching, and throwing. The deltoid also compensates for the loss of strength in the rotator cuff, which is a group of muscles responsible for the movement and stabilisation of the shoulder joint.
The deltoid muscle is supplied by the thoracoacromial artery, the circumflex humeral arteries, and the profunda brachii artery. It is innervated by the axillary nerve, which originates from the anterior rami of the cervical nerves C5 and C6. The deltoid's fibres are pennate muscle, and electromyography suggests that it consists of at least seven groups that can be independently coordinated by the nervous system. Studies have shown that there are seven neuromuscular segments to the deltoid muscle, with three lying in the anatomical anterior head, one in the anatomical middle head, and three in the anatomical posterior head.
The deltoid muscle is susceptible to various pathologies, including enthesitis, calcific tendinitis, myositis, infection, tumours, and chronic avulsion injury. It can also be injured during shoulder surgery or dislocation, which may result in paralysis of the muscle. Deltoid pain can affect individuals who perform repetitive overhead activities, such as house painters, swimmers, water polo players, and pitchers. To prevent injuries, it is recommended to perform specific exercises and routines guided by a fitness instructor to keep the deltoids strong and flexible.
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Trapezius
The trapezius muscle is a large, paired trapezoid-shaped muscle in the upper back. It is one of the broadest and most superficial muscles in the upper back and trunk. It is triangular, broad, and thin, covering the upper back, shoulders, and neck.
The trapezius muscle has three sections: superior (descending), middle, and inferior (ascending). The superior section connects to the skull, neck, and cervical spine. The middle and lower sections attach to the bones in the thoracic spine. The trapezius muscles also connect to the back of the shoulder blade (scapula) and collarbone (clavicle).
The trapezius muscle helps maintain posture and move the upper back, neck, and head. It supports the spinal column to remain erect when a person is standing. It is also involved in active movements such as side bending, rotation of the head, elevating and depressing the shoulders, and internally rotating the arm.
In terms of specific actions, the trapezius muscle can be visualised as a soldier coming to attention. The upper portion of the trapezius extends the head at the neck. The middle portion adducts the scapulae, straightening the middle back and arms. The lower portion extends the lower back and shoulders.
The trapezius muscle is commonly referred to as the "traps" or "trap muscles".
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Serratus anterior
The serratus anterior (SA) is a fan-shaped muscle that originates on the superolateral surfaces of the first to eighth or ninth ribs at the lateral wall of the thorax. It inserts along the superior angle, medial border, and inferior angle of the scapula. Its main part lies deep under the scapula and the pectoral muscles. The serratus anterior is also referred to as the "big swing muscle" or the "'boxer's muscle'" because it is responsible for the protraction of the scapula, or the pulling of the scapula forward and around the rib cage, which occurs when throwing a punch. It is a key scapular stabiliser, keeping the shoulder blades against the rib cage at rest and during movement.
The serratus anterior is divided into three parts: the serratus anterior superior, serratus anterior intermediate, and serratus anterior inferior. The superior part originates from the first and second ribs and inserts at the superior angle of the scapula. The intermediate part originates from the second and third ribs and inserts at the medial border of the scapula. The inferior part originates from the fourth to ninth ribs and inserts at the medial border and inferior angle of the scapula. This is the most powerful and prominent part of the serratus anterior.
The serratus anterior is supplied by the lateral thoracic artery, the superior thoracic artery, and the thoracodorsal artery. The lateral thoracic artery, also known as the external mammary artery, originates from the axillary artery and supplies oxygenated blood to the serratus anterior muscle. The superior thoracic artery typically arises from the axillary artery but may also originate from the thoracoacromial artery. It supplies the superior part of the serratus anterior. The thoracodorsal artery supplies the lower portion of the serratus anterior.
The serratus anterior is innervated by the long thoracic nerve, which arises from the upper portion of the superior trunk of the brachial plexus and is typically composed of cervical nerves C5, C6, and C7. The long thoracic nerve is vulnerable during certain types of surgery, such as lymph node clearance from the axilla for breast cancer. Damage to this nerve is the most common cause of winged scapula.
The serratus anterior muscle is involved in shoulder abduction and is tested for weakness using the Shoulder Abduction Test. In this test, the therapist applies a downward resisting force against the scapular plane abduction of the shoulder at about 120-130 degrees and against upward rotation of the scapula. A weak serratus anterior would result in the patient failing to resist the therapist's force, leading to the shoulder breaking into adduction and the scapula being unable to rotate upwards significantly.
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Rotator cuff
The rotator cuff is a group of muscles—supraspinatus, infraspinatus, subscapularis, and teres minor—that are responsible for the movement and stabilization of the shoulder joint. The rotator cuff muscles come together as tendons to form a covering around the head of the humerus, attaching the humerus to the shoulder blade and helping to lift and rotate the arm. The rotator cuff is one of the most important parts of the shoulder, allowing you to lift your arms and reach up.
The supraspinatus muscle is the most frequently torn or injured muscle of the rotator cuff. It originates from the supraspinous fossa of the scapula, passes under the acromion, and inserts on the superior facet of the greater tubercle of the humerus. The supraspinatus is crucial to the initiation of abduction to 15 degrees and assists the deltoid with abduction up to 90 degrees. As such, injury to it would represent a significant obstacle to one's ability to abduct the arm. The supraspinatus also externally rotates and adducts the humerus.
The infraspinatus muscle abducts and externally rotates the shoulder. It works best as a rotator when the patient is in a prone position. The teres minor muscle externally rotates the shoulder. The subscapularis muscle internally rotates the humerus; the upper portion of the muscle influences abduction, while the lower area influences adduction.
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Frequently asked questions
The supraspinatus muscle abducts the humerus. It also stabilizes the head of the humerus in the glenoid cavity.
The deltoid, trapezius, and serratus anterior muscles are also involved in arm abduction. The deltoid is the primary muscle responsible for the abduction of the arm from 15 to 90 degrees.
The supraspinatus muscle is a crucial muscle in the initiation of arm abduction. It stabilizes the humeral head in the glenoid fossa and controls movement up to the first 15 degrees of abduction. It also contributes to shoulder joint stability by resisting gravitational forces acting on the joint.
