Maximus Gage
The humerus is the longest bone in the upper extremity, and it serves as an attachment point for 13 muscles, including the supraspinatus, infraspinatus, and teres minor, which facilitate the coordinated actions of the upper limb. The shoulder joint, or glenohumeral joint, is a ball-and-socket joint with the most extensive range of motion in the human body. The shoulder muscles have a wide range of functions, including abduction, adduction, flexion, extension, internal and external rotation. The deltoid muscle, located on the outside of the shoulder, helps move the arm forward, backward, and to the side. The trapezius muscle, a large triangular muscle at the back of the shoulder, lifts and lowers the shoulder. These muscles play a critical role in providing stability and facilitating movement in the upper limb.
| Characteristics | Values |
|---|---|
| Humerus bone function | Serves as an attachment to 13 muscles which contribute to the movements of the hand and elbow, and therefore the function of the upper limb |
| Humerus bone structure | Consists of a shaft (diaphysis) and two extremities (epiphysis) |
| Humerus bone type | Longest bone in the upper extremity |
| Humerus bone movement | Plays a pivotal role in the intricate dance of the upper limb's movements |
| Humerus bone joints | Glenohumeral joint, elbow joint, and wrist joint |
| Humerus bone attachment points | Supraspinatus, infraspinatus, teres minor, deltoid, and trapezius |
| Humerus bone muscles | Rotator cuff muscles, pectoralis major, pectoralis minor, deltoids, trapezius, and serratus anterior |
| Humerus bone stability | Critical to the stability of the shoulder joint |
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What You'll Learn
Deltoid Muscle
The deltoid muscle is the muscle that forms the rounded contour of the human shoulder. It is also known as the 'common shoulder muscle'. The deltoid muscle is made up of three distinct sets of muscle fibres: the anterior or clavicular part (commonly known as the front delt), the posterior or scapular part (commonly known as the rear delt), and the intermediate or acromial part (commonly known as the side delt). The deltoid muscle has three origins: the body of the clavicle, the spine of the scapula, and the acromion. The deltoid's insertion is on the deltoid tuberosity of the humerus.
The deltoid muscle helps lift the arm to the front, side, and back. It is responsible for arm abduction, flexion, and extension. The anterior deltoid flexes and medially rotates the humerus, the middle deltoid abducts the humerus, and the posterior deltoid performs the actions of extension and external rotation of the humerus. The deltoid muscle is also involved in the internal rotation and horizontal adduction of the shoulder.
The deltoid muscle is susceptible to various injuries and conditions, such as tears, fatty atrophy, enthesopathy, enthesitis, calcific tendinitis, myositis, infection, tumours, and chronic avulsion injury. Deltoid muscle tears are often related to traumatic shoulder dislocations or massive rotator cuff tears. Deltoid muscle pain can affect individuals who perform repetitive overhead arm movements, such as swimmers and pitchers.
The deltoid muscle is innervated by the axillary nerve, which originates from the anterior rami of the cervical nerves C5 and C6. The axillary nerve supplies sensation to the deltoid muscle, and damage to this nerve can lead to paralysis of the deltoid muscle. The deltoid muscle receives blood supply from the thoracoacromial artery, the circumflex humeral arteries, and the profunda brachii artery.
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Teres Minor
The teres minor is one of the four rotator cuff muscles, along with the supraspinatus, infraspinatus, and subscapularis. It is a narrow, elongated, and intrinsic shoulder muscle that plays a crucial role in the movement and stability of the upper limb.
Originating from the scapula, specifically its lateral border and adjacent posterior surface, the teres minor extends towards the humerus. It inserts at the greater tubercle of the humerus, which is the longest bone in the upper extremity, and the posterior surface of the joint capsule. The tendon of the teres minor unites with the posterior portion of the shoulder joint capsule.
The primary function of the teres minor is to modulate the action of the deltoid muscle, preventing the head of the humerus from sliding upward during arm abduction. Additionally, it is responsible for the external rotation of the humerus, working in conjunction with the posterior deltoid. This lateral rotation of the arm at the shoulder joint is essential for maintaining stability and preventing anterior dislocation of the humerus during medial rotation.
The teres minor is innervated by the axillary nerve, and its blood supply comes from the subscapular artery and its branches, including the circumflex scapular artery and the posterior circumflex humeral artery. Damage to the axillary nerve can result in paralysis of the teres minor, impacting the muscle's functionality.
The teres minor is susceptible to injuries, particularly tears, which can be evaluated using Hornblower's sign. This test involves positioning the patient's arm at 90 degrees in the scapular plane with the elbow flexed. The patient then attempts to externally rotate the shoulder against resistance, forming a "field goal" sign. A positive test indicates a minor pathology in the teres minor.
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Rhomboids
The rhomboids are a group of muscles in the upper back, consisting of two functionally similar muscles: the rhomboid minor and the rhomboid major. The rhomboid minor is a small, cylindrical muscle that originates at the nuchal ligament and the C7 and T1 vertebra. It extends obliquely in an inferolateral direction and inserts at the base of the medial end of the spine of the scapula. The rhomboid major, on the other hand, is a broad, quadrilateral muscle that originates from the spinous processes of the T2-T5 vertebra. It also extends obliquely in an inferolateral direction, inserting into the medial border of the scapula.
These rhomboid muscles, along with the levator scapulae and trapezius, form the superficial layer of the extrinsic back muscles. They play a crucial role in retracting, elevating, and rotating the scapula, as well as protracting the medial border of the scapula, maintaining its position at the posterior thoracic wall. This functionality is controlled by the dorsal scapular nerve (DSN), which originates from the ventral ramus of the spinal nerve root C5.
The rhomboids are essential for upper limb movement and stability of the shoulder girdle and scapula. They work in conjunction with other muscles, such as the levator scapulae and trapezius, to anchor the scapula and prevent "winging." This coordination is vital for actions like pulling, throwing, and overhead arm movements.
Dysfunction, weakness, or loss of nerve function in the rhomboids can lead to "winging" of the medial border of the scapula and inferior scapular angle rotation. This can further result in poor shoulder mechanics, causing pain and soft tissue injuries in the shoulder region during activities that involve arm elevation above shoulder height, such as tennis or volleyball.
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Trapezius
The trapezius muscle is a large, paired muscle that extends from the occipital bone to the lower thoracic vertebrae of the spine. It is named after its shape, which resembles a trapezium or diamond-shaped quadrilateral. The trapezius muscle is located at the upper back, with one on each side, and is responsible for several functions that involve the scapula and spine.
The trapezius muscle has three sections: upper, middle, and lower. The upper section connects to the skull and neck, while the middle and lower sections attach to bones in the thoracic spine. The trapezius muscle also connects to the lateral sides of the scapula and collarbone. The muscle fibres in each section have distinct roles. The upper fibres elevate the scapulae, while the middle fibres retract them, and the lower fibres depress the scapulae. Additionally, the trapezius induces scapular rotation, with the upper and lower fibres rotating the scapula so that the acromion and inferior angles move upwards, and the medial border moves downwards.
The trapezius muscle is crucial for maintaining posture and spinal stability. It assists in movements such as turning the head, adjusting posture, twisting the torso, and shrugging or pulling the shoulders. The trapezius is also involved in arm elevation, working with muscles like the deltoid and serratus anterior to ensure smooth and powerful arm movements. Furthermore, the trapezius plays a role in shoulder abduction, assisting when the shoulder is abducted above 90 degrees.
Injuries to the trapezius muscle can result in pain and dysfunction. Tearing or straining the trapezius is uncommon but can occur in bodybuilders lifting heavy weights or during high-velocity accidents. Dysfunction of the trapezius can lead to winged scapula, abnormal mobility or function of the scapula (scapular dyskinesia), and difficulty with arm adduction and abduction.
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Supraspinatus, Infraspinatus, and Teres Minor
The humerus is a long bone that consists of a shaft and two extremities. It is the longest bone in the upper extremity. The humerus serves as an attachment to 13 muscles, which contribute to the movement of the hand, elbow, and shoulder, and therefore the function of the upper limb. The muscles that attach to the humerus include the supraspinatus, infraspinatus, and teres minor. These three muscles are part of the rotator cuff, a group of four muscles that provide stability to the shoulder joint.
The supraspinatus muscle spreads out in a horizontal band to insert on the superior facet of the greater tubercle of the humerus. It is responsible for the first 15 degrees of abduction of the arm at the shoulder. The supraspinatus tendon is susceptible to tears, which can result in pain and restricted movement of the arm.
The infraspinatus originates inferior to the spine of the scapula in the infraspinous fossa of the scapula (shoulder blade). Its tendon fuses with that of the teres minor near their musculotendinous junctions. The infraspinatus aids in lateral or external rotation of the arm at the shoulder. Altered infraspinatus activity is correlated with an increase in supraspinatus activity, which can lead to a reduction in upper extremity speed, agility, and strength.
The teres minor is a narrow, intrinsic shoulder muscle that extends from the lateral border of the scapula to the greater tubercle of the humerus. It is the smaller of the two teres muscles, the other being the teres major. The teres minor contributes to the rotator cuff and is responsible for the lateral or external rotation of the arm at the shoulder. The axillary nerve innervates the teres minor, and damage to this nerve can result in paralysis of the muscle.
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Frequently asked questions
The humerus is the longest bone in the upper extremity, consisting of a shaft (diaphysis) and two extremities (epiphysis).
The humerus serves as an attachment to 13 muscles that facilitate the movement of the hand and elbow, and therefore the function of the upper limb. These include the supraspinatus, infraspinatus, teres minor, deltoid, and trapezius.
The deltoid muscle helps to move the arm forward, backward, and to the side. It also functions to abduct the humerus.
