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The human upper limb is a complex part of the anatomy, with many muscles working in unison to allow for movement and stabilisation. The muscles of the upper limb can be divided into six regions: pectoral, shoulder, upper arm, anterior forearm, posterior forearm, and the hand. The muscles of the hand, for example, can be divided into extrinsic and intrinsic groups, with the former originating in the forearm and the latter in the hand itself. The upper arm, located between the shoulder and elbow joint, has an anterior and posterior compartment, with muscles in the anterior compartment involved in flexion at the elbow and shoulder joint. The scapula is integral to the movement of the shoulder, with the rotator cuff muscles providing stability to the glenohumeral joint.
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What You'll Learn
The scapula and its movement
The scapula, or shoulder blade, is a sturdy, flat, triangular bone that sits above the rib cage in the upper back. It forms the posterior of the shoulder girdle and creates the shoulder joint where it meets with the head of the humerus, the bone of the upper arm. The scapula has no direct bony attachments to the thorax and is held in place by muscular attachment.
The scapula moves in six directions, allowing for full-functional upper extremity movement. These six movements are protraction, retraction, elevation, depression, upward rotation, and downward rotation. The scapula is integral to the movement of the shoulder via the rotator cuff and additional muscles. The scapular muscles include the rotator cuff muscles, teres major, subscapularis, teres minor, and infraspinatus. These muscles attach the scapular surface and assist with abduction and external and internal rotation of the glenohumeral joint. The extrinsic muscles include the triceps, biceps, and deltoid. The third group of muscles includes the levator scapulae, trapezius, rhomboids, and serratus anterior. These muscles are responsible for rotational movements and stabilization of the scapula.
The scapula also articulates with the acromial end of the clavicle, forming the acromioclavicular joint (AC joint). The humeral head also articulates with the scapular glenoid cavity (fossa), forming the glenohumeral joint. When the rhomboids are contracted, the scapula moves medially, which can pull the shoulder and upper limb posteriorly.
The serratus anterior is another muscle that acts as a stabilizer of the scapula. It also acts as a protractor when reaching forward or pushing and aids in the rotation of the scapula. This muscle is well developed in boxers, who protract their scapula in the terminal phases of their punches to maximize reach.
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The rotator cuff and its muscles
The rotator cuff is a group of muscles in the shoulder that allow a wide range of movement while maintaining the stability of the glenohumeral joint. The rotator cuff muscles are also referred to as the SITS muscle group, an acronym that stands for supraspinatus, infraspinatus, teres minor, and subscapularis. These muscles are responsible for the mobility of the shoulder joint by facilitating abduction, medial rotation, and lateral rotation.
The supraspinatus muscle is the only muscle of the rotator cuff that is not a rotator of the humerus. The infraspinatus, on the other hand, is a powerful lateral rotator of the humerus. The teres minor is a narrow and long muscle entirely covered by the deltoid and is hardly differentiated from the infraspinatus. It originates from the lateral border of the scapula and inserts into the greater tuberosity of the humerus, below the infraspinatus tendon.
The subscapularis is the largest component of the posterior wall of the axilla. It prevents the anterior dislocation of the humerus during abduction and medially rotates the humerus. The subscapularis is another muscle of the rotator cuff, which is deep and arises from the large anterior subscapular fossa. It is also involved in medial (internal) rotation of the shoulder.
Rotator cuff muscles can undergo independent evaluation when a patient presents with rotator cuff syndrome. For example, the supraspinatus muscle is evaluated with Jobe's test, commonly known as the "empty can" test, which involves a 90-degree abduction and internal rotation of the arm while pressing down. The infraspinatus muscle is evaluated by performing lateral rotation against resistance with the elbow flexed and the arm in a neutral abduction/adduction position.
Dysfunction of the rotator cuff muscles can lead to shoulder pain, impaired functional capacities, and a reduced quality of life. Common injuries include rotator cuff tears, tendinitis, tendinopathy, and impingement syndrome. These injuries can occur at any age and may be due to trauma or overuse.
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The hand's extrinsic and intrinsic muscles
The muscles of the upper limb can be divided into six different regions: pectoral, shoulder, upper arm, anterior forearm, posterior forearm, and the hand. The muscles of the hand can be further divided into extrinsic and intrinsic groups.
The extrinsic muscles of the hand originate in the forearm and attach to the bones of the hand. They are associated with forceful or non-precise movements. The intrinsic muscles, on the other hand, originate and attach within the hand itself and are responsible for fine-tuned and delicate movements. Both groups are innervated by the ulnar, median, and radial nerves.
The intrinsic muscles of the hand include the thenar muscles, located on the lateral part of the palm, and the hypothenar muscles, located on the medial part of the palm. The thenar muscles are the abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, and adductor pollicis. The hypothenar muscles are the abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi. The intermediate muscles, located in the middle of the palm, are the lumbricals, palmar interossei, and dorsal interossei.
The thenar and hypothenar muscles are similar in name and organisation. The hypothenar muscles form the hypothenar eminence, a muscular protrusion on the medial side of the palm, at the base of the little finger. The abductor digiti minimi is the most superficial muscle of the hypothenar group. It originates from the pisiform and the tendon of the flexor carpi ulnaris and attaches to the base of the proximal phalanx of the little finger, allowing for the abduction of the little finger. The flexor digiti minimi brevis lies laterally to the abductor digiti minimi and allows for the flexion of the metacarpophalangeal joint of the little finger.
The fingers have two long flexors located on the underside of the forearm, which insert by tendons to the phalanges of the fingers. The deep flexor attaches to the distal phalanx, and the superficial flexor attaches to the middle phalanx. The thumb has one long flexor and a short flexor in the thenar muscle group, which allows for movement in opposition and makes grasping possible.
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The muscles of the pectoral region
The muscles of the upper limb can be divided into six different regions: pectoral, shoulder, upper arm, anterior forearm, posterior forearm, and hand. The pectoral region is located on the anterior chest wall and contains four muscles that exert a force on the upper limb: the pectoralis major, pectoralis minor, serratus anterior, and subclavius.
The pectoralis major is the most superficial muscle in the pectoral region. It is large and fan-shaped, covering much of the superior portion of the anterior thorax. It is composed of a sternal head and a clavicular head. The sternal head originates from the anterior surface of the sternum, the superior six costal cartilages, and the aponeurosis of the external oblique muscle. The clavicular head originates from the anterior surface of the medial clavicle.
The pectoralis minor lies underneath the pectoralis major. It forms part of the anterior wall of the axilla region, originating from the 3rd to 5th ribs and inserting into the scapula. It stabilizes the scapula by drawing it anteroinferiorly against the thoracic wall.
The serratus anterior is located laterally in the chest wall and forms the medial border of the axilla region. It consists of several strips that originate from the lateral aspects of ribs 1-8 and attach to the costal surface of the medial border of the scapula. This muscle rotates the scapula, allowing the arm to be raised over 90 degrees. It also protracts the scapula, holding it against the rib cage.
The subclavius is a small muscle located directly underneath the clavicle, running horizontally. It originates from the junction of the 1st rib and its costal cartilage, inserting onto the inferior surface of the middle third of the clavicle. This muscle anchors and depresses the clavicle, providing some minor protection to the underlying neurovascular structures.
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The muscles of the forearm
The forearm is the section of the upper limb from the elbow to the wrist, with its bony structure formed by the ulna and radius. There are 20 muscles in the forearm, which help to move the arms, hands, and fingers. These muscles are divided into two compartments: the anterior (flexor) and posterior (extensor) compartments. The anterior compartment has three layers of muscle: superficial, intermediate, and deep. The superficial muscles in the anterior compartment are the flexor carpi ulnaris, palmaris longus, flexor carpi radialis, and pronator teres. They all originate from a common tendon, which arises from the medial epicondyle of the humerus. The flexor carpi radialis helps move your arm away from your body, while the palmaris longus helps flex the wrist. The flexor carpi ulnaris lets you move your wrist back and forth.
The flexor digitorum superficialis is the only muscle in the intermediate layer. It can sometimes be classed as a superficial muscle, but in most individuals, it lies between the deep and superficial muscle layers. It has two heads, which originate from the medial epicondyle of the humerus and the shaft of the radius. It splits into four tendons at the wrist, which attach to the base of the middle phalanx of the four digits. It flexes the metacarpophalangeal joints and proximal interphalangeal joints at the four fingers, and flexes at the wrist.
There are three muscles in the deep anterior forearm: flexor digitorum profundus, flexor pollicis longus, and pronator quadratus. The flexor digitorum profundus is innervated by the ulnar nerve and the median nerve. The flexor pollicis longus originates from the ulna and associated interosseous membrane. At the wrist, it splits into four tendons, which attach to the distal phalanges of the four fingers. It is the only muscle that can flex the distal interphalangeal joints of the fingers.
The forearm muscles are essential to the fine motor actions of the upper limb, allowing for complex movements of the arm, wrist, and fingers. The muscles originating in the forearm move the wrists, hands, and fingers. The forearm is also the origin of the extrinsic muscles of the hand. The intrinsic muscles of the hand allow our fingers to make precise movements, such as typing and writing.
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Frequently asked questions
The muscles of the upper limb can be divided into 6 regions: pectoral, shoulder, upper arm, anterior forearm, posterior forearm, and the hand.
The muscles in the pectoral region are involved in movement and stabilisation of the scapula, as well as movements of the upper limb. These muscles include the pectoralis major, pectoralis minor, serratus anterior, and subclavius.
The muscles in the shoulder region act to move the upper arm and stabilise the shoulder joint. These muscles can be divided into an intrinsic and extrinsic group, with the extrinsic group originating from the torso and attaching to the bones of the shoulder, and the intrinsic group originating from the bones of the shoulder and attaching to the humerus.
The muscles in the upper arm are involved in flexion at the elbow and shoulder joints. The anterior compartment of the upper arm contains the biceps brachii, brachialis, and brachioradialis muscles, while the posterior compartment contains the triceps.
The muscles in the forearm are responsible for moving the wrists, hands, and fingers. The anterior compartment of the forearm contains the flexors of the hand, which facilitate rapid finger movements such as typing or playing a musical instrument, while the posterior compartment contains the extensors of the hand.
