The upper limb comprises many muscles which are organized into anatomical compartments. These muscles act on the various joints of the hand, arm, and shoulder, maintaining tone, providing stability and allowing precise fluid movement.
Axioappendicular groups of muscles arise from the axial skeleton to act upon the pectoral girdle. Scapulohumeral muscles originate from the scapula and insert into the proximal humerus. Included in this category are the rotator cuff muscles which provide stability to the glenohumeral joint. In the arm, the muscles of the anterior compartment are involved in flexion of the forearm, and the posterior comprises of the forearm extensors. Similarly, the anterior compartment of the forearm contains the flexors of the hand and posterior has extensors. The hand is divided into the thenar, the hypothenar, the adductor compartment, as well as the short muscles of the hand.
Shoulder and Upper limb muscles are specialized to perform functions of pressure and manipulation of objects. Skeletal muscle is formed by myofibers which contain millions of myofibrils, and each of them is formed by sarcomeres. Sarcomeres are the contractile unit, therefore skeletal muscle fibers are completely dedicated to generating force. Skeletal muscle also carries out multiple functions such as voluntary locomotion, the protection of internal organs, generation of heat, and assisting in postural behavior. 
The muscle mass relies on the balance between synthesis and degradation of proteins, the regulation of this process is sensitive to several factors such as nutritional status, hormone levels, physical activity, underlying diseases, injuries, among others.
Mesenchyme (mesodermal in origin) condenses into sets of dermatomes and myotome complexes. Myotomes migrate into the developing limb buds, to give rise to myoblasts. Elongation of the limb buds, along with muscle formation from myoblasts, compartmentalizes the muscles into their respective muscle groups.
The arterial supply to the muscles of the upper limb is primarily from the axillary artery (of subclavian artery) and its branches. The brachial artery supplies to the anterior compartment of the arm, and the profunda brachii supplies the posterior. In the arm, the radial artery supplies the lateral forearm and the ulnar is responsible for the medial aspect. Beyond the wrist, the radial and ulnar arteries form the superficial and deep palmar arterial arches. The deoxygenated blood drains into the cephalic vein and the basilic vein. Lymphatics of the right upper limb drain into the right lymphatic duct, and the left drains into the thoracic duct.
Anterior Axioappendicular Muscles (Thoracoappendicular Muscles)
Posterior Axioappendicular Muscles
Scapulohumeral (Intrinsic Shoulder Muscles)
*Rotator cuff muscles: supraspinatus, infraspinatus, teres minor, subscapularis
Muscles of Anterior Compartment of Arm (Flexors of Arm)
Muscles of Posterior Compartment of Arm (Extensors of Arm)
Muscles of Anterior Compartment of Forearm (Flexors of Forearm)
Flexor carpi radialis
Flexor carpi ulnaris
Flexor digitorum superficialis
Flexor digitorum profundus
Flexor pollicis longus
Muscles of Posterior Compartment of Forearm
Extensor carpi radialis longus
Extensor carpi radialis brevis
Extensor digiti minimi
Extensor carpi ulnaris
Abductor pollicus longus
Extensor pollicus longus
Extensor pollicus brevis
Intrinsic Muscles of Hand
Abductor pollicus brevis
Flexor pollicus brevis
Abductor digiti minimi
Flexor digiti minimi brevis
Opponens digiti minimi
The biceps brachii is one of the most variable muscles in the human body, this muscle can have more than two heads arising from humerus at the insertion of the coracobrachialis or neck of the humerus. Some reports describe supernumerary bicipital heads fluctuating from 3 to 7 in different groups, being the 3 heads variant the most common type.
Palmaris longus variations have also been described. The reversed palmaris longus is a polymorphic variation where the muscle is fleshy distally and tendinous proximally. Other case reports have documented a variation in its distal attachment of the muscle to the tendon of the adductor pollicis brevis, and long flexor; or to the flexor carpis ulnaris muscle, and muscle of the hypothenar eminence; as well as to the fascias of the forearm or the hypothenar eminence, and at the metacarpophalangeal joint. 
Upper extremity injuries are usually treated in non-operative modalities such as medications, splints, injection, physical therapy. The surgical consideration of the muscles of the upper limb depends on the underlying condition.
The surgical purpose in upper limb dysfunction due to brain injury is to decrease muscle spasticity, correct joint contractures, as well as enhance the appearance, and function of the extremity. Some of the approaches to increase or decrease the muscle tone imply the peripheral nerve surgery or lengthening hyperactive muscles.
In patients with spinal cord injuries some of the surgical techniques are aimed to restore elbow rectification, and grasp of the upper limb.
Some injuries where fractures and destruction of soft tissues (skin and muscles) are present, a multidisciplinary approach is required in which the orthopedic surgeons work with plastic surgeons to treat open fractures of the extremities, osteomyelitis, or unstable scars correctly.
Rotator Cuff Disease
Involves impingement, tendonitis, as well as tearing of the tendons of the muscles of the rotator cuff. Majority of the cases involve the tendon of the supraspinatus muscle. This is thought to be due to its poor blood supply. The patient complains of pain, especially while lying down on the affected arm or when doing overarm activities. In the clinic, it can be tested by the Hawking’s test and Neer test. The drop test is confirmatory. An MRI is also advised to rule out or confirm a tendon tear. Treatment depends on severity. Management involves NSAIDs, physiotherapy, and arthroscopic repair.
This is the congenital ipsilateral absence or hypoplasia of pectoralis major and pectoralis minor muscles with hypoplasia of the corresponding ribs. It is hypothesized to be caused by an in-utero defect of blood supply to the developing chest. Poland syndrome is commonly associated with defects in breast and/or upper limb development.
Winging of the Scapula
Denervation of the serratus anterior muscle causes palsy of the long thoracic nerve. This causes lateral and posterior movement of the scapula, away from the underlying ribs, giving it a wing-like appearance.
Lateral epicondylitis (tennis elbow) is caused by a combination of repetitive or sustained contraction of the extensor muscles of the forearm leading to inflammation of the common extensor origin. Medial epicondylitis (golfers elbow) is due to repetitive or sustained contraction of the flexor muscles of the forearm leading to inflammation of the common flexor origin. Patients present with pain and tenderness over the affected epicondyle that worsens with extension (in the case of lateral epicondylitis) or flexion (when suspecting medial epicondylitis). Treatment involves avoiding exacerbating activities, physical therapy, and pain relief.
Traumatic injury, malignancy, infection, or congenital deformity of the upper extremity can lead to amputation. Surgeons have different surgical options to improve the potential of using prosthetic technologies for this group of patients. Targeted muscle reinnervation is a surgical procedure to enhance the control of myoelectric upper limb prostheses, and it helps to prevent and treat painful postamputation neuromas. This technique was originally described for transhumeral amputations and shoulder disarticulations but nowadays it has also been applied in the treatment of transtibial, transfemoral, transradial, and partial hand amputees. 
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