Shoulder and arm
The shoulder is one of the four major segments that constitute the upper limb (shoulder, arm, forearm, and hand). It is the most proximal segment of the limb and it overlaps with the thorax, back, and the lower lateral part of the neck.
The shoulder is subdivided into regions that include the pectoral, scapular, deltoid and lateral aspect of the lateral cervical region. It overlies half of the pectoral girdle, a set of bones that connects the arm to the axial skeleton on each side.
Bones and joints
The shoulder is a complex structural unit of bones, joints, ligaments and muscles. The three bones are the scapula and clavicle, which form the pectoral girdle, and the proximal end of the humerus. The main articulations are also three in number, and they are the acromioclavicular, sternoclavicular, and the glenohumeral joint, the most mobile joint in the body.
Some of the shoulder muscles include the trapezius, levator scapulae, and rhomboid muscles. They connect the scapula and clavicle to the trunk. Other muscles connect these two to the proximal end of the humerus, and they are the pectoralis major, pectoralis minor, latissimus dorsi, teres major, and deltoid. The trapezius and deltoid are the most superficial of shoulder muscles; they provide its characteristic contour.
The most important shoulder muscles, however, are that of the rotator cuff. These muscles are the supraspinatus, infraspinatus, subscapularis and teres minor. They arise from the scapula to attach to the humerus, and provide support for the glenohumeral joint.
The muscles and joints of the shoulder allow for a remarkable range of motion. The shoulder has various functions including, abduction, adduction, medial and lateral rotation, full 360° rotation in a sagittal plane, arm elevation, and elbow flexion and extension. This wide range of movement makes it unstable, and far more prone to dislocation.
The arm is considered the first and longest segment of the free upper limb (the more mobile part of the upper limb that is independent of the trunk). It extends from the shoulder to the elbow and consists of anterior and posterior regions that are centered around the humerus.
Humerus and muscles of the arm
Humerus is the bone of the arm, its distal end articulates with the bones of the forearm, the radius and ulna, at the elbow joint. The four major muscles of the arm are separated by fascia, bone and ligaments, into anterior (flexors) and posterior (extensors) compartments. The medial and lateral intermuscular septa are continuous with the deep fascia enclosing the arm and attach to the sides of the humerus.
The anterior compartment contains the three flexor muscles: the biceps brachii, brachialis, and coracobrachialis. They are supplied by the musculocutaneous nerve. The posterior compartment has only one extensor muscle, the triceps brachii, and it is supplied by the radial nerve. The anconeus is a small triangular muscle that partially blends with the triceps at the posterolateral aspect of the elbow, aiding it in forearm extension.
Muscles in the arm mainly act on moving the forearm at the elbow joint. The flexor muscles of the anterior compartment are stronger in all positions than that of the extensor muscles in the posterior compartment. This makes Humans better pullers than pushers. However, this doesn’t diminish from the importance of elbow extensors.