The triceps brachii is a large, thick muscle on the dorsal part of the upper arm. It often appears as the shape of a horseshoe on the posterior aspect of the arm. The main function of the triceps is the extension of the elbow joint. It is composed of three heads (tri = three, cep = head): a long head, a lateral head, and a medial head. The tendons all have different origins, but the three heads combine to form a single tendon distally. The long head originates from the infraglenoid tubercle of the scapula while both the lateral head and the medial head both originate from the humerus. The three heads converge into a single tendon, and this tendon attaches to the proximal portion of the olecranon process (the bony prominence of the elbow) located on the upper portion of the ulna.
The triceps' primary function is extending the forearm at the elbow joint, which opposes the action of the flexors such as the biceps brachii. At rest, with the arm slightly bent with flexion, the biceps brachii overpowers the triceps brachii. Along with extending the forearm at the elbow joint, the triceps can also stabilize the elbow joint when the forearm and hand are doing fine movements such as writing.
The triceps brachii is located in the dorsal compartment of the arm. The lateral intermuscular septum is what separates the dorsal part from the arm from the ventral part which is where the flexors of the arm are (biceps, brachialis, and brachioradialis).
Long head—The origin of the long head is the infra-glenoid tubercle of the scapula. Because it attaches the scapula, the long head not only extends the elbow, but will also have a small action on the glenohumeral, or shoulder joint. With the arm adducted, the triceps muscle acts to hold the head of the humerus in the glenoid cavity. This action helps prevent any displacement of the humerus. The long head also assists with the extension and adduction of the arm at the shoulder joint. The lateral head is also active during extension forearm at the elbow joint when the forearm is supinated or pronated.
Medial head—The origin of the medial head is at the dorsal humerus, inferior to the radial groove, and connecting to the intermuscular septum. The medial head does not attach to the scapula and therefore has no action on the glenohumeral joint whether that be with stabilization or movement. The medial head is active, however during extension of the forearm at the elbow joint when the forearm is supinated or pronated.
Lateral head—The lateral head originates at the dorsal humerus as well, but unlike the medial head, it is superior to the radial groove where it fuses to the lateral intermuscular septum. This head is considered to be the strongest head of the three. It is active during extension of the forearm at the elbow joint when the forearm is supinated or pronated.
The mesoderm develops during the third week following the gastrulation process. At the third week, the paraxial mesodermal layer begins to organize into segments called somitomeres. These consist of concentric layers of mesodermal cells formed with cranial to caudal progression. Subsequently, they organize in somites. The somites appear with a frequency of about three pairs a day so that at the fifth week are recognizable 42-44 pairs of somites of which: four occipital pairs, seven cervical, twelve thoracic, five lumbar, five sacral and between eight and ten coccyx pairs.
The triceps brachii begins to develop during the 5th week of embryogenesis. The muscle originates from the dorsal muscle mass of the upper limb bud and is derived from the paraxial mesodermal leaflet.
The arterial supply to the triceps is provided by the deep brachial artery, which is a branch of the brachial artery and is supplied by ulnar collateral arteries. The venous drainage is the brachial vein with runs with the deep brachial artery.
Lymphatic vessels, like lymph nodes, are divided into superficial and deep. The superficial lymphatic collectors are the cutaneous lymphatic capillaries, especially of the hand, from which they course proximally in the forearm to form three groups: medial, lateral, and anterior collectors. In the arm then they continue to get to the armpit to the axilla.
The provision of nerve supply to the triceps is by the radial nerve (root C6, C7, and C8). However, according to the cadaveric study, it was found that the medial head of the triceps brachii could be innervated partially by the ulnar nerve. Following that line of investigation, other research also reveals that the long head of the triceps brachii also can be innervated partially by the axillary nerve.
The radial nerve originates from the posterior cord of the brachial plexus in the anterior axilla. The nerve then courses along with the profunda brachii artery (the artery that supplies blood to the posterior arm) travels through the arm inferolaterally. It passes behind the humerus and through the radial groove between both the lateral and medial heads of the triceps brachii.
There have been reports of a fourth head in a few cadaver studies. This accessory muscle can originate from the humerus, shoulder capsule, coracoid processes, or muscles in close proximity. These accessory muscles can potentially compress the radial and ulnar nerves. It is therefore essential for surgeons and physicians to keep in mind that although rare these variations exist, which can be helpful in diagnosing cases of nerve entrapment and other pathologic causes that may not be explained by any other typical factors.
Axillary nerve damage can have an impact on the long head of the triceps brachii (LTB). Therefore, people with axillary nerve damage should undergo assessment for the function of the LTB. If they demonstrate lost function, then this shows a poor prognosis and early repair at three months is the recommendation.
The triceps muscle can undergo reinnervation through a distal nerve transfer. Commonly used nerves for reinnervation include the flexor carpi ulnaris fascicle of the ulnar nerve and the posterior branch of the axillary nerve. Both of these nerves have been shown to recover the function of the triceps muscle
Additionally, various heads of the triceps can be used as flap coverage in traumatic injury. The medial and lateral heads are commonly harvested as flaps. These procedures are indicated during nerve injuries, shoulder or elbow defects, or muscular reanimation. 
The triceps reflex, elicited by sharply striking the triceps tendon, is often used to test the function of the nerves of the arm. This reflex tests spinal nerves C6 and C7, predominately C7. The reflex is tested by abducting the patient's shoulder and elbow to 90 degrees. Then the triceps tendon is tapped using a reflex hammer just proximal to the olecranon. 
Triceps tendinitis can occur with an overload of the muscle. It commonly manifests as chronic posterior elbow pain that is worsened with active extension. The condition is commonly seen in men aged 30-40 years who are throwing athletes. The condition is typically managed with rest and pain management.
Ruptures of the triceps muscle are rare, and typically only occur in anabolic steroid users. Distal triceps ruptures are also relatively uncommon. The reason they are not common revolves primarily around the anatomy of the triceps muscle. If a rupture would to occur, it would take place at the tendon-bone junction, and it would result from an eccentric contraction of the muscle. The injury typically occurs due to either a fall on an outstretched hand or a direct blow to the triceps tendon. The patient presents after a painful popping sensation with pain and swelling over the posterior elbow with the inability to extend the elbow against resistance. Surgical consideration for a partial rupture is controversial, but it is necessary for a complete rupture. If the rupture is chronic, then along with surgical repair, remodeling is also indicated.
The triceps brachii can undergo training in a variety of different ways. It can be worked out in isolation or with compound elbow extension movements. It can also be contracted statically to keep the arm straight among resistance.
Examples of isolation movements include lying triceps extensions, behind the back arm extensions, cable push-downs, and standing triceps "kickbacks." The compound exercises that work out the triceps include any pressing movements like push-ups, bench press, close grip bench press, tricep dips, and military presses. The closer the grip on these exercises the more they isolate the triceps. The further the grip, the more they work the outer chest.
Static contraction movements include pullovers, straight-arm pull-downs, and bent-over lateral raises, which are also used to build the deltoids and latissimus dorsi.
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