Biceps

Biceps brachii
Location of biceps. Two different colors represent two different bundles which compose biceps.   Short head   Long head
Details
Latin	musculus biceps brachii
Origin	Short head: coracoid process of the scapula. Long head: supraglenoid tubercle
Insertion	Radial tuberosity and bicipital aponeurosis into deep fascia on medial part of forearm
Artery	Brachial artery
Nerve	Musculocutaneous nerve (C5–C6)
Actions	Flexes elbow, flexes and abducts shoulder and supinates radioulnar joint in the forearm
Antagonist	Triceps brachii muscle
Identifiers
Gray's	p.443
Dorlands /Elsevier	m_22/12548475
TA	A04.6.02.013
FMA	37670
Anatomical terms of muscle

In human anatomy, the biceps brachii (/ˈbaɪsɛps ˈbræki.aɪ/), commonly known as the biceps, is a two-headed muscle that lies on the upper arm between the shoulder and the elbow. Both heads arise on the scapula and join to form a single muscle belly which is attached to the upper forearm. While the biceps crosses both the shoulder and elbow joints, its main function is at the latter where it flexes the forearm at the elbow and supinates the forearm. Both these movements are used when opening a bottle with a corkscrew: first biceps unscrews the cork (supination), then it pulls the cork out (flexion).^[1]

Structure

Deep (left) and superficial (right) muscles of the chest and arm; the short head of the biceps originates from the coracoid process at the top of the scapula. The long head originates from the supraglenoid tubercle just below the shoulder joint, from where its tendon passes up along the intertubercular groove of the humerus into the joint capsule of the shoulder joint. ^[1]

When the humerus is in motion, the tendon of the long head is held firmly in place in the intertubercular groove by the greater and lesser tubercles and the overlying transverse humeral ligament. During the motion from external to internal rotation, the tendon is forced medially against the lesser tubercle and superiorly against the transverse ligament. ^[2] The tendon of the short head runs adjacent to the tendon of the coracobrachialis and likewise attaches to the coracoid process.

Both heads join on the middle of the humerus, usually near the insertion of the deltoid, to form a common muscle belly. Distally, biceps ends in two tendons: the stronger attaches to (inserts into) the radial tuberosity on the radius, while the other, the bicipital aponeurosis, radiates into the ulnar part of the antebrachial fascia. ^[3]

The tendon that attaches to the radial tuberosity is partially or completely surrounded by a bursa; the bicipitoradial bursa, which ensures frictionless motion between the biceps tendon and the proximal radius during pronation and supination of the forearm.^[4]

Two additional muscles lie underneath the biceps brachii. These are the coracobrachialis muscle, which like the biceps attaches to the coracoid process of the scapula, and the brachialis muscle which connects to the ulna and along the mid-shaft of the humerus. Besides those, the brachioradialis muscle is adjacent to the biceps and also inserts on the radius bone, though more distally.

Variation

Traditionally described as a two-headed muscle, biceps brachii is one of the most variable muscles of the human body and has a third head arising from the humerus in 10% of cases (normal variation) — most commonly originating near the insertion of the coracobrachialis and joining the short head — but four, five, and even seven supernumerary heads have been reported in rare cases. ^[5] The distal biceps tendons are completely separated in 40% and bifurcated in 25% of cases. ^[6]

Innervation

Biceps brachii is innervated by the musculocutaneous nerve together with coracobrachialis and brachialis; like the latter, from fibers of the fifth and sixth cervical nerves. ^[7]

Function

Flexed arm in the pronated position (left); with the biceps partially contracted and in a supinated position with the biceps more fully contracted, approaching minimum length (right.)

The biceps is tri-articulate, meaning that it works across three joints.^[8] The most important of these functions is to supinate the forearm and flex the elbow.

These joints and the associated actions are listed as follows in order of importance:^[9]

• Proximal radioulnar joint (upper forearm) – Contrary to popular belief, the biceps brachii is not the most powerful flexor of the forearm, a role which actually belongs to the deeper brachialis muscle. The biceps brachii functions primarily as a powerful supinator of the forearm (turns the palm upwards). This action, which is aided by the supinator muscle, requires the elbow to be at least partially flexed. If the elbow, or humeroulnar joint, is fully extended, supination is then primarily carried out by the supinator muscle.
• Humeroulnar joint (elbow) – The biceps brachii also functions as an important flexor of the forearm, particularly when the forearm is supinated. Functionally, this action is performed when lifting an object, such as a bag of groceries or when performing a biceps curl. When the forearm is in pronation (the palm faces the ground), the brachialis, brachioradialis, and supinator function to flex the forearm, with minimal contribution from the biceps brachii.
• Glenohumeral joint (shoulder) – Several weaker functions occur at the glenohumeral, or shoulder, joint. The biceps brachii weakly assists in forward flexion of the shoulder joint (bringing the arm forward and upwards). It may also contribute to abduction (bringing the arm out to the side) when the arm is externally (or laterally) rotated. The short head of the biceps brachii also assists with horizontal adduction (bringing the arm across the body) when the arm is internally (or medially) rotated. Finally, the short head of the biceps brachii, due to its attachment to the scapula (or shoulder blade), assists with stabilization of the shoulder joint when a heavy weight is carried in the arm.

Clinical significance

The split between the short head (front) and the long head (back) of the bicep muscle

The proximal tendons of the biceps brachii are commonly involved in pathological processes and are a frequent cause of anterior shoulder pain.^[10] Disorders of the distal biceps brachii tendon typically result from partial and complete tears of the muscle. Partial tears are usually characterized by enlargement and abnormal contour of the tendon.^[11] Complete tears generate a soft-tissue mass in the anterior aspect of the arm, the so-called Popeye sign, which paradoxically leads to a decreased strength during flexion and supination of the forearm.^[12] Tears of the biceps brachii occur in athletic activities and corrective surgery repairs biceps brachii tendon tears. Proximal ruptures of the long head of the biceps tendon can be surgically repaired by two different techniques. Biceps tenodesis is resurfacing the tendon by screw fixation on the humerus and biceps tenotomy is trimming the long head of the biceps tendon promoting the muscle origination from the coracoid process. Preexisting degeneration in the tendon can cause partial tears called lesions and are rarely associated with a traumatic event. The most common symptom of a biceps tear is pain. It will be the most severe in the muscle, but may stretch to the shoulders and elbows as well. Treatment of a biceps tear depends on the severity of the injury. In most cases, the muscle will heal over time with no corrective surgery. Applying cold pressure and using anti-inflammatory medications will ease pain and reduce swelling. More severe injuries require surgery and post-op physical therapy to regain strength and functionality in the muscle. Corrective surgeries of this nature are typically reserved for elite athletes who rely on a complete recovery.^[13]

Society and culture

The biceps are usually attributed as representative of masculinity within cultures.

Etymology and grammar

The biceps brachii muscle is the one that gave all muscles their name: it comes from the Latin musculus, "little mouse", because the appearance of the flexed biceps resembles the back of a mouse. The same phenomenon occurred in Greek, in which μῦς, mȳs, means both "mouse" and "muscle".

The term biceps brachii is a Latin phrase meaning "two-headed [muscle] of the arm", in reference to the fact that the muscle consists of two bundles of muscle, each with its own origin, sharing a common insertion point near the elbow joint. The proper plural form of the Latin adjective biceps is bicipites, a form not in general English use. Instead, biceps is used in both singular and plural (i.e., when referring to both arms).

The English form bicep [sic], attested from 1939, is a back formation derived from interpreting the s of biceps as the English plural marker -s.^[14][15] While common even in professional contexts, it is often considered incorrect.^[16]

Training

A fitness model showing her biceps muscle

The biceps can be strengthened using weight and resistance training. Examples of well known biceps exercises are the chin-up and biceps curl.

To isolate the biceps brachii in elbow flexion, place the shoulder in hyperextension.

In training the biceps brachii, it is important to distinguish between the long head and the short head of the biceps. The long head is the outer portion of the muscle. The short head is the inner portion of the muscle. If you look at the additional images below, you will see a picture that highlights each of the biceps heads for you.

There is much debate over the best biceps workouts for targeting each of these heads.

The first theory for targeting is based on the proximity of the arms in relation to the body. It is said that when the elbows are pulled back behind the body, this targets the long head more. To target the short head, the elbows should be in front of the body.

The second theory uses grip placement and angle as the primary factor in targeting each head. For instance, to target the long head when using dumbbells or cables, the grip should be semi-supinated (hammer) grip where the palms face each other. If using a barbell (EZ grip or straight), the grip should be inside of shoulder width. To target the short head when using dumbbells or cables, grip should be supinated, where the palms are facing up completely. If using a barbell (EZ grip or straight), grip should be outside of shoulder width.^[17]

History

Leonardo da Vinci expressed the original idea of the biceps acting as a supinator in a series of annotated drawings made between 1505 and 1510 (referred to as his Milanese period); in which the principle of the biceps as a supinator, as well as its role as a flexor to the elbow were devised. However, this function remained undiscovered by the medical community as da Vinci was not regarded as a teacher of anatomy, nor were his results publicly released. It was not until 1713 that this movement was re-discovered by William Cheselden and subsequently recorded for the medical community. It was rewritten several times by different authors wishing to present information to different audiences. The most notable recent expansion upon Cheselden's recordings was written by Guillaume Duchenne in 1867, in a journal named Physiology of Motion. To this day it remains one of the major references on supination action of the biceps brachii.

Other species

Neanderthals

In Neanderthals, the radial bicipital tuberosities were larger than in modern humans, which suggests they were probably able to use their biceps for supination over a wider range of pronation-supination. It is possible that they relied more on their biceps for forceful supination without the assistance of the supinator muscle like in modern humans, and thus that they used a different movement when throwing. ^[18]

Horses

In the horse, the biceps' function is to extend the shoulder and flex the elbow. It is composed of two short-fibred heads separated longitudinally by a thick internal tendon which stretches from the origin on the supraglenoid tubercle to the insertion on the medial radial tuberosity. This tendon can withstand very large forces when the biceps is stretched. From this internal tendon a strip of tendon, the lacertus fibrosus, connects the muscle with the extensor carpi radialis -- an important feature in the horse's stay apparatus (through which the horse can rest and sleep whilst standing.) ^[19]

Additional images

• 

  Position of biceps (shown in red)

• 

    Short head

    Long head
• 

  Still image

• 

  Biceps and triceps.

• 

  The middle and distal third of the left biceps brachii

• 

  The proximal part of the left biceps brachii

• 

  Long head tendon of the biceps brachii

• 

  Movement of biceps and triceps when arm is flexing

See also

This article uses anatomical terminology; for an overview, see anatomical terminology.

• Triceps

References

External links

Wikimedia Commons has media related to Biceps brachii muscle.

Look up biceps in Wiktionary, the free dictionary.

• Biceps brachii muscle at GPnotebook
• Anatomy photo:06:05-0102 at the SUNY Downstate Medical Center
• "Bicep Exercises". Elite Men's Guide Health.