| Biceps brachii | |
|---|---|
| Location | |
| Latin | musculus biceps brachii |
| Gray's | subject #124 443 |
| 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 and supinates forearm |
| Antagonist | Triceps brachii muscle |
In human anatomy, the biceps brachii, or simply biceps in common parlance, is, as the name implies, a two-headed muscle located on the upper arm. 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 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]
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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.[2][3] While common even in professional contexts, it is often considered incorrect.[4]
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".
Proximally (towards the body), the short head of the biceps attaches to (originates from) the coracoid process at the top of the scapula. The long head originates on the supraglenoid tubercle just above the shoulder joint from where its tendon passes down 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. [5]
Both heads join on the middle of the humerus, usually near the insertion of the deltoid, to form a common muscle belly. Distally (towards the fingers), 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. [6]
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.
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. [7] The distal biceps tendons are completely separated in 40% and bifurcated in 25% of cases. [8]
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. [9]
The biceps is tri-articulate, meaning that it works across three joints.[10] 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:[11]
The biceps can be strengthened using weight and resistance training. An example of a well known biceps exercise is the simple biceps curl.
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. [12]
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 is capable to 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.) [13]
The conventional radiography can illustrate the bony channel for the long head tendon. This bicipital sulcus is delineated by the major and minor humeral tuberosities. Fractures with subsequent deformation of the sulcus may lead to painful internal or external rotation. A conventional radiography can clearly depict these deformations.[14]
De Maeseneer described in the publicly accessible paper the specific tendon imaging in the following way: “The tendon of the long head of the biceps muscle attaches to the antero-superior aspect of the glenoid rim. The attachment of the biceps tendon may demonstrate four components, including fibers that attach to the antero-superior labrum, the postero-superior labrum, the supraglenoid tubercle, and the base of the coracoid process. From its site of attachment, the biceps tendon courses laterally and exits the gleno-humeral joint through the intertubercular groove, where it is secured by the transverse ligament. The labral-bicipital complex is well visualized on transverse CT or MR arthrograms as well as on coronal MR arthrograms and reconstructed images from coronal CT arthrograms.”[15]
At the ingress through the sulcus intertubercularis the so called pulley system stabilizes the tendon. The pulley system is formed by the coracohumeral and superior glenohumeral ligaments. The intraarticular part of the long head of the biceps brachii muscle accounts to the superior stabilization of the humerus head. In the last few years, injuries of the biceps tendon anchor have been considered as a reason for chronic shoulder pain. Especially in overhead athletes.[16]
[17]
Only about 5% of biceps ruptures are distal. They only occur in men and usually around the age of 40 to 60 years, typically after a sudden extension force at a 90° flexed arm in supination. In 8 out of 10 cases the dominant arm is affected. A predisposition for tendon ruptures in general is a degeneration due to overuse or steroid taking. Predispositive are also systemic diseases like hyperparathyroidism, diabetes mellitus, gout or non-systemic inflammatory thickening of the biceps tendon or bone hypertrophy at the radial tuberosity due to the impingement. Most tendon injuries are located at the tuberositas radialis, which could be explained with a found hypovascularisation of the tendon 1cm more proximally. [18]
Radiological signs for a complete rupture:
With sufficient experience of the examiner, which is an essential factor for a successful ultrasonographic assessment in general, the ultrasound is the method of choice for distal biceps tendon ruptures. This makes an expensive MR imaging mostly unnecessary.
Ultrasonographic findings on longitudinal and transversal plane, ventral humero-radial in supination: Hypoechoic hematoma at the insertion area of the tuberositas radii, instead of a parallel echogenic insertion. Changes of the contour, retraction of the M. biceps brachii and hemarthrosis.[19]
Axial MRI sequences are necessary to determine the exact degree of tendon injury. They can show a proximally thickened and signal increased distal biceps tendon including a surrounding irritative tissue reaction. More distal the tendon profile is not depictable, because it is completely detached from the osseous attachment.[20]
Sagittal sequences are useful for assessing the degree of tendon retraction. A bone marrow edema in the tuberositas radialis and fluid in the bursa bicipito-radialis indicate a partial rupture and facilitate the discrimination to a tendinosis. An isolated bursitis is often clinically not clearly distinguishable from a biceps tendinosis. A biceps tendinosis does affect the tendon’s biomechanical tensile strength negatively. Hematoma or large effusion in the bursa could cause nerve compression of the N. medianus, N. radialis or Nn. interosseus posteriores.
A conservative treatment leads to a loss of 30-40% strength in flexion and supination. It is therefore recommended to surgically re-fix the tendon to the tuberositas radialis within the first 2 weeks after the trauma, especially in physically active patients with high expectations. Within the first 2 weeks the muscle gets neither scarred nor retracted.
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