| Triceps Brachii | |
|---|---|
| Triceps brachii | |
| Muscles on the dorsum of the scapula, and the Triceps brachii. | |
| Latin | musculus triceps brachii |
| Gray's | subject #124 444 |
| Origin | long head: infraglenoid tubercle of scapula lateral head: posterior humerus medial head: posterior humerus |
| Insertion | olecranon process of ulna |
| Artery | deep brachial artery (Profunda brachii) |
| Nerve | radial nerve and axillary nerve (long head) |
| Actions | extends forearm, long head adducts shoulder |
| Antagonist | Biceps brachii muscle |
The triceps brachii muscle (Latin for "three-headed arm muscle") is the large muscle on the back of the upper limb of many vertebrates. It is the muscle principally responsible for extension of the elbow joint (straightening of the arm).
Contents |
It is sometimes called a three-headed muscle because there are three bundles of muscles, each of different origins, joining together at the elbow. Though a similarly named muscle, the triceps surae, is found on the lower leg, the triceps brachii is commonly called the "triceps".
Historically, the plural form of the adjective triceps was tricipites, a form not in general use today; instead triceps is used in both singular and plural (i.e., when referring to both arms). The triceps also make up approximately 2/3 of the muscle mass in the arm.
The long head arises from the infraglenoid tubercle of the scapula. It extends distally anterior to the teres minor and posterior to the teres major.[1]
The medial head arises distally from the groove of the radial nerve; from the dorsal (back) surface of the humerus; from the medial intermuscular septum; and its distal part also arises from the lateral intermuscular septum. The medial head is mostly covered by the lateral and long heads, and is only visible distally on the humerus.[1]
The lateral head arises from the dorsal surface of the humerus, lateral and proximal to the groove of the radial nerve, from the greater tubercle down to the region of the lateral intermuscular septum.[1]
Each of the three fascicles has its own motorneuron subnucleus in the motor column in the spinal cord. The medial head is formed predominantly by small type I fibers and motor units, the lateral head of large type IIb fibers and motor units and the long head of a mixture of fiber types and motor units.[2] It has been suggested that each fascicle "may be considered an independent muscle with specific functional roles."[2]
All three heads of the triceps brachii are classically believed to be innervated by the radial nerve.[3] However, a study conducted in 2004 determined that, in 20 cadaveric specimens and 15 surgical dissections on participants, the long head was innervated by a branch of the axillary nerve in all cases.[4]
The fibers converge to a single tendon to insert onto the olecranon process of the ulna (though some research indicates that there may be more than one tendon)[5] and to the posterior wall of the capsule of the elbow joint where bursae (cushion sacks) are often found. Parts of the common tendon radiates into the fascia of the forearm and can almost cover the anconeus. [1]
The triceps is an extensor muscle of the elbow joint, and is an antagonist of the biceps and brachialis muscles. It can also fixate the elbow joint when the forearm and hand are used for fine movements, e.g., when writing. It has been suggested that the long head fascicle is employed when sustained force generation is demanded, or when there is a need for a synergistic control of the shoulder and elbow or both. The lateral head is used for movements requiring occasional high-intensity force, while the medial fascicle enables more precise, low-force movements.[2]
With its origin on the scapula, the long head also acts on the shoulder joint and is also involved in retroversion and adduction of the arm.[1]
A tendinous arch is frequently the origin of the long head and the tendon of latissimus dorsi. In rare cases, the long head can originate from the lateral margin of the scapula and from the capsule of the shoulder joint. [1]
The triceps can be worked through either isolation or compound elbow extension movements, and can contract statically to keep the arm straightened against resistance.
Isolation movements include cable push-downs, lying triceps extensions and arm extensions behind the back. Examples of compound elbow extension include pressing movements like the push up, bench press, close grip bench press (flat, incline or decline), military press and dips. A closer grip targets the triceps more than wider grip movements.
Static contraction movements include pullovers, straight-arm pulldowns, and bent-over lateral raises, which are also used to build the deltoids and latissimus dorsi.
Elbow extension is important to many athletic activities. As the biceps is often worked more for aesthetic purposes, this is usually a mistake for fitness training. While it is important to maintain a balance between the biceps and triceps for postural and effective movement purposes, what the balance should be and how to measure it is disputed. Pushing and pulling movements on the same plane are often used to measure this ratio.
In the horse, 84%, 15%, and 3% of the total muscle weight correspond to the long, lateral, and medial heads, respectively.[6]
Many mammals such as dogs, cows, and pigs have a fourth head, the "Accessory head", which lies between the Lateral and Medial heads.[2] In humans, the Anconeus is sometimes loosely called "the fourth head of the triceps brachii".
[7]
Injuries of the distal triceps tendon are rare. Beyer et al. found a higher rate of partial tears in contrast to Beletani et al. who described full thickness tears as more common. Children and adults are affected. Tendinous injuries are typically found in the area of the olecranon insertion. Full thickness tears mostly include a bony avulsion.
MR images show an explicit large fluid-filled gap between the distal triceps tendon and the olecranon with a large amount of edema in the surrounding subcutaneous tissue. The distal edges of the torn triceps tendon are frayed and have a heterogeneous signal intensity.[8] Partial ruptures affect mainly the middle section of the tendon above the olecranon. Rupture at the musculo-tendinous transition has been described, but is extremely rare. A forced flexion of the elbow against resistance, e.g. during a fall on the outstretched arm, is a common causing mechanism. Especially seen in athletes, triceps tendon tears can be caused by direct hits on the tendon itself or by extreme contractions for example while weightlifting. Predispositions for tendon injuries are: local injection of anabolic steroids, years of therapy with corticoids followed by a tendinitis, bursitis dorsally of the olecranon, hyperparathyroidism, chronic renal failure and Marfan’s syndrome.[9]
The clinical diagnosis can be hindered because of pain. Hence the MRI, besides sonography, has a great clinical significance. The investigation is essential, because a complete rupture requires an immediate surgical repair.
Recommended are T2-weighted fat suppressed axial and sagittal planes.It has to be added that the sagittal plane is at an eventual “postero-lateral rotary instability”, for example after dislocation, a good choice to illustrate the centering of the radius head or changes at the olecranon and the tripecs tendon. All tendons of the elbow are hypointense, except the triceps tendon. It shows, just like the quadriceps tendon at the area of the distal thigh, signal increased stripes in between two collagen bundles. Those are fatty inter-positions and should not be misinterpreted as tears.[10]
Small sesamoid bones in the triceps tendon (patellae cubitalis) are very rare, and their genesis remains unclear. Whether if a trauma in the patients history or an ossification disorder of the olecranon apophysis is the root cause, remains unclear and is still academically discussed. More common is an ossicle, 1-2 cm large, ventral to the triceps tendon in the subsynovial fat tissue or even intraarticular in the fossa olecrani. A so-called os supratrochleare dorsalis. Large ossicles could lead to an extension deficit. This variety of the norm is to distinguish from an olecranon fracture, because fractures at that area are often combined with a dehiscence, respectively, a dislocation of an osseous fragment due to traction of the triceps muscle.[11][12]
Distortion or direct trauma can cause injuries to the muscles around the elbow. A rupture of muscular fibers leads to an intramuscular hematoma. There has no specific predilection site been found. Muscular injuries can best be depicted on T2-weighted fat suppressed MR images. They show the extent of the hematoma and the intramuscular edema, which can have a streaky or a macular arrangement. First the pathology appears signal weak and develops later on, with the development of methaemoglobin, a strong signal intensity. A subacute or chronic strain indicates the additional use of contrast agent in order to show a circumscribed enhancement at the lesion. Recommended are fat suppressed T1- and T2-weighted MR images, before and after injection of the contrast agent.[13]
|
|
||||||||||||||||||||||||||||||||||||||||||||||||||||
