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The triceps brachii muscle (Latin for “three-headed” muscle of the arm, it is called a three headed muscle because there are three bundles of muscle, each of different origin, joining together at the elbow) is the large muscle on the back of the human upper limb. It is the muscle principally responsible for extension of the elbow joint (i.e. straightening of the arm). Though a similarly-named muscle, the triceps surae, is found on the lower leg, the triceps brachii is commonly called simply the “triceps”.
Historically, in a now-extinct dialect of English, the plural form of the adjective triceps was tricipes, a form not in general use today, instead triceps is used in both singular and plural (i.e., when referring to both arms).
The three heads have the following names and origins:
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.)
In the horse, 84%, 15%, and 3% of the total muscle weight correspond to the long, lateral, and medial heads.
Many mammals such as dogs, cows, and pigs have a fourth head, the “Accessory head”, which lies between the Lateral and Medial heads. In humans, the Anconeus is sometimes loosely called “the fourth head of the triceps brachii”.
Each of the three fascicle has its own motorneuron subnucleus in the motor column in the spinal cord. The medial head being formed predominantly by small type I fibers and motor units, the lateral fascicle of large type IIb fibers and motor units and the long head of a mixture of fiber types and motor units. It has been suggested that each fascicle “may be considered an independent muscle with specific functional roles”.
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.
The triceps accounts for approximately 60 percent of the upper arm’s muscle mass.
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 a conflicted area. Pushing and pulling movements on the same plane are often used to measure this ratio.

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