What is the name of the large buttock and thigh muscle?
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Buttocks Muscle
The buttocks (singular: buttock) are two rounded portions of the anatomy, located on the posterior of the pelvic region of apes (including humans), and many other bipeds or quadrupeds, and comprise a layer of fat superimposed on the gluteus maximus and gluteus medius muscles.  Physiologically, the buttocks enable weight to be taken off the feet while sitting. In many cultures, they play a role in sexual attraction.  Many cultures have also used the buttocks as a primary target for corporal punishment,  as the buttocks' layer of subcutaneous fat offers protection against injury while still allowing for the infliction of pain. There are several connotations of buttocks in art, fashion, culture and humor, and the English language is replete with many popular synonyms. In humans the buttocks are located between the lower back and the perineum.

Anatomy

The buttocks are formed by the masses of the gluteal muscles or "glutes" (the gluteus maximus muscle and the gluteus medius muscle) superimposed by a layer of fat. The superior aspect of the buttock ends at the iliac crest, and the lower aspect is outlined by the horizontal gluteal crease. The gluteus maximus has two insertion points:  superior portion of the linea aspera of the femur, and the superior portion of the iliotibial tractus. The masses of the gluteus maximus muscle are separated by an intermediate intergluteal cleft or "crack" in which the anus is situated.

The buttocks allow primates to sit upright without needing to rest their weight on their feet as four-legged animals do. Females of certain species of baboon have red buttocks that blush to attract males. In the case of humans, females tend to have proportionally wider and thicker buttocks due to higher subcutaneous fat and proportionally wider hips. In humans they also have a role in propelling the body in a forward motion and aiding bowel movement.  

Some baboons and all gibbons, though otherwise fur-covered, have characteristic naked callosities on their buttocks. While human children generally have smooth buttocks, mature males and females have varying degrees of hair growth, as on other parts of their body. Females may have hair growth in the gluteal cleft (including around the anus), sometimes extending laterally onto the lower aspect of the cheeks. Males may have hair growth over some or all of the buttocks.

Society and culture

Connotations

The English word of Greek origin "callipygian" indicates someone who has beautiful buttocks. However, the qualities that make buttocks beautiful or well-formed are not fixed, as sexual aesthetics of the buttocks vary considerably from culture to culture, from one period of fashion to another and even from person to person.
    
Depending on the context, exposure of the buttocks in non-intimate situations can cause feelings of embarrassment or humiliation, and embarrassment or amusement in an onlooker (see pantsing).

Willfully exposing one's own bare buttocks as a protest, a provocation, or just for fun is called mooning.

In many punitive traditions, the buttocks are a common target for corporal punishment, which can be meted out with no risk of long-term physical harm compared with the dangers of applying it to other parts of the body, such as the hands, which could easily be damaged.  The buttocks have often been described as "the place provided by nature" for this purpose.

In Western and some other cultures, many comedians, writers and others rely on the buttocks as a source of amusement, camaraderie and fun.

In American English, phrases use the buttocks or synonyms (especially "butt" and "ass") as a synecdoche or pars pro toto for a whole person, often with a negative connotation. For example, terminating an employee may be described as "firing his ass". One might say "move your ass" or "haul ass" as an exhortation to greater haste or urgency. Expressed as a function of punishment, defeat or assault becomes "kicking one's ass". Such phrases also may suggest a person's characteristics, e.g. difficult people are termed "hard asses". In America an annoying person or any source of frustration may be termed "a pain in the ass" (a synonym for "a pain in the neck"). People deemed excessively puritanical or proper may be termed "tight asses" (in Australia and New Zealand, "tight arse" refers to someone who is excessively miserly).

In the 2010s, the reality TV star Kim Kardashian is famous for her buttocks, which have appeared on magazine covers. Kardashian appeared on the cover, and in a pictorial in Papers winter issue in 2014 photographed by Jean-Paul Goude.  On the cover, her nude buttocks are featured above the caption: "Break the Internet", which generated considerable comment in both social and traditional media.  

Certain physical dispositions of the buttocks—particularly size—are sometimes identified, controversially, as a racial characteristic (see race). A famous example was the case of Saartjie Baartman, the so-called "Hottentot Venus".

Synonyms

See WikiSaurus:buttocks for a list of synonyms and slang words for buttocks in many languages.  For unrelated homophones of butt(ocks), see also butt (disambiguation) and bud (disambiguation)

The Latin name for the buttocks is nates (modern pronunciation,  classical pronunciation nătes ) which is plural; the singular, natis (buttock), is rarely used. There are many colloquial terms to refer to them, including:

* Backside, posterior, behind and its derivates (hind-quarters, hinder or the childish diminutive "heinie" (US usage only), strictly the whole body behind the hind leg-trunk attachment), rear or rear-end, derrière (French for "behind")—all strictly positional descriptions, as the inaccurate use of rump (as in 'rump roast', after a 'hot' spanking), thighs, upper legs; analogous are:
** Aft, stern and poop, naval in origin; in  nautical jargon, buttocks also designates the aftermost portion of a hull above the water line and in front of the rudder, merging with the run below the water line
** Caboose, originally a ship's galley in wooden cabin on deck; also the "rear end" car of a freight train, considered a cute synonym suitable for any audience
** Bottom (and the shortening "bot" as well as childish diminutives "bottie" or "botty"), but the use of similar-sounding "booty" or "bootie" may be related.
** Tail (strictly anatomically a zoomorphism, humans only have a tail-bone, yet the illogical "tail feather" was popularized by musicians. When used to refer to a woman or to women in general, the term is derogatory; also used for the even more sensual phallus) and tail-end
** Trunk, in American English, particularly when describing large buttocks: "junk in the trunk"
* Apple, referring to the similar shape of the fruit, derived from the 1970s. Also likened to an upside-down heart, attributed from various, popular ads of the 1970s.
* Arse or ass, arsehole or asshole, and (butt-)hole: a pars pro toto (strictly only the actual body cavity and directly adjoining anal region); also used as an insult for a person. The term arse or ass is Anglo-Saxon, and over a thousand years old.
* Badonkadonk: onomatopoeic US slang meaning the voluptuously bouncing, large yet firm buttocks of a woman
* Booty, US slang, used in the popular slang expression "booty call".  It has been suggested that the word derives from a Bambara (West African) word for anus, buda. 
* Breech, a metaphorical sense derived from on older form of the garment breeches (as the French culotte meaning pantoloons, via cul from Latin culus "butt"), so 'bare breech' means without breeches, i.e., trouserless butt
* Bum: in British English, used frequently in the United Kingdom, Ireland, Canada, Australia, New Zealand and many other English-speaking Commonwealth countries, also historically in the United States, is a mild often humorous term for buttocks, not necessarily in a vulgar or sexual context: "I've a boil on my bum, thrice as large as my thumb" (The Judge With The Sore Rump, St. George Tucker). A bum boy is an insulting term for a male homosexual.
* Bumpy: a euphemistic term for the buttocks, used primarily with children
* Buns, from Gaelic bun "bottom, base", mounds (cfr. Butte, a geographical mound, known since 1805 in American English, from (Old) French butte "mound, knoll") and orbs—shape-metaphors.
* Bund: derived from Punjabi
* Bunda: Brazilian Portuguese slang for buttocks, from Kimbundu mbunda, with same meaning.
* Butt: the common term for a pair of buttocks in the US (singular, as one body-part; cognate but neither its root nor an abbreviation), used in everyday speech.
* Cakes: slang word for buttocks
* Can (a container) had an unusual development: the slang meaning "toilet" is recorded c. 1900, said to be a shortening of piss-can, the meaning "buttocks" from c. 1910, and the verb meaning "fire an employee" (to flush=dump?) from 1905.
* Cheeks, a shape-metaphor within human anatomy, but also used in the singular: left cheek and right cheek; sounds particularly naughty because of the homonym and the adjective cheeky, lending themselves to word puns
* Culo: (From Spanish/Italian) slang, usually meaning a woman's voluptuous, round and firm buttocks. Derived from a term for booty; in Spanish the term is considered vulgar and offensive, but less so in Spain than in Latin America.
* Duffs: Ulster Irish origin
* Dumper sometimes denotes the buttocks, especially when they are large.
* Fanny: a socially acceptable term in print, in Canada and the United States at least, for many years before some of the bolder terms came along; and a subject of jokes, since "Fannie" can be a woman's name, diminutive of "Frances"; however, in British English fanny refers to the female genitals or vulva and is considered vulgar. The figure of a bare-bottomed lass named Fanny is ubiquitous in Provence (the southeast of France) wherever pétanque is played: traditionally when a player loses 13 to 0 it is said that “il est fanny” (he's fanny), and he has to kiss the bottom of a girl called Fanny; as there is rarely an obliging Fanny, there is always a substitute picture, woodcarving or pottery so that Fanny’s bottom is always available. 
* Fourth point of contact: in military slang, because of the sequence of textbook parachute jump landing
* Fundament (literally "foundation", not common in this general sense in English, but for the buttocks since 1297)
* Gand or Gaand: a Hindi derivative
* Hams, like buttocks generally as a plural, after the meat cut from the analogous part of a hog ; pressed ham refers to mooning against a window; brawn, a singular derived from the Frankish for ham or roast, is also used for both a muscular body part (but either on arms or legs) or boar meat, especially roast
* Hurdies: Scots, origin unknown, also applied to the whole rump
* Haunches
* Moon was a common shape-metaphor for the butt in English since 1756, and the verb to moon meant 'to expose to (moon)light' since 1601, long before they were combined in US student slang in the verb (al expression) mooning "to flash the buttocks" in 1968.
* Prat (British English, origin unknown; as in pratfall, a music hall term; also a term of abuse for a person)
* Seat (of the trousers; or metaphorically): another long-standing socially acceptable term, referring to the use for sitting—but compare the sarcastic use of seat of wisdom and similar expressions, such as 'seat of learning', referring to use as target for an 'educational' spanking.
* Sit-upon; has various independent counterparts in other languages, e.g., Dutch zitvlak ("sitting plain"), German Gesäß  Italian sedere
* Six; in military terminology, particularly in the United States Navy, it refers to the term "six o'clock", i.e., a point directly behind the referenced person.
* Tuchis: Yiddish.
* Tush or tushy (from the Yiddish language "tuchis" or "tochis" meaning "under" or "beneath")
* Ultimatum (Latin, literally 'the furthest part') was used in slang c.1820s.

Related terms

* The word "callipygian" is sometimes used to describe someone with notably attractive buttocks. The term comes from the Greek kallipygos, (first used for the Venus Kallipygos) which literally means "beautiful buttocks"; the prefix is also a root of "calligraphy" (beautiful writing) and "calliope" (beautiful voice); callimammapygian means having both beautiful breasts and buttocks.
* Both the English (in) tails and the Dutch billentikker ('tapping the buttocks') are ironic terms for very formal coats with a significantly longer tail end as part of festive (especially wedding party) dress
* Macropygia means 'heaving large buttocks, hindquarter', and occurs in biological species names,
* A pygopag(ous) (from the Greek pygè 'buttock' and pagein 'attached') was a monster in Ancient (Greek) mythology consisting of two bodies joint by common buttocks, now a medical term for 'Siamese' twins thus joint back-to-back
* Pygophilia is sexual arousal or excitement caused by seeing, playing with or touching the buttocks; people who have strong attraction to buttocks are called pygophilists.
* Pygoscopia means observing someone's rear; pygoscopophobia a pathological fear to be its unwilling object
* Pygalgia is soreness in the buttocks, i.e. a pain in the rump.
* Steatopygia is a marked accumulation of fat in and around the buttocks.
* Uropygial in ornithology means situated on or belonging to the uropygium, i.e. the rump of a bird.
* "Bubble butt" has at least two connotations, which are at odds with each other: either a small, round and firm pair of buttocks resembling a pair of soap bubbles next to each other, or a large rear end, seemingly about to burst from the strain. In both cases, the term implies an appealing shapeliness about the buttocks.

Fashion

The 1880s were well known for the fashion trend among women called the bustle, which made even the smallest buttocks appear huge. The popularity of this fashion is shown in the famous Georges Seurat painting A Sunday Afternoon on the Island of La Grande Jatte in the two women to the far left and right. Like long underwear with the ubiquitous "butt flap" (used to allow baring only the bottom with a simple gesture, as for hygiene), this clothing style was acknowledged in popular media such as cartoons and comics for generations afterward.

More recently, the cleavage of the buttocks is sometimes exposed by some women, deliberately or accidentally, as fashion dictated trousers be worn lower, as with hip-hugger pants.

An example of another attitude in an otherwise hardly exhibitionist culture is the Japanese fundoshi.

In popular culture

*In 1966 Yoko Ono made a roughly 90 minute-long experimental film called No. 4, which is colloquially known as Bottoms. It consists of footage of human buttocks in motion while the person walks on a turntable.
*The 1984 Heavy metal music mockumentary This Is Spinal Tap contained the song "Big Bottom" featuring the lyrics Big bottom, big bottom, Talk about bum cakes, my girl's got 'em, Big bottom drive me out of my mind, How could I leave this behind? The band performs the song using three simultaneous bass guitars, a pun on the song title, as the bass is often colloquially referred to as the "bottom end" of the rock sound.
*Numerous songs have been released which glorify this body part.  As early as 1961, the American Folk band The Limeliters recorded the song "Vicki Dougan" [sic], which pays a humorous tribute to the pin-up star Vikki Dougan who wore infamously low-cut, backless dresses. A risque song for the time, the lyrics  mention her "callipygian cleft" by name. Other English language examples include:
** "Bootylicious" by Destiny's Child
** "(Shake, Shake, Shake) Shake Your Booty" by KC and the Sunshine Band
** The 1978 singles "Fat Bottomed Girls" and "Bicycle Race" by Queen
** "I See You Baby (Shakin' That Ass)" by Groove Armada
** "Baby Got Back" by Sir Mix-a-Lot (1992)
** "All About That Bass" by Meghan Trainor (2014)
*Rolling Stone magazine named the 1990s the "Decade of the Butt" because many of these songs were released in that decade.
*"My Humps" (in the back as well as in the front) by The Black Eyed Peas
*"Big Bottom" by Spinal Tap
*"Pop That Booty" by Marques Houston
*"Ms. New Booty" by Bubba Sparxxx and Ying Yang Twins
*In the United Kingdom there is an annual award given to the male and female winners of the celebrity voted to have the Rear of the Year for that year.  Past Winners have included Sarah Lancashire and Jane Danson.
*In 2008, Hong Kong lifestyle retail store G.O.D. collaborated with Kee Wah Bakery to design mooncakes for the Chinese Mid-Autumn Festival. They presented the traditional treats in the shape of bottoms in eight different designs, but still filled with traditional white lotus seed paste and salted yolks. 

Studio Jean Jacques Lequeu.jpg|Jean-Jacques Lequeu (c. 1785).
Étude de fesses.jpg|Félix Vallotton (c. 1884). Muscle is a soft tissue found in most animals. Muscle cells contain protein filaments of actin and myosin  that slide past one another, producing a contraction that changes both the length and the shape of the cell. Muscles function to produce force and motion. They are primarily responsible for maintaining and changing posture, locomotion, as well as movement of internal organs, such as the contraction of the heart and the movement of food through the digestive system via peristalsis.

Muscle tissues are derived from the mesodermal layer of embryonic germ cells in a process known as myogenesis.  There are three types of muscle, skeletal or striated, cardiac, and smooth. Muscle action can be classified as being either voluntary or involuntary. Cardiac and smooth muscles contract without conscious thought and are termed involuntary, whereas the skeletal muscles contract upon command.  Skeletal muscles in turn can be divided into fast and slow twitch fibers.

Muscles are predominantly powered by the oxidation of fats and carbohydrates, but anaerobic chemical reactions are also used, particularly by fast twitch fibers. These chemical reactions produce adenosine triphosphate (ATP) molecules that are used to power the movement of the myosin heads. 

The term muscle is derived from the Latin musculus meaning "little mouse" perhaps because of the shape of certain muscles or because contracting muscles look like mice moving under the skin.  

Anatomy 

The anatomy of muscles includes gross anatomy, which comprises all the muscles of an organism, and microanatomy, which comprises the structures of a single muscle.

Types of tissue 

Muscle tissue is a soft tissue, and is one of the four fundamental types of tissue present in animals. There are three types of muscle tissue recognized in vertebrates:
* Skeletal muscle or "voluntary muscle" is anchored by tendons (or by aponeuroses at a few places) to bone and is used to effect skeletal movement such as locomotion and in maintaining posture. Though this postural control is generally maintained as an unconscious reflex, the muscles responsible react to conscious control like non-postural muscles. An average adult male is made up of 42% of skeletal muscle and an average adult female is made up of 36% (as a percentage of body mass).
* Smooth muscle or "involuntary muscle" is found within the walls of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin (in which it controls erection of body hair). Unlike skeletal muscle, smooth muscle is not under conscious control.
* Cardiac muscle (myocardium), is also an "involuntary muscle" but is more akin in structure to skeletal muscle, and is found only in the heart.

Cardiac and skeletal muscles are "striated" in that they contain sarcomeres that are packed into highly regular arrangements of bundles; the myofibrils of smooth muscle cells are not arranged in sarcomeres and so are not striated. While the sarcomeres in skeletal muscles are arranged in regular, parallel bundles, cardiac muscle sarcomeres connect at branching, irregular angles (called intercalated discs). Striated muscle contracts and relaxes in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions.

Skeletal (voluntary) muscle is further divided into two broad types: slow twitch and fast twitch:

* Type I, slow twitch, or "red" muscle, is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and sustain aerobic activity using fats or carbohydrates as fuel. Slow twitch fibers contract for long periods of time but with little force.
* Type II, fast twitch muscle, has three major subtypes (IIa, IIx, and IIb) that vary in both contractile speed and force generated. Fast twitch fibers contract quickly and powerfully but fatigue very rapidly, sustaining only short, anaerobic bursts of activity before muscle contraction becomes painful. They contribute most to muscle strength and have greater potential for increase in mass.  Type IIb is anaerobic, glycolytic, "white" muscle that is least dense in mitochondria and myoglobin. In small animals (e.g., rodents) this is the major fast muscle type, explaining the pale color of their flesh.

The density of mammalian skeletal muscle tissue is about 1.06 kg/liter. This can be contrasted with the density of adipose tissue (fat), which is 0.9196 kg/liter. This makes muscle tissue approximately 15% denser than fat tissue.

Histogenesis 

All muscles are derived from paraxial mesoderm. The paraxial mesoderm is divided along the embryo's length into somites, corresponding to the segmentation of the body (most obviously seen in the vertebral column. Each somite has 3 divisions, sclerotome (which forms vertebrae), dermatome (which forms skin), and myotome (which forms muscle). The myotome is divided into two sections, the epimere and hypomere, which form epaxial and hypaxial muscles, respectively. The only epaxial muscles in humans are the erector spinae and small intervertebral muscles, and are innervated by the dorsal rami of the spinal nerves. All other muscles, including those of the limbs are hypaxial, and inervated by the ventral rami of the spinal nerves.

During development, myoblasts (muscle progenitor cells) either remain in the somite to form muscles associated with the vertebral column or migrate out into the body to form all other muscles. Myoblast migration is preceded by the formation of connective tissue frameworks, usually formed from the somatic lateral plate mesoderm. Myoblasts follow chemical signals to the appropriate locations, where they fuse into elongate skeletal muscle cells.

Microanatomy 

Skeletal muscles are sheathed by a tough layer of connective tissue called the epimysium. The epimysium anchors muscle tissue to tendons at each end, where the epimysium becomes thicker and collagenous. It also protects muscles from friction against other muscles and bones. Within the epimysium are multiple bundles called fascicles, each of which contains 10 to 100 or more muscle fibers collectively sheathed by a perimysium. Besides surrounding each fascicle, the perimysium is a pathway for nerves and the flow of blood within the muscle. The threadlike muscle fibers are the individual muscle cells (myocytes), and each cell is encased within its own endomysium of collagen fibers. Thus, the overall muscle consists of fibers (cells) that are bundled into fascicles, which are themselves grouped together to form muscles.  At each level of bundling, a collagenous membrane surrounds the bundle, and these membranes support muscle function both by resisting passive stretching of the tissue and by distributing forces applied to the muscle. Scattered throughout the muscles are muscle spindles that provide sensory feedback information to the central nervous system. (This grouping structure is analogous to the  organization of nerves which uses epineurium, perineurium, and endoneurium).

This same bundles-within-bundles structure is replicated within the muscle cells. Within the cells of the muscle are myofibrils, which themselves are bundles of protein filaments.  The term "myofibril" should not be confused with "myofiber", which is a simply another name for a muscle cell. Myofibrils are complex strands of several kinds of protein filaments organized together into repeating units called sarcomeres. The striated appearance of both skeletal and cardiac muscle results from the regular pattern of sarcomeres within their cells. Although both of these types of muscle contain sarcomeres, the fibers in cardiac muscle are typically branched to form a network. Cardiac muscle fibers are interconnected by intercalated discs, giving that tissue the appearance of a syncytium.

The filaments in a sarcomere are composed of actin and myosin.

Gross anatomy 

The gross anatomy of a muscle is the most important indicator of its role in the body.  There is an important distinction seen between pennate muscles and other muscles. In most muscles, all the fibers are oriented in the same direction, running in a line from the origin to the insertion. However, In pennate muscles, the individual fibers are oriented at an angle relative to the line of action, attaching to the origin and insertion tendons at each end. Because the contracting fibers are pulling at an angle to the overall action of the muscle, the change in length is smaller, but this same orientation allows for more fibers (thus more force) in a muscle of a given size. Pennate muscles are usually found where their length change is less important than maximum force, such as the rectus femoris.

Skeletal muscle is arranged in discrete muscles, an example of which is the biceps brachii (biceps). The tough, fibrous epimysium of skeletal muscle is both connected to and continuous with the tendons.  In turn, the tendons connect to the periosteum layer surrounding the bones, permitting the transfer of force from the muscles to the skeleton.  Together, these fibrous layers, along with tendons and ligaments, constitute the deep fascia of the body.

Muscular system 

The muscular system consists of all the muscles present in a single body.  There are approximately 650 skeletal muscles in the human body, but an exact number is difficult to define. The difficulty lies partly in the fact that different sources group the muscles differently and partly in that some muscles, such as palmaris longus, are not always present.

A muscular slip is a narrow length of muscle that acts to augment a larger muscle or muscles.

The muscular system is one component of the musculoskeletal system, which includes not only the muscles but also the bones, joints, tendons, and other structures that permit movement.

Physiology 

The three types of muscle (skeletal, cardiac and smooth) have significant differences. However, all three use the movement of actin against myosin to create contraction. In skeletal muscle, contraction is stimulated by electrical impulses transmitted by the nerves, the  motoneurons (motor nerves) in particular. Cardiac and smooth muscle contractions are stimulated by internal pacemaker cells which regularly contract, and propagate contractions to other muscle cells they are in contact with. All skeletal muscle and many smooth muscle contractions are facilitated by the neurotransmitter acetylcholine.

Function 

The action a muscle generates is determined by the origin and insertion locations. The cross-sectional area of a muscle (rather than volume or length) determines the amount of force it can generate by defining the number of sarcomeres which can operate in parallel. The amount of force applied to the external environment is determined by lever mechanics, specifically the ratio of in-lever to out-lever. For example, moving the insertion point of the biceps more distally on the radius (farther from the joint of rotation) would increase the force generated during flexion (and, as a result, the maximum weight lifted in this movement), but decrease the maximum speed of flexion. Moving the insertion point proximally (closer to the joint of rotation) would result in decreased force but increased velocity. This can be most easily seen by comparing the limb of a mole to a horse - in the former, the insertion point is positioned to maximize force (for digging), while in the latter, the insertion point is positioned to maximize speed (for running).

Energy consumption 

Muscular activity accounts for much of the body's energy consumption. All muscle cells produce adenosine triphosphate (ATP) molecules which are used to power the movement of the myosin heads. Muscles have a short-term store of energy in the form of creatine phosphate which is generated from ATP and can regenerate ATP when needed with creatine kinase. Muscles also keep a storage form of glucose in the form of glycogen. Glycogen can be rapidly converted to glucose when energy is required for sustained, powerful contractions. Within the voluntary skeletal muscles, the glucose molecule can be metabolized anaerobically in a process called glycolysis which produces two ATP and two lactic acid molecules in the process (note that in aerobic conditions, lactate is not formed; instead pyruvate is formed and transmitted through the citric acid cycle). Muscle cells also contain globules of fat, which are used for energy during aerobic exercise. The aerobic energy systems take longer to produce the ATP and reach peak efficiency, and requires many more biochemical steps, but produces significantly more ATP than anaerobic glycolysis. Cardiac muscle on the other hand, can readily consume any of the three macronutrients (protein, glucose and fat) aerobically without a 'warm up' period and always extracts the maximum ATP yield from any molecule involved. The heart, liver and red blood cells will also consume lactic acid produced and excreted by skeletal muscles during exercise.

At rest, skeletal muscle consumes 54.4 kJ/kg (13.0 kcal/kg) per day. This is larger than adipose tissue (fat) at 18.8 kJ/kg (4.5 kcal/kg), and bone at 9.6 kJ/kg (2.3 kcal/kg).

Nervous control 

Efferent leg 

The efferent leg of the peripheral nervous system is responsible for conveying commands to the muscles and glands, and is ultimately responsible for voluntary movement.  Nerves move muscles in response to voluntary and autonomic (involuntary) signals from the brain. Deep muscles, superficial muscles, muscles of the face and internal muscles all correspond with dedicated regions in the primary motor cortex of the brain, directly anterior to the central sulcus that divides the frontal and parietal lobes.

In addition, muscles react to reflexive nerve stimuli that do not always send signals all the way to the brain. In this case, the signal from the afferent fiber does not reach the brain, but produces the reflexive movement by direct connections with the efferent nerves in the spine. However, the majority of muscle activity is volitional, and the result of complex interactions between various areas of the brain.

Nerves that control skeletal muscles in mammals correspond with neuron groups along the primary motor cortex of the brain's cerebral cortex. Commands are routed though the basal ganglia and are modified by input from the cerebellum before being relayed through the pyramidal tract to the spinal cord and from there to the motor end plate at the muscles. Along the way, feedback, such as that of the extrapyramidal system contribute signals to influence muscle tone and response.

Deeper muscles such as those involved in posture often are controlled from nuclei in the brain stem and basal ganglia.

Afferent leg 

The afferent leg of the peripheral nervous system is responsible for conveying sensory information to the brain, primarily from the sense organs like the skin. In the muscles, the muscle spindles convey information about the degree of muscle length and stretch to the central nervous system to assist in maintaining posture and joint position. The sense of where our bodies are in space is called proprioception, the perception of body awareness. More easily demonstrated than explained, proprioception is the "unconscious" awareness of where the various regions of the body are located at any one time. This can be demonstrated by anyone closing their eyes and waving their hand around. Assuming proper proprioceptive function, at no time will the person lose awareness of where the hand actually is, even though it is not being detected by any of the other senses.

Several areas in the brain coordinate movement and position with the feedback information gained from proprioception. The cerebellum and red nucleus in particular continuously sample position against movement and make minor corrections to assure smooth motion.

Efficiency 

The efficiency of human muscle has been measured (in the context of rowing and cycling) at 18% to 26%. The efficiency is defined as the ratio of mechanical work output to the total metabolic cost, as can be calculated from oxygen consumption. This low efficiency is the result of about 40% efficiency of generating ATP from food energy, losses in converting energy from ATP into mechanical work inside the muscle, and mechanical losses inside the body. The latter two losses are dependent on the type of exercise and the type of muscle fibers being used (fast-twitch or slow-twitch). For an overall efficiency of 20 percent, one watt of mechanical power is equivalent to 4.3 kcal per hour. For example, one manufacturer of rowing equipment calibrates its rowing ergometer to count burned calories as equal to four times the actual mechanical work, plus 300 kcal per hour, this amounts to about 20 percent efficiency at 250 watts of mechanical output.  The mechanical energy output of a cyclic contraction can depend upon many factors, including activation timing, muscle strain trajectory, and rates of force rise & decay.  These can be synthesized experimentally using work loop analysis.

Strength 

A display of "strength" (e.g. lifting a weight) is a result of three factors that overlap: physiological strength (muscle size, cross sectional area, available crossbridging, responses to training), neurological strength (how strong or weak is the signal that tells the muscle to contract), and mechanical strength (muscle's force angle on the lever, moment arm length, joint capabilities).

Physiological strength 

Vertebrate muscle typically produces approximately  of force per square centimeter of muscle cross-sectional area when isometric and at optimal length.   Some invertebrate muscles, such as in crab claws, have much longer sarcomeres than vertebrates, resulting in many more sites for actin and myosin to bind and thus much greater force per square centimeter at the cost of much slower speed.  The force generated by a contraction can be measured non-invasively using either mechanomyography or phonomyography, be measured in vivo using tendon strain (if a prominent tendon is present), or be measured directly using more invasive methods.

The strength of any given muscle, in terms of force exerted on the skeleton, depends upon length, shortening speed, cross sectional area, pennation, sarcomere length, myosin isoforms, and neural activation of motor units.  Significant reductions in muscle strength can indicate underlying pathology, with the chart at right used as a guide.

The "strongest" human muscle 

Since three factors affect muscular strength simultaneously and muscles never work individually, it is misleading to compare strength in individual muscles, and state that one is the "strongest". But below are several muscles whose strength is noteworthy for different reasons.

* In ordinary parlance, muscular "strength" usually refers to the ability to exert a force on an external object—for example, lifting a weight. By this definition, the masseter or jaw muscle is the strongest. The 1992 Guinness Book of Records records the achievement of a bite strength of 4337 N for 2 seconds. What distinguishes the masseter is not anything special about the muscle itself, but its advantage in working against a much shorter lever arm than other muscles.
* If "strength" refers to the force exerted by the muscle itself, e.g., on the place where it inserts into a bone, then the strongest muscles are those with the largest cross-sectional area. This is because the tension exerted by an individual skeletal muscle fiber does not vary much. Each fiber can exert a force on the order of 0.3 micronewton. By this definition, the strongest muscle of the body is usually said to be the quadriceps femoris or the gluteus maximus.
* Because muscle strength is determined by cross-sectional area, a shorter muscle will be stronger "pound for pound" (i.e., by weight) than a longer muscle of the same cross-sectional area. The myometrial layer of the uterus may be the strongest muscle by weight in the female human body. At the time when an infant is delivered, the entire human uterus weighs about 1.1 kg (40 oz). During childbirth, the uterus exerts 100 to 400 N (25 to 100 lbf) of downward force with each contraction.
* The external muscles of the eye are conspicuously large and strong in relation to the small size and weight of the eyeball. It is frequently said that they are "the strongest muscles for the job they have to do" and are sometimes claimed to be "100 times stronger than they need to be." However, eye movements (particularly saccades used on facial scanning and reading) do require high speed movements, and eye muscles are exercised nightly during rapid eye movement sleep.
* The statement that "the tongue is the strongest muscle in the body" appears frequently in lists of surprising facts, but it is difficult to find any definition of "strength" that would make this statement true. Note that the tongue consists of eight muscles, not one.
* The heart has a claim to being the muscle that performs the largest quantity of physical work in the course of a lifetime. Estimates of the power output of the human heart range from 1 to 5 watts. This is much less than the maximum power output of other muscles; for example, the quadriceps can produce over 100 watts, but only for a few minutes. The heart does its work continuously over an entire lifetime without pause, and thus does "outwork" other muscles. An output of one watt continuously for eighty years yields a total work output of two and a half gigajoules.

Health 

Humans are genetically predisposed with a larger percentage of one type of muscle group over another. An individual born with a greater percentage of Type I muscle fibers would theoretically be more suited to endurance events, such as triathlons, distance running, and long cycling events, whereas a human born with a greater percentage of Type II muscle fibers would be more likely to excel at sprinting events such as 100 meter dash. 

Exercise 

Exercise is often recommended as a means of improving motor skills, fitness, muscle and bone strength, and joint function. Exercise has several effects upon muscles, connective tissue, bone, and the nerves that stimulate the muscles.  One such effect is muscle hypertrophy, an increase in size.  This is used in bodybuilding.

Various exercises require a predominance of certain muscle fiber utilization over another. Aerobic exercise involves long, low levels of exertion in which the muscles are used at well below their maximal contraction strength for long periods of time (the most classic example being the marathon). Aerobic events, which rely primarily on the aerobic (with oxygen) system, use a higher percentage of Type I (or slow-twitch) muscle fibers, consume a mixture of fat, protein and carbohydrates for energy, consume large amounts of oxygen and produce little lactic acid. Anaerobic exercise involves short bursts of higher intensity contractions at a much greater percentage of their maximum contraction strength. Examples of anaerobic exercise include sprinting and weight lifting. The anaerobic energy delivery system uses predominantly Type II or fast-twitch muscle fibers, relies mainly on ATP or glucose for fuel, consumes relatively little oxygen, protein and fat, produces large amounts of lactic acid and can not be sustained for as long a period as aerobic exercise. Many exercises are partially aerobic and partially anaerobic; for example, soccer and rock climbing involve a combination of both.

The presence of lactic acid has an inhibitory effect on ATP generation within the muscle; though not producing fatigue, it can inhibit or even stop performance if the intracellular concentration becomes too high. However, long-term training causes neovascularization within the muscle, increasing the ability to move waste products out of the muscles and maintain contraction. Once moved out of muscles with high concentrations within the sarcomere, lactic acid can be used by other muscles or body tissues as a source of energy, or transported to the liver where it is converted back to pyruvate. In addition to increasing the level of lactic acid, strenuous exercise causes the loss of potassium ions in muscle and causing an increase in potassium ion concentrations close to the muscle fibres, in the interstitium. Acidification by lactic acid may allow recovery of force so that acidosis may protect against fatigue rather than being a cause of fatigue.

Delayed onset muscle soreness is pain or discomfort that may be felt one to three days after exercising and generally subsides two to three days later. Once thought to be caused by lactic acid build-up, a more recent theory is that it is caused by tiny tears in the muscle fibers caused by eccentric contraction, or unaccustomed training levels. Since lactic acid disperses fairly rapidly, it could not explain pain experienced days after exercise.

Hypertrophy 

Independent of strength and performance measures, muscles can be induced to grow larger by a number of factors, including hormone signaling, developmental factors, strength training, and disease.  Contrary to popular belief, the number of muscle fibres cannot be increased through exercise.  Instead, muscles grow larger through a combination of muscle cell growth as new protein filaments are added along with additional mass provided by undifferentiated satellite cells alongside the existing muscle cells.

Biological factors such as age and hormone levels can affect muscle hypertrophy. During puberty in males, hypertrophy occurs at an accelerated rate as the levels of growth-stimulating hormones produced by the body increase. Natural hypertrophy normally stops at full growth in the late teens. As testosterone is one of the body's major growth hormones, on average, men find hypertrophy much easier to achieve than women. Taking additional testosterone or other anabolic steroids will increase muscular hypertrophy.

Muscular, spinal and neural factors all affect muscle building.  Sometimes a person may notice an increase in strength in a given muscle even though only its opposite has been subject to exercise, such as when a bodybuilder finds her left biceps stronger after completing a regimen focusing only on the right biceps.  This phenomenon is called cross education.

Atrophy 

Inactivity and starvation in mammals lead to atrophy of skeletal muscle, a decrease in muscle mass that may be accompanied by a smaller number and size of the muscle cells as well as lower protein content. Muscle atrophy may also result from the natural aging process or from disease.

In humans, prolonged periods of immobilization, as in the cases of bed rest or astronauts flying in space, are known to result in muscle weakening and atrophy. Atrophy is of particular interest to the manned spaceflight community, because the weightlessness experienced in spaceflight results is a loss of as much as 30% of mass in some muscles.  Such consequences are also noted in small hibernating mammals like the golden-mantled ground squirrels and brown bats.

During aging, there is a gradual decrease in the ability to maintain skeletal muscle function and mass, known as sarcopenia. The exact cause of sarcopenia is unknown, but it may be due to a combination of the gradual failure in the "satellite cells" that help to regenerate skeletal muscle fibers, and a decrease in sensitivity to or the availability of critical secreted growth factors that are necessary to maintain muscle mass and satellite cell survival. Sarcopenia is a normal aspect of aging, and is not actually a disease state yet can be linked to many injuries in the elderly population as well as decreasing quality of life.

There are also many diseases and conditions that cause muscle atrophy. Examples include cancer and AIDS, which induce a body wasting syndrome called cachexia. Other syndromes or conditions that can induce skeletal muscle atrophy are congestive heart disease and some diseases of the liver.

Disease 

Neuromuscular diseases are those that affect the muscles and/or their nervous control. In general, problems with nervous control can cause spasticity or paralysis, depending on the location and nature of the problem. A large proportion of neurological disorders, ranging from cerebrovascular accident (stroke) and Parkinson's disease to Creutzfeldt-Jakob disease, can lead to problems with movement or motor coordination.

Symptoms of muscle diseases may include weakness, spasticity, myoclonus and myalgia. Diagnostic procedures that may reveal muscular disorders include testing creatine kinase levels in the blood and electromyography (measuring electrical activity in muscles). In some cases, muscle biopsy may be done to identify a myopathy, as well as genetic testing to identify DNA abnormalities associated with specific myopathies and dystrophies.

A non-invasive elastography technique that measures muscle noise is undergoing experimentation to provide a way of monitoring neuromuscular disease. The sound produced by a muscle comes from the shortening of actomyosin filaments along the axis of the muscle. During contraction, the muscle shortens along its longitudinal axis and expands across the transverse axis, producing vibrations at the surface.

Evolution 

The evolutionary origin of muscle cells in metazoans is a highly debated topic. In one line of thought scientists have believed that muscle cells evolved once and thus all animals with muscles cells have a single common ancestor. In the other line of thought, scientists believe muscles cells evolved more than once and any morphological or structural similarities are due to convergent evolution and genes that predate the evolution of muscle and even the mesoderm - the germ layer from which many scientists believe true muscle cells derive.

Schmid and Seipel argue that the origin of muscle cells is a monophyletic trait that occurred concurrently with the development of the digestive and nervous systems of all animals and that this origin can be traced to a single metazoan ancestor in which muscle cells are present.  They argue that molecular and morphological similarities between the muscles cells in cnidaria and ctenophora are similar enough to those of bilaterians that there would be one ancestor in metazoans from which muscle cells derive. In this case, Schmid and Seipel argue that the last common ancestor of bilateria, ctenophora, and cnidaria was a triploblast or an organism with three germ layers and that diploblasty, meaning an organism with two germ layers, evolved secondarily due to their observation of the lack of mesoderm or muscle found in most cnidarians and ctenophores. By comparing the morphology of cnidarians and ctenophores to bilaterians, Schmid and Seipel were able to conclude that there were myoblast-like structures in the tentacles and gut of some species of cnidarians and in the tentacles of ctenophores. Since this is a structure unique to muscle cells, these scientists determined based on the data collected by their peers that this is a marker for striated muscles similar to that observed in bilaterians. The authors also remark that the muscle cells found in cnidarians and ctenophores are often contests due to the origin of these muscle cells being the ectoderm rather than the mesoderm or mesendoderm. The origin of true muscles cells is argued by others to be the endoderm portion of the mesoderm and the endoderm. However, Schmid and Seipel counter this skepticism about whether or not the muscle cells found in ctenophores and cnidarians are true muscle cells by considering that cnidarians develop through a medusa stage and polyp stage. They observe that in the hydrozoan medusa stage there is a layer of cells that separate from the distal side of the ectoderm to form the striated muscle cells in a way that seems similar to that of the mesoderm and call this third separated layer of cells the ectocodon. They also argue that not all muscle cells are derived from the mesendoderm in bilaterians with key examples being that in both the eye muscles of vertebrates and the muscles of spiralians these cells derive from the ectodermal mesoderm rather than the endodermal mesoderm. Furthermore, Schmid and Seipel argue that since myogenesis does occur in cnidarians with the help of molecular regulatory elements found in the specification of muscles cells in bilaterians that there is evidence for a single origin for striated muscle.

In contrast to this argument for a single origin of muscle cells, Steinmetz et al. argue that molecular markers such as the myosin II protein used to determine this single origin of striated muscle actually predate the formation of muscle cells.  This author uses an example of the contractile elements present in the porifera or sponges that do truly lack this striated muscle containing this protein. Furthermore, Steinmetz et al. present evidence for a polyphyletic origin of striated muscle cell development through their analysis of morphological and molecular markers that are present in bilaterians and absent in cnidarians, ctenophores, and bilaterians. Steimetz et al. showed that the traditional morphological and regulatory markers such as actin, the ability to couple myosin side chains phosphorylation to higher concentrations of the positive concentrations of calcium, and other MyHC elements are present in all metazoans not just the organisms that have been shown to have muscle cells. Thus, the usage of any of these structural or regulatory elements in determining whether or not the muscle cells of the cnidarians and ctenophores are similar enough to the muscle cells of the bilaterians to confirm a single lineage is questionable according to Steinmetz et al. Furthermore, Steinmetz et al. explain that the orthologues of the MyHc genes that have been used to hypothesize the origin of striated muscle occurred through a gene duplication event that predates the first true muscle cells (meaning striated muscle), and they show that the MyHc genes are present in the sponges that have contractile elements but no true muscle cells. Furthermore, Steinmetz et all showed that the localization of this duplicated set of genes that serve both the function of facilitating the formation of striated muscle genes and cell regulation and movement genes were already separated into striated myhc and non-muscle myhc. This separation of the duplicated set of genes is shown through the localization of the striated myhc to the contractile vacuole in sponges while the non-muscle myhc was more diffusely expressed during developmental cell shape and change. Steinmetz et al. found a similar pattern of localization in cnidarians with except with the cnidarian N. vectensis having this striated muscle marker present in the smooth muscle of the digestive track. Thus, Steinmetz et al. argue that the pleisiomorphic trait of the separated orthologues of myhc cannot be used to determine the monophylogeny of muscle, and additionally argue that the presence of a striated muscle marker in the smooth muscle of this cnidarian shows a fundamentally different mechanism of muscle cell development and structure in cnidarians.

Steinmetz et al. continue to argue for multiple origins of striated muscle in the metazoans by explaining that a key set of genes used to form the troponin complex for muscle regulation and formation in bilaterians is missing from the cnidarians and ctenophores, and of 47 structural and regulatory proteins observed, Steinmetz et al. were not able to find even on unique striated muscle cell protein that was expressed in both cnidarians and bilaterians. Furthermore, the Z-disc seemed to have evolved differently even within bilaterians and there is a great deal diversity of proteins developed even between this clade, showing a large degree of radiation for muscle cells. Through this divergence of the Z-disc, Steimetz et al. argue that there are only four common protein components that were present in all bilaterians muscle ancestors and that of these for necessary Z-disc components only an actin protein that they have already argued is an uninformative marker through its pleisiomorphic state is present in cnidarians. Through further molecular marker testing, Steinmetz et al. observe that non-bilaterians lack many regulatory and structural components necessary for bilaterians muscle formation and do not find any unique set of proteins to both bilaterians and cnidarians and ctenophores that are not present in earlier, more primitive animals such as the sponges and amoebozoans. Through this analysis the authors conclude that due to the lack of elements that bilaterians muscles are dependent on for structure and usage, nonbilaterian muscles must be of a different origin with a different set regulatory and structural proteins.

In another take on the argument, Andrikou and Arnone use the newly available data on gene regulatory networks to look at how the hierarchy of genes and morphogens and other mechanism of tissue specification diverge and are similar among early deuterostomes and protostomes. By understanding not only what genes are present in all bilaterians but also the time and place of deployment of these genes, Andrikou and Arnone discuss a deeper understanding of the evolution of myogenesis.

In their paper Andrikou and Arnone argue that to truly understand the evolution of muscle cells the function of transcriptional regulators must be understood in the context of other external and internal interactions.  Through their analysis, Andrikou and Arnone found that there were conserved orthologues of the gene regulatory network in both invertebrate bilaterians and in cnidarians. They argue that having this common, general regulatory circuit allowed for a high degree of divergence from a single well functioning network. Andrikou and Arnone found that the orthologues of genes found in vertebrates had been changed through different types of structural mutations in the invertebrate deuterostomes and protostomes, and they argue that these structural changes in the genes allowed for a large divergence of muscle function and muscle formation in these species. Andrikou and Arnone were able to recognize not only any difference due to mutation in the genes found in vertebrates and invertebrates but also the integration of species specific genes that could also cause divergence from the original gene regulatory network function. Thus, although a common muscle patterning system has been determined, they argue that this could be due to a more ancestral gene regulatory network being coopted several times across lineages with additional genes and mutations causing very divergent development of muscles. Thus it seems that myogenic patterning framework may be an ancestral trait. However, Andrikou and Arnone explain that the basic muscle patterning structure must also be considered in combination with the cis regulatory elements present at different times during development. In contrast with the high level of gene family apparatuses structure, Andrikou and Arnone found that the cis regulatory elements were not well conserved both in time and place in the network which could show a large degree of divergence in the formation of muscle cells. Through this analysis, it seems that the myogenic GRN is an ancestral GRN with actual changes in myogenic function and structure possibly being linked to later coopts of genes at different times and places.

Evolutionarily, specialized forms of skeletal and cardiac muscles predated the divergence of the vertebrate/arthropod evolutionary line. This indicates that these types of muscle developed in a common ancestor sometime before 700 million years ago (mya). Vertebrate smooth muscle was found to have evolved independently from the skeletal and cardiac muscle types.
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Gluteus maximus
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