For other uses of "muscle", see muscle (disambiguation).

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A top-down view of skeletal muscle

Muscle is a soft tissue of animals. Muscle cells contain protein filaments 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 cause motion. They are primarily responsible for maintenance of and changes in posture, locomotion of the organism itself, as well as movement of internal organs, such as the contraction of the heart and movement of food through the digestive system via peristalsis.

Muscle tissues are derived from the mesodermal layer of embryonic germ cells. They are classified as skeletal, cardiac, or smooth muscles. Cardiac and smooth muscle contraction occurs without conscious thought and is necessary for survival. Voluntary contraction of the skeletal muscles is used to move the body and can be finely controlled. Examples are movements of the eye, or gross movements like the quadriceps muscle of the thigh.

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 which 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.[1][2]

## Anatomy[edit]

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

### Types of tissue[edit]

Main article: Muscle tissue
Types of muscle (shown at different magnifications)

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).[3]
- 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 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 and are packed into highly regular arrangements of bundles; smooth muscle has neither. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects 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.[4] 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[5] and force generated.[4] 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.[6] This can be contrasted with the density of adipose tissue (fat), which is 0.9196?kg/liter.[7] This makes muscle tissue approximately 15% denser than fat tissue.

### Histogenesis[edit]

Main articles: Histogenesis and Mesoderm
A chicken embryo, showing the paraxial mesoderm on both sides of the neural fold. The anterior (forward) portion has begun to form somites (labeled "primitive segments").

All muscles derive from paraxial mesoderm.[8] 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.[8] Each somite has 3 divisions, sclerotome (which forms vertebrae), dermatome (which forms skin), and myotome (which forms muscle).[8] The myotome is divided into two sections, the epimere and hypomere, which form epaxial and hypaxial muscles, respectively.[8] Epaxial muscles in humans are only the erector spinae and small intervertebral muscles, and are innervated by the dorsal rami of the spinal nerves.[8] All other muscles, including limb muscles, are hypaxial muscles, formed from the hypomere, and inervated by the ventral rami of the spinal nerves.[8]

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.[8] Myoblast migration is preceded by the formation of connective tissue frameworks, usually formed from the somatic lateral plate mesoderm.[8] Myoblasts follow chemical signals to the appropriate locations, where they fuse into elongate skeletal muscle cells.[8]

### Microanatomy[edit]

Main articles: Myocyte and Sarcomere
Internal microanatomy of a muscle.

Skeletal muscles are sheathed by a tough layer of connective tissue called the epimysium.[9] 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.[9] The threadlike muscle fibers are the individual muscle cells (myocytes), and each cell is encased within its own endomysium of collagen fibers.[9] 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.[9] Scattered throughout the muscles are muscle spindles that provide sensory feedback information to the central nervous system.

Portion of a myofibril, showing sarcomere structure:
S = Sarcomere, unit of muscle function
A = A-band, region of myosin
I = I-band, region of actin
H = H-zone, region of just myosin
Z = Z-line, sarcomere boundary
M = M-line, sarcomere center

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,[10] giving that tissue the appearance of a syncytium.

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

### Gross anatomy[edit]

The gross anatomy of a muscle is the most important indicator of its role in the body. One particularly important aspect of gross anatomy of muscles is pennation or lack thereof. In most muscles, all the fibers are oriented in the same direction, running in a line from the origin to the insertion. 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. 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[edit]

Main articles: Muscular system and List of muscles of the human body

The muscular system consists of all the muscles present in a single body. There are approximately 650 skeletal muscles in the human body,[11] 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.

Muscles, anterior view (See Gray's muscle pictures for detailed pictures) | Muscles, posterior view (See Gray's muscle pictures for detailed pictures)

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[edit]

Main article: muscle contraction

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[edit]

Muscle fibers in relaxed (above) and contracted (below) positions

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[edit]

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 conserve 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).[12]

### Nervous control[edit]

For more details on this topic, see Nervous system.
Simplified schema of basic nervous system function. Signals are picked up by sensory receptors and sent to the spinal cord and brain via the afferent leg of the peripheral nervous system, whereupon processing occurs that results in signals sent back to the spinal cord and then out to motor neurons via the efferent leg.

#### Efferent leg[edit]

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[edit]

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[edit]

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 overal efficiency of 20 percent, one watt of mechanical power is equivalent to 4.3 kcal per hour. For example, a manufacturer of rowing equipment shows burned calories as four times the actual mechanical work, plus 300 kcal per hour,[13] which 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[edit]

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[edit]

Main article: Physical strength

Grading of muscle strength
Grade 0 | No contraction
Grade 1 | Trace of contraction, but no movement at the joint
Grade 2 | Movement at the joint with gravity eliminated
Grade 3 | Movement against gravity, but not against added resistance
Grade 4 | Movement against external resistance, but less than normal
Grade 5 | Normal strength

Vertebrate muscle typically produces approximately 25 N (5.6 lbf) 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. The force generated by a contraction can be measured using either mechanomyography or phonomyography.

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[edit]

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 4,337 N (975 lbf) 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.
- A shorter muscle will be stronger "pound for pound" (i.e., by weight) than a longer muscle. 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.[14] 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.[14]

## Health[edit]

Jogging is one form of aerobic exercise.

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 anaerobic events such as a 200 meter dash, or weightlifting.[citation needed]

### Exercise[edit]

Main article: Physical 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 involves 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.[15]

Delayed onset muscle soreness is pain or discomfort that may be felt one to three days after exercising and subsides generally within two to three days later. Once thought to be caused by lactic acid buildup, 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.[16]

### Hypertrophy[edit]

Main article: Muscle 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.[11] Muscle fibres have a limited capacity for growth through hypertrophy and some believe they split through hyperplasia if subject to increased demand.[citation needed]

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.[citation needed]

### Atrophy[edit]

Main article: Muscle atrophy
Prisoner of war exhibiting muscle loss as a result of malnutrition. Muscles may atrophy as a result of malnutrition, physical inactivity, aging, or disease.

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.[17] 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, since the weightlessness experienced in spaceflight results is a loss of as much as 30% of mass in some muscles.[18][19] Such consequences are also noted in small hibernating mammals like the golden-mantled ground squirrels and brown bats.[20]

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" which help to regenerate skeletal muscle fibers, and a decrease in sensitivity to or the availability of critical secreted growth factors which 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.[21]

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

### Disease[edit]

Main article: Neuromuscular disease
In muscular dystrophy, the affected tissues become disorganized and the concentration of dystrophin (green) is greatly reduced.

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.[22]

## Evolution[edit]

Muscles have evolved at least twice, once in cnidaria and once in bilateria; they seem to have evolved from contractile cells in sponge-grade organisms.[23]

Evolutionarily, specialized forms of skeletal and cardiac muscles predated the divergence of the vertebrate/arthropod evolutionary line.[24][dead link] 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 muscles.

## See also[edit]

- Electroactive polymers?materials that behave like muscles, used in robotics research
- Hand strength
- Meat
- Muscle memory
- Myotomy
- Preflexes
- Rohmert's law?pertaining to muscle fatigue

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21. ^ Roche, Alex F. (1994). "Sarcopenia: A critical review of its measurements and health-related significance in the middle-aged and elderly". American Journal of Human Biology 6: 33. doi:10.1002/ajhb.1310060107.
22. ^ Dum?, Belle (18 May 2007). "'Muscle noise' could reveal diseases' progression". NewScientist.com news service.
23. ^ Steinmetz, P. R. H.; Kraus, J. E. M.; Larroux, C.; Hammel, J. R. U.; Amon-Hassenzahl, A.; Houliston, E.; Wörheide, G.; Nickel, M.; Degnan, B. M.; Technau, U. (2012). "Independent evolution of striated muscles in cnidarians and bilaterians". Nature 487 (7406): 231–234. doi:10.1038/nature11180. PMC 3398149. PMID 22763458. edit
24. ^ "Evolution of muscle fibers" (PDF).

## External links[edit]

| Look up muscle in Wiktionary, the free dictionary.

- Media related to muscles at Wikimedia Commons
- University of Dundee article on performing neurological examinations (Quadriceps "strongest")
- Muscle efficiency in rowing
- Human Muscle Tutorial (clear pictures of main human muscles and their Latin names, good for orientation)
- Microscopic stains of skeletal and cardiac muscular fibers to show striations. Note the differences in myofibrilar arrangements.

Biological tissue
Animals | Epithelium Connective Muscular Nervous
Plants | Dermal Vascular Ground

Muscular system (TA A04.0, GA 4.361)
Topics | Muscular tissue Muscles of the human body
Types of muscles | Cardiac muscle Skeletal muscle Smooth muscle
Other | Fusiform muscle Unipennate muscle Bipennate muscle Origin Insertion Fascia (Superficial fascia, Deep fascia, Visceral fascia) Tendon/Aponeurosis Fascial compartment
M: MUS, DF+DRCT | anat (h/n, u, t/d, a/p, l)/phys/devp/hist | noco (m, s, c)/cong (d)/tumr, sysi/epon, injr | proc, drug (M1A/3)

Histology: Muscle tissue (TH H2.00.05, H3.3)
Smooth muscle | Calmodulin · Vascular smooth muscle
Striated muscle |
Skeletal muscle |
Costamere/ DAPC |
Membrane/ extracellular | DAP: Sarcoglycan (SGCA, SGCB, SGCD, SGCE, SGCG, SGCZ) · Dystroglycan Sarcospan · Laminin, alpha 2
Intracellular | Dystrophin · Dystrobrevin (A, B) · Syntrophin (A, B1, B2, G1, G2) · Syncoilin · Dysbindin · Synemin/desmuslin related: NOS1 · Caveolin 3
Sarcomere/ (a, i, and h bands; z and m lines) | Myofilament (thin filament/actin, thick filament/myosin, elastic filament/titin, nebulin) Tropomyosin Troponin (T, C, I)
Connective tissue | Epimysium · Fascicle · Perimysium · Endomysium · Connective tissue in skeletal muscle
General | Neuromuscular junction · Motor unit · Muscle spindle · Excitation-contraction coupling · Sliding filament mechanism
Cardiac muscle | Myocardium · Intercalated disc · Nebulette
Both |
Fiber | Muscle fiber (intrafusal, extrafusal) · Myofibril · Microfilament/Myofilament · Sarcomere
Cells | Myoblast/Myocyte · Myosatellite cell
Other | Desmin · Sarcoplasm · Sarcolemma (T-tubule) · Sarcoplasmic reticulum
Other/ ungrouped | Myotilin · Telethonin · Dysferlin · Fukutin · Fukutin-related protein
M: MUS, DF+DRCT | anat (h/n, u, t/d, a/p, l)/phys/devp/hist | noco (m, s, c)/cong (d)/tumr, sysi/epon, injr | proc, drug (M1A/3)

List of muscles of head and neck: the head (TA A04.1, GA 4.378)
Extraocular (CN III, IV, VI) | oblique (inferior, superior) · rectus (superior, inferior, medial, lateral) · levator palpebrae superioris (superior tarsal)
Mastication (CN V3) | masseter · temporalis (sphenomandibularis) · pterygoid (lateral, medial) fascia: Masseteric fascia · Temporal fascia · Deep portion: cementomaxillary tendon · Superficial portion: cementomandibular tendon
Facial (CN VII) |
Ear | auricular (anterior, superior, posterior) · temporoparietalis
Scalp/eyelid | occipitofrontalis (occipitalis, frontalis) · orbicularis oculi (depressor supercilii) · corrugator supercilii fascia: Galea aponeurotica
Nose | procerus · nasalis (dilatator naris) · depressor septi nasi · levator labii superioris alaeque nasi
Mouth | levator anguli oris · levator labii superioris · zygomaticus (major, minor) orbicularis oris · risorius · buccinator depressor anguli oris · depressor labii inferioris · mentalis
Palate/fauces (CN IX, X, XI) (except TVP=V3) | veli palatini (tensor, levator) · musculus uvulae · palatopharyngeus (to pharynx) · palatoglossus (to tongue)
Tongue (CN XII) | extrinsic (genioglossus, hyoglossus/chondroglossus, styloglossus, and palatoglossus [CN X]) · intrinsic (superior longitudinal, inferior longitudinal, transverse, vertical)
M: MUS, DF+DRCT | anat (h/n, u, t/d, a/p, l)/phys/devp/hist | noco (m, s, c)/cong (d)/tumr, sysi/epon, injr | proc, drug (M1A/3)

List of muscles of head and neck: the neck (TA A04.2, GA 4.387)
Cervical | CN VII (superficial, PC): platysma CN XI (deep): sternocleidomastoid C1–C6 (anterior): Prevertebral muscles: longus (capitis, colli) C3–C8 (lateral): scalene (anterior, medius, posterior)
Suboccipital | C1: rectus capitis posterior (major, minor) · obliquus capitis (inferior, superior) C1–C6 (anterior): Prevertebral muscles: rectus capitis (anterior, lateralis)
Suprahyoid | CN V3 (medial): mylohyoid · anterior belly of digastric CN VII (lateral): stylohyoid · posterior belly of digastric C1 (deep): geniohyoid
Infrahyoid/strap | C1: thyrohyoid C1–C3: sternohyoid · sternothyroid · omohyoid
Fasciae | Deep cervical fascia (Pretracheal fascia, Prevertebral fascia, Investing layer) · Carotid sheath · Alar fascia pharynx: Buccopharyngeal fascia · Pharyngobasilar fascia Palatine aponeurosis Pharyngeal raphe
Pharynx | pharyngeal constrictor (superior, middle, inferior) · longitudinal (stylopharyngeus, salpingopharyngeus)
Larynx | cricothyroid · cricoarytenoid (posterior, lateral) · arytenoid (oblique arytenoid/aryepiglottic, transverse arytenoid) · thyroarytenoid (vocal, thyroepiglottic)
M: MUS, DF+DRCT | anat (h/n, u, t/d, a/p, l)/phys/devp/hist | noco (m, s, c)/cong (d)/tumr, sysi/epon, injr | proc, drug (M1A/3)

List of muscles of thorax and back (TA A04.3–4, GA 4.397)
Back | splenius (capitis, cervicis) · erector spinae (iliocostalis, longissimus, spinalis) · latissimus dorsi transversospinales: (semispinalis dorsi, semispinalis cervicis, semispinalis capitis, multifidus, rotatores) · interspinales · intertransversarii vertebral column: trapezius · latissimus dorsi · rhomboid (major, minor) · levator scapulae fascia: thoracolumbar fascia
Thorax | intercostales (external, internal, innermost) · subcostales · transversus thoracis · levatores costarum · serratus posterior (inferior, superior) · diaphragm thoracic cavity: pectoralis major · pectoralis minor · subclavius · serratus anterior fascia: pectoral fascia · clavipectoral fascia
M: MUS, DF+DRCT | anat (h/n, u, t/d, a/p, l)/phys/devp/hist | noco (m, s, c)/cong (d)/tumr, sysi/epon, injr | proc, drug (M1A/3)

List of muscles of abdominopelvic cavity (TA A04.5, GA 4.408)
Abdomen/ wall |
Anterior/ lateral |
Muscle | Abdominal external oblique · Transversus abdominis/Conjoint tendon · Rectus sheath (rectus abdominis, pyramidalis) · Arcuate line · Tendinous intersection Cremaster · Abdominal internal oblique
Fascia | Fascia/abdominal fascia: panniculus adiposus (Fascia of Camper) · stratum membranosum (Fascia of Scarpa) · Transversalis fascia (Interfoveolar ligament) Linea alba · Linea semilunaris · Inguinal triangle Inguinal canal (Deep inguinal ring, Superficial inguinal ring, Intercrural fibers, Crura of superficial inguinal ring) Inguinal ligament (Pectineal ligament, Lacunar ligament, Reflected ligament)
Posterior |
Muscle | quadratus lumborum · psoas major/psoas minor · iliacus
Fascia | iliopsoas fascia (Iliac fascia/Iliopectineal arch)
Pelvis |
Muscle | levator ani (iliococcygeus, pubococcygeus, puborectalis) · coccygeus
Fascia | fascia/pelvic fascia visceral (Rectovaginal fascia, Rectoprostatic fascia) · parietal (Obturator fascia/Tendinous arch, Piriformis fascia) floor/diaphragm: Superior fascia of pelvic diaphragm (Pubovesical ligament, Puboprostatic ligament) · Inferior fascia of pelvic diaphragm Anococcygeal body
M: MUS, DF+DRCT | anat (h/n, u, t/d, a/p, l)/phys/devp/hist | noco (m, s, c)/cong (d)/tumr, sysi/epon, injr | proc, drug (M1A/3)

List of muscles of upper limbs (TA A04.6, GA 4.432)
Shoulder | deltoid · rotator cuff (supraspinatus, infraspinatus, teres minor, subscapularis) · teres major fascia: deltoid fascia · supraspinous fascia · infraspinous fascia
Arm (compartments) |
anterior | coracobrachialis · biceps brachii · brachialis
posterior | triceps brachii · anconeus · articularis cubiti
fascia | axillary sheath · axillary fascia · brachial fascia · intermuscular septa (lateral, medial)
other | spaces (quadrangular space, triangular space, triangular interval)
Forearm |
anterior | superficial: pronator teres · palmaris longus · flexor carpi radialis · flexor carpi ulnaris · flexor digitorum superficialis deep: pronator quadratus · flexor digitorum profundus · flexor pollicis longus
posterior | superficial: mobile wad (brachioradialis, extensor carpi radialis longus and brevis) · extensor digitorum · extensor digiti minimi · extensor carpi ulnaris deep: supinator · anatomical snuff box (abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus) · extensor indicis
fascia | bicipital aponeurosis · common tendons (extensor, flexor) · antebrachial fascia
other | cubital tunnel
Hand |
lateral volar | thenar (opponens pollicis, flexor pollicis brevis, abductor pollicis brevis) · adductor pollicis
medial volar | hypothenar (opponens digiti minimi, flexor digiti minimi brevis, abductor digiti minimi) · palmaris brevis
intermediate | lumbrical · interossei (dorsal, palmar)
fascia | posterior: extensor retinaculum · extensor expansion anterior: flexor retinaculum · palmar aponeurosis
M: MUS, DF+DRCT | anat (h/n, u, t/d, a/p, l)/phys/devp/hist | noco (m, s, c)/cong (d)/tumr, sysi/epon, injr | proc, drug (M1A/3)

List of muscles of lower limbs (TA A04.7, GA 4.465)
Iliac region / Iliopsoas | psoas major/psoas minor · iliacus
Buttocks | gluteals: (maximus, medius, minimus) · tensor fasciae latae lateral rotator group: quadratus femoris · inferior gemellus · obturator internus · superior gemellus · piriformis
Thigh / compartments |
Anterior | sartorius · quadriceps (rectus femoris, vastus lateralis, vastus intermedius, vastus medialis) · articularis genu
Posterior/hamstring | biceps femoris · semitendinosus · semimembranosus
Medial | pectineus · obturator externus · gracilis · adductor (longus, brevis, magnus, minimus)
Fascia | Femoral sheath (Femoral canal) · Femoral ring Adductor canal · Adductor hiatus fascia lata (Iliotibial tract, Lateral intermuscular septum of thigh, Medial intermuscular septum of thigh, Fascia cribrosa/Saphenous opening) · Muscular lacuna
Leg/ Crus/ compartments |
Anterior | tibialis anterior · extensor hallucis longus · extensor digitorum longus · fibularis tertius
Posterior | superficial · triceps surae (gastrocnemius, soleus, accessory soleus, Achilles tendon) · plantaris deep · tarsal tunnel (flexor hallucis longus, flexor digitorum longus, tibialis posterior) · popliteus
Lateral | fibularis muscles (longus, brevis)
Fascia | Pes anserinus crural fascia (Anterior crural intermuscular septum, Posterior crural intermuscular septum, Transverse intermuscular septum)
Foot |
Dorsal | extensor hallucis brevis · extensor digitorum brevis
Plantar | 1st layer (abductor hallucis, flexor digitorum brevis, abductor digiti minimi) · 2nd layer (quadratus plantae, lumbrical muscle) · 3rd layer (flexor hallucis brevis, adductor hallucis, flexor digiti minimi brevis) · 4th layer (dorsal interossei, plantar interossei)
Fascia | Plantar fascia retinacula (Fibular, Inferior extensor, Superior extensor, Flexor)
M: MUS, DF+DRCT | anat (h/n, u, t/d, a/p, l)/phys/devp/hist | noco (m, s, c)/cong (d)/tumr, sysi/epon, injr | proc, drug (M1A/3)