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[[Image:Skeletal muscle.jpg|right|300px|thumbnail|A top-down view of skeletal muscle]]

'''Muscle''' (from [[Latin]] ''musculus'' "little mouse", referring to muscles like the biceps which pop up as though a mouse were scurrying about under the skin [http://www.askoxford.com/concise_oed/muscle]) is [[Muscle contraction|contractile]] [[tissue (biology)|tissue]] of the body and is derived from the [[Germ layer|mesodermal layer]] of embryonic germ cells. Its function is to produce [[force]] and cause [[motion (physics)|motion]], either locomotion or movement within [[internal organs]]. Much of muscle contraction occurs without [[conscious]] thought and is necessary for survival, like the contraction of the [[heart]] or [[peristalsis]], which pushes food through the [[digestive system]]. Voluntary muscle contraction is used to move the body and can be finely controlled, such as movements of the finger or gross movements like the [[quadriceps muscle]] of the [[thigh]].

== Types ==
[[Image:Illu muscle tissues.jpg|thumb|350px|Types of muscle]]
{{expert}}
There are three types of muscle:
* [[Skeletal muscle]] or "voluntary muscle" is anchored by [[tendons]] to the [[bone]] and is used to affect [[skeleton|skeletal]] movement such as [[locomotion]].
* [[Smooth muscle]] or "involuntary muscle" is found within the walls of organs and structures such as the [[esophagus]], [[stomach]], [[intestine]]s, [[bronchi]], [[uterus]], [[ureter]]s, [[bladder]], and [[blood vessel]]s, and unlike skeletal muscle, smooth muscle is not under conscious control.
* [[Cardiac muscle]] is also an "involuntary muscle" but it's a specialized kind of muscle found only within the [[heart]].

Cardiac and skeletal muscle are "striated" in that they contain [[sarcomere]] 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. Striated muscle contracts and relaxes in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions.

Skeletal muscle is further divided into several subtypes:
* Type I, slow oxidative, ''slow twitch'', or "red" muscle is dense with [[capillary|capillaries]] and is rich in [[mitochondria]] and [[myoglobin]], giving the muscle tissue its characteristic red color. It can carry more [[oxygen]] and sustain [[aerobic metabolism|aerobic]] activity.
* Type II, ''fast twitch'', muscle has three major kinds that are, in order of increasing contractile speed:<ref>{{cite journal
| last = Larsson | first = L | coauthors = Edstrom L, Lindegren B, Gorza L, Schiaffino S
| year = 1991 | month = July
| title = MHC composition and enzyme-histochemical and physiological properties of a novel fast-twitch motor unit type
| journal = The American Journal of Physiology | volume = 261 | issue = 1 pt 1 | pages = C93&ndash;101
| id = PMID 1858863 | url = http://ajpcell.physiology.org/cgi/reprint/261/1/C93
| accessdate = 11 June 2006
}}</ref>
**a) Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red.
**b) Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin. This is the fastest muscle type in humans. It can contract more quickly and with a greater amount of force than oxidative muscle, but can sustain only short, [[Fermentation (biochemistry)|anaerobic]] bursts of activity before muscle contraction becomes painful (often incorrectly attributed to a build-up of [[lactic acid]]). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB.<ref>{{cite journal
| last = Smerdu | first = V | coauthors = Karsch-Mizrachi I, Campione M, Leinwand L, Schiaffino S
| year = 1994 | month = December
| title = Type IIx myosin heavy chain transcripts are expressed in type IIb fibers of human skeletal muscle
| journal = The American Journal of Physiology | volume = 267 | issue = 6 pt 1 | pages = C1723&ndash;1728
| id = PMID 7545970 | url = http://ajpcell.physiology.org/cgi/reprint/267/6/C1723
| accessdate = 11 June 2006
}} ''Note: Access to full text requires subscription; abstract freely available''</ref>
**c) Type IIb, which is anaerobic, [[glycolysis|glycolytic]], "white" muscle that is even less dense in mitochondria and myoglobin. In small animals like rodents or rabbits this is the major fast muscle type, explaining the pale color of their meat.

== Anatomy ==

Muscle is composed of muscle [[cell (biology)|cells]] (sometimes known as "[[muscle fiber]]s"). Within the cells are [[myofibril]]s; myofibrils contain [[sarcomere]]s, which are composed of [[actin]] and [[myosin]]. Individual muscle cells are lined with [[endomysium]]. Muscle cells are bound together by [[perimysium]] into bundles called [[fascicles]]; the bundles are then grouped together to form muscle, which is lined by [[epimysium]]. [[muscle spindle|Muscle spindles]] are distributed throughout the muscles and provide sensory feedback information to the [[central nervous system]].

[[Skeletal muscle]], which involves muscles from the skeletal tissue, is arranged in discrete groups, examples of which include the ''[[biceps brachii muscle|biceps brachii]]''. It is connected by [[tendon]]s to processes of the [[skeleton]]. In contrast, smooth muscle occurs at various scales in almost every organ, from the [[skin]] (in which it controls erection of [[body hair]]) to the [[blood vessel]]s and [[digestive tract]] (in which it controls the caliber of a [[Lumen (anatomy)|lumen]] and [[peristalsis]]).

There are approximately 650 skeletal muscles in the human body (see [[list of muscles of the human body]]). Contrary to popular belief, the number of muscle fibers cannot be increased through [[exercise]]; instead the muscle cells simply get bigger. It is however believed that myofibrils have a limited capacity for growth through [[hypertrophy]] and will split [[hyperplasia]] if subject to increased demand.

== Physiology ==
{{main|muscle contraction}}

The three (skeletal, cardiac and smooth) types of muscle have significant differences. However, all three use the movement of [[actin]] against [[myosin]] to create [[Muscle contraction|contraction]]. In skeletal muscle, contraction is stimulated by [[action potential|electrical impulses]] transmitted by the [[nerve]]s, the motor nerves and [[motoneuron]]s in particular. Cardiac and smooth muscle contractions are stimulated by internal pacemaker cells who regularly contract, and propogate contractions to other muscle cells they are in contact with. All skeletal muscle and many smooth muscle contractions are facilitated by the [[neurotransmitter]] [[acetylcholine]].

Muscular activity accounts for most of the body's [[energy]] consumption. Muscles store energy for their own use in the form of [[glycogen]], which represents about 1% of their mass. This can be rapidly converted to [[glucose]] when energy is required for sustained, powerful contractions. Within the voluntary skeletal muscles, the glucose molecule is split into two-three carbon molecules known as [[Lactic acid]], producing two [[Adenosine triphosphate]] (ATP) molecules in the process, which are then used to power the movement of the [[myosin]] heads. Muscle cells also contain globules of fat, which are used for energy during aerobic exercise. The aerobic energy systems takes longer to produce the ATP and reach peak efficiency, and requires many more biochemical steps, but produces significantly more ATP than a glucose molecule. Cardiac muscle on the other hand, can readily consume any of the three macronutrients (protein, glucose and fat) without a 'warm up' period and always extracts the maximum ATP yield from any molecule involved. The heart will also consume lactic acid produced and excreted by skeletal muscles during exercise.

== Nervous control ==
=== [[Efferent nerve fiber|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. [[Nerve]]s move muscles in response to [[somatic nervous system|voluntary]] and [[autonomic nervous system|autonomic]] (involuntary) signals from the [[brain]]. Deep muscles, superficial muscles, [[Template:Muscles of head|muscles of the face]] and internal muscles all correspond with dedicated regions in the primary motor cortex of the [[human brain|brain]], directly anterior to the central sulcus that divides the frontal and parietal lobes.

In addition, muscles react to [[reflex action|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 [[Spinal cord|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 [[mammal]]s 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 loops 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 nerve fiber|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 spindle|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. Sometimes known as [[muscle memory]], 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 [[nucleus ruber]] in particular continuously sample position against movement and make minor corrections to assure smooth motion.

== Role in health and disease ==
=== Exercise ===

[[Exercise]] is often recommended as a means of improving [[motor skill]]s, [[physical fitness|fitness]], muscle and bone strength, and joint function. Exercise has several effects upon muscles, [[connective tissue]] and [[bone]], and the nerves that stimulate the muscles.

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 [[Strength training| weight lifting]]. The anaerobic energy delivery system uses predominantly Type II muscle fibers, or (fast-twitch) muscle fibers, rely mainly on [[Adenosine triphosphate]] or glucose for fuel, consume relatively little oxygen, protein and fat, produces larger amounts of lactic acid and can not be sustained for as long a period as 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 weight lifting. People with high overall musculation and balanced muscle type percentage engage in sports such as [[Rugby]], or [[Boxing]], and often engage on other sports just to increase their performance on the former.

[[Delayed onset muscle soreness]] is the pain or discomfort often felt 24 to 76 hours after exercising and subsides generally within 2 to 3 days. Once thought to be caused by [[lactic acid]] buildup, a more recent theory is that it is caused by tiny tears in the muscle fibres caused by [[eccentric contraction]], or unaccustomed training levels. The reason for the demise of the lactic acid theory was that since lactic acid disperses fairly rapidly, it could not explain pain felt the next day{{citation needed}}.

=== Disease ===

{{main|Neuromuscular disease}}

There are many diseases and conditions which cause a decrease in muscle mass, known as [[atrophy]]. For example diseases such as cancer and AIDS induce a body wasting syndrome called "cachexia", which is notable for the severe [[muscle atrophy]] seen. Other syndromes or conditions which can induce skeletal [[muscle atrophy]] are congestive heart disease and liver disease.

During aging, there is a gradual decrease in the ability to maintain skeletal muscle function and mass. This condition is called "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.

In addition to the simple loss of muscle mass ([[atrophy]]), or the age-related decrease in muscle function (sarcopenia), there are other diseases which may be caused by structural defects in the muscle (the dystrophies), or by inflammatory reactions in the body directed against muscle (the myopathies).

Symptoms of muscle disease may include [[weakness]] or [[spasticity]]/rigidity, [[myoclonus]] (twitching) and [[myalgia]] (muscle pain). 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.

[[Neuromuscular disease]]s 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 disorder]]s leads to problems with movement, ranging from [[cerebrovascular accident]] (stroke) and [[Parkinson's disease]] to [[Creutzfeldt-Jakob disease]].

== The strongest human muscle ==

A display of "strength" (eg 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).

Since these three factors exist simultaneously, and muscles never work individually, it is unrealistic for us to believe that one could accurately compare strength in individual muscles, and crown one "strongest".
*All three factors must be assessed individually if we are to compare strength, and this is just not possible.
*One muscle can not be isolated for us to accuately measure its total display of individual "strength"

The following is one author's thoughts regarding different perspectives on the "strongest muscle".
Please keep in mind that the statements that follow offer important perspectives on "strength", but can not be absolutely accurate for reasons mentioned above.

'''Perspectives on Strength:'''

Depending on what definition of "strongest" is used, many different muscles in the human body can be characterized as being the "strongest."

In ordinary parlance, muscular "strength" usually refers to the ability to exert a [[force]] on an external object&mdash;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 975 [[lbf]] (4337 [[Newton|N]]) for two 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 at their belly. This is because the tension exerted by an individual skeletal (striated) [[muscle fiber]] does not vary much, either from muscle to muscle, or with length. 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|quadriceps femoris]] or the [[gluteus maximus]].

Again taking strength to mean only "force" (in the [[physicist]]'s sense, and as contrasted with "[[energy]]" or "[[Power (physics)|power]]"), then a shorter muscle will be stronger "pound for pound" (i.e., by [[mass|weight]]) than a longer muscle. The [[uterus]] may be the strongest muscle by weight in the human body. At the time when an [[infant]] is delivered, the human uterus weighs about 40 oz (1.1 kg). During childbirth, the uterus exerts 25 to 100 lbf (100 to 400 N) 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". Eye movements, however, probably do "need" to be exceptionally fast.

The unexplained 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 sixteen muscles, not one. The tongue may possibly be the strongest muscle at birth.

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 seventy years yields a total work output of 2 to 3 &times;10<sup>9</sup> [[joule]]s.

== Efficiency ==

The [[mechanical efficiency|efficiency]] of human muscle has been measured (in the context of [[Watercraft rowing|rowing]] and [[cycling]]) at 14% to 27%. The efficiency is defined as the ratio of [[mechanical work]] done to the total energy output (heat plus work). This can be improved by using muscles with machines, a human riding a [[bicycle]] is very efficient (more than a combustion powered vehicle), travelling great distances with relatively little fuel{{fact}}.

== Muscle evolution ==

Evolutionarily, specialized forms of [[Skeletal muscle|skeletal]] and [[cardiac muscle]]s predated the divergence of the [[vertebrate]]/[[arthropod]] evolutionary line<ref name="evolution">[http://www.umbi.umd.edu/~collins/myoinformatics/muscle-evolution.pdf Evolution of muscle fibers]</ref>. This indicates that these types of muscle developed in a common [[ancestor]] sometime before 700 million years ago (mya). Vertebrate smooth muscle (smooth muscle found in humans) was found to have evolved independently from the skeletal and cardiac muscles.

{{section-stub}}

== References ==

* Costill, David L and Wilmore, Jack H. (2004). ''Physiology of Sport and Exercise.'' Champaign, Illinois: Human Kinetics. ISBN 0-7360-4489-2.
* Phylogenetic Relationship of Muscle Tissues Deduced from Superimposition of Gene Trees, Satoshi OOta and Naruya Saitou, Mol. Biol. Evol. 16(6) 856-7, 1999
* [[Johnson George B.]] (2005) "Biology, Visualizing Life." Holt, Rinehart, and Winston. ISBN 0-03-016723-X

=== Notes ===
<References/>

== External links ==
*[http://hypertextbook.com/facts/2003/IradaMuslumova.shtml Physics factbook] (Heart output 1.3 to 5 watts, lifetime output 2 to 3 &times;10<sup>9</sup> joules)
*[http://www.dundee.ac.uk/medther/StrokeSSM/ClinExamNeuro.htm University of Dundee] article on performing neurological examinations (Quadriceps "strongest")
*[http://www.coachesinfo.com/category/rowing/77/ Muscle efficiency in rowing]
*[http://www.gssiweb.com/reflib/attachment.cfm?id=11 "Gatorade Sports Science Institute" on muscle efficiency in cyclists (PDF)]
*[http://www.gwc.maricopa.edu/class/bio201/muscle/mustut.htm Human Muscle Tutorial] (clear pictures of main human muscles and their latin names, good for orientation)
*[http://www.northland.cc.mn.us/biology/AP2Online/Fall2002/AP2PowerPoint/AP2Brainlecture_files/v3_document.htm MUSCLE STRUCTURE & FUNCTION]

== See also ==
{{Wiktionary}}
{{commonscat|muscles}}

*[[Bodybuilding]]
*[[List of muscles of the human body]]
*[[Myopathy]] (Pathology of muscle cells)
*[[Myotomy]]
*[[Rapid plant movement]]
*[[Atrophy]]
*[[Muscle atrophy]]
*[[Muscle tone]] (residual muscle tension)
*[[Skeletal Muscle]]
*[[Electroactive polymers]] (materials that behave like muscles, used in robotics research)
*[[Characteristics of muscle]]
{{Muscular system}}
{{Biological tissue}}

[[Category:Muscular system]]
[[Category:Tissues]]
[[Category:Exercise physiology]]

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Revision as of 07:28, 18 October 2006

this pasge dusnt help me with anything omg HELPPP IS DUE TOMOROO im hornii

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