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Muscle

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	Topics Muscle Topic Home Muscle (from LatinFacts About Latin Latin is an ancient Indo-European language originally spoken in Latium, the region immediately surrounding Rome.... musculus, diminutive of mus "mouse") is contractileMuscle contraction A muscle contraction occurs when a muscle cell shortens.... tissue of the body and is derived from the mesodermal layerGerm layer A germ layer is a collection of cells, formed during animal embryogenesis.... of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to produce forceForce In physics, force is that which changes or tends to change the state of rest or motion of a body.... and cause motionMotion (physics) In physics, motion means a continuous change in the position of a body relative to a reference point, as measured by a parti... . Muscles can cause either locomotion of the organism itself or movement of internal organs. Cardiac and smooth muscle contraction occurs without consciousFacts About Consciousness Consciousness is a quality of the mind generally regarded to comprise qualities such as subjectivity, self-awareness, sentie... thought and is necessary for survival. Examples are the contraction of the heartHeart The heart is a hollow, muscular organ in vertebrates, responsible for pumping blood through the blood vessels by repeated, r... and peristalsisPeristalsis In the digestive tract, the muscularis externa propels the material along the alimentary canal.... which pushes food through the digestive system. 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 muscleQuadriceps muscle The quadriceps femoris includes the four remaining muscles on the front of the thigh.... of the thighThigh In humans the thigh is the area between the pelvis and buttocks and the knee.... . There are two broad types of voluntary muscle fibers: slow twitch and fast twitch. Slow twitch fibers contract for long periods of time but with little force while fast twitch fibers contract quickly and powerfully but fatigue very rapidly. Types There are three types of muscle: Skeletal muscleSkeletal muscle Skeletal muscle is a type of striated muscle, attached to the skeleton.... or "voluntary muscle" is anchored by tendons to boneBone Bone, also called osseous tissue, is a type of hard endoskeletal connective tissue found in many vertebrate animals.... and is used to affect skeletalFacts About Skeleton In biology, the skeleton or skeletal system is the biological system providing physical support in living organisms.... movement such as locomotionLocomotion In a general sense, locomotion simply means active movement or travel, applying not just to biological individuals.... and in maintaining posture. Though this postural control is generally maintained as a subconscious reflex, the muscles responsible react to conscious control like non-postural muscles. An average adult male is made up of 40–50% of skeletal muscle and an average adult female is made up of 30–40% (as a percentage of body mass). Smooth muscleFacts About Smooth muscle Smooth muscle is a type of non-striated muscle, found within the "walls" of hollow organs; such as the bladder, the uterus, ... or "involuntary muscle" is found within the walls of organs and structures such as the esophagusEsophagus The esophagus, or gullet is the muscular tube in vertebrates through which ingested food passes from the mouth area to... , stomachStomach Summary In anatomy, the stomach is an organ in the gastrointestinal tract used to digest food.... , intestineIntestine In anatomy, the intestine is the portion of the alimentary canal extending from the stomach to the anus and, in humans and o... s, bronchi, uterusUterus The uterus or womb is the major female reproductive organ of most mammals, including humans.... , urethraUrethra In anatomy, the urethra is a tube which connects the urinary bladder to the outside of the body.... , bladderUrinary bladder Summary In the anatomy of mammals, the urinary bladder is the organ that collects urine excreted by the kidneys prior to disposal by... , blood vesselBlood vessel Overview The blood vessels are part of the circulatory system and function to transport blood throughout the body.... s, and even the skin (in which it controls erection of body hair). Unlike skeletal muscle, smooth muscle is not under conscious control. Cardiac muscleCardiac muscle Cardiac muscle is a type of involuntary mononucleated, or uninucleated, striated muscle found exclusivly within the heart.... 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 sarcomereSarcomere A sarcomere is the basic unit of a cross striated muscle's myofibril.... s 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 muscle is further divided into several subtypes: Type I, slow oxidative, slow twitch, or "red" muscle is dense with capillariesCapillary Capillaries, are the smallest of a body's blood vessels, measuring 5-10 m.... and is rich in mitochondria and myoglobinMyoglobin Myoglobin is a single-chain protein of 153 amino acids, containing a heme group in the center.... , giving the muscle tissue its characteristic red color. It can carry more oxygenOxygen Oxygen is a chemical element with the chemical symbol O and atomic number 8.... and sustain aerobic activity. Type II, fast twitch muscle, has three major kinds that are, in order of increasing contractile speed: Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red. 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, anaerobicFermentation (biochemistry) Fermentation is a process that is important in anaerobic conditions when there is no oxidative phosphorylation to maintain ... bursts of activity before muscle contraction becomes painful (often incorrectly attributed to a build-up of lactic acidLactic acid Summary Lactic acid, also known as milk acid, is a chemical compound that plays a role in several biochemical processes.... ). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB. Type IIb, which is anaerobic, glycolyticGlycolysis Glycolysis is a series of biochemical reactions by which a molecule of glucose is oxidized to two molecules of pyruvic acid... , "white" muscle that is even less dense in mitochondria and myoglobin. In small animals like rodents this is the major fast muscle type, explaining the pale color of their flesh. Anatomy The anatomy of muscles includes both gross anatomyGross anatomy Gross anatomy is the study of anatomy at the macroscopic level.... , comprising all the muscles of an organism, and, on the other hand, microanatomy, which comprises the structures of a single muscle. Gross anatomy The gross anatomy of a muscle is the most important indicator of its role in the body. 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). 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. There are approximately 639 skeletal muscles in the human body. However, the exact number is difficult to define because different sources group muscles differently. Microanatomy Muscle is mainly composed of muscle cellsFacts About Cell (biology) The cell is the structural and functional unit of all living organisms, and is sometimes called the "building block of life.... . Within the cells are myofibrilMyofibril Myofibrils are cylindrical organelles, found within muscle cells.... s; myofibrils contain sarcomeres, which are composed of actinActin Actin is a globular structural protein that polymerizes in a helical fashion to form an actin filament.... and myosinFacts About Myosin Myosins are a large family of motor proteins found in eukaryotic tissues.... . Individual muscle fibres are surrounded by endomysiumEndomysium The endomysium, literally meaning within the muscle, is a layer of connective tissue that ensheaths a muscle fiber and is co... . Muscle fibers are bound together by perimysiumPerimysium Summary Perimysium is a sheath of connective tissue which groups individual muscle fibers into bundles or fascicles.... into bundles called fascicles; the bundles are then grouped together to form muscle, which is enclosed in a sheath of epimysiumEpimysium Epimysium is a layer of connective tissue which ensheaths the entire muscle.... . Muscle spindleMuscle spindle A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons.... s are distributed throughout the muscles and provide sensory feedback information to the central nervous systemCentral nervous system Summary The central nervous system represents the largest part of the nervous system, including the brain and the spinal cord.... . Skeletal muscle is arranged in discrete muscles, an example of which is the biceps brachiiBiceps brachii muscle In human anatomy, the biceps brachii is a muscle located on the upper arm.... . It is connected by tendonTendon Summary A tendon is a tough band of fibrous connective tissue that connects muscle to bone, or muscle to muscle.... s to processes of the skeleton. Cardiac muscle is similar to skeletal muscle in both composition and action, being comprised of myofibrils of sarcomeres, but anatomically different in that the muscle fibers are typically branched like a tree and connect to other cardiac muscle fibers through intercalcated discs, and form the appearance of a syncytiumSyncytium In biology, a syncytium is a large region of cytoplasm that contains many nuclei.... . Physiology The three (skeletal, cardiac and smooth) types of muscle have significant differences. However, all three use the movement of actin against myosin to create contractionMuscle contraction A muscle contraction occurs when a muscle cell shortens.... . In skeletal muscle, contraction is stimulated by electrical impulsesAction potential Summary An action potential is a wave of electrical discharge that travels along the membrane of a cell.... transmitted by the nerveNerve A nerve is an enclosed, cable-like bundle of nerve fibers or axons, which includes the glia that ensheath the axons in... s, the motor nerves and motoneurons 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 neurotransmitterNeurotransmitter Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between a neuron and another... acetylcholineAcetylcholine The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified.... . Muscular activity accounts for much of the body's energyEnergy In general, the concept of energy refers to "the potential for causing changes." The word is used in several different conte... consumption. All muscle cells produce adenosine triphosphateAdenosine triphosphate Adenosine 5'-triphosphate , discovered in 1929 by Karl Lohmann, is a multifunctional nucleotide primarily known in biochemis... (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 kinaseCreatine kinase Creatine kinase, also known as phosphocreatine kinase or creatine phosphokinase is an enzyme expressed by variou... . Muscles also keep a storage form of glucose in the form of glycogenGlycogen Glycogen is a polysaccharide that is the principal storage form of glucose in animal and human cells.... . Glycogen can be rapidly converted to glucoseGlucose Glucose , a monosaccharide , is one of the most important carbohydrates in biology.... 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 cycleCitric acid cycle Summary The citric acid cycle is a series of chemical reactions of central importance in all living cells that utilize oxygen as pa... ). Muscle cells also contain globules of fat, which are used for energy during aerobic exerciseAerobic exercise In physical exercise, aerobic exercise is complementary to anaerobic 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. Nervous control Efferent leg The efferent leg of the peripheral nervous systemPeripheral nervous system The peripheral nervous system or PNS, is part of the nervous system, and consists of the nerves and neurons that resid... is responsible for conveying commands to the muscles and glands, and is ultimately responsible for voluntary movement. NerveNerve A nerve is an enclosed, cable-like bundle of nerve fibers or axons, which includes the glia that ensheath the axons in... s move muscles in response to voluntarySomatic nervous system The somatic nervous system is that part of the peripheral nervous system associated with the voluntary control of body movem... and autonomicAutonomic nervous system The autonomic nervous system is the part of the nervous system that is not under conscious control.... (involuntary) signals from the brainBrain In animals, the brain, or encephalon , is the control center of the central nervous system.... . Deep muscles, superficial muscles, and internal muscles all correspond with dedicated regions in the primary motor cortex of the brainHuman brain The human brain is the anteriormost part of the central nervous system in humans as well as the primary control center for t... , directly anterior to the central sulcus that divides the frontal and parietal lobes. In addition, muscles react to reflexiveReflex action Summary A reflex action is an automatic neuromuscular action elicited by a defined stimulus.... 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 spineSpinal cord In vertebrates, the spinal cord is the part of the central nervous system that is enclosed in and protected by the vertebral... . 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 mammalMammal The mammals are the class of vertebrate animals characterized by the presence of mammary glands, which in females produce mi... s correspond with neuron groups along the primary motor cortexPrimary motor cortex The primary motor cortex works in association with pre-motor areas to plan and execute movements.... of the brain's cerebral cortexCerebral cortex The cerebral cortex is a brain structure in vertebrates.... . Commands are routed though the basal gangliaBasal ganglia The basal ganglia are a group of nuclei in the brain associated with motor and learning functions.... and are modified by input from the cerebellumCerebellum The cerebellum is a region of the brain that plays an important role in the integration of sensory perception and motor out... before being relayed through the pyramidal tract to the spinal cordSpinal cord In vertebrates, the spinal cord is the part of the central nervous system that is enclosed in and protected by the vertebral... and from there to the motor end plate at the muscles. Along the way, feedback, such as that of the extrapyramidal systemExtrapyramidal system In human anatomy, the extrapyramidal system is a neural network located in the brain that is part of the motor system involv... contribute signals to influence muscle tone and response. Deeper muscles such as those involved in posturePosture Posture or posturing may refer to:... often are controlled from nuclei in the brain stemBrain stem The brain stem is the lower part of the brain, adjoining and structurally continuous with the spinal cord.... 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 spindleMuscle spindle Overview A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons.... s convey information about the degree of muscle length and stretch to the central nervous system to assist in maintaining posture and joint position. The senseSense Senses are the physiological methods of perception.... of where our bodies are in space is called proprioceptionProprioception Proprioception is the sense of the relative position of neighbouring parts of the body.... , 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 nucleusRed nucleus The red nucleus is a structure in the rostral midbrain involved in motor coordination.... in particular continuously sample position against movement and make minor corrections to assure smooth motion. Exercise Exercise is often recommended as a means of improving motor skillMotor skill A motor skill is a skill that regards the ability of an organism to utilise skeletal muscles effectively.... s, fitnessPhysical fitness Summary The notion of physical fitness is used in two close meanings.... , muscle and bone strength, and joint function. Exercise has several effects upon muscles, connective tissueConnective tissue Connective tissue is one of the four types of tissue in traditional classifications It is largely a category of exclusion ra... , 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 marathonMarathon The marathon is a long-distance road running event of 42.195 km . ... ). 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. 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 neovascularizationNeovascularization Overview Neovascularization is the formation of functional microvascular networks with red blood cell perfusion.... 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. The ability of the body to export lactic acid and use it as a source of energy depends on training level. 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. People with high overall musculation and balanced muscle type percentage engage in sports such as rugbyRugby football Rugby football, often just referred to as rugby, refers to sports descended from a common form of football developed a... or boxingBoxing Boxing, also called Western Boxing, prizefighting or the sweet science , is a sport and martial art in wh... and often engage in other sports to increase their performance in the former. Delayed onset muscle sorenessDelayed onset muscle soreness Delayed Onset Muscle Soreness is the pain or discomfort often felt 24 to 76 hours after exercising and subsides generally wi... 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 contractionMuscle contraction A muscle contraction occurs when a muscle cell shortens.... , or unaccustomed training levels. Since lactic acid disperses fairly rapidly, it could not explain pain experienced days after exercise. Disease Symptoms of muscle diseases may include weaknessWeakness Weakness can mean:The opposite of strength... , spasticitySpasticity Spasticity* is a disorder of the body's motor system in which certain muscles are continuously contracted.... , myoclonusMyoclonus Myoclonus is brief, involuntary twitching of a muscle or a group of muscles.... and myalgiaMyalgia Myalgia means "muscle pain" and is a symptom of many diseases and disorders.... . Diagnostic procedures that may reveal muscular disorders include testing creatine kinase levels in the blood and electromyographyElectromyography Electromyography is a medical technique for evaluating and recording physiologic properties of muscles at rest and while con... (measuring electrical activity in muscles). In some cases, muscle biopsyMuscle biopsy In medicine, a muscle biopsy is a procedure in which a piece of muscle tissue is removed from an organism and examined micro... may be done to identify a myopathyMyopathy In medicine, a myopathy is a neuromuscular disease in which the muscle fibers dysfunction for any one of many reasons, resul... , as well as genetic testingFacts About Genetic testing Genetic testing allows the genetic diagnosis of vulnerabilities to inherited diseases, and can also be used to determine a p... to identify DNADNA Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions for the biological development of a cellu... abnormalities associated with specific myopathies and dystrophiesMuscular dystrophy The muscular dystrophies are a group of genetic and hereditary muscle diseases; characterized by progressive skeletal muscle... . Neuromuscular diseaseNeuromuscular disease Neuromuscular disease is a very broad term that encompasses many diseases and ailments that either directly or indirectly im... s are those that affect the muscles and/or their nervous control. In general, problems with nervous control can cause spasticity or paralysisParalysis Paralysis is the complete loss of muscle function for one or more muscle groups.... , depending on the location and nature of the problem. A large proportion of neurological disorders leads to problems with movement, ranging from cerebrovascular accident (stroke) and Parkinson's diseaseParkinson's disease Parkinson's disease is a degenerative disorder of the central nervous system that affects the control of muscles, and so may... to Creutzfeldt-Jakob diseaseCreutzfeldt-Jakob disease Overview Creutzfeldt-Jakob disease is a very rare and incurable degenerative neurological disorder that is ultimately fatal.... . A non-invasive elastographyElastography Elastography is an emerging method in which stiffness or strain ... 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 filamentsFacts About Protein filament In biology, a filament is a long chain of proteins, such as those found in hair, muscle, or in flagella.... along the axis of the muscle. During contractionContraction Contraction can mean:* Contraction, a contraction during childbirth... , the muscle shortens along its longitudinal axis and expands across the transverse axisTransverse plane The transverse or axial plane is an imaginary plane that divides the body into superior and inferior parts.... , producing vibrations at the surface. Atrophy There are many diseases and conditions which cause a decrease in muscle mass, known as muscle atrophyMuscle atrophy When a muscle atrophies, it necessarily becomes weaker, since the ability to exert force is related to mass.... . Example include cancerCancer Summary Cancer is a class of diseases or disorders characterized by uncontrolled division of cells and the ability of these cells to... and AIDSAIDS Acquired Immunodeficiency Syndrome or Acquired Immune Deficiency Syndrome is a collection of symptoms and infections i... , which induce a body wasting syndrome called cachexiaCachexia Cachexia is loss of weight, muscle atrophy, fatigue, weakness and anorexia in someone who is not actively trying to lose wei... . Other syndromes or conditions which can induce skeletal muscle atrophy are congestive heart disease and some diseases of the liverHepato-biliary diseases =Liver diseases=... . During aging, there is a gradual decrease in the ability to maintain skeletal muscle function and mass, known as sarcopeniaSarcopenia Sarcopenia is the degenerative loss of skeletal muscle mass and strength in senescence.... . 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. Physical inactivity and atrophy Inactivity and starvation in rodents and mammals lead to atrophy of skeletal muscle, accompanied by a smaller number and size of the muscle cells as well as lower protein content. 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. Such consequences are also noted in small hibernating mammals like the golden-mantled ground squirrels and brown bats. Representatives of the Ursid species make for an interesting exception to this expected norm. Bears are famous for their ability to survive unfavorable environmental conditions of low temperatures and limited nutrition availability during winter by means of hibernation. During that time Ursids go through a series of physiological, morphological and behavioral changes. Their ability to maintain skeletal muscle number and size at time of disuse is of a significant importance. During hibernation bears spend four to seven months of inactivity and anorexia without undergoing muscle atrophy and protein loss. There are a few known factors that contribute to the sustaining of muscle tissue. During the summer period, Ursids take advantage of the nutrition availability and accumulate muscle protein. The protein balance of bears at time of dormancy is also maintained by lower levels of protein breakdown during the winter time. At times of immobility, muscle wasting in Ursids is also suppressed by a proteolytic inhibitor that is released in circulation. Another factor that contributes to the sustaining of muscle strength in hibernating bears is the occurrence of periodic voluntary contractions and involuntary contractions from shivering during torpor. The three to four daily episodes of muscle activity are responsible for the maintenance of muscle strength and responsiveness in bears during hibernation. 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). Contrary to popular belief, the number of muscle fibres cannot be increased through exercisePhysical exercise Physical exercise is the performance of some activity in order to develop or maintain physical fitness and overall health.... ; instead the muscle cells simply get bigger. Muscle fibres have a limited capacity for growth through hypertrophy and some believe they split through hyperplasiaHyperplasia Hyperplasia is a general term for an increase in the number of the cells of an organ or tissue causing it to increase in siz... if subject to increased demand. 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 jawJaw The jaw is either of the two opposable structures forming, or near the entrance to, the mouth.... muscle is the strongest. The 1992 Guinness Book of Records records the achievement of a bite strength of 4337 NNewton The newton is the SI unit of force.... (975 lbfPound-force A pound-force is a unit of force. One pound-force is the force equivalent to that exerted on a mass of one avoirdupois pound... ) 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 fiberMuscle fiber A muscle fiber or muscle fibre is a single cell of a muscle.... 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 weightMass Mass is a property of a physical object that quantifies the amount of matter and energy it is equivalent to.... ) than a longer muscle. The myometrial layer of the uterus may be the strongest muscle by weight in the human body. At the time when an infantFacts About Infant Infant is a formal term for the word baby, the youngest category of a child.... 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 saccadeSaccade A saccade is a fast movement of an eye, head or other part of an animal's body or device.... s 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 tongueTongue The tongue is the large bundle of skeletal muscles on the floor of the mouth that manipulates food for chewing and swallowin... 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 heartFacts About Heart The heart is a hollow, muscular organ in vertebrates, responsible for pumping blood through the blood vessels by repeated, r... 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 wattWatt The watt is the SI derived unit of power, equal to one joule per second.... s. This is much less than the maximum power output of other muscles; for example, the quadricepsQuadriceps muscle The quadriceps femoris includes the four remaining muscles on the front of the thigh.... 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 gigajouleGigajoule A gigajoule is 1,000,000,000 joules. It is a unit of energy. ... s. Efficiency The efficiencyMechanical efficiency Overview In physics, mechanical efficiency is the effectiveness of a machine and is defined as... of human muscle has been measured (in the context of rowingWatercraft rowing With regard to watercraft, rowing is propulsion, usually of a small boat, by forcing one or more oars mounted on the craft v... and cyclingCycling Cycling is a recreation, a sport and a means of transport across land.... ) at 14% to 27%. The efficiency is defined as the ratio of mechanical workMechanical work Mechanical work is a force applied through a distance, defined mathmatically as the line integral of a scalar product of for... output to the total metabolic costMetabolism Metabolism is the biochemical modification of chemical compounds in living organisms and cells.... . Density of muscle tissue compared to adipose tissue The densityDensity Density is a measure of mass per unit of volume.... of mammalian skeletal muscle tissue is about 1.06 kg/liter. This can be contrasted with the density of adipose tissueAdipose tissue Adipose tissue is an anatomical term for loose connective tissue composed of adipocytes.... (fat), which is 0.9196 kg/liter. This makes muscle tissue approximately 15% denser than fat tissue. Muscle evolution Evolutionarily, specialized forms of skeletal and cardiac muscleCardiac muscle Cardiac muscle is a type of involuntary mononucleated, or uninucleated, striated muscle found exclusivly within the heart.... s predated the divergence of the vertebrateVertebrate Vertebrata is a subphylum of chordates, specifically, those with backbones or spinal columns.... /arthropodArthropod Arthropods are the largest phylum of animals and include the insects, arachnids, crustaceans, and others.... evolutionary line. This indicates that these types of muscle developed in a common ancestorAncestor An ancestor is a parent or the parent of an ancestor.... sometime before 700 million years ago (mya)Mya (unit) Summary In astronomy, geology, and paleontology, mya or "m.y.a." is an acronym for million years ago.... . Vertebrate smooth muscle (smooth muscle found in humans) was found to have evolved independently from the skeletal and cardiac muscles. See also FasciaFascia Fascia is specialized connective tissue layer which surrounds muscles, bones, and joints, providing support and protection a... BodybuildingBodybuilding Bodybuilding is the process of developing muscle fibres through the combination of weight training, increased caloric intake... List of muscles of the human bodyList of muscles of the human body This is a list of muscles of the human anatomy.... MyopathyFacts About Myopathy In medicine, a myopathy is a neuromuscular disease in which the muscle fibers dysfunction for any one of many reasons, resul... (pathology of muscle cells) MyotomyMyotomy Myotomy describes a surgical procedure in which muscle is cut.... Rapid plant movementRapid plant movement Rapid plant movement encompasses movement in plant structures occurring over a very short period of time, usually under one ... AtrophyAtrophy Atrophy is the partial or complete wasting away of a part of the body.... Muscle atrophyMuscle atrophy When a muscle atrophies, it necessarily becomes weaker, since the ability to exert force is related to mass.... Muscle toneMuscle tone Muscle tone is the continuous and passive partial contraction of the muscles.... (residual muscle tension) Electroactive polymersElectroactive polymers Electroactive Polymers or EAPs are polymers whose shape is modified when a voltage is applied to them.... (materials that behave like muscles, used in robotics research) Muscle memoryMuscle memory Muscle memory is a common term for neuromuscular facilitation, which is the process of the neuromuscular system memorizing m... Musculoskeletal systemMusculoskeletal system The musculoskeletal system is an organ system that gives animals the ability to physically move, by using the muscles and sk... Hand strengthHand strength Hand strength measurements are of interest to study pathology of the hand that involves loss of muscle strength.... External links (Heart output 1.3 to 5 watts, lifetime output 2 to 3 ×10⁹ joules) article on performing neurological examinations (Quadriceps "strongest") (clear pictures of main human muscles and their Latin names, good for orientation) Microscopic stains of and muscular fibers to show striations. Note the differences in myofibrilar arrangements.