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Muscle is a contractile
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...

 tissue
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

 of animals and is derived from the mesodermal layer
Germ layer
A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...

 of embryonic germ cells. Muscle cells
Myocyte
A myocyte is the type of cell found in muscles. They arise from myoblasts.Each myocyte contains myofibrils, which are long, long chains of sarcomeres, the contractile units of the cell....

 contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...

, cardiac
Cardiac muscle
Cardiac muscle is a type of involuntary striated muscle found in the walls and histologic foundation of the heart, specifically the myocardium. Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth muscle...

, or smooth
Smooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...

 muscles. Their function is to produce force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

 and cause motion
Motion (physics)
In physics, motion is a change in position of an object with respect to time. Change in action is the result of an unbalanced force. Motion is typically described in terms of velocity, acceleration, displacement and time . An object's velocity cannot change unless it is acted upon by a force, as...

. Muscles can cause either locomotion of the organism itself or movement of internal organs
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

. Cardiac and smooth muscle contraction
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...

 occurs without conscious
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...

 thought and is necessary for survival. Examples are the contraction of the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 and peristalsis
Peristalsis
Peristalsis is a radially symmetrical contraction and relaxation of muscles which propagates in a wave down the muscular tube, in an anterograde fashion. In humans, peristalsis is found in the contraction of smooth muscles to propel contents through the digestive tract. Earthworms use a similar...

 which pushes food through the digestive system
Digestion
Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....

. 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
Thigh
In humans the thigh is the area between the pelvis and the knee. Anatomically, it is part of the lower limb.The single bone in the thigh is called the femur...

. 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.

Muscles are predominantly powered by the oxidation
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 of fat
Fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...

s and carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

s, but anaerobic
Anaerobic respiration
Anaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen...

 chemical reactions are also used, particularly by fast twitch fibers. These chemical reactions produce adenosine triphosphate
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

 (ATP) molecules which are used to power the movement of the myosin
Myosin
Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...

 heads.

Embryology


All muscles derive from paraxial mesoderm
Paraxial mesoderm
Paraxial mesoderm is the area of mesoderm that forms just lateral to the neural tube on both sides.It differentiates rostrally into somatomeres and caudally into somites.It gives rise to the somitomeres/somites and mesoderm of the branchial arches....

. The paraxial mesoderm is divided along the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

's length into somite
Somite
A somite is a division of the body of an animal. In vertebrates this is mainly discernible in the embryo stage; in arthropods it is a characteristic of a hypothetical ancestor.- In vertebrates :...

s, corresponding to the segmentation
Segmentation (biology)
Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...

 of the body (most obviously seen in the vertebral column
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...

. Each somite has 3 divisions, sclerotome
Sclerotome
A sclerotome is part of a somite, a structure in vertebrate embryonic development. Sclerotomes eventually differentiate into the vertebrae and most of the skull...

 (which forms vertebrae), dermatome (which forms skin), and myotome
Myotome
In vertebrate embryonic development, a myotome is a group of tissues formed from somites that develop into the body wall muscle.Each myotome divides into a dorsal epaxial part and a ventral hypaxial part....

 (which forms muscle). The myotome is divided into two sections, the epimere and hypomere, which form epaxial
Epaxial and Hypaxial muscles
Trunk muscles can be broadly divided into hypaxial muscle, which lie ventral to the horizontal septum of the vertebrae and epaxial muscles, which lie dorsal to the septum. Hypaxial muscles include some vertebral muscles, the diaphragm, the abdominal muscles, and all limb muscles...

 and hypaxial
Epaxial and Hypaxial muscles
Trunk muscles can be broadly divided into hypaxial muscle, which lie ventral to the horizontal septum of the vertebrae and epaxial muscles, which lie dorsal to the septum. Hypaxial muscles include some vertebral muscles, the diaphragm, the abdominal muscles, and all limb muscles...

 muscles, respectively. Epaxial muscles in humans are only the erector spinae and small intervertebral muscles, and are innervated by the dorsal rami of the spinal nerve
Spinal nerve
The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body...

s. All other muscles, including limb muscles, are hypaxial muscles, formed from the hypomere, and inervated by the ventral rami of the spinal nerve
Spinal nerve
The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body...

s.

During development, myoblast
Myoblast
A myoblast is a type of embryonic progenitor cell that gives rise to muscle cells .The muscle cells can be skeletal muscle, smooth muscle, and cardiac muscle....

s (muscle progenitor cells) either remain in the somite to form muscles associated with the vertebral column or migrate out into the body to form all other muscles. Myoblast migration is preceded by the formation of connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

 frameworks, usually formed from the somatic lateral plate mesoderm
Lateral plate mesoderm
Lateral plate mesoderm is a type of mesoderm that is found at the periphery of the embryo.-Division into layers:It will split into two layers, the somatic layer/mesoderm and the splanchnic layer/mesoderm...

. Myoblasts follow chemical signals to the appropriate locations, where they fuse into elongate skeletal muscle cells.

Types



There are three types of muscle:
  • Skeletal muscle
    Skeletal muscle
    Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...

     or "voluntary muscle" is anchored by tendon
    Tendon
    A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other...

    s (or by aponeuroses
    Aponeurosis
    Aponeuroses are layers of flat broad tendons. They have a shiny, whitish-silvery color, are histologically similar to tendons, and are very sparingly supplied with blood vessels and nerves. When dissected, aponeuroses are papery, and peel off by sections...

     at a few places) to bone
    Bone
    Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

     and is used to effect skeletal
    Skeleton
    The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...

     movement such as locomotion
    Animal locomotion
    Animal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, swimming, jumping and flying. Animals move for a variety of reasons, such as to find food, a mate, or a suitable microhabitat, and to escape predators...

     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 42% of skeletal muscle and an average adult female is made up of 36% (as a percentage of body mass).
  • Smooth muscle
    Smooth muscle
    Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...

     or "involuntary muscle" is found within the walls of organs and structures such as the esophagus
    Esophagus
    The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...

    , stomach
    Stomach
    The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

    , intestine
    Intestine
    In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...

    s, bronchi
    Bronchus
    A bronchus is a passage of airway in the respiratory tract that conducts air into the lungs. The bronchus branches into smaller tubes, which in turn become bronchioles....

    , uterus
    Uterus
    The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...

    , urethra
    Urethra
    In anatomy, the urethra is a tube that connects the urinary bladder to the genitals for the removal of fluids out of the body. In males, the urethra travels through the penis, and carries semen as well as urine...

    , bladder
    Urinary bladder
    The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...

    , blood vessel
    Blood vessel
    The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

    s, 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
    Cardiac muscle
    Cardiac muscle is a type of involuntary striated muscle found in the walls and histologic foundation of the heart, specifically the myocardium. Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth 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 sarcomere
Sarcomere
A sarcomere is the basic unit of a muscle. Muscles are composed of tubular muscle cells . Muscle cells are composed of tubular myofibrils. Myofibrils are composed of repeating sections of sarcomeres, which appear under the microscope as dark and light bands...

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 capillaries
    Capillary
    Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...

     and is rich in mitochondria and myoglobin
    Myoglobin
    Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. The only time myoglobin is found in the...

    , giving the muscle tissue its characteristic red color. It can carry more oxygen
    Oxygen
    Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

     and sustain aerobic activity.
    • Type I muscle fiber are sometimes broken down into Type I and Type Ic categories, as a result of recent research.
  • 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, anaerobic
      Fermentation (biochemistry)
      Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...

       bursts of activity before muscle contraction becomes painful (often incorrectly attributed to a build-up of lactic acid
      Lactic acid
      Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...

      ). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB.
    • Type IIb, which is anaerobic, glycolytic
      Glycolysis
      Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...

      , "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 anatomy
Gross anatomy
Gross anatomy is the study of anatomy at the macroscopic level. The term gross distinguishes it from other areas of anatomical study, including microscopic anatomy, which is studied on a microscopic scale, typically with a microscope....

, 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
Pennate muscle
A pennate or pinnate muscle is a muscle with fascicles that attach obliquely to its tendon...

 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 and some muscles, such as palmaris longus, are variably present in humans.

Microanatomy


Muscle is mainly composed of muscle cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

. Within the cells are myofibril
Myofibril
A myofibril is a basic unit of a muscle. Muscles are composed of tubular cells called myocytes or myofibers. Myofibers are composed of tubular myofibrils. Myofibrils are composed of long proteins such as actin, myosin, and titin, and other proteins that hold them together...

s; myofibrils contain sarcomeres, which are composed of actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...

 and myosin
Myosin
Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...

. Individual muscle fibres are surrounded by endomysium
Endomysium
The endomysium, meaning within the muscle, is a layer of connective tissue that ensheaths a muscle fiber and is composed mostly from reticular fibers. It also contains capillaries, nerves, and lymphatics...

. Muscle fibers are bound together by perimysium
Perimysium
Perimysium is a sheath of connective tissue that groups muscle fibers into bundles or fascicles.Recent advances in muscle physiology suggest that the perimysium plays a role in transmitting lateral contractile movements...

 into bundles called fascicles
Muscle fascicle
In anatomy, a fascicle is a bundle of skeletal muscle fibers surrounded by perimysium, a type of connective tissue.Specialized muscle fibers in the heart that transmit electrical impulses from the Atrioventricular Node to the Purkinje Fibers are fascicles, also referred to as bundle branches...

; the bundles are then grouped together to form muscle, which is enclosed in a sheath of epimysium
Epimysium
Epimysium is a layer of connective tissue, which ensheaths the entire muscle. It is composed of dense irregular connective tissue. It is continuous with fascia and other connective tissue wrappings of muscle including the endomysium, and perimysium. It is also continuous with tendons where it...

. Muscle spindle
Muscle spindle
Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts...

s are distributed throughout the muscles and provide sensory feedback information to the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

.

Skeletal muscle is arranged in discrete muscles, an example of which is the biceps brachii
Biceps brachii muscle
In human anatomy, the biceps brachii, or simply biceps in common parlance, is, as the name implies, a two-headed muscle located on the upper arm. Both heads arise on the scapula and join to form a single muscle belly which is attached to the upper forearm...

. It is connected by tendon
Tendon
A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other...

s to processes of the skeleton. Cardiac muscle is similar to skeletal muscle in both composition and action, being made up 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 syncytium
Syncytium
In biology, a syncytium is a large cell-like structure; filled with cytoplasm and containing many nuclei. Most cells in eukaryotic organisms have a single nucleus; syncytia are specialized forms used by various organisms.The term may also refer to cells that are connected by specialized membrane...

.

Physiology


The three types of muscle (skeletal, cardiac and smooth) have significant differences. However, all three use the movement of actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...

 against myosin
Myosin
Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...

 to create contraction
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...

. In skeletal muscle, contraction is stimulated by electrical impulses
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

 transmitted by the nerve
Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...

s, 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
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...

 acetylcholine
Acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...

.

Muscular activity accounts for much of the body's energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 consumption. All muscle cells produce adenosine triphosphate
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

 (ATP) molecules which are used to power the movement of the myosin
Myosin
Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...

 heads. Muscles conserve energy in the form of creatine phosphate which is generated from ATP and can regenerate ATP when needed with creatine kinase
Creatine kinase
Creatine kinase , also known as creatine phosphokinase or phospho-creatine kinase , is an enzyme expressed by various tissues and cell types. CK catalyses the conversion of creatine and consumes adenosine triphosphate to create phosphocreatine and adenosine diphosphate...

. Muscles also keep a storage form of glucose in the form of glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...

. Glycogen can be rapidly converted to glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 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
Citric acid cycle
The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...

). Muscle cells also contain globules of fat, which are used for energy during aerobic exercise
Aerobic exercise
Aerobic exercise is physical exercise of relatively low intensity that depends primarily on the aerobic energy-generating process. Aerobic literally means "living in air", and refers to the use of oxygen to adequately meet energy demands during exercise via aerobic metabolism...

. 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.

Efferent leg


The efferent leg of the peripheral nervous system
Peripheral nervous system
The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain...

 is responsible for conveying commands to the muscles and glands, and is ultimately responsible for voluntary movement. Nerve
Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...

s move muscles in response to voluntary
Somatic nervous system
The somatic nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles...

 and autonomic
Autonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...

 (involuntary) signals from the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

. Deep muscles, superficial muscles, muscles of the face and internal muscles all correspond with dedicated regions in the primary motor cortex of the brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...

, directly anterior to the central sulcus that divides the frontal and parietal lobes.

In addition, muscles react to reflexive
Reflex action
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...

 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
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

. 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
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s correspond with neuron groups along the primary motor cortex
Primary motor cortex
The primary motor cortex is a brain region that in humans is located in the posterior portion of the frontal lobe. Itworks in association with pre-motor areas to plan and execute movements. M1 contains large neurons known as Betz cells, which send long axons down the spinal cord to synapse onto...

 of the brain's cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

. Commands are routed though the basal ganglia
Basal ganglia
The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...

 and are modified by input from the cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...

 before being relayed through the pyramidal tract to the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

 and from there to the motor end plate at the muscles. Along the way, feedback, such as that of the extrapyramidal system
Extrapyramidal system
In human anatomy, the extrapyramidal system is a neural network located in the brain that is part of the motor system involved in the coordination of movement. The system is called "extrapyramidal" to distinguish it from the tracts of the motor cortex that reach their targets by traveling through...

 contribute signals to influence muscle tone and response.

Deeper muscles such as those involved in posture
Human position
Human positions refers to the different positions that the human body can take.There are several synonyms that refer to the human position, often used interchangeably, but having specific flavors....

 often are controlled from nuclei in the brain stem
Brain stem
In vertebrate anatomy the brainstem is the posterior part of the brain, adjoining and structurally continuous with the spinal cord. The brain stem provides the main motor and sensory innervation to the face and neck via the cranial nerves...

 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 spindle
Muscle spindle
Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts...

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 sense
Sense
Senses are physiological capacities of organisms that provide inputs for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology , and philosophy of perception...

 of where our bodies are in space is called proprioception
Proprioception
Proprioception , from Latin proprius, meaning "one's own" and perception, is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement...

, 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
Red nucleus
The red nucleus is a structure in the rostral midbrain involved in motor coordination. It comprises a caudal magnocellular and a rostral parvocellular part.-Function:...

 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 skill
Motor skill
A motor skill is a learned sequence of movements that combine to produce a smooth, efficient action in order to master a particular task. The development of motor skill occurs in the motor cortex, the region of the cerebral cortex that controls voluntary muscle groups.- Development of motor skills...

s, fitness
Physical fitness
Physical fitness comprises two related concepts: general fitness , and specific fitness...

, muscle and bone strength, and joint function. Exercise has several effects upon muscles, connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

, bone, and the nerves that stimulate the muscles. One such effect is muscle hypertrophy
Muscle hypertrophy
Muscle hypertrophy is an increase in the size of muscle cells. It differs from muscle hyperplasia, which is the formation of new muscle cells.-Hypertrophy stimuli:A range of stimuli can increase the volume of muscle cells...

, an increase in size. This is used in bodybuilding
Bodybuilding
Bodybuilding is a form of body modification involving intensive muscle hypertrophy. An individual who engages in this activity is referred to as a bodybuilder. In competitive and professional bodybuilding, bodybuilders display their physiques to a panel of judges, who assign points based on their...

.

Various exercises require a predominance of certain muscle fiber utilization over another. Aerobic exercise
Aerobic exercise
Aerobic exercise is physical exercise of relatively low intensity that depends primarily on the aerobic energy-generating process. Aerobic literally means "living in air", and refers to the use of oxygen to adequately meet energy demands during exercise via aerobic metabolism...

 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
Marathon
The marathon is a long-distance running event with an official distance of 42.195 kilometres , that is usually run as a road race...

). 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
Anaerobic exercise
Anaerobic exercise is exercise intense enough to trigger anaerobic metabolism. It is used by athletes in non-endurance sports to promote strength, speed and power and by body builders to build muscle mass...

 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
Weight training
Weight training is a common type of strength training for developing the strength and size of skeletal muscles. It uses the weight force of gravity to oppose the force generated by muscle through concentric or eccentric contraction...

. 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
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...

 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
Neovascularization
Neovascularization is the formation of functional microvascular networks with red blood cell perfusion. Neovascularization differs from angiogenesis in that angiogenesis is mainly characterized by the protrusion and outgrowth of capillary buds and sprouts from pre-existing blood vessels.In...

 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.
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.

Delayed onset muscle soreness
Delayed onset muscle soreness
Delayed onset muscle soreness , also called muscle fever, is the pain and stiffness felt in muscles several hours to days after unaccustomed or strenuous exercise. The soreness is felt most strongly 24 to 72 hours after the exercise. It is caused by eccentric exercise...

 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.

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
Cross Education
Cross education is a neurophysiological phenomenon where an increase in strength is witnessed within an untrained limb following unilateral strength training in the opposite, contralateral limb....

.

Disease


Symptoms of muscle diseases may include weakness
Muscle weakness
Muscle weakness or myasthenia is a lack of muscle strength. The causes are many and can be divided into conditions that have true or perceived muscle weakness...

, spasticity
Spasticity
Spasticity is a feature of altered skeletal muscle performance in muscle tone involving hypertonia, which is also referred to as an unusual "tightness" of muscles...

, myoclonus
Myoclonus
Myoclonus is brief, involuntary twitching of a muscle or a group of muscles. It describes a medical sign and, generally, is not a diagnosis of a disease. Brief twitches are perfectly normal. The myoclonic twitches are usually caused by sudden muscle contractions; they also can result from brief...

 and myalgia
Myalgia
Myalgia means "muscle pain" and is a symptom of many diseases and disorders. The most common causes are the overuse or over-stretching of a muscle or group of muscles. Myalgia without a traumatic history is often due to viral infections...

. Diagnostic procedures that may reveal muscular disorders include testing creatine kinase levels in the blood and electromyography
Electromyography
Electromyography is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle...

 (measuring electrical activity in muscles). In some cases, muscle biopsy
Muscle biopsy
In medicine, a muscle biopsy is a procedure in which a piece of muscle tissue is removed from an organism and examined microscopically. A biopsy needle is usually inserted into a muscle, wherein a small amount of tissue remains. Alternatively, an "open biopsy" can be performed by obtaining the...

 may be done to identify a myopathy
Myopathy
In medicine, a myopathy is a muscular disease in which the muscle fibers do not function for any one of many reasons, resulting in muscular weakness. "Myopathy" simply means muscle disease...

, as well as genetic testing
Genetic testing
Genetic testing is among the newest and most sophisticated of techniques used to test for genetic disorders which involves direct examination of the DNA molecule itself. Other genetic tests include biochemical tests for such gene products as enzymes and other proteins and for microscopic...

 to identify DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 abnormalities associated with specific myopathies and dystrophies
Muscular dystrophy
Muscular dystrophy is a group of muscle diseases that weaken the musculoskeletal system and hamper locomotion. Muscular dystrophies are characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle cells and tissue.In the 1860s, descriptions of boys who...

.

Neuromuscular disease
Neuromuscular disease
Neuromuscular disease is a very broad term that encompasses many diseases and ailments that either directly, via intrinsic muscle pathology, or indirectly, via nerve pathology, impair the functioning of the muscles....

s are those that affect the muscles and/or their nervous control. In general, problems with nervous control can cause spasticity or paralysis
Paralysis
Paralysis is loss of muscle function for one or more muscles. Paralysis can be accompanied by a loss of feeling in the affected area if there is sensory damage as well as motor. A study conducted by the Christopher & Dana Reeve Foundation, suggests that about 1 in 50 people have been diagnosed...

, depending on the location and nature of the problem. A large proportion of neurological disorder
Neurological disorder
A neurological disorder is a disorder of the body's nervous system. Structural, biochemical or electrical abnormalities in the brain, spinal cord, or in the nerves leading to or from them, can result in symptoms such as paralysis, muscle weakness, poor coordination, loss of sensation, seizures,...

s leads to problems with movement, ranging from cerebrovascular accident (stroke) and Parkinson's disease
Parkinson's disease
Parkinson's disease is a degenerative disorder of the central nervous system...

 to Creutzfeldt-Jakob disease
Creutzfeldt-Jakob disease
Creutzfeldt–Jakob disease or CJD is a degenerative neurological disorder that is incurable and invariably fatal. CJD is at times called a human form of mad cow disease, given that bovine spongiform encephalopathy is believed to be the cause of variant Creutzfeldt–Jakob disease in humans.CJD...

.

A non-invasive elastography
Elastography
Elastography is a non-invasive method in which stiffness or strain images of soft tissue are used to detect or classify tumors. A tumor or a suspicious cancerous growth is normally 5-28 times stiffer than the background of normal soft tissue. When a mechanical compression or vibration is applied,...

 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
Protein filament
In biology, a filament is a "long chain of proteins, such as those found in hair, muscle, or in flagella". They are often bundled together for strength and rigidity. Some cellular examples include:*Actin filaments*Microtubules*Intermediate filaments...

 along the axis of the muscle. During contraction
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...

, the muscle shortens along its longitudinal axis and expands across the transverse axis
Transverse plane
The transverse plane is an imaginary plane that divides the body into superior and inferior parts. It is perpendicular to the coronal and sagittal planes....

, producing vibrations at the surface.

Atrophy


There are many diseases and conditions which cause a decrease in muscle mass, known as muscle atrophy
Muscle atrophy
Muscle atrophy, or disuse atrophy, is defined as a decrease in the mass of the muscle; it can be a partial or complete wasting away of muscle. When a muscle atrophies, this leads to muscle weakness, since the ability to exert force is related to mass...

. Examples include cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

 and AIDS
AIDS
Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...

, which induce a body wasting syndrome called cachexia
Cachexia
Cachexia or wasting syndrome is loss of weight, muscle atrophy, fatigue, weakness, and significant loss of appetite in someone who is not actively trying to lose weight...

. Other syndromes or conditions which can induce skeletal muscle atrophy are congestive heart disease and some diseases of the liver
Hepato-biliary diseases
Hepato-biliary diseases affect the liver and/or biliary tract, and are studied in the branch of medicine called Hepatology.-Viral hepatitis:* Acute hepatitis A* Acute hepatitis B...

.

During aging, there is a gradual decrease in the ability to maintain skeletal muscle function and mass, known as sarcopenia
Sarcopenia
Sarcopenia is the degenerative loss of skeletal muscle mass and strength associated with aging...

. 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.

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.

Physical inactivity and atrophy


Inactivity and starvation in 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.

Bear
Bear
Bears are mammals of the family Ursidae. Bears are classified as caniforms, or doglike carnivorans, with the pinnipeds being their closest living relatives. Although there are only eight living species of bear, they are widespread, appearing in a wide variety of habitats throughout the Northern...

s are an exception to this rule; species in the family Ursidae are famous for their ability to survive unfavorable environmental conditions of low temperatures and limited nutrition availability during winter by means of hibernation
Hibernation
Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...

. During that time, bears 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, bears take advantage of the nutrition availability and accumulate muscle protein. The protein balance at time of dormancy is also maintained by lower levels of protein breakdown during the winter time. At times of immobility, muscle wasting in bears 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
Torpor
Torpor, sometimes called temporary hibernation is a state of decreased physiological activity in an animal, usually characterized by a reduced body temperature and rate of metabolism. Animals that go through torpor include birds and some mammals such as mice and bats...

. 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 exercise
Physical exercise
Physical exercise is any bodily activity that enhances or maintains physical fitness and overall health and wellness. It is performed for various reasons including strengthening muscles and the cardiovascular system, honing athletic skills, weight loss or maintenance, as well as for the purpose of...

; instead the muscle cells simply get bigger. Muscle fibres have a limited capacity for growth through hypertrophy and some believe they split through hyperplasia
Hyperplasia
Hyperplasia means increase in number of cells/proliferation of cells. It may result in the gross enlargement of an organ and the term is sometimes mixed with benign neoplasia/ benign tumor....

 if subject to increased demand.

Physiological strength


Vertebrate muscle typically produces approximately 25 N 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 sarcomere
Sarcomere
A sarcomere is the basic unit of a muscle. Muscles are composed of tubular muscle cells . Muscle cells are composed of tubular myofibrils. Myofibrils are composed of repeating sections of sarcomeres, which appear under the microscope as dark and light bands...

s than vertebrates, resulting in many more sites for actin and myosin to bind and thus much greater force per square centimeter.

The strength of any given muscle, in terms of force exerted on the skeleton, depends upon length, shortening speed, cross sectional area, pennation
Pennate muscle
A pennate or pinnate muscle is a muscle with fascicles that attach obliquely to its tendon...

, sarcomere
Sarcomere
A sarcomere is the basic unit of a muscle. Muscles are composed of tubular muscle cells . Muscle cells are composed of tubular myofibrils. Myofibrils are composed of repeating sections of sarcomeres, which appear under the microscope as dark and light bands...

 length, and myosin
Myosin
Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...

 isoform.

The "strongest" human muscle


Since three factors affect muscular strength simultaneously and muscles never work individually, it is misleading to compare strength in individual muscles, and state that one is the "strongest". But below are several muscles whose strength is noteworthy for different reasons.
  • In ordinary parlance, muscular "strength" usually refers to the ability to exert a force on an external object—for example, lifting a weight. By this definition, the masseter or jaw
    Jaw
    The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...

     muscle is the strongest. The 1992 Guinness Book of Records records the achievement of a bite strength of 4337 N for 2 seconds. What distinguishes the masseter is not anything special about the muscle itself, but its advantage in working against a much shorter lever arm than other muscles.
  • If "strength" refers to the force exerted by the muscle itself, e.g., on the place where it inserts into a bone, then the strongest muscles are those with the largest cross-sectional area. This is because the tension exerted by an individual skeletal muscle fiber does not vary much. Each fiber can exert a force on the order of 0.3 micronewton. By this definition, the strongest muscle of the body is usually said to be the quadriceps femoris or the gluteus maximus.
  • A shorter muscle will be stronger "pound for pound" (i.e., by weight
    Mass
    Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

    ) 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
    Infant
    A newborn or baby is the very young offspring of a human or other mammal. A newborn is an infant who is within hours, days, or up to a few weeks from birth. In medical contexts, newborn or neonate refers to an infant in the first 28 days after birth...

     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 saccade
    Saccade
    A saccade is a fast movement of an eye, head or other part of an animal's body or device. It can also be a fast shift in frequency of an emitted signal or other quick change. Saccades are quick, simultaneous movements of both eyes in the same direction...

    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 tongue
    Tongue
    The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...

     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
    Heart
    The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

     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 watt
    Watt
    The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

    s. 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.

Efficiency


The efficiency
Mechanical efficiency
Mechanical efficiency measures the effectiveness of a machine in transforming the energy and power that is input to the device into an output force and movement...

 of human muscle has been measured (in the context of rowing
Watercraft rowing
Watercraft rowing is the act of propelling a boat using the motion of oars in the water. The difference between paddling and rowing is that with rowing the oars have a mechanical connection with the boat whereas with paddling the paddles are hand-held with no mechanical connection.This article...

 and cycling
Cycling
Cycling, also called bicycling or biking, is the use of bicycles for transport, recreation, or for sport. Persons engaged in cycling are cyclists or bicyclists...

) at 18% to 26%. The efficiency is defined as the ratio of mechanical work
Mechanical work
In physics, work is a scalar quantity that can be described as the product of a force times the distance through which it acts, and it is called the work of the force. Only the component of a force in the direction of the movement of its point of application does work...

 output to the total metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 cost, as can be calculated from oxygen consumption. This low efficiency is the result of about 40% efficiency of generating ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

 from food energy
Food energy
Food energy is the amount of energy obtained from food that is available through cellular respiration.Food energy is expressed in food calories or kilojoules...

, 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, 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
Work Loop
The work loop technique is used in muscle physiology to evaluate the mechanical work and power output of skeletal or cardiac muscle contractions via in vitro muscle testing of whole muscles, fiber bundles or single muscle fibers...

.

Density of muscle tissue compared to adipose tissue


The density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 of mammalian skeletal muscle tissue is about 1.06 kg/liter. This can be contrasted with the density of adipose tissue
Adipose tissue
In histology, adipose tissue or body fat or fat depot or just fat is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblasts...

 (fat), which is 0.9196 kg/liter. This makes muscle tissue approximately 15% denser than fat tissue.

Resting energy expenditure of muscle


At rest, skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...

 consumes 54.4 kJ/kg (13.0 kcal/kg) per day. This is larger than adipose tissue
Adipose tissue
In histology, adipose tissue or body fat or fat depot or just fat is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblasts...

 (fat) at 18.8 kJ/kg (4.5 kcal/kg), and bone at 9.6 kJ/kg (2.3 kcal/kg).

Muscle evolution


Evolutionarily, specialized forms of skeletal and cardiac muscle
Cardiac muscle
Cardiac muscle is a type of involuntary striated muscle found in the walls and histologic foundation of the heart, specifically the myocardium. Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth muscle...

s predated the divergence of the vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

/arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

 evolutionary line. This indicates that these types of muscle developed in a common ancestor
Ancestor
An ancestor is a parent or the parent of an 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


  • Atrophy
    Atrophy
    Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations , poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply to the target organ, disuse or lack of exercise or disease intrinsic to the tissue itself...

  • Bodybuilding
    Bodybuilding
    Bodybuilding is a form of body modification involving intensive muscle hypertrophy. An individual who engages in this activity is referred to as a bodybuilder. In competitive and professional bodybuilding, bodybuilders display their physiques to a panel of judges, who assign points based on their...

  • Cross education
    Cross Education
    Cross education is a neurophysiological phenomenon where an increase in strength is witnessed within an untrained limb following unilateral strength training in the opposite, contralateral limb....

  • Electroactive polymers
    Electroactive polymers
    Electroactive Polymers, or EAPs, are polymers that exhibit a change in size or shape when stimulated by an electric field. The most common applications of this type of material are in actuators and sensors. A typical characteristic property of an EAP is that they will undergo a large amount of...

     (materials that behave like muscles, used in robotics research)
  • Fascia
    Fascia
    A fascia is a layer of fibrous tissue that permeates the human body. A fascia is a connective tissue that surrounds muscles, groups of muscles, blood vessels, and nerves, binding those structures together in much the same manner as plastic wrap can be used to hold the contents of sandwiches...

  • Hand strength
    Hand strength
    Hand strength measurements are of interest to study pathology of the hand that involves loss of muscle strength. Examples of these pathologies are carpal tunnel syndrome, nerve injury, tendon injuries of the hand, and neuromuscular disorders....

  • In vitro muscle testing
    In vitro muscle testing
    In vitro muscle testing is a method used to characterize properties of living muscle tissue after having removed the tissue from an organism. This allows more extensive and precise quantification of muscle properties than in vivo testing...

  • List of muscles of the human body
  • List of weight training exercises
  • Muscle atrophy
    Muscle atrophy
    Muscle atrophy, or disuse atrophy, is defined as a decrease in the mass of the muscle; it can be a partial or complete wasting away of muscle. When a muscle atrophies, this leads to muscle weakness, since the ability to exert force is related to mass...

  • Muscle memory
    Muscle memory
    Muscle memory has been used synonymously with motor learning, which is a form of procedural memory that involves consolidating a specific motor task into memory through repetition. When a movement is repeated over time, a long-term muscle memory is created for that task, eventually allowing it to...

  • Muscle tone
    Muscle tone
    In physiology, medicine, and anatomy, muscle tone is the continuous and passive partial contraction of the muscles, or the muscle’s resistance to passive stretch during resting state. It helps maintain posture, and it declines during REM sleep.-Purpose:Unconscious nerve impulses maintain the...

     (residual muscle tension)
  • Musculoskeletal system
    Musculoskeletal system
    A musculoskeletal system is an organ system that gives animals the ability to move using the muscular and skeletal systems...

  • Muscular system
    Muscular system
    The muscular system is the anatomical system of a species that allows it to move. The muscular system in vertebrates is controlled through the nervous system, although some muscles can be completely autonomous.- Muscles :...

  • Myopathy
    Myopathy
    In medicine, a myopathy is a muscular disease in which the muscle fibers do not function for any one of many reasons, resulting in muscular weakness. "Myopathy" simply means muscle disease...

     (pathology of muscle cells)
  • Myotomy
    Myotomy
    Myotomy describes a surgical procedure in which muscle is cut.A common example of a myotomy is the Heller myotomy....

  • Phonomyography
    Phonomyography
    Phonomyography is a technique to measure the force of muscle contraction by recording the low frequency sounds created during muscular activity.Although less precise than the more traditional mechanomyography, it is considerably easier to set up...

  • Preflexes
    Preflexes
    Preflexes are the latent capacities in the musculoskeletal system that auto-stabilize movements through the use of the nonlinear visco-elastic properties of muscles when they contract. The term "preflex" for such a zero-delay, intrinsic feedback loop was coined by Loeb...

  • Rapid plant movement
    Rapid plant movement
    Rapid plant movement encompasses movement in plant structures occurring over a very short period of time, usually under one second. For example, the Venus Flytrap closes its trap in about 100 milliseconds. The Dogwood Bunchberry's flower opens its petals and fires pollen in less than 0.5 milliseconds...

  • Rohmert's law
    Rohmert's law
    Widely used in the human factors and ergonomics field, Rohmert's law states that the maximum force one's muscles can exert decreases exponentially from the time one begins continuously exerting the said force...

  • Soft tissue
    Soft tissue
    In anatomy, the term soft tissue refers to tissues that connect, support, or surround other structures and organs of the body, not being bone. Soft tissue includes tendons, ligaments, fascia, skin, fibrous tissues, fat, and synovial membranes , and muscles, nerves and blood vessels .It is sometimes...


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(Heart output 1.3 to 5 watts, lifetime output 2 to 3 ×109 joules)