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Skeletal muscles (commonly referred to as muscles) are
organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma Parenchyma () is the bulk of functional ...
of the
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...
muscular system The muscular system is an organ system An organ system is a biological system A biological system is a complex biological network, network which connects several biologically relevant entities. Biological organization spans several scales and ...

muscular system
that are mostly attached by
tendon A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant ...

tendon
s to
bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...

bone
s of the
skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consu ...

skeleton
. The
muscle cell A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell Cardiac muscle cells or cardiomyocytes (also known as myocardiocytes or cardiac myocytes) are the muscle cells (myocyte A muscle cell is also known as a m ...
s of skeletal muscles are much longer than in the other types of
muscle tissue Muscle tissues are soft tissue of a tendon A tendon or sinew is a tough band of fibrous connective tissue that connects muscle to bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton i ...

muscle tissue
, and are often known as
muscle fibers A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell Cardiac muscle cells or cardiomyocytes (also known as myocardiocytes or cardiac myocytes) are the muscle cells (myocyte A muscle cell is also known as a m ...
. The muscle tissue of a skeletal muscle is – having a striped appearance due to the arrangement of the
sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue Striated muscle tissue is a muscle tissue Muscle tissue is a soft tissue that composes muscles in animal bodie ...

sarcomere
s. Skeletal muscles are voluntary muscles under the control of the
somatic nervous system The somatic nervous system (SNS), or voluntary nervous system is the part of the peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In Biology, biology, the nervous syste ...
. The other types of muscle are
cardiac muscle Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biol ...

cardiac muscle
which is also striated and
smooth muscle Smooth muscle is an involuntary non-striated muscle Striated muscle tissue is a muscle tissue Muscle tissue is a soft tissue that composes muscles in animal bodies, and gives rise to muscles' ability to contract. It is also referred to as myo ...

smooth muscle
which is non-striated; both of these types of muscle tissue are classified as involuntary, or, under the control of the
autonomic nervous system The autonomic nervous system (ANS), formerly the vegetative nervous system, is a division of the peripheral nervous system that supplies smooth muscle and glands, and thus influences the function of viscera, internal organs. The autonomic nervous ...

autonomic nervous system
. A skeletal muscle contains multiple fascicles – bundles of muscle fibers. Each individual fiber, and each muscle is surrounded by a type of
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
layer of
fascia A fascia (; plural fasciae ; adjective fascial; from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Thro ...
. Muscle fibers are formed from the fusion of developmental
myoblasts A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell Cardiac muscle cells or cardiomyocytes (also known as myocardiocytes or cardiac myocytes) are the muscle cells (myocyte A muscle cell is also known as a m ...

myoblasts
in a process known as
myogenesis Myogenesis is the formation of skeletal muscle, skeletal muscular tissue, particularly during embryonic development. Skeletal muscle#Skeletal muscle fibers, Muscle fibers generally form through the fusion of precursor cell, precursor myoblasts int ...
resulting in long
multinucleate Multinucleate cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a roo ...
d cells. In these cells the
nuclei ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nuclei
termed ''myonuclei'' are located along the inside of the
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...
. Muscle fibers also have multiple
mitochondria A mitochondrion (; ) is a double-membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, i ...

mitochondria
to meet energy needs. Muscle fibers are in turn composed of
myofibrils s in parallel, and sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the complicated unit of striated muscle tissue. It is the repeating unit between two Z lines. Skeletal muscles are composed of tubular mu ...
. The myofibrils are composed of
actin Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments. It is found in essentially all Eukaryote, eukaryotic cells, where it may be present at a concentration of over 100 Micromolar, μ ...
and
myosin Myosins () are a superfamily SUPERFAMILY is a database and search platform of structural and functional annotation for all proteins and genomes. It classifies amino acid sequences into known structural domains, especially into SCOP superfamilie ...

myosin
filaments called
myofilament Myofilaments are the two protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inte ...

myofilament
s, repeated in units called sarcomeres, which are the basic functional, contractile units of the muscle fiber necessary for
muscle contraction Muscle contraction is the activation of tension Tension may refer to: Science * Psychological stress * Tension (physics), a force related to the stretching of an object (the opposite of compression) * Tension (geology), a stress which stretches ...

muscle contraction
. Muscles are predominantly powered by the
oxidation Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter ...

oxidation
of
fat In nutrition Nutrition is the biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided ...

fat
s and
carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a ...
s, but
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: *Anaerobic adhesive, a bonding ag ...

anaerobic
chemical reactions are also used, particularly by fast twitch fibers. These chemical reactions produce
adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properti ...

adenosine triphosphate
(ATP) molecules that are used to power the movement of the
myosin head The myosin head is the part of the thick myofilament Myofilaments are the two protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, c ...
s.


Structure


Gross anatomy

There are more than 600 skeletal muscles in the human body, making up around 40% to 50% of body weight. Most muscles occur in bilaterally-placed pairs to serve both sides of the body. Muscles are often classed as groups of muscles that work together to carry out an action. In the
torso The torso or trunk is an anatomical terminology, anatomical term for the central part, or the core (anatomy), core, of the body (biology), body of many animals (including humans), from which the head, neck, limb (anatomy), limbs, tail and other a ...

torso
there are several major muscle groups including the pectoral, and
abdominal muscles The abdomen (colloquially called the belly, tummy, midriff or stomach) is the part of the body between the thorax The thorax or chest is a part of the anatomy of humans, mammals, other tetrapod animals located between the neck and the abdomen. I ...

abdominal muscles
; intrinsic and extrinsic muscles are subdivisions of muscle groups in the
hand A hand is a prehensile, multi- fingered appendage located at the end of the forearm The forearm is the region of the upper limb between the Elbow-joint, elbow and the wrist. The term forearm is used in anatomy to distinguish it from the arm, ...

hand
,
foot The foot (plural: feet) is an anatomical Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemis ...

foot
,
tongue The tongue is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Brita ...

tongue
, and
extraocular muscles The extraocular muscles are the six muscles that control Eye movement (sensory), movement of the eye and one muscle that controls eyelid elevation (Levator palpebrae superioris muscle, levator palpebrae). The actions of the six muscles responsible f ...

extraocular muscles
of the
eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light L ...

eye
. Muscles are also grouped into compartments including four groups in the arm, and the four groups in the leg. Apart from the contractile part of a muscle consisting of its fibers, a muscle contains a non-contractile part of dense fibrous connective tissue that makes up the
tendon A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant ...

tendon
at each end. The tendons attach the muscles to bones to give skeletal movement. The length of a muscle includes the tendons.
Connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
is present in all muscles as
deep fascia Deep fascia (or investing fascia) is a fascia A fascia (; plural fasciae ; adjective fascial; from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken ...
. Deep fascia specialises within muscles to enclose each muscle fiber as
endomysium The endomysium, meaning ''within the muscle'', is a wispy layer of areolar connective tissue Loose connective tissue (also called areolar connective tissue) is a category of connective tissue Connective tissue is one of the four basic types o ...
; each muscle fascicle as
perimysium Perimysium is a sheath of connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () i ...
, and each individual muscle as
epimysium Epimysium (plural ''epimysia'') (Greek ''epi-'' for on, upon, or above + Greek ''mys'' for muscle) is the fibrous tissue envelope that surrounds skeletal muscle. It is a layer of dense irregular connective tissue which ensheaths the entire muscle a ...
. Together these layers are called ''mysia''. Deep fascia also separates the groups of muscles into muscle compartments. Two types of
sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * ...
s found in muscles are
muscle spindle Muscle spindles are stretch receptor Stretch receptors are mechanoreceptors A mechanoreceptor, also called mechanoceptor, is a sensory neuron, sensory cell that responds to mechanical pressure or distortion. There are four main types of mechanore ...
s, and
Golgi tendon organ The Golgi tendon organ (GTO) (also called Golgi organ, tendon organ, neurotendinous organ or neurotendinous spindle) is a proprioceptor Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement and bod ...
s. Muscle spindles are
stretch receptor Stretch receptors are mechanoreceptors A mechanoreceptor, also called mechanoceptor, is a sensory neuron, sensory cell that responds to mechanical pressure or distortion. There are four main types of mechanoreceptors in glabrous, or hairless, mammal ...
s located in the muscle belly. Golgi tendon organs are
proprioceptor Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense A sense is a biological system used by an organism for sensation, the process of gathering information about the world and responding to Stimulus (physiology ...
s located at the myotendinous junction that inform of a muscle's tension.


Skeletal muscle fibers

Skeletal muscle cells are the individual contractile cells within a muscle, and are often termed as muscle fibers. A single muscle such as the
biceps The biceps ( la, musculus biceps brachii, "two-headed muscle of the arm") is a large muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life ...

biceps
in a young adult male contains around 253,000 muscle fibers. Skeletal muscle fibers are the only muscle cells that are
multinucleate Multinucleate cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a roo ...
d with the
nuclei ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nuclei
often referred to as myonuclei. This occurs during
myogenesis Myogenesis is the formation of skeletal muscle, skeletal muscular tissue, particularly during embryonic development. Skeletal muscle#Skeletal muscle fibers, Muscle fibers generally form through the fusion of precursor cell, precursor myoblasts int ...
with the fusion of myoblasts each contributing a nucleus. Fusion depends on muscle-specific proteins known as fusogens called ''myomaker'' and ''myomerger''. Many nuclei are needed by the skeletal muscle cell for the large amounts of proteins and enzymes needed to be produced for the cell's normal functioning. A single muscle fiber can contain from hundreds to thousands of nuclei. A muscle fiber for example in the human biceps with a length of 10 cm can have as many as 3000 nuclei. Unlike in a non-muscle cell where the nucleus is centrally positioned, the myonucleus is elongated and located close to the
sarcolemma The sarcolemma (''sarco'' (from ''sarx'') from Greek; flesh, and ''lemma'' from Greek; sheath) also called the myolemma, is the cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasm ...
. The myonuclei are quite uniformly arranged along the fiber with each nucleus having its own ''myonuclear domain'' where it is responsible for supporting the volume of cytoplasm in that particular section of the myofiber. A group of muscle
stem cell In multicellular organisms Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties ...
s known as
myosatellite cell Myosatellite cells, also known as satellite cells or muscle stem cells, are small multipotent cells with very little cytoplasm In cell biology, the cytoplasm is all of the material within a Cell (biology), cell, enclosed by the cell membrane, ...
s, also ''satellite cells'' are found between the
basement membrane The basement membrane is a thin, pliable sheet-like type of extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processe ...
and the sarcolemma of muscle fibers. These cells are normally quiescent but can be activated by exercise or pathology to provide additional myonuclei for muscle growth or repair.


Attachment to tendons

Muscles attach to
tendon A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant ...

tendon
s in a complex interface region known as the musculotendinous junction also known as the myotendinous junction, an area specialised for the primary transmission of force. At the muscle-tendon interface, force is transmitted from the sarcomeres in the muscle cells to the tendon. Muscles and tendons develop in close association, and after their joining at the myotendinous junction they constitute a dynamic unit for the transmission of force from muscle contraction to the skeletal system.


Arrangement of muscle fibers

Muscle architecture refers to the arrangement of muscle fibers relative to the axis of force generation, which runs from a to its . The usual arrangements are types of
parallel Parallel may refer to: Computing * Parallel algorithm In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their a ...

parallel
, and types of
pennate muscle A pennate or pinnate muscle (also called a penniform muscle) is a type of skeletal muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system that are mostly attached by tendons to ...
. In parallel muscles the fascicles run parallel to the axis of force generation, but the fascicles can vary in their relationship to one another, and to their tendons. These variations are seen in
fusiform Fusiform means having a spindle-like shape that is wide in the middle and tapers at both ends. It is similar to the lemon-shape, but often implies a focal broadening of a structure that continues from one or both ends, such as an aneurysm An ...
,
strap A strap, sometimes also called strop, is an elongated flap Flap may refer to: Arts, entertainment, and media * ''Flap'' (film), a 1970 American film * Flap, a boss character in the arcade game ''Gaiapolis'' * Flap, a minor character in the fi ...

strap
, and convergent muscles. A convergent muscle has a triangular or fan-shape as the fibers converge at its insertion and are fanned out broadly at the origin. A less common example of a parallel muscle is a circular muscle such as the
orbicularis oculi The orbicularis oculi is a muscle in the face that closes the eyelids. It arises from the nasal part of the frontal bone, from the frontal process of the maxilla in front of the lacrimal groove, and from the anterior surface and borders of a short f ...
, in which the fibers are longitudinally arranged, but create a circle from origin to insertion.Lieber, Richard L. (2002) ''Skeletal muscle structure, function, and plasticity''. Wolters Kluwer Health. These different architectures, can cause variations in the tension that a muscle can create between its tendons. The fibers in
pennate muscle A pennate or pinnate muscle (also called a penniform muscle) is a type of skeletal muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system that are mostly attached by tendons to ...
s run at an angle to the axis of force generation. This pennation angle reduces the effective force of any individual fiber, as it is effectively pulling off-axis. However, because of this angle, more fibers can be packed into the same muscle volume, increasing the
physiological cross-sectional area 300px, Figure 1 Pennate muscle fiber arrangements. The green lines represent PCSA; the blue lines represent ACSA In muscle physiology, physiological cross-sectional area (PCSA) is the area of the cross section of a muscle perpendicular to its fibe ...
(PCSA). This effect is known as fiber packing, and in terms of force generation, it more than overcomes the efficiency-loss of the off-axis orientation. The trade-off comes in overall speed of muscle shortening and in the total excursion. Overall muscle shortening speed is reduced compared to fiber shortening speed, as is the total distance of shortening. All of these effects scale with pennation angle; greater angles lead to greater force due to increased fiber packing and PCSA, but with greater losses in shortening speed and excursion. Types of pennate muscle are
unipennate Muscle architecture is the physical arrangement of muscle fibers at the macroscopic level that determines a muscle’s mechanical function. There are several different muscle architecture types including: parallel, pennate and hydrostats. Force prod ...
,
bipennate Muscle architecture is the physical arrangement of muscle fibers at the macroscopic level that determines a muscle’s mechanical function. There are several different muscle architecture types including: parallel, pennate and hydrostats. Force prod ...
, and multipennate. A unipennate muscle has similarly angled fibers that are on one side of a tendon. A bipennate muscle has fibers on two sides of a tendon. Multipennate muscles have fibers that are oriented at multiple angles along the force-generating axis, and this is the most general and most common architecture.


Muscle fiber growth

Muscle fibers grow when exercised and shrink when not in use. This is due to the fact that exercise stimulates the increase in
myofibrils s in parallel, and sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the complicated unit of striated muscle tissue. It is the repeating unit between two Z lines. Skeletal muscles are composed of tubular mu ...
which increase the overall size of muscle cells. Well exercised muscles can not only add more size but can also develop more
mitochondria A mitochondrion (; ) is a double-membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, i ...

mitochondria
,
myoglobin Myoglobin (symbol Mb or MB) is an iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their ...

myoglobin
,
glycogen Glycogen is a multibranched polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant found in . They are long chain carbohydrates composed of units bound together by . This carbohydrate can react with water () usi ...

glycogen
and a higher density of
capillaries A capillary is a small blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biologic ...

capillaries
. However muscle cells cannot divide to produce new cells, and as a result there are fewer muscle cells in an adult than in a newborn.


Muscle naming

There are a number of terms used in the naming of muscles including those relating to size, shape, action, location, their orientation, and their number of heads. ;By size: ''brevis'' means short; ''longus'' means long; ''longissimus'' means longest; ''magnus'' means large; ''major'' means larger; ''maximus'' means largest; ''minor'' means smaller, and ''minimus'' smallest; ''latissimus'' means widest, and ''vastus'' means huge. These terms are often used after the particular muscle such as
gluteus maximus The gluteus maximus is the main extensor muscle of the hip. It is the largest and outermost of the three gluteal muscles and makes up a large part of the shape and appearance of each side of the hips. Its thick fleshy mass, in a quadrilateral sh ...

gluteus maximus
, and
gluteus minimus The gluteus minimus, or glutæus minimus, the smallest of the three gluteal muscles, is situated immediately beneath the gluteus medius. Origin and insertion It is fan-shaped, arising from the outer surface of the ilium, between the anterior ...

gluteus minimus
. ;By relative shape: ''deltoid'' means triangular; ''quadratus'' means having four sides; ''rhomboideus'' means having a
rhomboid Traditionally, in two-dimensional geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, ...

rhomboid
shape; ''teres'' means round or cylindrical, and ''trapezius'' means having a
trapezoid In Euclidean geometry Euclidean geometry is a mathematical system attributed to Alexandrian Greek mathematics , Greek mathematician Euclid, which he described in his textbook on geometry: the ''Euclid's Elements, Elements''. Euclid's meth ...

trapezoid
shape; ''serratus'' means saw-toothed; ''orbicularis'' means circular; ''pectinate'' means comblike; ''piriformis'' means pear-shaped; ''platys'' means flat and ''gracilis'' means slender. Examples are the
pronator teres The pronator teres is a muscle (located mainly in the forearm) that, along with the Pronator quadratus muscle, pronator quadratus, serves to Anatomical terms of motion#Pronation and supination, pronate the forearm (turning it so that the palm faces ...

pronator teres
, and the
pronator quadratus Pronator quadratus is a square-shaped muscle Muscle is a soft tissue found in most animals. Muscle cells contain protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residu ...
. ;By action: '' abductor'' moving away from the midline; '' adductor'' moving towards the midline; '' depressor'' moving downwards; ''
elevator An elevator (North American English North American English (NAmE, NAE) is the most generalized variety (linguistics), variety of the English language as spoken in the United States and Canada. Because of their related histories and ...
'' moving upwards; '' flexor'' moving that decreases an angle; ''
extensor Motion (physics), Motion, the process of movement, is described using specific anatomical terminology, anatomical terms. Motion includes movement of Organ (anatomy), organs, joints, limbs, and specific sections of the body. The terminology used d ...
'' moving that increase an angle or straightens; ''
pronator Motion (physics), Motion, the process of movement, is described using specific anatomical terminology, anatomical terms. Motion includes movement of Organ (anatomy), organs, joints, limbs, and specific sections of the body. The terminology used d ...

pronator
'' moving ; ''
supinator In human anatomy, the supinator is a broad muscle in the posterior compartment of the forearm The posterior compartment of the forearm (or extensor compartment) contains twelve muscles which are chiefly responsible for extension of the wrist I ...

supinator
'' moving ; '' internal rotator''
rotating A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotation plane'', and the imaginary line extending from the center an ...

rotating
towards the body; '' external rotator'' rotating away from the body; ''
sphincter A sphincter is a circular muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A give ...
'' decreases the size , and ''tensor'' gives tension to; '' fixator muscles'' serve to fix a joint in a given position by stabilizing the prime mover whilst other joints are moving. ;By number of heads:''biceps'' two; ''triceps'' three and ''quadriceps'' four. ;By location: named after the near main structure such as the
temporal muscle In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of concerned with the study of the structure of s and their parts. Anatomy is a branch of natural science which deals with the structural organization of living things. It i ...

temporal muscle
(temporalis) near to the
temporal bone The temporal bones are situated at the sides and base of the skull The skull is a bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function ...

temporal bone
. Also ''supra-'' above; ''infra-'' below, and ''sub-'' under. ;By fascicle orientation: Relative to the midline, ''rectus'' means parallel to the midline; ''transverse'' means perpendicular to the midline, and ''oblique'' means diagonal to the midline. Relative to the axis of the generation of force – types of ''parallel'', and types of ''pennate'' muscles.


Fiber types

Broadly there are two types of muscle fiber: Type I, which is slow, and Type II which are fast. Type II has two divisions of type IIA (oxidative), and type IIX (glycolytic), giving three main fiber types. These fibers have relatively distinct metabolic, contractile, and
motor unit A motor unit is made up of a motor neuron and all of the Skeletal muscle#Skeletal muscle fibers, skeletal muscle fibers, also known as sarcomere innervated by the neuron's axon terminals. Groups of motor units often work together as a Motor pool (n ...

motor unit
properties. The table below differentiates these types of properties. These types of properties—while they are partly dependent on the properties of individual fibers—tend to be relevant and measured at the level of the motor unit, rather than individual fiber.


Fiber color

Traditionally, fibers were categorized depending on their varying color, which is a reflection of
myoglobin Myoglobin (symbol Mb or MB) is an iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their ...

myoglobin
content. Type I fibers appear red due to the high levels of myoglobin. Red muscle fibers tend to have more mitochondria and greater local capillary density. These fibers are more suited for endurance and are slow to fatigue because they use
oxidative metabolism 300px, Typical eukaryote, eukaryotic cell Cellular respiration is a set of metabolism, metabolic reactions and processes that take place in the cell (biology), cells of organisms to convert Energy#Energy and life, chemical energy from oxygen mo ...
to generate ATP (
adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properti ...

adenosine triphosphate
). Less oxidative Type II fibers are white due to relatively low myoglobin and a reliance on glycolytic enzymes.


Twitch speed

Fibers can also be classified on their twitch capabilities, into fast and slow twitch. These traits largely, but not completely, overlap the classifications based on color, ATPase, or MHC. Some authors define a fast twitch fiber as one in which the myosin can split ATP very quickly. These mainly include the ATPase type II and MHC type II fibers. However, fast twitch fibers also demonstrate a higher capability for electrochemical transmission of action potentials and a rapid level of calcium release and uptake by the sarcoplasmic reticulum. The fast twitch fibers rely on a well-developed,
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: *Adhesive#Anaerobic, Anaerobic ad ...
, short term, glycolytic system for energy transfer and can contract and develop tension at 2–3 times the rate of slow twitch fibers. Fast twitch muscles are much better at generating short bursts of strength or speed than slow muscles, and so fatigue more quickly. The slow twitch fibers generate energy for ATP re-synthesis by means of a long term system of
aerobic Aerobic means "requiring Earth's atmosphere, air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Cellular respiration#Aerobic r ...
energy transfer. These mainly include the ATPase type I and MHC type I fibers. They tend to have a low activity level of ATPase, a slower speed of contraction with a less well developed glycolytic capacity. Fibers that become slow-twitch develop greater numbers of mitochondria and capillaries making them better for prolonged work. ;Type distribution: Individual muscles tend to be a mixture of various fiber types, but their proportions vary depending on the actions of that muscle. For instance, in humans, the quadriceps muscles contain ~52% type I fibers, while the soleus is ~80% type I. The orbicularis oculi muscle of the eye is only ~15% type I. Motor units within the muscle, however, have minimal variation between the fibers of that unit. It is this fact that makes the size principal of
motor unit recruitment Motor unit recruitment refers to the activation of additional motor unit A motor unit is made up of a motor neuron and the skeletal muscle fibers innervated by that motor neuron's axon terminal, axonal terminals. Groups of motor units often work tog ...

motor unit recruitment
viable. The total number of skeletal muscle fibers has traditionally been thought not to change. It is believed there are no sex or age differences in fiber distribution; however, proportions of fiber types vary considerably from muscle to muscle and person to person. Among different species there is a much variation in the proportions of muscle fiber types. Sedentary men and women (as well as young children) have 45% type II and 55% type I fibers. People at the higher end of any sport tend to demonstrate patterns of fiber distribution e.g. endurance athletes show a higher level of type I fibers. Sprint athletes, on the other hand, require large numbers of type IIX fibers. Middle-distance event athletes show approximately equal distribution of the two types. This is also often the case for power athletes such as throwers and jumpers. It has been suggested that various types of exercise can induce changes in the fibers of a skeletal muscle. It is thought that if you perform endurance type events for a sustained period of time, some of the type IIX fibers transform into type IIA fibers. However, there is no consensus on the subject. It may well be that the type IIX fibers show enhancements of the oxidative capacity after high intensity endurance training which brings them to a level at which they are able to perform oxidative metabolism as effectively as slow twitch fibers of untrained subjects. This would be brought about by an increase in mitochondrial size and number and the associated related changes, not a change in fiber type.


Fiber typing methods

There are numerous methods employed for fiber-typing, and confusion between the methods is common among non-experts. Two commonly confused methods are
histochemical Histology, also known as microscopic anatomy or microanatomy, is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Mol ...
staining for
myosin ATPase Myosin ATPase ATPases (, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, SV40 T-antigen, adenosine 5'-triphosphatase, ATP hydrolase, complex V (mitochondrial electron transport), (Ca2+ + Mg2+)-ATPase, HCO3−-ATPase, adenosine tr ...
activity and
immunohistochemical targeting the protein CD10. Image:HSP IF IgA.jpg, 200px, Immunofluorescence of human skin using an anti-IgA antibody. The skin is from a patient with Henoch–Schönlein purpura: IgA deposits are found in the walls of small superficial capillarie ...
staining for
myosin heavy chain Myosins () are a Protein superfamily, superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are adenosine triphosphate, ATP-dependent and responsible for ac ...
(MHC) type. Myosin ATPase activity is commonly—and correctly—referred to as simply "fiber type", and results from the direct assaying of ATPase activity under various conditions (e.g. ).
Myosin heavy chain Myosins () are a Protein superfamily, superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are adenosine triphosphate, ATP-dependent and responsible for ac ...
staining is most accurately referred to as "MHC fiber type", e.g. "MHC IIa fibers", and results from determination of different MHC
isoforms A protein isoform, or "protein variant", is a member of a set of highly similar proteins Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystal ...

isoforms
. These methods are closely related physiologically, as the MHC type is the primary determinant of ATPase activity. However, neither of these typing methods is directly metabolic in nature; they do not directly address oxidative or glycolytic capacity of the fiber. When "type I" or "type II" fibers are referred to generically, this most accurately refers to the sum of numerical fiber types (I vs. II) as assessed by myosin ATPase activity staining (e.g. "type II" fibers refers to type IIA + type IIAX + type IIXA ... etc.). Below is a table showing the relationship between these two methods, limited to fiber types found in humans. Subtype capitalization is used in fiber typing vs. MHC typing, and some ATPase types actually contain multiple MHC types. Also, a subtype B or b is not expressed in humans by either method. Early researchers believed humans to express a MHC IIb, which led to the ATPase classification of IIB. However, later research showed that the human MHC IIb was in fact IIx, indicating that the IIB is better named IIX. IIb is expressed in other mammals, so is still accurately seen (along with IIB) in the literature. Non human fiber types include true IIb fibers, IIc, IId, etc. Further fiber typing methods are less formally delineated, and exist on more of a spectrum. They tend to be focused more on metabolic and functional capacities (i.e., oxidative vs. glycolytic, fast vs. slow contraction time). As noted above, fiber typing by ATPase or MHC does not directly measure or dictate these parameters. However, many of the various methods are mechanistically linked, while others are correlated ''in vivo''. For instance, ATPase fiber type is related to contraction speed, because high ATPase activity allows faster crossbridge cycling. While ATPase activity is only one component of contraction speed, Type I fibers are "slow", in part, because they have low speeds of ATPase activity in comparison to Type II fibers. However, measuring contraction speed is not the same as ATPase fiber typing.


Microanatomy

Skeletal muscle exhibits a distinctive banding pattern when viewed under the microscope due to the arrangement of two contractile proteins
myosin Myosins () are a superfamily SUPERFAMILY is a database and search platform of structural and functional annotation for all proteins and genomes. It classifies amino acid sequences into known structural domains, especially into SCOP superfamilie ...

myosin
, and
actin Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments. It is found in essentially all Eukaryote, eukaryotic cells, where it may be present at a concentration of over 100 Micromolar, μ ...
– that are two of the
myofilament Myofilaments are the two protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inte ...

myofilament
s in the
myofibril s in parallel, and sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the complicated unit of striated muscle tissue. It is the repeating unit between two Z lines. Skeletal muscles are composed of tubular mu ...

myofibril
s. The myosin forms the thick filaments, and actin forms the thin filaments, and are arranged in repeating units called
sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue Striated muscle tissue is a muscle tissue Muscle tissue is a soft tissue that composes muscles in animal bodie ...

sarcomere
s. The interaction of both proteins results in muscle contraction. The sarcomere is attached to other organelles such as the mitochondria by
intermediate filament Intermediate filaments (IFs) are cytoskeleton, cytoskeletal structural components found in the cells of vertebrates, and many invertebrates. Homologues of the IF protein have been noted in an invertebrate, the cephalochordate ''Branchiostoma' ...

intermediate filament
s in the cytoskeleton. The
costamere The costamere is a structural-functional component of striated muscle cells which connects the sarcomere of the muscle to the cell membrane (i.e. the sarcolemma).20: 2327-2331 Costameres are sub-sarcolemmal protein assemblies circumferentially al ...
attaches the sarcomere to the sarcolemma. Every single organelle and macromolecule of a muscle fiber is arranged to ensure that it meets desired functions. The
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

cell membrane
is called the sarcolemma with the cytoplasm known as the
sarcoplasm Sarcoplasm is the cytoplasm In cell biology, the cytoplasm is all of the material within a Cell (biology), cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear envelo ...
. In the sarcoplasm are the myofibrils. The myofibrils are long protein bundles about one micrometer in diameter. Pressed against the inside of the sarcolemma are the unusual flattened myonuclei. Between the myofibrils are the
mitochondria A mitochondrion (; ) is a double-membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, i ...

mitochondria
. While the muscle fiber does not have smooth endoplasmic cisternae, it contains
sarcoplasmic reticulum The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic reticulum in other Cell (biology), cells. The main function of the SR is to store calcium ions (Ca2+). Calcium ion le ...
. The sarcoplasmic reticulum surrounds the myofibrils and holds a reserve of the
calcium ions Calcium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...
needed to cause a muscle contraction. Periodically, it has dilated end sacs known as terminal cisternae. These cross the muscle fiber from one side to the other. In between two terminal cisternae is a tubular infolding called a transverse tubule (T tubule).
T tubule T-tubules (transverse tubules) are extensions of the cell membrane cell membrane vs. Prokaryotes A prokaryote is a typically unicellular organism that lacks a nuclear membrane-enclosed cell nucleus, nucleus. The word ''prokaryote'' comes fr ...
s are the pathways for action potentials to signal the sarcoplasmic reticulum to release calcium, causing a muscle contraction. Together, two terminal cisternae and a transverse tubule form a
triad A triad, meaning a "group of 3, three". Triad or triade may refer to: Associations * Triad (organized crime), Chinese organized-crime societies * Lexington Triad, a group of three fraternities founded at colleges in Lexington, Virginia * Miam ...
.


Development

All muscles are derived from
paraxial mesoderm Paraxial mesoderm, also known as presomitic or somitic mesoderm is the area of mesoderm In all bilaterian animals, the mesoderm is one of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside la ...
. During
embryonic development An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...
in the process of
somitogenesis Somitogenesis is the process by which somite The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmente ...
the paraxial mesoderm is divided along the
embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

embryo
's length to form
somite The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm Paraxial mesoderm, also known as presomitic or somitic mesoderm is the area of mesoderm In all bilaterian animals, the mesoderm is one o ...

somite
s, corresponding to the
segmentation Segment or segmentation may refer to: Biology *Segmentation (biology), the division of body plans into a series of repetitive segments **Segmentation in the human nervous system *Internodal segment, the portion of a nerve fiber between two Nodes of ...
of the body most obviously seen in the
vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton Axial may refer to: * one of the describing relationships in an animal body * and submarine volcano off Oregon, USA * , a ghost town * In geometry: :* ...

vertebral column
. Each somite has three divisions,
sclerotome The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryogenesis, embryonic stage of somitogenesis, along the head-to-tail axis in segmentation (biology), segmented animals. ...
(which forms
vertebrae In the vertebrate spinal column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform c ...
), dermatome (which forms skin), and
myotome A myotome is the group of muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given or ...
(which forms muscle). The myotome is divided into two sections, the epimere and hypomere, which form
epaxial and hypaxial muscles In adult animals, trunk muscles can be broadly divided into hypaxial muscles, which lie ventral to the horizontal septum In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical s ...
, respectively. The only epaxial muscles in humans are the
erector spinae The erector spinae ( ) or spinal erectors is a set of skeletal muscles, muscles that straighten and rotate the human back, back. Structure The erector spinae is not just one muscle, but a group of muscles and tendons which run more or less the len ...
and small vertebral muscles, and are innervated by the dorsal rami of the
spinal nerve A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into the ...

spinal nerve
s. All other muscles, including those of the limbs are hypaxial, and innervated by the
ventral rami The ventral ramus (pl. ''rami'') (Latin for ''branch'') is the anterior division of a spinal nerve A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are ...
of the spinal nerves. During development,
myoblast Myogenesis is the formation of skeletal muscle, skeletal muscular tissue, particularly during embryonic development. Skeletal muscle#Skeletal muscle fibers, Muscle fibers generally form through the fusion of precursor cell, precursor myoblasts int ...
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 is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
frameworks, usually formed from the somatic
lateral plate mesoderm Lateral is a geometric term of location {{unreferenced, date=March 2014 Geometric terms of location describe directions or positions relative to the shape of an object. These terms are used in descriptions of engineering, physics, and other scien ...
. Myoblasts follow chemical signals to the appropriate locations, where they fuse into elongated multinucleated skeletal muscle cells. Between the tenth and the eighteenth weeks of gestation, all muscle cells have fast myosin heavy chains; two myotube types become distinguished in the developing
fetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism tha ...

fetus
– both expressing fast chains but one expressing fast and slow chains. Between 10 and 40 per cent of the fibers express the slow myosin chain. Fiber types are established during embryonic development and are remodelled later in the adult by neural and hormonal influences. The population of satellite cells present underneath the basal lamina is necessary for the postnatal development of muscle cells.


Function

The primary function of muscle is . Following contraction, skeletal muscle functions as an
endocrine The endocrine system is a messenger system comprising feedback loops of the hormone A hormone (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Re ...
organ by secreting
myokineA myokine is one of several hundred cytokine Cytokines are a broad and loose category of small protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residue ...
s – a wide range of
cytokine Cytokines are a broad and loose category of small protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and ...

cytokine
s and other
peptide Peptides (from Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the dialects of the Greek language spoken ...
s that act as signalling molecules. Myokines in turn are believed to mediate the health benefits of
exercise Exercise is any bodily activity that enhances or maintains physical fitness Physical fitness is a state of health Health is a state of physical, mental and social well-being Well-being, also known as ''wellness'', ''prudential value ...

exercise
. Myokines are secreted into the bloodstream after muscle contraction.
Interleukin 6 Interleukin 6 (IL-6) is an interleukin that acts as both a pro-inflammatory cytokine and an anti-inflammatory myokine. In humans, it is encoded by the ''IL6'' gene. In addition, osteoblasts secrete IL-6 to stimulate osteoclast formation. Smooth ...
(IL-6) is the most studied myokine, other muscle contraction-induced myokines include
BDNF Brain-derived neurotrophic factor (BDNF), or abrineurin, is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max P ...
,
FGF21 Fibroblast growth factor 21 is a protein that in mammals is encoded by the ''FGF21'' gene. The protein encoded by this gene is a member of the fibroblast growth factor (FGF) family and specifically a member of the endocrine subfamily which includes ...
, and
SPARC SPARC (Scalable Processor Architecture) is a reduced instruction set computing (RISC) instruction set architecture originally developed by Sun Microsystems. Its design was strongly influenced by the experimental Berkeley RISC system developed i ...

SPARC
. Muscle also functions to produce body heat. Muscle contraction is responsible for producing 85% of the body's heat. This heat produced is as a by-product of muscular activity, and is mostly wasted. As a
homeostatic In biology, homeostasis is the state of steady internal, physics, physical, and chemistry, chemical conditions maintained by organism, living systems. This is the condition of optimal functioning for the organism and includes many variables, such ...
response to extreme cold, muscles are signaled to trigger contractions of
shivering Shivering (also called shuddering) is a bodily function in response to cold and extreme fear Fear is an intensely unpleasant emotion Emotions are mental state, psychological states brought on by neurophysiology, neurophysiological change ...
in order to generate heat.


Contraction

Contraction is achieved by the muscle's structural unit the muscle fiber, and by its functional unit, the
motor unit A motor unit is made up of a motor neuron and all of the Skeletal muscle#Skeletal muscle fibers, skeletal muscle fibers, also known as sarcomere innervated by the neuron's axon terminals. Groups of motor units often work together as a Motor pool (n ...

motor unit
. Muscle fibers are excitable cells stimulated by
motor neuron A motor neuron (or motoneuron or efferent neuron) is a neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * ...
s. The motor unit consists of a motor neuron and the many fibers that it makes contact with. A single muscle is stimulated by many motor units. Muscle fibers are subject to
depolarization In biology, depolarization or hypopolarization is a change within a cell (biology), cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolar ...
by the neurotransmitter
acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter—a chemical message released by nerve cells to send signals to other cells, such as neurons, musc ...

acetylcholine
, released by the motor neurons at the
neuromuscular junction A neuromuscular junction (or myoneural junction) is a chemical synapse Chemical synapses are biological junctions through which neuron A neuron or nerve cell is an electrically excitable cell that communicates with other cells via spe ...

neuromuscular junction
s. In addition to the
actin Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments. It is found in essentially all Eukaryote, eukaryotic cells, where it may be present at a concentration of over 100 Micromolar, μ ...
and
myosin Myosins () are a superfamily SUPERFAMILY is a database and search platform of structural and functional annotation for all proteins and genomes. It classifies amino acid sequences into known structural domains, especially into SCOP superfamilie ...

myosin
myofilament Myofilaments are the two protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inte ...

myofilament
s in the
myofibril s in parallel, and sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the complicated unit of striated muscle tissue. It is the repeating unit between two Z lines. Skeletal muscles are composed of tubular mu ...

myofibril
s that make up the contractile
sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue Striated muscle tissue is a muscle tissue Muscle tissue is a soft tissue that composes muscles in animal bodie ...

sarcomere
s, there are two other important regulatory proteins –
troponin image:Troponin Ribbon Diagram.png, 400px, Ribbon representation of the human cardiac troponin core complex (52 kDa core) in the calcium-saturated form. Blue = troponin C; green = troponin I; magenta = troponin T.; ; rendered with PyMOL Troponin, or ...
and
tropomyosin Tropomyosin is a two-stranded alpha-helical, coiled coil A coiled coil is a structural motif In a polymer, chain-like biological molecule, such as a protein or nucleic acid, a structural motif is a common three-dimensional structure that ...

tropomyosin
, that make muscle contraction possible. These proteins are associated with actin and cooperate to prevent its interaction with myosin. Once a cell is sufficiently stimulated, the cell's
sarcoplasmic reticulum The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic reticulum in other Cell (biology), cells. The main function of the SR is to store calcium ions (Ca2+). Calcium ion le ...
releases ionic calcium (Ca2+), which then interacts with the regulatory protein troponin. Calcium-bound troponin undergoes a conformational change that leads to the movement of tropomyosin, subsequently exposing the myosin-binding sites on actin. This allows for myosin and actin ATP-dependent cross-bridge cycling and shortening of the muscle.


Excitation-contraction coupling

Excitation contraction coupling Muscle contraction is the activation of tension Tension may refer to: Science * Psychological stress * Tension (physics), a force related to the stretching of an object (the opposite of compression) * Tension (geology), a stress which stretches ...
is the process by which a muscular action potential in the muscle fiber causes the
myofibril s in parallel, and sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the complicated unit of striated muscle tissue. It is the repeating unit between two Z lines. Skeletal muscles are composed of tubular mu ...

myofibril
s to contract. This process relies on a direct coupling between the
sarcoplasmic reticulum The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic reticulum in other Cell (biology), cells. The main function of the SR is to store calcium ions (Ca2+). Calcium ion le ...
calcium release channel
RYR1 Ryanodine receptor 1 (RYR-1) also known as skeletal muscle calcium release channel or skeletal muscle-type ryanodine receptor is one of a class of ryanodine receptor Ryanodine receptors (RyR for short) form a class of intracellular calcium channel ...
(ryanodine receptor 1), and voltage-gated L-type calcium channels (identified as dihydropyridine receptors, DHPRs). DHPRs are located on the sarcolemma (which includes the surface sarcolemma and the transverse tubules), while the RyRs reside across the SR membrane. The close apposition of a transverse tubule and two SR regions containing RyRs is described as a triad and is predominantly where excitation–contraction coupling takes place. Excitation–contraction coupling occurs when depolarization of skeletal muscle cell results in a muscle action potential, which spreads across the cell surface and into the muscle fiber's network of
T-tubule T-tubules (transverse tubules) are extensions of the sarcolemma, cell membrane that penetrate into the centre of Skeletal muscle#Skeletal muscle fibers, skeletal and cardiac muscle cells. With membranes that contain large concentrations of ion ...
s, thereby depolarizing the inner portion of the muscle fiber. Depolarization of the inner portions activates dihydropyridine receptors in the terminal cisternae, which are in close proximity to ryanodine receptors in the adjacent
sarcoplasmic reticulum The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic reticulum in other Cell (biology), cells. The main function of the SR is to store calcium ions (Ca2+). Calcium ion le ...
. The activated dihydropyridine receptors physically interact with ryanodine receptors to activate them via foot processes (involving conformational changes that allosterically activates the ryanodine receptors). As the ryanodine receptors open, is released from the sarcoplasmic reticulum into the local junctional space and diffuses into the bulk cytoplasm to cause a calcium spark. Note that the sarcoplasmic reticulum has a large
calcium bufferingCalcium buffering describes the processes which help stabilise the concentration of free calcium ions within cells, in a similar manner to how pH buffers maintain a stable concentration of hydrogen ions. The majority of calcium ions within the cell ...
capacity partially due to a
calcium-binding protein Calcium-binding proteins are protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew ...
called
calsequestrin Calsequestrin is a calcium-binding protein that acts as a calcium buffer within the sarcoplasmic reticulum. The protein helps hold calcium in the cisterna of the sarcoplasmic reticulum after a muscle contraction Muscle contraction is the ac ...
. The near synchronous activation of thousands of
calcium sparksA calcium spark is the microscopic release of calcium (Ca2+) from a store known as the sarcoplasmic reticulum (SR), located within muscle cells A myocyte is a muscle cell of the heart ( cardiac muscle cell) or of smooth muscle. There are two spe ...
by the action potential causes a cell-wide increase in calcium giving rise to the upstroke of the calcium transient. The released into the cytosol binds to
Troponin C Troponin C is a protein which is part of the troponin image:Troponin Ribbon Diagram.png, 400px, Ribbon representation of the human cardiac troponin core complex (52 kDa core) in the calcium-saturated form. Blue = troponin C; green = troponin I; ma ...
by the
actin filamentsImage:MEF microfilaments.jpg, Actin cytoskeleton of house mouse, mouse embryo fibroblasts, stained with Fluorescein isothiocyanate-phalloidin, 250px Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic ce ...
, to allow crossbridge cycling, producing force and, in some situations, motion. The (SERCA) actively pumps back into the sarcoplasmic reticulum. As declines back to resting levels, the force declines and relaxation occurs.


Muscle movement

The efferent leg of the
peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, ...
is responsible for conveying commands to the muscles and glands, and is ultimately responsible for voluntary movement.
Nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term diale ...

Nerve
s move muscles in response to
voluntary Voluntary may refer to: * Voluntary (music)In music a voluntary is a piece of music, usually for an organ, that is played as part of a church service. In English-speaking countries, the music played before and after the service is often called a 'v ...
and (involuntary) signals from the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tis ...

brain
. Deep muscles, superficial muscles, muscles of the face and internal muscles all correspond with dedicated regions in the primary motor cortex of the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tis ...

brain
, directly anterior to the central sulcus that divides the frontal and parietal lobes. In addition, muscles react to reflexive nerve stimuli that do not always send signals all the way to the brain. In this case, the signal from the afferent fiber does not reach the brain, but produces the reflexive movement by direct connections with the efferent nerves in the . 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 Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s correspond with neuron groups along the
primary motor cortex The primary motor cortex (Brodmann area 4) is a Human brain, brain region that in humans is located in the dorsal portion of the frontal lobe. It is the primary cortex, primary region of the motor system and works in association with other motor a ...
of the brain's
cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting of a ...
. Commands are routed through the
basal ganglia #REDIRECT Basal ganglia The basal ganglia (or basal nuclei) are a group of subcortical The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue Nervous tissue, also called neural tissue, is the main tissue ...

basal ganglia
and are modified by input from the
cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain The hindbrain or rhombencephalon is a developmental Development of the human body is the process of growth to maturity. The process begins with fertilization ...

cerebellum
before being relayed through the
pyramidal tract The pyramidal tracts include both the corticobulbar tract The corticobulbar (or corticonuclear) tract is a two-neuron white matter motor pathway connecting the motor cortex in the cerebral cortex to the medullary pyramids, which are part of the ...
to the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. It encloses the central canal of the spinal cord, which contain ...

spinal cord
and from there to the motor end plate at the muscles. Along the way, feedback, such as that of the
extrapyramidal system In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any indi ...
contribute signals to influence
muscle tone In physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studies life and living organisms, including their anatomy, physi ...
and response. Deeper muscles such as those involved in Human position, posture often are controlled from nuclei in the brain stem and basal ganglia.


Proprioception

In skeletal muscles,
muscle spindle Muscle spindles are stretch receptor Stretch receptors are mechanoreceptors A mechanoreceptor, also called mechanoceptor, is a sensory neuron, sensory cell that responds to mechanical pressure or distortion. There are four main types of mechanore ...
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 of where our bodies are in space is called proprioception, the perception of body awareness, the "unconscious" awareness of where the various regions of the body are located at any one time. Several areas in the brain coordinate movement and position with the feedback information gained from proprioception. The cerebellum and red nucleus in particular continuously sample position against movement and make minor corrections to assure smooth motion.


Energy consumption

Muscular activity accounts for much of the body's energy consumption. All muscle cells produce
adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properti ...

adenosine triphosphate
(ATP) molecules which are used to power the movement of the
myosin head The myosin head is the part of the thick myofilament Myofilaments are the two protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, c ...
s. Muscles have a short-term store of energy in the form of creatine phosphate which is generated from ATP and can regenerate ATP when needed with creatine kinase. Muscles also keep a storage form of glucose in the form of
glycogen Glycogen is a multibranched polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant found in . They are long chain carbohydrates composed of units bound together by . This carbohydrate can react with water () usi ...

glycogen
. Glycogen can be rapidly converted to glucose when energy is required for sustained, powerful contractions. Within the voluntary skeletal muscles, the glucose molecule can be metabolized anaerobically in a process called glycolysis which produces two ATP and two lactic acid molecules in the process (note that in aerobic conditions, lactate is not formed; instead pyruvate is formed and transmitted through the citric acid cycle). Muscle cells also contain globules of fat, which are used for energy during aerobic exercise. The aerobic energy systems take longer to produce the ATP and reach peak efficiency, and requires many more biochemical steps, but produces significantly more ATP than anaerobic glycolysis. Cardiac muscle on the other hand, can readily consume any of the three macronutrients (protein, glucose and fat) aerobically without a 'warm up' period and always extracts the maximum ATP yield from any molecule involved. The heart, liver and red blood cells will also consume lactic acid produced and excreted by skeletal muscles during exercise. Skeletal muscle uses more calories than other organs. At rest it consumes 54.4 kJ/kg (13.0 kcal/kg) per day. This is larger than adipose tissue (fat) at 18.8 kJ/kg (4.5 kcal/kg), and bone at 9.6 kJ/kg (2.3 kcal/kg).


Efficiency

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


Muscle strength

Muscle strength is a result of three overlapping factors: ''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). Vertebrate muscle typically produces approximately of force per square centimeter of muscle cross-sectional area when isometric and at optimal length. Some invertebrate muscles, such as in crab claws, have much longer
sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue Striated muscle tissue is a muscle tissue Muscle tissue is a soft tissue that composes muscles in animal bodie ...

sarcomere
s than vertebrates, resulting in many more sites for actin and myosin to bind and thus much greater force per square centimeter at the cost of much slower speed. The force generated by a contraction can be measured non-invasively using either mechanomyography or phonomyography, be measured in vivo using tendon strain (if a prominent tendon is present), or be measured directly using more invasive methods. The strength of any given muscle, in terms of force exerted on the skeleton, depends upon Muscle contraction#Force-length and force-velocity relationships, length, shortening speed, cross sectional area, Pennate muscle, pennation,
sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue Striated muscle tissue is a muscle tissue Muscle tissue is a soft tissue that composes muscles in animal bodie ...

sarcomere
length,
myosin Myosins () are a superfamily SUPERFAMILY is a database and search platform of structural and functional annotation for all proteins and genomes. It classifies amino acid sequences into known structural domains, especially into SCOP superfamilie ...

myosin
isoforms, and neural activation of
motor unit A motor unit is made up of a motor neuron and all of the Skeletal muscle#Skeletal muscle fibers, skeletal muscle fibers, also known as sarcomere innervated by the neuron's axon terminals. Groups of motor units often work together as a Motor pool (n ...

motor unit
s. Significant reductions in muscle strength can indicate underlying pathology, with the chart at right used as a guide.


The "strongest" human muscle

Since three factors affect muscular strength simultaneously and muscles never work individually, it is misleading to compare strength in individual muscles, and state that one is the "strongest". But below are several muscles whose strength is noteworthy for different reasons. * In ordinary parlance, muscular "strength" usually refers to the ability to exert a force on an external object—for example, lifting a weight. By this definition, the masseter or jaw muscle is the strongest. The 1992 Guinness Book of Records records the achievement of a bite strength of 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, quadriceps femoris or the
gluteus maximus The gluteus maximus is the main extensor muscle of the hip. It is the largest and outermost of the three gluteal muscles and makes up a large part of the shape and appearance of each side of the hips. Its thick fleshy mass, in a quadrilateral sh ...

gluteus maximus
. * Because muscle strength is determined by cross-sectional area, a shorter muscle will be stronger "pound for pound" (i.e., by mass, weight) than a longer muscle of the same cross-sectional area. The myometrial layer of the uterus may be the strongest muscle by weight in the female human body. At the time when an infant is delivered, the entire human uterus weighs about 1.1 kg (40 oz). During childbirth, the uterus exerts 100 to 400 N (25 to 100 lbf) of downward force with each contraction. * The external muscles of the eye are conspicuously large and strong in relation to the small size and weight of the human eyeball, eyeball. It is frequently said that they are "the strongest muscles for the job they have to do" and are sometimes claimed to be "100 times stronger than they need to be." However, eye movements (particularly saccades used on facial scanning and reading) do require high speed movements, and eye muscles are exercised nightly during rapid eye movement sleep. * The statement that "the tongue is the strongest muscle in the body" appears frequently in lists of surprising facts, but it is difficult to find any definition of "strength" that would make this statement true. Note that the tongue consists of eight muscles, not one.


Force generation

Muscle architecture#Force generation, Muscle force is proportional to physiologic cross-sectional area (PCSA), and muscle velocity is proportional to muscle fiber length. The torque around a joint, however, is determined by a number of biomechanical parameters, including the distance between muscle insertions and pivot points, muscle size and architectural gear ratio. Muscles are normally arranged in opposition so that when one group of muscles contracts, another group relaxes or lengthens. Antagonism in the transmission of nerve impulses to the muscles means that it is impossible to fully stimulate the contraction of two Antagonist (muscle), antagonistic muscles at any one time. During ballistic motions such as throwing, the antagonist muscles act to 'brake' the Agonist (muscle), agonist muscles throughout the contraction, particularly at the end of the motion. In the example of throwing, the chest and front of the shoulder (anterior deltoid) contract to pull the arm forward, while the muscles in the back and rear of the shoulder (posterior deltoid) also contract and undergo eccentric contraction to slow the motion down to avoid injury. Part of the training process is learning to relax the antagonist muscles to increase the force input of the chest and anterior shoulder. Contracting muscles produce vibration and sound. Slow twitch fibers produce 10 to 30 contractions per second (10 to 30 Hz). Fast twitch fibers produce 30 to 70 contractions per second (30 to 70 Hz).
Peak Performance – Endurance training: understanding your slow twitch muscle fibers will boost performance
The vibration can be witnessed and felt by highly tensing one's muscles, as when making a firm fist. The sound can be heard by pressing a highly tensed muscle against the ear, again a firm fist is a good example. The sound is usually described as a rumbling sound. Some individuals can voluntarily produce this rumbling sound by contracting the tensor tympani muscle of the middle ear. The rumbling sound can also be heard when the neck or jaw muscles are highly tensed.


Signal transduction pathways

Skeletal muscle fiber-type phenotype in adult animals is regulated by several independent signaling pathways. These include pathways involved with the Ras subfamily, Ras/mitogen-activated protein kinase (MAPK) pathway, calcineurin, calcium/calmodulin-dependent protein kinase IV, and the peroxisome proliferator γ coactivator 1 (PGC-1). The MAPK/ERK pathway, Ras/MAPK signaling pathway links the motor neurons and signaling systems, coupling excitation and transcription regulation to promote the nerve-dependent induction of the slow program in regenerating muscle. Calcineurin, a Ca2+/calmodulin-activated phosphatase implicated in nerve activity-dependent fiber-type specification in skeletal muscle, directly controls the phosphorylation state of the transcription factor NFAT, allowing for its translocation to the nucleus and leading to the activation of slow-type muscle proteins in cooperation with myocyte enhancer factor 2 (MEF2) proteins and other regulatory proteins. Ca2+/calmodulin-dependent protein kinase activity is also upregulated by slow motor neuron activity, possibly because it amplifies the slow-type calcineurin-generated responses by promoting MEF2 transactivator functions and enhancing oxidative capacity through stimulation of mitochondrial biogenesis. Contraction-induced changes in intracellular calcium or reactive oxygen species provide signals to diverse pathways that include the MAPKs, calcineurin and calcium/calmodulin-dependent protein kinase IV to activate transcription factors that regulate gene expression and enzyme activity in skeletal muscle. PGC1-α (PPARGC1A), a transcriptional coactivator of nuclear receptors important to the regulation of a number of mitochondrial genes involved in oxidative metabolism, directly interacts with MEF2 to synergistically activate selective slow twitch (ST) muscle genes and also serves as a target for calcineurin signaling. A peroxisome proliferator-activated receptor δ (PPARδ)-mediated transcriptional pathway is involved in the regulation of the skeletal muscle fiber phenotype. Mice that harbor an activated form of PPARd display an "endurance" phenotype, with a coordinated increase in oxidative enzymes and mitochondrial biogenesis and an increased proportion of ST fibers. Thus—through functional genomics—calcineurin, calmodulin-dependent kinase, PGC-1α, and activated PPARδ form the basis of a signaling network that controls skeletal muscle fiber-type transformation and metabolic profiles that protect against insulin resistance and obesity. The transition from aerobic to anaerobic metabolism during intense work requires that several systems are rapidly activated to ensure a constant supply of ATP for the working muscles. These include a switch from fat-based to carbohydrate-based fuels, a redistribution of blood flow from nonworking to exercising muscles, and the removal of several of the by-products of anaerobic metabolism, such as carbon dioxide and lactic acid. Some of these responses are governed by transcriptional control of the fast twitch (FT) glycolytic phenotype. For example, skeletal muscle reprogramming from an ST glycolytic phenotype to an FT glycolytic phenotype involves the Six1/Eya1 complex, composed of members of the Six protein family. Moreover, the hypoxia-inducible factor 1-α (HIF1A) has been identified as a master regulator for the expression of genes involved in essential hypoxic responses that maintain ATP levels in cells. Ablation of HIF-1α in skeletal muscle was associated with an increase in the activity of rate-limiting enzymes of the mitochondria, indicating that the citric acid cycle and increased fatty acid oxidation may be compensating for decreased flow through the glycolytic pathway in these animals. However, hypoxia-mediated HIF-1α responses are also linked to the regulation of mitochondrial dysfunction through the formation of excessive reactive oxygen species in mitochondria. Other pathways also influence adult muscle character. For example, physical force inside a muscle fiber may release the transcription factor serum response factor from the structural protein titin, leading to altered muscle growth.


Exercise

Exercise is often recommended as a means of improving motor skills, physical fitness, fitness, muscle and bone strength, and joint function. Exercise has several effects upon muscles,
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
, bone, and the nerves that stimulate the muscles. One such effect is muscle hypertrophy, an increase in size of muscle due to an increase in the number of muscle fibers or cross-sectional area of myofibrils. Muscle changes depend on the type of exercise used. Generally, there are two types of exercise regimes, aerobic and anaerobic. Aerobic exercise (e.g. marathons) involves low intensity, but long duration activities during which, the muscles used are below their maximal contraction strength. Aerobic activities rely on the aerobic respiration (i.e. citric acid cycle and electron transport chain) for metabolic energy by consuming fat, protein, carbohydrates, and oxygen. Muscles involved in aerobic exercises contain a higher percentage of Type I (or slow-twitch) muscle fibers, which primarily contain mitochondrial and oxidation enzymes associated with aerobic respiration. On the contrary, anaerobic exercise is associated with short duration, but high intensity exercise (e.g. sprinting and weight training, weight lifting). The anaerobic activities predominately use Type II, fast-twitch, muscle fibers. Type II muscle fibers rely on glucogenesis for energy during anaerobic exercise. During anaerobic exercise, type II fibers consume little oxygen, protein and fat, produces large amounts of lactic acid and are fatigable. Many exercises are partially aerobic and anaerobic; for example, soccer and rock climbing. The presence of lactic acid has an inhibitory effect on ATP generation within the muscle. It can even stop ATP production if the intracellular concentration becomes too high. However, endurance training mitigates the buildup of lactic acid through increased capillarization and myoglobin. This increases the ability to remove waste products, like lactic acid, out of the muscles in order to not impair muscle function. Once moved out of muscles, 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 results in the loss of potassium ions in muscle. This may facilitate the recovery of muscle function by protecting against fatigue. Delayed onset muscle soreness is pain or discomfort that may be felt one to three days after exercising and generally subsides two to three days after which. Once thought to be caused by lactic acid build-up, a more recent theory is that it is caused by tiny tears in the muscle fibers caused by eccentric contraction, or unaccustomed training levels. Since lactic acid disperses fairly rapidly, it could not explain pain experienced days after exercise.


Clinical significance


Muscle disease

Diseases of skeletal muscle are termed myopathies, while diseases of nerves are called neuropathies. Both can affect muscle function or cause muscle pain, and fall under the umbrella of neuromuscular disease. The cause of many myopathies is attributed to mutations in the various associated muscle proteins. Some inflammatory myopathies include polymyositis and inclusion body myositis Neuromuscular diseases affect the muscles and their nervous control. In general, problems with nervous control can cause spasticity or paralysis, depending on the location and nature of the problem. A number of movement disorders are caused by neurological disorders such as Parkinson's disease and Huntington's disease where there is central nervous system dysfunction. Symptoms of muscle diseases may include Muscle weakness, weakness, spasticity, myoclonus and myalgia. Diagnostic procedures that may reveal muscular disorders include testing Creatine kinase#Laboratory testing, creatine kinase levels in the blood and electromyography (measuring electrical activity in muscles). In some cases, muscle biopsy may be done to identify a myopathy, as well as genetic testing to identify DNA abnormalities associated with specific myopathies and Muscular dystrophy, dystrophies. A non-invasive elastography technique that measures muscle noise is undergoing experimentation to provide a way of monitoring neuromuscular disease. The sound produced by a muscle comes from the shortening of actomyosin Protein filament, filaments along the axis of the muscle. During Muscle contraction, contraction, the muscle shortens along its length and expands across its width, producing vibrations at the surface.


Hypertrophy

Independent of strength and performance measures, muscles can be induced to grow larger by a number of factors, including hormone signaling, developmental factors, strength training, and disease. Contrary to popular belief, the number of muscle fibres cannot be increased through Physical exercise, exercise. Instead, muscles grow larger through a combination of muscle cell growth as new protein filaments are added along with additional mass provided by undifferentiated satellite cells alongside the existing muscle cells. Biological factors such as age and hormone levels can affect muscle hypertrophy. During puberty in males, hypertrophy occurs at an accelerated rate as the levels of growth-stimulating hormones produced by the body increase. Natural hypertrophy normally stops at full growth in the late teens. As testosterone is one of the body's major growth hormones, on average, men find hypertrophy much easier to achieve than women. Taking additional testosterone or other anabolic steroids will increase muscular hypertrophy. Muscular, spinal and neural factors all affect muscle building. Sometimes a person may notice an increase in strength in a given muscle even though only its opposite has been subject to exercise, such as when a bodybuilder finds her left biceps stronger after completing a regimen focusing only on the right biceps. This phenomenon is called cross education.


Atrophy

Every day between one and two percent of muscle is broken down and rebuilt. Physical inactivity, Inactivity, malnutrition, disease, and aging can increase the breakdown leading to muscle atrophy or sarcopenia. Sarcopenia is commonly an age-related process that can cause frailty syndrome, frailty and its consequences. A decrease in muscle mass may be accompanied by a smaller number and size of the muscle cells as well as lower protein content. Spaceflight, Human spaceflight, involving prolonged periods of immobilization and weightlessness is known to result in muscle weakening and atrophy resulting in a loss of as much as 30% of mass in some muscles. Such consequences are also noted in some mammals following hibernation. Many diseases and conditions including cancer, AIDS, and heart failure can cause muscle loss known as cachexia.


Research

Myopathies have been modeled with cell culture systems of muscle from healthy or diseased tissue Biopsy, biopsies. Another source of skeletal muscle and progenitors is provided by the directed differentiation of Induced pluripotent stem cells, pluripotent stem cells. Research on skeletal muscle properties uses many techniques. Electrical muscle stimulation is used to determine force and contraction speed at different frequencies related to fiber-type composition and mix within an individual muscle group. In vitro muscle testing is used for more complete characterization of muscle properties. The electrical activity associated with muscle contraction is measured via electromyography (EMG). Skeletal muscle has two physiological responses: relaxation and contraction. The mechanisms for which these responses occur generate electrical activity measured by EMG. Specifically, EMG can measure the action potential of a skeletal muscle, which occurs from the Hyperpolarization (biology), hyperpolarization of the motor axons from nerve impulses sent to the muscle. EMG is used in research for determining if the skeletal muscle of interest is being activated, the amount of force generated, and an indicator of muscle fatigue. The two types of EMG are intra-muscular EMG and the most common, surface EMG. The EMG signals are much greater when a skeletal muscle is contracting verses relaxing. However, for smaller and deeper skeletal muscles the EMG signals are reduced and therefore are viewed as a less valued technique for measuring the activation. In research using EMG, a muscle contraction, maximal voluntary contraction (MVC) is commonly performed on the skeletal muscle of interest, to have reference data for the rest of the EMG recordings during the main experimental testing for that same skeletal muscle.


See also

* Electroactive polymers, robotics research materials that behave like muscles * Facioscapulohumeral muscular dystrophy * Hill's muscle model * In vitro muscle testing * Musculoskeletal injury * Muscle relaxant * Microtrauma * Muscle memory * Myotomy * Preflexes * Rohmert's law, pertains to muscle fatigue


References

{{DEFAULTSORT:Skeletal Muscle Skeletal muscle, Muscular system Somatic motor system Muscle tissue