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which may contribute to muscle fatigue this occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle c aerobic respiration is the breakdown of glucose in the presence of oxygen o2 to produce carbon dioxide water and atp approximately 95 percent of the atp required for resting or moderately active muscles is provided by aerobic respiration which takes place in mitochondria muscular activity accounts for much of the body s energy consumption all muscle cells produce adenosine triphosphate atp molecules which are used to power the movement of the myosin heads muscles 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 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 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 require 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 81 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 82 efficiency edit this section does not cite any sources please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed june 2021 learn how and when to remove this message the efficiency of human muscle has been measured in the context of rowing and cycling at 18 to 26 the efficiency is defined as the ratio of mechanical work output to the total metabolic cost as can be calculated from oxygen consumption this low efficiency is the result of about 40 efficiency of generating atp from food energy losses in converting energy from atp into mechanical work inside the muscle and mechanical losses inside the body the latter two losses are dependent on the type of exercise and the type of muscle fibers being used fast twitch or slow twitch for an 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 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 and decay these can be synthesized experimentally using work loop analysis muscle strength edit this section needs more citations please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed march 2016 learn how and when to remove this message main article physical strength grading of muscle strength grade 0 no contraction grade 1 trace of contraction but no movement at the joint grade 2 movement at the joint with gravity eliminated grade 3 movement against gravity but not against added resistance grade 4 movement against external resistance but less than normal grade 5 normal strength 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 25 33 n 5 6 7 4 lb f of force per square centimeter of muscle cross sectional area when isometric and at optimal length 83 some invertebrate muscles such as in crab claws have much longer sarcomeres than vertebrates resulting in many more sites for actin and myosin to bind and thus much greater force per square centimeter 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 length shortening speed cross sectional area pennation sarcomere length myosin isoforms and neural activation of motor units significant reductions in muscle strength can indicate underlying pathology with the chart at right used as a guide the maximum holding time for a contracted muscle depends on its supply of energy and is stated by rohmert s law to exponentially decay from the beginning of exertion since three factors affect muscular strength simultaneously and muscles never work individually no one muscle can be claimed to be the strongest there are several muscles with noteworthy strength the masseter a jaw muscle is claimed to be the strongest the 1992 guinness book of records records the achievement of a bite strength of 4 337 n 975 lb f for 2 seconds what distinguishes the masseter is its advantage in working against a much shorter lever arm than other muscles strength can refer to the force exerted by the muscle itself such as 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 can be said to be either the quadriceps femoris or the gluteus maximus because muscle strength is determined by cross sectional area a shorter muscle will be stronger by 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 body at the time when an infant is delivered the entire uterus weighs about 1 1 kg 40 oz during childbirth the uterus exerts 100 to 400 n 25 to 100 lbf of downward force with each contraction the external muscles of the eye are conspicuously large and strong in relation to the small size and weight of the eyeball and are sometimes claimed to be a 100 times stronger than they need to be 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 tongue is sometimes claimed to be the strongest muscle in the body but a definition is lacking and the tongue consists of eight muscles not one force generation edit muscle force is proportional to physiological cross sectional area pcsa and muscle velocity is proportional to muscle fiber length 84 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 85 antagonism in the transmission of nerve impulses to the muscles means that it is impossible to fully stimulate the contraction of two antagonistic muscles at any one time during ballistic motions such as throwing the antagonist muscles act to brake the 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 86 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 87 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 citation needed signal transduction pathways edit this section does not cite any sources please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed may 2021 learn how and when to remove this message skeletal muscle fiber type phenotype in adult animals is regulated by several independent signaling pathways these include pathways involved with the ras mitogen activated protein kinase mapk pathway calcineurin calcium calmodulin dependent protein kinase iv and the peroxisome proliferator γ coactivator 1 pgc 1 the 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 ca 2 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 exercise induced signaling pathways in skeletal muscle that determine specialized characteristics of slow and fast twitch muscle fibers 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 pparδ 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 edit main articles exercise and epigenetics of physical exercise jogging is one form of aerobic exercise physical exercise is often recommended as a means of improving motor skills fitness muscle and bone strength and joint function exercise has several effects upon muscles connective tissue bone and the nerves that stimulate the muscles one such effect is muscle hypertrophy an increase in size of muscle due to an increase in the number of muscle fibers or cross sectional area of myofibrils 88 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 activities of low intensity but long duration during which the muscles used are below their maximal contraction strength aerobic activities rely on 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 89 90 on the contrary anaerobic exercise ...