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this is chapter 10, module 3--types of muscle contractions and types of muscle fibers. the learning objectivesof this module are one, distinguish betweenthe different types of muscle contractions, isometricand isotonic, including concentric and eccentric, andtwo, relate types of muscle fibers to muscular performance. when sarcomeres shortenin a contraction, they shorten the muscle fiber.

this shortening exerts tensionon the connective tissue fibers attached to the muscle fiber. the tension produced by anindividual muscle can vary. the amount of tensionproduced by the muscle as a whole is the sum ofthe tensions generated by individual fibers, sincethey are all pulling together. for this reason, theamount of tension produced by a skeletal muscle canbe controlled by the number of muscle fibers it stimulates.

a typical skeletalmuscle contains thousands of muscle fibers. some motor neurons controla few muscle fibers, but most controlhundreds of them. all the muscle fibers controlledby a single motor neuron constitute a motor unit. the size of a motorunit is an indication of how fine the controlof the movement can be. in the muscles of theeye, where precise control

is extremely important,a motor neuron may control four tosix muscle fibers. we have much less precisecontrol over our leg muscles, where a single motor neuron maycontrol 1,000 to 2,000 muscle fibers. in any skeletal muscle,some motor units are always active, even whenthe entire muscle is not contracting. their contractions do notproduce enough tension

to cause movement, but theydo tense and firm the muscle. this resting tensionin a skeletal muscle is called muscle tone. a muscle with little muscletone appears limp and flaccid, whereas one with moderatemuscle tone is firm and solid. resting muscle tonestabilizes the positions of bones and joints. for example, in order tomaintain body position, there must be enough motorunits stimulated to produce

enough tension to do so. we classify muscle contractionsas isotonic or isometric. isotonic contraction iswhere the tension rises and the skeletalmuscles' length changes. lifting an object off thedesk, walking, and running involve isotonic contractions. two types ofisotonic contractions exist-- concentricand eccentric. in a concentric contraction,the muscle tension

exceeds the load andthe muscle shortens. in an eccentric contraction,the peak tension developed is less than theload, and the muscle elongates due to thecontraction of another muscle or the pull of gravity. think of a tug-of-war teamtrying to stop a moving car. although everyone pullsas hard as they can, the rope slipsthrough their fingers. during physicaltraining, people commonly

perform cycles of concentricand eccentric contractions, as when you do bicep curls byholding a weight in your hand and slowly flex andextend your elbow. during flexion,your biceps involve concentric contractions. during extension, your bicepsare still actively contracting, but now involveeccentric contraction. in an isometric contraction,the muscle as a whole does not change length andthe tension produced never

exceeds the load. examples of isometriccontractions include carryinga bag of groceries and holding our heads up. when you perform anisometric contraction, the contractingmuscle bulges, but not as much as it does duringan isotonic contraction. after a contraction,a muscle fiber must return to itsoriginal length.

however, there are no activemechanisms for muscle fibers to elongate. therefore, the muscle fiberreturns to its original length through a combination ofelastic forces, opposing muscle contractions, and gravity. the tendons and the cellfiber will gradually return the muscle's fiber to itsoriginal resting length because of its elasticity. the contraction ofopposing muscles

can return a muscleto its resting length more quickly than theelastic factors can. for example, when the bicepsbrachii muscle contracts, the triceps brachiimuscle extends the elbow and stretches the muscle fibersof the biceps brachii muscle to the original length. and gravity may assistopposing muscle groups in quickly returning amuscle to its resting length after a contraction.

the elbow's bent withthe forearm elevated. when the muscle relaxes, gravitywill pull the forearm down and stretch the muscle. the demand for atp in acontracting muscle fiber is so high that it wouldbe impossible to have all the necessaryenergy available as atp before the contraction begins. instead, a resting musclefiber contains only enough atp to sustain a contractionuntil additional atp can

be generated. throughout the restof the contraction, the muscle fiber will generateatp at roughly the same rate as it is used. muscle performance capabilitiesdepend on muscle fiber type and physical condition. a muscle's performanceinvolves force, which is the maximumamount of tension produced by a particularmuscle, and endurance,

which is the amount of timeduring which the individual can perform a particular activity. several factors determinethe performance capabilities of any skeletalmuscle-- the type of distribution and size ofmuscle fibers in the muscle, as well as physicalconditioning and training. the human body hasthree major types of skeletal muscle fibers--fast fibers, slow fibers, and intermediate fibers.

most of the skeletalmuscle fibers in the body are called fastfibers, because they can reach peak twitchtension very quickly. fast fibers are large indiameter and contain densely packed myofibrils, largereserves of glycogen, and few mitochondria. muscles dominated by fast fibersproduce powerful contractions. but the fibers fatigue rapidly,because their contractions use atp in massive amounts.

with so fewmitochondria, they are unable to generate atp tosupport prolonged activity. slow fibers have only abouthalf the diameter of fast fibers and take three times aslong to reach peak tension after stimulation. they are able tocontinue contracting longer than a fastfiber, because of the extensive capillarynetwork that supplies oxygen to support the activityof the mitochondria.

slow fibers also containthe red pigment myoglobin, which is similar to hemoglobin. myoglobin helps todeliver oxygen molecules. skeletal musclesdominated by slow fibers are dark red,because slow fibers have both an extensive capillarysupply and a high concentration of myoglobin. intermediate fibers mostclosely resemble fast fibers, for they containlittle myoglobin

and are relatively pale. they do have an intermediatecapillary network and mitochondrialsupply around them and are more resistant tofatigue than are fast fibers. muscles dominated byfast fibers appear pale and are often called whitemuscles or white meat. chicken breastscontain white meat because chickens use theirwings only for brief intervals, as when fleeing a predator.

muscles dominated byslow fibers are known as red muscles or red meat. chickens walk aroundall day, so their legs contain dark meat, which containthe capillaries and myoglobin. most human muscles containa mixture of fiber types, and so appear pink. many back and calf musclesare dominated by slow fibers, and these muscles contractalmost continuously to help us remain upright.

our genes determinethe percentage of fast versus slowfibers in each muscle. athletic trainingcan also increase the ratio of intermediatefibers to fast fibers. as a result of repeatedextensive stimulation, muscle fibers develop moremitochondria, a higher concentration ofglycolytic enzymes, which are enzymes that breakdown glycogen into glucose, and larger glycogen reserves.

the effect of thisstimulation is hypertrophy, or an enlargementof the stimulated muscle. the number of muscle fibersdoes not change significantly, but the muscle as a wholeenlarges because each muscle fiber increases in diameter. a skeletal muscle that isnot regularly stimulated by a motor neuron losesmuscle tone and mass. the muscle becomes flaccidand the muscle fibers become smaller and weaker.

this reduction in musclesize, tone, and power is called atrophy. individuals paralyzed by spinalcord injuries or other damage to the nervous system graduallylose muscle tone and size in the areas affected. muscle atrophy isreversible at first, but dying muscle fibersare not replaced. that is why physical therapyis crucial for people who are temporarilyunable to move normally.

direct electrical stimulationby an external device can substitute fornerve stimulation and prevent or reducemuscle atrophy. this ends chapter 10, module3, types of muscle contractions