How Is The Makeup Of Fibers Determined In A Person

Human skeletal muscle is composed of a heterogenous collection of muscle fiber types. ^one–3 This range of muscle fiber types allows for the wide variety of capabilities that human muscles brandish. In addition, muscle fibers tin adjust to irresolute demands by changing size or fiber type composition. This plasticity serves as the physiologic ground for numerous physical therapy interventions designed to increase a patient's force development or endurance. Changes in fiber blazon composition also may be partially responsible for some of the impairments and disabilities seen in patients who are deconditioned because of prolonged inactivity, limb immobilization, or musculus denervation. ² Over the by several decades, the number of techniques available for classifying muscle fibers has increased, resulting in several nomenclature systems. The objective of this update is to provide the basic knowledge necessary to read and interpret enquiry on human skeletal musculus.

Muscle cobweb types tin can be described using histochemical, biochemical, morphological, or physiologic characteristics; however, classifications of muscle fibers past different techniques do not always agree. ¹ Therefore, muscle fibers that may be grouped together by one classification technique may be placed in dissimilar categories using a different classification technique. A basic understanding of muscle construction and physiology is necessary to empathize the muscle fiber classification techniques.

Review of Muscle Fiber Beefcake and Physiology

Musculus fibers are composed of functional units called sarcomeres. ³ Within each sarcomere are the myofibrillar proteins myosin (the thick filament) and actin (the thin filament). The interaction of these 2 myofibrillar proteins allows muscles to contract (Fig. 1). ⁴ Several classification techniques differentiate fibers based on different myosin structures (isoforms) or physiologic capabilities. ^one,2,5 The myosin molecule is composed of 6 polypeptides: 2 heavy bondage and iv light chains (2 regulatory and 2 alkali). A regulatory and an alkali light concatenation are associated with each of the heavy chains. The heavy chains contain the myosin heads that interact with actin and allow musculus to contract (Fig. i). ⁴ The myosin heavy concatenation in the head region too contains an adenosine triphosphate (ATP) binding site and serves equally the enzyme (adenosinetriphosphatase [ATPase]) for hydrolyzing ATP into adenosine diphosphate (ADP) and inorganic phosphate (P_I), which provides the free energy necessary for musculus contraction. The thin filament is fabricated of actin and two regulatory proteins, troponin and tropomyosin. ³ When the muscle fiber receives a stimulus in the grade of an activity potential, Ca²⁺ is released from the sarcoplasmic reticulum. The calcium then binds to troponin and, through tropomyosin, exposes a myosin binding site on the actin molecule (Fig. i). ⁴ In the presence of ATP, the myosin caput binds to actin and pulls the thin filament forth the thick filament, assuasive the sarcomere to shorten. As long as Ca²⁺ and ATP are present, the myosin heads will adhere to the actin molecules, pull the actin, release, and reattach. This procedure is known as cantankerous-bridge cycling. The speed at which cantankerous-bridge cycling tin can occur is limited predominantly by the rate that the ATPase of the myosin head tin hydrolyze ATP.

Figure 1

Regulatory role of troponin and tropomyosin. Troponin is a pocket-size globular protein with iii subunits (TnT, TnI, TnC). (A) Resting condition: Tropomyosin under resting conditions blocks the active sites of actin, preventing actin and myosin from binding. (B) Wrinkle: When troponin binds with Ca^two+, it undergoes a conformational change and pulls tropomyosin from the blocking position on the actin filament, allowing myosin heads to course cross-bridges with actin. From Plowman SA, Smith DL. Exercise Physiology for Health, Fitness, and Performance. Boston, Mass: Allyn & Bacon; 1997:433. Copyright 1997 past Allyn & Bacon. Reprinted/adapted past permission.

Effigy one

Regulatory role of troponin and tropomyosin. Troponin is a modest globular protein with 3 subunits (TnT, TnI, TnC). (A) Resting condition: Tropomyosin under resting weather blocks the active sites of actin, preventing actin and myosin from binding. (B) Contraction: When troponin binds with Ca²⁺, it undergoes a conformational modify and pulls tropomyosin from the blocking position on the actin filament, allowing myosin heads to course cantankerous-bridges with actin. From Plowman SA, Smith DL. Practise Physiology for Health, Fitness, and Performance. Boston, Mass: Allyn & Bacon; 1997:433. Copyright 1997 past Allyn & Bacon. Reprinted/adapted by permission.

Muscle Fiber Typing

Initially, whole muscles were classified as being fast or tiresome based on speeds of shortening. ^three This division too corresponded to a morphological difference, with the fast muscles actualization white in some species, notably birds, and the slow muscles actualization red. The redness is the result of loftier amounts of myoglobin and a high capillary content. ⁱⁱⁱ The greater myoglobin and capillary content in crimson muscles contributes to the greater oxidative capacity of red muscles compared with white muscles. Histological assay shows that there is a correlation between myosin ATPase activity and the speed of muscle shortening. ⁶ This histochemical analysis led to the original sectionalization of muscle fibers into type I (slow) and type 2 (fast). Currently, muscle fibers are typed using 3 different methods: histochemical staining for myosin ATPase, myosin heavy chain isoform identification, and biochemical identification of metabolic enzymes.

Myosin ATPase Staining

In humans, myosin ATPase hydrolysis rates for fast fibers are 2 to 3 times greater than those of irksome fibers. ⁷ However, myosin ATPase histochemical staining, which is widely used for classifying musculus fibers, does not evaluate myosin ATPase hydrolysis rates. ¹ Fibers are separated based solely on staining intensities considering of differences in pH sensitivity, not because of the relative hydrolysis rates of ATPases. ¹ Advances in the histochemical staining technique used to evaluate myosin ATPase have led to 7 recognized man muscle fiber types (Fig. ii). ⁱ Originally, fibers were identified as type I, IIA, or IIB. ^1,v More recently, types IC, IIC, IIAC, and IIAB, which have intermediate myosin ATPase staining characteristics, have been identified. The slowest fiber, blazon IC, has staining characteristics more than like those of type I fibers, whereas the fastest fiber, type IIAC, stains more similar type IIA. Type IIAB fibers have intermediate staining characteristics between type IIA and IIB fibers. Considering these delineations are based on qualitative analysis of stained fibers, it remains possible that more fiber types will exist identified in the future. In summary, the vii homo muscle fiber types, as identified by myosin ATPase histochemical staining are (from slowest to fastest): types I, IC, IIC, IIAC, IIA, IIAB, and IIB (Fig. 2). ^1,iii,5 These divisions are based on the intensity of staining at different pH levels, and, as such, whatever given fiber could be grouped differently past different researchers. Furthermore, not all studies use all 7 fiber types. Some researchers place all musculus fibers into just the original 3 cobweb types.

Figure 2

Comparison of 3 unlike skeletal muscle fiber type classifications: histochemical staining for myosin adenosinetriphosphatase (mATPase), myosin heavy chain identification, and biochemical identification of metabolic enzymes. Note: in humans, MHCIIb are now more than accurately referred to as MHCIIx/d. The question marks indicate the poor correlation between biochemical and myosin heavy chain or mATPase cobweb type classification schemes.

Figure 2

Comparison of 3 different skeletal muscle fiber type classifications: histochemical staining for myosin adenosinetriphosphatase (mATPase), myosin heavy concatenation identification, and biochemical identification of metabolic enzymes. Annotation: in humans, MHCIIb are now more than accurately referred to as MHCIIx/d. The question marks indicate the poor correlation betwixt biochemical and myosin heavy chain or mATPase fiber type nomenclature schemes.

Myosin Heavy Concatenation Identification

Identification of unlike myosin heavy chain isoforms also allows for fiber blazon nomenclature (Fig. 2). ⁱ The different myosin ATPase-based fibers correspond to unlike myosin heavy concatenation isoforms. ^1,8 This is not surprising because the myosin heavy chains contain the site that serves as the ATPase. The fact that each muscle fiber tin can incorporate more than 1 myosin heavy chain isoform explains the existence of myosin ATPase fiber types other than the pure type I, type IIA, and blazon IIB fibers. Although the human being genome contains at least x genes for myosin heavy chains, merely 3 are expressed in adult homo limb muscles. ^one Myosin heavy chain isoforms can be identified by immunohistochemical analysis using antimyosin antibodies or by sodium dodecyl sulfate-polyacrylamide gel electrophoretic (SDS-Page) separation. ⁵

The three myosin isoforms that were originally identified were MHCI, MHCIIa, and MHCIIb, and they corresponded to the isoforms identified by myosin ATPase staining as types I, IIA, and IIB, respectively. ^ane,iii,5 Human mixed fibers almost always comprise myosin heavy chain isoforms that are"neighbors" (ie, MHCI and MHCIIa or MHCIIa and MHCIIb). ² Consequently, the histochemical myosin ATPase type IC, IIC, and IIAC fibers coexpress the MHCI and MHCIIa genes to varying degrees, whereas the blazon IIAB fibers coexpress the MHCIIa and MHCIIb genes. ⁱ Considering of its quantitative nature, identifying myosin heavy chain isoforms using single-fiber electrophoretic separation (SDS-Page technique) probably represents the best method for muscle fiber typing. Electrophoretic separation allows for the relative concentrations of different myosin heavy chain isoforms to exist detected in a mixed cobweb. ^five,8

I indicate regarding human myosin heavy chain isoforms and fiber type identification may testify confusing to someone trying to read research literature in this area. In small-scale mammals, a 4th myosin heavy chain isoform, MHCIIx or MHCIId, is present that has an intermediate contractile speed betwixt the MHCIIa and MHCIIb isoform. ⁹ Based on several types of evidence, extending to the level of DNA analysis, what was originally identified in humans as MHCIIb is actually homologous to MHCIIx/d of small mammals. ^two,v,9 As a event, what has been called MHCIIb in humans is actually MHCIIx/d, and humans exercise not limited the fastest myosin heavy chain isoform (MHCIIb). ⁵ Because the histochemical myosin ATPase fiber blazon classification was developed using man musculus, blazon IIB fibers, which nosotros at present know correspond to the MHCIIx/d myosin heavy chain isoform, are non probable to be renamed blazon IIX. ^ane Consequently, depending on the author, histochemical myosin ATPase-based human blazon IIB fibers may exist associated with either MHCIIb or MHCIIx/d isoforms. It is of import to remember that in human limb muscles only 3 myosin heavy chain isoforms are present (from slowest to fastest): MHCI, MHCIIa, and MHCIIx/d (formerly erroneously identified every bit MHCIIb). ^one Humans practice not express the fastest myosin heavy chain isoform, MHCIIb. ^nine We will acquaintance MHCIIx/d in humans with the histochemical myosin ATPase-based type IIB fiber in the residuum of this article.

Biochemical

A third classification scheme that is often used to classify musculus fibers combines information on muscle fiber myosin ATPase histochemistry and qualitative histochemistry for certain enzymes that reflect the energy metabolism of the fiber (Fig. ii). ² Histochemical myosin ATPase fiber typing is used to classify muscle fibers equally type I or type Two, which are known to correspond to tiresome and fast muscle fibers, respectively. ² The enzymes that are analyzed reverberate metabolic pathways that are either aerobic/oxidative or anaerobic/glycolytic. ⁵ This classification technique leads to iii fiber types: fast-twitch glycolytic (FG), fast-twitch oxidative (FOG), and slow-twitch oxidative (SO). ^2,3 Although a good correlation exists betwixt blazon I and SO fibers, the correlations between type IIA and FOG and type IIB and FG fibers are more varied. ^three,10 Therefore, the type IIB fibers do not always rely primarily on anaerobic/glycolytic metabolism, nor do the type IIA fibers e'er rely primarily on aerobic/oxidative metabolism. ⁵ Although, in general, fibers at the type I finish of the continuum depend on aerobic/oxidative energy metabolism and fibers at the type IIB end of the continuum depend on anaerobic/glycolytic metabolism, the correlation is not strong plenty for type IIB and FG or type IIA and FOG to be used interchangeably. ^two,five

Myosin Light Bondage

The light chains of the myosin molecule also exist in dissimilar isoforms, slow and fast, that bear upon the contractile properties of the muscle fiber. ^3,eleven Muscle fibers that are homogeneous for a myosin heavy concatenation isoform (ie, a pure fiber) may be heterogenous in regard to myosin lite concatenation isoforms, although, in general, fast myosin heavy concatenation isoforms associate with fast myosin low-cal chain isoforms and wearisome myosin heavy chain isoforms acquaintance with slow myosin light chain isoforms. ^ii,5,12 In that location is expert evidence that additional proteins in musculus fibers are coexpressed so that the diverse"fast" proteins are expressed with one another and the various"slow" proteins are expressed with one another, which suggests"a fiber blazon specific program of gene expression." ^2,11,12

Motor Unit Classification

Although we have been discussing fiber types, the true functional unit of the neuromuscular arrangement is the motor unit of measurement. ^13,xiv A motor unit is an alpha motoneuron (originating in the spinal string) and all of the muscle fibers that it innervates. Based on myosin ATPase histochemistry and qualitative histochemistry for enzymes that reflect the energy metabolism of the fiber, all of the muscle fibers of a motor unit have similar characteristics. ¹⁵ Motor units can be divided into groups based on the contractile and fatigue characteristics of the musculus fibers. ^3,14 Based on contractile speed, motor units are classified equally either slow-twitch (S) or fast-twitch (F). ¹⁴ The F motor units are further subdivided into fast-twitch fatigue-resistant (FR), fast-twitch fatigue-intermediate (Fint), and fast-twitch fatigable (FF). ^16,17

Motor Unit of measurement/Muscle Cobweb Plasticity

Regardless of the classification scheme used to group muscle fibers, there is overwhelming evidence that muscle fibers—and therefore motor units—not only change in size in response to demands, but they can also convert from one type to some other. ^2,18,19 This plasticity in contractile and metabolic backdrop in response to stimuli (eg, training and rehabilitation) allows for adaptation to unlike functional demands. ^two Cobweb conversions between type IIB and type IIA are the near common, but type I to blazon Ii conversions are possible in cases of severe deconditioning or spinal cord injury (SCI). ^2,20

Less evidence exists for the conversion of type Two to type I fibers with training or rehabilitation, considering only studies that employ denervated muscle that is chronically activated with electrical stimulation have consistently demonstrated that such a conversion is possible. ²¹

Changes in the muscle fiber types are likewise responsible for some of the loss of part associated with deconditioning. ^two Experiments in animals involving hind-limb intermission, which unloads hind-limb muscles, and observations of humans and rats post-obit microgravity exposure during spaceflight have demonstrated a shift from slow to fast muscle fiber types. ² In addition, numerous studies on animals and humans with SCI have demonstrated a shift from slow to fast fibers. ^2,20 In humans, detraining (ie, a decrease in muscle use from a previously loftier activeness level) has been shown to lead to the same slow to fast conversion, with shifts from MHCIIa to MHCIIx/d and maybe MHCI to MHCIIa. ² In that location is also a concomitant decrease in the enzymes associated with aerobic-oxidative metabolism. ² In summary, decreased utilise of skeletal muscle can lead to a conversion of musculus fiber types in the tedious to fast direction.

Interestingly, some of the loss of muscle operation (eg, decreased strength product) due to aging does non appear to be only due to the conversion of muscle fibers from 1 blazon to another, but largely due to a selective atrophy of sure populations of muscle fiber types. ^22,23 With aging, at that place is a progressive loss of musculus mass and maximal oxygen uptake, leading to a reduction in musculus functioning and presumably some of the loss of role (eg, decreased ability to perform activities of daily living) seen in elderly people. ^ane,22,23

Historic period-related loss of musculus mass results primarily from a decrease in the total number of both type I and blazon II fibers and, secondarily, from a preferential atrophy of type 2 fibers. ^22,24 Atrophy of blazon II fibers leads to a larger proportion of slow type muscle mass in anile muscle, as evidenced by slower contraction and relaxation times in older muscle. ^25,26 In addition, the loss of alpha motoneurons with age results in some reinnervation of"abased" muscle fibers by adjacent motor units that may exist of a dissimilar type. ^22,27 This may facilitate fiber type conversion, as the reinnervated muscle fibers take on the backdrop of the new"parent" motor unit. ^3,22 Recent prove in anile muscle suggests that fiber type conversion may occur, because in that location is a much larger coexpression of myosin heavy chain in older adults as compared with young individuals. ²⁸ Older muscle was plant to take a greater pct of fibers that coexpress MHCI and MHCIIa (28.v%) compared with younger muscle (5%–x%). ²⁸

Fortunately, concrete therapy interventions can touch muscle fiber types leading to improvements in musculus functioning. In the context of this update, physical therapy interventions can be broadly divided into those designed to increase the patient's resistance to fatigue and those designed to increase the patient'south force product. It has been known for some time that grooming that places a loftier metabolic need on the muscle (endurance training) will increase the oxidative capacity of all muscle fiber types, mainly through increases in the amount of mitochondria, aerobic/oxidative enzymes, and capillarization of the trained muscle. ^29,30 Using the metabolic enzyme–based nomenclature organization, this would atomic number 82 to a transition from FG to FOG muscle fibers without, necessarily, a conversion of myosin heavy chain isoforms. ^two

The myosin heavy chain composition of a muscle fiber can change when subjected to endurance training. ¹⁹ Within type II fibers there is a transformation from IIB to IIA, with more MHCIIa being expressed, at the expense of MHCIIx/d. ^two,19 Consequently, the percentage of pure type IIB fibers decreases and the percentages of blazon IIAB and pure type IIA fibers increase. Evidence is defective to demonstrate that type 2 fibers convert to blazon I with endurance training, ^xix although there does appear to be an increase in the mixed type I and IIA fiber populations. ² Researchers accept found that type I fibers go faster with endurance practice and slower with deconditioning in humans. ^31,32 This alter in contractile speed is not because of a conversion of fiber types, but rather because of changes in the myosin low-cal chain isoforms from wearisome to fast isoforms and from fast to slow isoforms, respectively. ^31,32 Because this change in musculus contractile speed does non occur by altering the myosin ATPase, it would not exist detectable by histochemical fiber typing. ⁱⁱ The shift from ho-hum to fast myosin light concatenation isoforms allows the slow fibers to contract at a rate fast enough for the given exercise (eg, running, cycling), withal retain efficient properties of energy use. ³⁰ In summary, muscle fiber adaptations to endurance exercise depend on cobweb type, although the oxidative chapters of all fibers is increased. Type I fibers may get faster through myosin light chain conversion, whereas blazon Ii fibers catechumen into slower, more oxidative types.

High-intensity resistance preparation (eg, loftier-load–low-repetition grooming) results in changes in fiber type like to those seen with endurance training, although musculus hypertrophy as well plays an essential office in producing force gains. ³³ Initial increases in force product with high-intensity resistance grooming programs are largely mediated by neural factors, rather than visible hypertrophy of muscle fibers, in adults with no pathology or impairments. ³⁴ Nonetheless, changes in musculus proteins, such as the myosin heavy chains, do begin after a few workouts, just visible hypertrophy of musculus fibers is not evident until training is conducted over a longer period of time (>8 weeks). ³³

Most researchers take found that high-intensity resistance training of sufficient duration (>8 weeks) causes an increase in MHCIIa composition and a corresponding decrease in MHCIIx/d composition. ^35–37 In many studies of high-intensity resistance preparation, researchers accept also reported concomitant increases in MHCI composition, ³⁷ although some researchers written report no changes in MHCI composition. ^38,39 Both endurance training and resistance training effect in similar reductions in myosin heavy chain coexpression, such that a greater number of"pure" fibers are present. ^xl Although the trends in fiber type conversions are similar for endurance training and resistance training, differences in physiological changes that occur with each type of do are too of import. Endurance training increases the oxidative capacity of muscle, whereas preparation to increase forcefulness production of sufficient intensity and duration promotes hypertrophy of muscle fibers by increasing the volume of contractile proteins in the fibers.

Knowing the differences between human skeletal musculus fiber types allows clinicians to understand more than completely the morphological and physiological ground for the effectiveness of concrete therapy interventions, such every bit endurance training and resistance grooming. In improver, this knowledge also offers some caption for the changes in muscle that occur with age, deconditioning, immobilization, and musculus denervation. Such knowledge is helpful for the optimal design of rehabilitation programs that target deficits in musculus morphology and physiology.

All authors provided concept/inquiry design and writing. Michael Higgins, Michael Lewek, Darcy Reisman, Scott Stackhouse, and Glenn Williams provided consultation (including review of the manuscript before submission).

Dr Binder–Macleod was supported by a grant from the National Institutes of Health (HD36787). Mr Scott and Ms Stevens were supported by a grooming grant from the National Institutes of Wellness (T32 HD07490).

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© 2001 American Physical Therapy Association

© 2001 American Physical Therapy Association

Source: https://academic.oup.com/ptj/article/81/11/1810/2857618

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