Muscles Alive Ch 1

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CONTENTS CHAPTER[APTER 20. Muscles of Mastication, Face, an d Neck 447[APTER 21. Extraocular Muscles and Muscles of

Middle Ear . 470(APTER 22. Motor Nerve Conduction Velocity andResidual Latency . 482eferences . 491idex ..... 553

IntroductionElectromyography is the study of muscle function through the inquiry

of the electrical signal the muscles emanate.Inherent movement is the prime sign of animal life. For this and many

other reasons, man has shown a perpetual curiosity about th e organs oflocomotion in his own body and in those of other creatures. Indeed,some of the earliest scientific experiments known to us concerned muscleand its functions.

With the reawakening of science during the Renaissance, interest inmuscles was inevitable. Leonardo da Vinci, fo r example, devoted muchof his thought to the analysis of muscles and their functions. So, too, didthe acknowledged "father" of modern anatomy, Andreas Vesalius, whoseinfluence through his monumental work, the Fabrica, extends down tothis day. In one sense, however, the heritage of Vesalius was unfortunatebecause it stressed the appearance and the geography of dead musclesrather than their dynamics (Fig. 1.1).

During subsequent years, a series of scientists gave life back to th emuscles. Th e first logical deduction of muscle-generated electricity wasdocumented by Italian Francesco Redi in 1666. He suspected that theshock of the electric ray fish was muscular in origin an d wrote, "I tappeared to me as if the painful action of the electric ray was located inthese two sickle-shaped bodies, or muscles, more than any other part"(Biederman, 1898). Th e relationship between electricity and musclecontraction was first observed by Luigti Galvani in 1791. In his epochmaking experiments, he depolarizeeM11e muscles of a frog's legs by -+touching them with metal rods (see Fig. 1.2). His concept of "animalelectricity" was enthusiastically received throughout Europe. Calvani'soriginal book, De Viribus Electricitatis, has been translated into Englishby Green (1953). This discovery is generally acknowledged as representing the birth of neurophysiology, thereby making Galvani the father ofthis field which continues to expand rapidly.Many rushed to confirm Galvani's results and praise his discovery.Among them was Alessandro Volta, who initially embraced the discoveryand in retrospect wrote "it contains one of the most beautiful an dsurprising discoveries an d the germ of many others" (Volta, 1816). But,within two years, in 1793, Volta questioned Calvani's findings by provingthat dissimilar metals in contact with an electrolyte (such as those presentin body tissues) would generate an electric current. In the following yearGalvani reaffirmed his concept when he found that a muscle contraction

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2 MUSCLES ALIVE INTRODUCTION 3>p .ANV. VES.ALI I VE COJ:'POlIS

Figure 1.1. A "muscle-man" from Vesalius' Fabrica. (From a rare 1555 edition inthe Library of Queen's University.)

Figure 1.2. Galvani's demonstrat ions of the effects of electricity on muscles offrogs and sheep. (From Fulton's reproduction of a plate in Galvani's De ViribusElectricitatis in Motu Museulari Commentarius, 1792.)could be elicited by placing the free en d of a nerve across a musclewithout the intervention of metals.

However, Volta's blow was so powerful that the concept of animalelectricity was no t discussed meaningfully for four decades. Th e engi-neering development of Volta, in providing a device fo r generatingelectrical currents and stimulating muscles conveniently, was no tmatched by a comparable engineering development in providing equip-ment and techniques to detect the electrical current in the muscles. In1820, Schweigger built the first practical galvanometer based on Oerst-ed's discoveries on magnetism. Five years later Nobili improved thesensitivity by compensating for the torque of the earth's magnetic field.Using this improved galvanometer, Carlo Matteucci in 1838 finallyproved that electrical currents did originate in muscles. In 1844 he


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4 5USCLES ALIVEwrote, "The interior of a muscle place in connection with any partwhatsoever of the same muscle. . .produces a current which goes in theanimal from the muscular part to that which is no t so."

The work of Matteucci attracted the interest of th e Frenchman DuBois-Reyrnond, who in 1849 was the first to report th e detection ofvoluntarily elicited electrical signals from human muscles. Dulsois-Reymond's achievement was an example of ingenuity and stalwartness. Hedevised a surface electrode which consisted of a wire attached to ablotting paper immersed in a jar of saline solution. Figure 1.3 is areproduction of Figure 147 in his book. This diagram demonstrates therecording apparatus. He found that when the fingers were immersed inthe saline solution, and the arms and hand were contracted (as shown inthe figure) the deflection on the galvanometer was minute (approxi-

Figure 1.3. Depiction of the first recorded detec tion of the EMG signal fromhuman muscles during voluntary contraction. (From Figure 147 of the book "UberThierische Elektricitat" by Du Bois-Reymond published in 1849.)

INTRODUCTIONmately 2 to 3). He realized that the impedance of the skin reduced thecurrent which he could detect to drive the galvanometer. He cjrcumvented this problem by inducing a blister in each forearm. Then heremoved the skin an d placed the open wounds in contact with th e salinesolution of the electrode. Upon contraction he measured a sizable deflection (65) on his galvanometer. He repeated the contraction threetimes fo r each arm an d always obtained similar results. To remove doubt,he repeated the whole experiment several weeks later, after the originalwounds had healed. He obtained the same results.

However, measurements from human musculature remained unwieldyuntil the metal surface electrode was employed by the German Piper(1907). The detection techniques were further simplified with the adventof the cathode ray tube, which was invented by Braun (1897) and wasfirst used to amplify action potentials in conjunction with a string galvanometer by Forbes and Thacher (1920). Tw o years later, Gasser an dErlanger (1922) used a cathode ray oscilloscope in place of th e galvanometer, which up to this time was the sole apparatus able to "show" th esignals from the muscles. This application, along with their wise interpretation of the action potentials which they were able to "see," earnedGasser an d Erlanger a Nobel Prize in 1944.

During the 19th century, th e capability of detecting th e electrornyographic or myoelectric signal from a human remained a sophisticatedan d delicate venture. As such, it was mastered only by a few, an d usefulachievements were slow in coming. On the other hand, the task ofelectrically stimulating a muscle by applying current through the skinwas relatively simple and gained wide attention. In fact, many unqualifiedcharlatans rushed to exploit its novelty by claiming that a "properly"applied dose of electricity could perform miraculous cures on a widevariety of ailments ranging from tic douloureux to chronic functionaldisorders (Holbrook, 1959).However, among this morass of deceptions flowered th e work of fewindividuals who remained true to the scientific inquiry. Towering abovethem was the Frenchman Duchenne, who in the middle of the pastcentury, skillfully applied electrical stimulation to investigate systernatically the dynamics an d functions of intact skeletal muscles (Figure 1.4).His immortal work, Physiologie des Mouvements, is now available in anEnglish edition (translated by E.B. Kaplan). No on e before or after hascontributed so much to ou r understanding of muscular function, although Beevors (1903) work cannot be ignored.

It was only natural that the business of detecting electric signals from,and applying electric currents to, muscles should attract the attention ofan electrical engineer. Such was the case for the Englishman Baines, whopublished his works in 1918. He argued that appropriate technicalconsiderations should be administered before obtaining or interpreting


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6 MUSCLES ALIVE INTRODUCTION 7

Figure 1.4. Duchenne's illustrationof electrical stimulation of muscles.data related to electrophysiological phenomena. This call still echoesamong the numerous abuses that have been promulgated throughoutthe past seven decades. Baines was the first to formalize the analogybetween the propagation of pulses in a nerve trunk an d an electric cable.This approach subsequently became known as the cable theory. Heinitiated the concept of modeling parts of th e nervous system withelectrical circuits in attempting to explain their behavior (see Fig. 1.5).It may be said that he was th e first biomedical engineer.

With the introduction of vacuum tube amplifiers, the task of detectingth e electromyographic signal was greatly simplified. Soon the new "art"of electromyography was put to practical usages in the clinical environment. Th e first successful attempt at detecting a signal from a dysfunctional muscle was made by Proebster (1928) who obtained "tracings"from a muscle with peripheral nerve paralysis.

However, th e impact on th e clinical community occurred after theintroduction of the needle electrode by Adrian and Bronk (1929). Thisapproach, for th e first time, enabled us to observe the electrical activityassociated with individual muscle fibers (or small groups of muscle fibers).Although Adrian an d Bronk's motivation was the investigation of themotor control schemes which acted on the muscle, the immediate impactwas on the clinical community. Th e use of the needle electrode wasmethodologically exploited by Buchthal an d his colleagues during the1950s and 1960s.As the quality an d availability of electronics apparatus improved;

Phyliological. Electrical.

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+11=-=:1'+ ,+51 !L HII ~ ~ I +

~ ~

Fig. 106. (A/ter HalliburlDn.) Fig. 107.PCC "" smaIl cells at the hue of IUJ"" low-tension condenaen.

the posterior cornu. bbb .. high-tension condenaen.ACC = large motor-cells of theanterior cornu.M = mU8Cular fibrell.PF = axon.CC :z cell of the cerebral greymatter.Figure 1.5. First known electrical model of the nervous system. Published byArthur E. Baines in 191B. He was possibly the first biomedical engineer. (Reproduced from Figures 106 and 107 of his book.)

anatomists, kinesiologists, an d orthopedic surgeons began to make increasing use of electromyography. Th e first study that gained wideacceptance was that of Inman et al (1944), who reported their work onthe movement of the shoulder region. However, kinesiological studiesproliferated only after the appearance of the electrically stable silver

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.E a4e'Y


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9USCLES ALlVEsilver chloride surface electrode and the nonobtrusive inserted wirelectrode which appeared on th e scene circa 1960.

In 1960, a group of Russian engineers led by Kobrinsky revealed th eesign of a hand prosthesis controlled by myoelectric signals detectedrom the forearm muscles. This demonstration excited the engineeringnd rehabilitation community. Engineers in Canada, England, Sweden,ustria, and the USA rushed to explore an d exploit this new area. Theirnterest was fueled by funds made available to provide replacement limbs

or numerous children who were born with undeveloped limbs due tohe ingestion of th e drug thalidomide by their mothers during pregnancy.lthough th e work of th e Russians generated the excitement, it should

be mentioned that the earliest recorded effort to employ electromyoraphic signals for controlling prostheses should be accorded to Reinholdeiter, who applied fo r a patent describing th e concept in 1945 inermany.Thus, we have seen that during th e past two centuries a wide variety

f individuals of diverse training have been fascinated by an d haveontributed to th e understanding of th e electrical phenomenon which isssociated with a muscle contraction. The contents of this book werehosen to describe the more recent developments in electromyography

Nhich ar e th e heirs of all th e efforts that have preceded.TERMINOLOGY

Sure signs of a progressive an d developing field are the evolution an dnodification of definitions an d standards. Tw o groups have offered such~ u i d e l i n e s : th e IFSECN at its Second International Congress (Guld et aI,1970), and the Second Congress of th e International Society of Electroihysiological Kinesiology (ISEK) in 1972. The latter group has revisedts definitions an d has published a manual (Winter et aI, 1980).

Some of th e following terminology an d se t of definitions ar e anibridged version of those found in th e ISEK manual "Units, Terms an ditandards in th e Reporting of EMG Research" (1980). They have been-xtracted with permission.llpha-motoneuron-The neural structure whose cell body is located in the anterior horn of the spinal cord and which. through its relatively large diameter

axon and terminal branches, innervates a group of muscle fibers.dotor unit (MU)---The term used to describe the single smallest controllablemuscular unit. Th e motor unit consists of a single alpha-motoneuron, itsneuromuscular junction, and the muscle fibers it innervates (as few as 3, asmany as 2000).dusd e fiber action potential or motor action potential (MAP)--- Th e name given tothe detected waveform resulting from the depolarization wave as it propagatesin both directions along each muscle fiber from its motor end plate.dotor unit action potential (MUAP)-The name given to the detected waveformconsisting of the spatiotemporal summation of individual muscle fiber action

INTRODUCTIONpotentials originating from muscle fibers in the vicinity of a given electrode orelectrode pair.Motor unit action potential train (MUAPT)--- Th e name given to a repetitivesequence of MUAPs from a given motor unit.Interpulse interval (IPI)-Th e time between adjacent discharges of a motor unit.It is a semir andom quantity.Instantaneous firing rate-The parameter which represents the inverse value ofthe interpulse interval.Averagefiring rate-The average firing rate of a motor unit over a given periodof time. It is measured in units of pulses per second.

Synchronization-The term which describes the tendency for a motor unit todischarge at or near the time that another motor unit discharges. It thereforedescribes the interdependence or entrainment of two or more motor units.Electromyographic (EMG) signal-The name given to the total signal detected byan electrode. It is the algebraic summation of all MUAPTs from all activemotor units within the pick-up area of the electrode.Myoelectric signal-An alternative nomenclature for the electromyographic signal.Amplitude-That quantity which expresses the level of the signal activity.Time duration-The amount of time over which a waveform presents detectableenergy.Phase-In electromyography, the net excursion of the amplitude of a signal ineither the positive or negative direction.Shape-The characteristics of a signal which remain unalte red with linear scalingin either the amplitude or time domains. An example of such characteristicsare the phases of an action potential.Waveform-The term which describes all aspects of the excursion of the potential,voltage, or current associated with a signal as a function of time. It incorporatesall the notions of shape, amplitude, and time duration.Decomposition-The process whereby individual MUAPs are extracted from theelectromyogr aphic signal. .Electrode-A device or unit through which an electrical current enters or leavesan electrolyte, gas, or vacuums.Detection surface-The portion of the electrode which is in direct contact withthe medium which is being sensed.Unipolar electrode-One which consists of one detection surface.Bipolar electrode-One which consists of two detection surfaces.Concentric electrode-A unipolar electrode in which the detection surface islocated in the center of a metallic shield (typically, the cannula of a needle),which in turn is connected to ground.Detection-The process of sensing the signal by the electrode.Recording-The p r o c ~ s s which creates a record of the detected signal on anymedia (CRT, paper, magnetic tape, etc.)Isometric contraction-One during which the length of the contracting muscleremains constant. Generally, the muscle length is assessed by monitoring theangle of the joint being affected,Anisometric contraction-One during which the length of the contracting musclemay vary.Ballistic contraction-One that is execut ed with the gr eatest spee d physiologically

possible.Maximal voluntary contraction (MVC)---Th e greatest amount of effort that anindividual may exert. Usually, the effort is concentrated on one muscle or on


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10 MUSCLES ALIVE INTRODUCTION 11one joint. It is generally measured by monitoring the force or torque output.Agonist muscle-One which initiates a contraction.Antagonist muscle-One which actively provides a negative contribution to aparticular function during a contraction.Synergist muscle-One which actively provides an additive contribution to aparticular function during a contraction.We would like to draw attention to the appellation of two terms:firingrate an d interpulse interval. These terms are used to describe parameters

which in the past have been referred to as firing frequency an d interspikeinterval. Such terminology has its origin in the early days of electrophysiology when the available electronics apparatus was considerably morelimited than today's versions an d could not provide either detailed orcorrect representation of the behavior of the MUAPT. Thus, earlierinvestigators could only observe the presence of spikes (deflections) in anoisy signal. Modern technology enables us to observe the individualaction potentials as distinguishable pulses. Thus, it is more proper todescribe the time interval as the interpulse interval. Th e use of the termfiring frequency is improper because the concept of frequency impliesperiodicity. It is now clear that the motor unit does no t discharge in aperiodic fashion, but rather in a semirandom fashion. I t is, therefore,more correct to use the terminology of stochastic processes, i.e., firingrate. Th e concept of rate provides information concerning the numberof firings pe r unit time without any restriction on the temporal regularityof th e discharges.

THE MOTOR UNITThe reader must have a clear knowledge of the structural an d func

tional units in striated muscles to appreciate fully much of the literaturein electromyography. Th e structural unit of contraction is, as everyoneknows, the muscle cell or muscle fibe r (Fig. 1.6). Best described as a veryfine thread, this muscle fiber has a length ranging from a few millimetersto 30 Co l an d a diameter of 10 to 100 J.Lm. On contracting it will shortento about 57% of its resting length (Haines, 1932, 1934).By looking at the intact normal muscle during contraction, one wouldbelieve, quite erroneously, that all the muscle fibers were in some sortof continuous smooth shortening. In fact, this is not true; instead, thereis a virtual buzzing of asynchronous activity in which the fibers areundergoing very rapid contractions an d relaxations.

In normal mammalian skeletal muscle, the fibers probably nevercon tract as individuals. Instead, small groups of them contract in concert.On investigation, one finds that all the members of each of these groupsof muscle fibers are supplied by the terminal branches of one nerve fiberor axon whose cell body is in the anterior horn of the spinal grey matter.Now, this nerve cell body, plus the long axon running down the motor

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L7:;';;1'---t --------------------f-----a.'t:' re s t - con tra .c t ing " ~

Figure 1.6. The structural unit of contraction is the muscle fiber. The greatestamount a whole muscle can actively shorten is dep end ent on the maximalcontraction of its contractile units. (From Basmajian, 1970, Williams & Wilkins,Baltimore.)nerve, plus its terminal branches an d all the muscle fibers supplied bythese branches, together constitute a motor unit (Fig. 1.7). Th e motorunit is, then, the functional unit of striated muscle, since an impulsedescending the motoneuron causes all the muscle fibers in one motorunit to contract almost simultaneously. Th e disparity in the time activation of different muscle fibers of the same motor unit has two causes.One is the variable delay introduced by the length and diameter of theindividual axon branches innervating individual muscle fibers. This delayis fixed for each muscle fiber. The other delay is introduced by therandom discharge of acetylcholine packets released at each neuromus-cular junction. Because this is a random process, the excitation of eachmuscle fiber of a motor unit is a random function of time. This randomexcitation of each muscle fiber appears as a jitter when the electricaldischarges of the individual muscle fibers are monitored. This phenom-enon was first observed by Ekstedt (1964), who coined the name. Innormal individuals the standard deviation of the jitter is about 20 J.LS.Th e chief use of this discovery has been in clinical diagnosis.

Th e termination of the axon of the muscle fiber defines an area knownas the endplate region. These endplates (neuromuscular junctions) areusually, but not always, located near the middle of the muscle fibers (Fig.1.8). This has been shown by Coers an d Woolf (1959) in human skeletalmuscle, by Gurkow an d Bast (1958) in the trapezius an d sternomastoidof the hamster, by Jarcho et al (1952) in the gracilis of the rat, an d by


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MUSCLES ALIVE2 INTRODUCTION 13

Spinalnerve

Cell bodyof neurone

MusclefibresFigure 1.7. Scheme of a motor unit. (Modified from Basmajian, 1955a.)

Dutta and Basmajian (1960) in th e pharyngeal constrictors of the rabbit.However, ou r own observations indicate that in the case of the humantibialis anterior, the endplate region is proximal to the middle of themuscle. In some muscles of some individuals, two aggregations of motorendplates or motor points may exist in a single muscle. Such is often thecase in the long head of the human biceps brachii.The number of muscle fibers that are' served by one axon, i.e., th enumber in a motor unit varies widely, but certain rules have beenestablished in recent years. Generally, it has been agreed that musclescontrolling fine movements an d adjustments (such as those attached tothe ossicles of the ear an d to the eyeball an d the larynx) have th e smallest[lumber of muscle fibers pe r motor unit. On the other hand, large coarse-acting muscles, i.e., those in th e limbs, have larger motor units. Themuscles that move the eye have small motor units with less than 10fibers/unit, as do the human tensor tympani muscle of the middle ear,.he laryngeal muscles an d th e pharyngeal muscles. These are all rathersmall delicate muscles which apparently control fine or delicate move-ments.Krnjevic an d Miledi (1958) report 7 to 17 fibers/motor unit in the rat

Figure 1.8. Bundle of parallel muscle fibers with endplates (dark dots) stainedby cholinesterase technique (From Coers and Woolf,


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14 MUSCLES ALIVE

Tergast (1873) estimated that the motor units of the sheep extraocularmuscles have 3 to 10 muscle fibers; Bors (1926) estimated 5 to 6 forhuman extraocular muscles. More particularly, Feinstein et al (1955)reported 9 muscle fibers/motor unit in the human lateral rectus, 25 inplatysma, 108 in the first lumbrical of the hand, and 2000 in the medialhead of gatrocnemius. Christensen (1959) reported 770 muscle fibers/motor unit in the biceps brachii of infants. Van Harreveld (1947)reported 100 to 125 muscle fibers/motor unit in the sartorius of therabbit; Berlendis an d De Caro (1955), 27 in the stapedius and 30 in thetensor tympani of the rabbit; Wersall (1958), lOin the human tensortympani; an d Ruedi (1959),2 to 3 muscle fibers/motor unit in the humanlaryngeal muscles.The above innervation ratios represent average values. In fact withina muscle, there exists a hierarchical arrangement of motor unit sizes.

Figure 1.10. Drawin g of a photog raph of nerve fibers ending on muscle fibers.(Magnification, about XSOO.) (From Dutta and Basmajian, 1960, AnatomicalRecords.)

INTRODUCTION 15

AutonomiCNervous 12/i.7.f.srem

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J{euroh 11mour-I 'i AC :1 0 Ii : ~ Lower MolorNeuro: ~ ' Muscle f ibre(maq5uppl/f u!' ro150 EtidPleie I (Exrra.fu8a.Vmuscle fibresJ : I)


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16 17USCLES ALIVEmay be considered to be approximately one-third of the cross-sectionalarea of the muscle. Buchthal an d his colleagues also showed that fibersof up to 30 different motor units may be located within the territoryoccupied by anyone motor unit.

MUSCLE FIBER TYPEIt is beyond the scope of this book to entertain a detailed discussionon the various classifications of muscle fibers, bu t some basic knowledge

is necessary to appreciate better the structure of muscles.During the past several decades there has been a growing tendency toestablish definable, meaningful categories describing the physiological

and biochemical properties of muscle fibers. During the decade of the1970s, numerous investigations were performed attempting to establishcriteria for the distinction of th e muscle fiber categories. Although suchdistinctions are undeniably useful, they may be self-defeating in somecases. For example, overemphasis on the differences among some populations of fiber types tends to make us forget that the physiological andbiochemical properties of muscle fibers ar e a continuum. Depending onwhich segment of the distribution of the continuum we look at, we seedistinctions from other segments. I f the segments ar e sufficiently farapart, the characteristics of th e fiber population described therein willbe definably different. Most of the work on fiber type categorization hasbeen performed on cats, with some relevant work on guinea pigs andrabbits. In these animals the typing distinction appears to be more clearlyapparent than in human muscle fibers.Muscles of higher-order mammals (including the human) consist ofmuscle fibers which vary widely in their physiological, morphological,and biochemical properties. Within anyone animal, different musclescontain varying amounts of th e different fiber types. For related detailson 36 human muscles, the reader is referred to the work of Polgar et al(1973) and Johnson et al (1973). However, the muscle fibers belongingto one motor unit show a remarkable homogeneity in their properties.

Fo r several decades muscle fibers have been categorized according totheir appearance. Some fibers have a pinkish or reddish visual appearancedue to the relatively large amount of blood that is supplied to them bytheir large vascularization; other fibers appear much paler in coloring,reflecting a less prolific vascularization. These two categories have beenreferred to as red an d white fibers. Numerous investigations have shownthat when the motoneuron of a motor unit consisting of re d fibers isstimulated, the resulting force twitch is slower rising an d longer lastingthan the force twitch which results when a motor unit consisting of whitefiber is stimulated. Thus, we have a situation in which red fibers are slowtwitch and white fibers are fast twitch.

During the 1960s an d 1970s these basic subdivisions of muscle fiber

INTRODUCTION

types were subjected to more detailed analysis, and additional subdivisions have been defined an d proposed. Engel (1962, 1974) proposedthat the fibers be identified as type 1 an d type 11. Histochemical tests-forkey enzymes which correspond closely to the physiological properties ofmuscle fibers have been performed. Th e myosin ATPase affinity is anindicator of the fibers' contractile speed (Barany, 1967). Based on thisfact, Brooke an d Kaiser (1970) suggested that the muscle fibers becategorized according to the different sensitivities of the myosin A'TPase,Th e type 1 fibers ar e acid stable an d alkaline labile, whereas the converseis true for the type 11 fibers. Th e intensity of staining for specific enzymesof the glycogenolytic and glycolytic metabolic pathways provides anindication of the fibers' capacity to perform work (usually in short bursts)in the absence of oxygen. This is known as the anaerobic capacity.Conversely, oxidative enzymes provide information concerning the capability of the contractile mechanism to use oxygen as its fuel. This isknown as aerobic capacity. A high aerobic capacity indicates that a musclefiber is resistant to fatigue as long as oxygen can be supplied to it via itsvascularization. I t also follows that a muscle fiber with a high aerobiccapacity would have a reddish appearance.

Th e terminology of Peter et al (1972) is commonly accepted to describethese biochemical-histological properties. Thus, FG stands for fast (highmyosin ATPase) glycolytic (high anaerobic and low aerobic capacity); Flstands fo r fast with high anaerobic an d "intermediate" aerobic capacity;FOG stands for fast ox idative (high aerobic capacity) and glycolytic (highanaerobic capacity); SO stands for slow (low myosin ATPase) with oxi

. dative capacity.An alternative approach for subdividing the categories of muscle fibers

has been proposed by Burke et al (1971). They suggested the followingclassification based on the mechanical response of all the muscle fibersof a motor unit when their motoneuron was stimulated by a singleelectrical pulse (contractile response) an d to a sustained train of stimuli(contractile fatigue response). They expanded the classification of theslow and fast twitch by introducing the evaluation of contractile fatigue,which may be measured by observing the time at which the amplitudeof the twitch response declines and/or the rate at which it declines. Theydenoted FF fo r fast (contracting), quickly fatigable units; F(lnt) for fast,intermediate fatigable units; FR for fast, fatigue-resistant units; an d Sfo r slow (contracting) fatigue-resistant units.

I t should be noted that these histochemical distinctions may be mademuch more easily in cat soleus an d gastrocnemius muscles than in humanmuscles. Saltin and Gollnick (1981) argue for the distinction amonghuman muscle fiber types to be made solely on the basis of the myofibrillar ATPase stability to low pH. They point ou t that the oxidativecapacity of fast twitch human muscle fibers is no t discrete. Furthermore,


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18 MUSCLES ALIVEthe contractile fatigue measures which have been described apply toartificial stimulus train, which provides a similar excitation to all themotor units. Natural, voluntary stimulus trains are not similar for thevarious motor units involved in generating a muscle contraction. Fo rexample, the firing rate of latter recruited motor units is lower than thatof earlier recruited motor units. Thus, the proposed classification pro-vides a description of the mosaic of the motor unit fatigability bu t mayno t provide a description of the motor unit fatigability during a voluntaryco n traction.

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CHAPTER 2Apparatus, Detection, and Recording-Techniquesr

l In this chapter we will discuss details concerning electrode types an dI configurations, as well as associated instrumentation that has a bearingI on the quality of the EMG signal that is detected an d subsequentlydisplayed, recorded, or processed. The process of sensing the signal by

the electrode is referred to as detection. Th e word recording is reservedfor describing the process that creates a record on any media (CRT,paper, magnetic tape, etc.).

Before beginning a substantive discussion on electrodes, it is necessaryto ensure some minimal knowledge concerning the concepts of "imped-ances" an d "filter functions."

CONCEPT OF IMPEDANCE AND FILTER FUNCTIONSAll forms of matter present an impedance to the transmission of an

electric current. Th e impedance function is a vector quantity, hence it isexpressed in terms of complex numbers, the real part of which denotesthe resistance and the imaginary part of which denotes the susceptance.This latter part exists due to the presence of capacitance and/or induct-ance, two basic electrical properties of matter. In media such as muscletissues, fatty tissue, and skin, the inductance is essentially unmeasurable.However, the capacitance is present in a significant amount and cannott be overlooked.

On e of the simplest expressions of an impedance function, which isuseful for conceptualizing the electrical characteristics of electrodes an dtissue, is the impedance of a resistance in s eries with a capacitor presentedin Figure 2.1. In this configuration the impedance function is expressedas a vector

Z(w) = R + . l ' where j is J=T, an imaginary quantityJWC 'R = the resistance (ohms), C = the capacitance (farads), W = 21rfandf=the frequency (Hz).A vector may also be expressed in terms of its magnitude an d phase(direction). The magnitude is the square root of the sum of the squaredreal part and the squared imaginary part.

2C 2R 2)1/2(1 + WIZ(w)/ = wC

Muscles Alive Ch 1

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