REGULATION OF SKELETAL MUSCLE FIBER SIZE, SHAPE ANTI FUNCTION V. REGctmEDGERTON*’ and ROUND R. ROY’ Depamnentof Kincsiology’ and Brain ResearchInstitute,’ University of California, Los Angcks. CA 90024-1527. U.S.A.

Abstmct-There is convincingevidence that the cross-sectionalama, the type of myosin expressed. thepotential for oxidativephosphorylationand the number of myonucki of a skektal musck fiber a closely intertkpcndent. Each of thesevtiables. as well as the shapeof the fiber. has idcntifiabk physiologicalconsequences.Further.it is suggestedthat the cytoplasmic 10 myonuckus ratio is a function of the myosin type and Ihe amount and/or mh of protein synthesisand dcgradadon.Although the neuromuscular activity (ekctromyogmphic activity) as well as the associatedmechanicaland meb&olic eventshave significantregulatory influencesonpnweinmetabolism ~ICIXam other importon regulatory factors independentof theseactivity-rclatcd events. Boththeactivity and non-activity ~kted~ulatorymcchPnisnrsprobablyoccurviaac~adeofcellulirrcv~u.Thespccificcombinrtionsofallular responsesthat occur may define ihc natu= of the modulatoryeffects onspecifc proteins.In spite of the compkxily of Ihe regulatory mechanismsof protein modularion and how thcx responsesarc structumlly integrated into or removedfrom functional fibers. it is suggestcd’that con~rolkdstudiaofhuman neuromuscularfunctioncan be more accumtcly defined and intcrpmted when fiber and muscle size and shape anzconsidered.

INTRODUCTION

The physiological properties of a muscle reflect the specific types and concentrationsof proteins within the musc1e.e.g..thepresenccof slow andfastmyosinisoforms. the concentration of mitochondrial proteins. etc. To an even greater extent. the physiological properties of a musclereflect 1) how theseproteinsare assembledwithin each muscle cell (Epstein and Fischman. 1991); 2) the anatomical arrangement of fibers within a single motor unit (Ounjian et al., 1991); and 3) the ensemble of anatomical features of all motor units within the muscle (Burke and Edgerton, 1975). The present paper addressesthe first point, i.e., the functional consequencesof varying architectural stmtegics in the assemblyof proteins, a processwhich defines the size (i.e.. fiber cross-sectionalarea, CSA) and shape (i.e.. fiber tapering) of a muscle fiber. Although numerous papershave addressedthe physiologicalconsequencesof Iiber length (Spector cf al.. 1980; Bodine et al.. 1982; Gans. 1982; Edgerton et al., 1987). CSA (Spector et al.. 1980; Powell er al., 1984, Wickiewicz er 01.. 1984) and more rarely, shape(Trotter. 1990: Trotter. 1991; Ounjian cl al.. 1991; Roy and Edgerton, In Press), the assembl) strategies which determine muscle fiber size and shape have been largely ignored. Recent evidence that the CSA. particularly of fast muscle libers, changesmarkedly over long distancesof the fiber (i.e., fiber tapering) emphasizes the impommce of fiber shape in defining its functional output. Since the CSA of muscle fibers hasbeen shown 10 be highly correlated with force. an obvious issueis which measure of CSA best representsthe force potential of a muscle fiber. Although the occurnnce of tapered fibers hasbeen well documented(Bardeen, 1903; Barrett, l%2; Burleigh. 1977; Loch eraf.. 1987; Richmond and Armstrong, 1988;Trotter, 199l;Ounjianer al., 1991). the physiological signi8cance of the shape of muscle fibers

I23

with respect to motor unit types has only been recently recognized (Chmann and Schelhom. 1988; Petit et& 1990~Emonet-Denander al.. 1990;Trotter, 1990, Ounjian ef al.. 1991; Roy and Edgerton, In Press). An understanding of the processeswhich regulate protein expressionis particularly important for studying the plasticityof multinucleatedskeletal musclefibers. For example, a I cm segment of a single muscle fiber of an adult mt solcuscontains approximately 1200 myonuclci (Tseng er al.. 1990). The implications for the degree of coordination of gene expression that must occur among this many myonuclei in a single fiber are apparent. Although there can be differential exp=ssion of specific proteinsalong the lengthof a myotubeor fibcr(Marlie and Sanes. 1985;Tidball CI al.. 1986; Rotundo. 1990; Dix and Eisenberg. 1990). in geneml. the quantity and kind of proteinsexpressedalong the length of a fiber appear10 be similar (Pette et al.. 1980; Edgerton er al.. 1990; Nemeth and Wilkinson. 1990). To achieve this level of coordinationofthepm~einexprcssioninamusclefibcr.allmyonuclei may expresssimilar genesor there may be a coordination ofcxpressionamongallmyonuclei.eachcxpressingunique combinationsof proteins.

MYONUCLEI STRATEGlES THAT DEFINE FIBER SIZE AND SHAPE

The volume and physiological propertiesof a muscle fiber and the propertiesof its cytoplasmare likely to reflect I) the number of myonuclei; 2) the potential of each myonucleusto expnss specific proteins;and 3) the concentration and turnover mte of each protein. Although therehave been no clear demonstrationsthat this indeedis truc,thereis indirectevidencethatthe numberof myonuclei and the kinds and amountsof specific proteinsexpressed by these myonuclei are closely associatedwith the CSA.

I21

V. R. bCiERlON

For example, Landing ef al. ( 1974) showed that the number of myonuclei in human isolated single fibers was directly correlated with fiber diameter. but no data were presented on the type of myosin. Burleigh (1977) reported that the cytoplasmic to nuclear ratio in fibers from a muscle consisting of predominantly fast fibers was about twice that observed from fibers of a muscle comprised predominantly ofslow fibers. Eisenberg er al. (I 989) have reported similar relative differences in the cytoplasmic volume to nuclear ratio of slow and fast fibers. Tseng cr al. (I 990) have shown a similar difference between fibers that express either slow or fast myosin heavy chains. An indication that myosin type is related to shape is provided by the findings in the cat tibialisanteriorthatall fibersofamotorunitwithinafascicle tapered usually at one end over a distance of about half the length of the fiber in 5 of 5 fast motor units, while in 2 of 2 slow motor units the fibers rarely tapered (Ounjian et nl., 1991). The combination of these data suggest that the assembly and disassembly of contractile proteins and the sarcomericorganization in skeletal muscle libers areclosely associated with the number of myonuclei. the type of motor unit, the type of myosin expressed and the overall shape (tapering) of a fiber. Although the type of myosin, i.e., slow or fast, is generally related to the CSA of a fiber, this relationship is extremely variable from muscle to muscle, species to species and from one physiological or pathological state to another. The fact remains, however, that generally those fibers in predominantly fast muscles, e.g.. the medial gastrocnemius, that express fast myosin tend to have a larger CSA than those that express slow myosin (see Armstrong and Phelps, 1984 for data on a large number of rat hindlimb muscles) (Fig. I). In contrast. fast fibers in a predominantly slow musc1e.e.g.. the rat soleus.are usually smaller than the slow fibers. Fibers that express fast myosin and maintain low concentrations of mitochondrial proteins (thus, have a low oxidative potential) are usually largerthan fast fibers that maintain highconcentrations of mitochondria (Fig. I). These differences in the relative size of the fiber types across muscles are consistent with the observation that succinate dehydrogenase (SDH) activity, a marker enzyme for how rapidly ATP can be phosphorylated oxidatively. is inversely related to CSA (Hauschka elnl.. 1987; Roy eral.. 1987). These data together are consistent with the hypothesis that the mitochondrial concentration may be more limiting with respect to CSA than myosin type. These combinations of properties, viz. fast myosin-low mitochondrial concentration, fast myosin-high mitochondrial concentration and slow myosin-high mitochondrial concentration, correspond to the commonly identified types of muscle fibers, i.e., fast glycolytic (FG), fast oxidative glycolytic (FOG) and slow oxidative (SO) fibers, respectively (Peter er cl., 1972). Although an interdependence of the myosin type and the metabolic characteristics within and across muscle fibers with respect to CSA have been demonstrated, these relationships have not been well defined. An initial effort to demonstrate these interactions is shown in Fig. 2. This illustration is based on measurements of succinate dehydrogenase (SDH) activity, and CSA of single fibers of the

and R. R. ROY

(00 0

0

fast myoaln low oxidative

i

!,

0

0

fast myosln hlgh oxldatlve

slow myosln high oxldatlve Pig. I. Schematic showing the interrelationship nmong relntive cross-sectional area, myosin type and nlntivc oxidative potential of fibers of a typical predominantly fast (mixed) muscle of a control animal. The relative size of n muscle fiber appears to be mlated to whether the fiber expmsses fast or slow myosin. i.e.. relatively huge fibers express fast myosin and relatively small fibers express slow myosin. Fiber size also is related to its oxidative potential, i.e., the capacity ofthe fiber to generate ATP via oxidative phosphorylation. Note that fibers expressing slow myosintend tohavealowercytoplasmtonucleusratiothan fibers expressing fast myosin. Note also that among those ribers expressing fast myosin. the cytoplasm to nucleus ratio is lower in

the more oxidative tibers.

cat plantaris. The myosin type expressed in the fiber is also shown. These data suggest a level of interdependence of three rather fundamental properties of a fiber that are very closely related to its physiological properties. For example, nlthough the small fibers have the largest range in SDH activities, the larger the fiber the higher the likelihood that the fibers will have a lower SDH activity. In addition, most of the larger fibers express fast myosin while the smaller fibers may express either slow or fast myosin. Thesedata are consistent with the hypothesis that the fibers that have the lowest rate of oxidative phosphotylation and express fast myosin have the greatest probability of having the largest CSA. On the other hand, there appear to be no restrictions with respect to oxidative metabolic properties and CSA of smaller fibers. It is

Control of skeletal muscle fiber size and shape

125

a

. b

2000 CSA (pm2) Fig. 2. The relationshipbetween fiber cross-sectionalarea (CSA) and succinatcdchydrogenase(SDH) activity for 294 fibers from the plantarismuscle,o typical predominantlyfast(mixed) muscle. of o controlcot is shown. SDH activity and CSA were determined on single fibers using quontitativc histochcmical onalyscs OS describedby Martin er al. (1985). Fibers that stain darkly (0) ond lightly (0) for myosin ATPose under alkaline prcincubation conditions(generally thoughtto be fast ond slow fibers. respectivcly) err illustrated (unpublished observations, Cholmcn. Roy and Edgerton).

evident that the rclativc importance of each of these proteins in determining muscle fiber size and shape need to bc more clearly defined. Future efforts focusing on this precise quantilication of the intcrdcpcndencc of proteins such as those shown in Fig. 2 should pmvidc a clearer understanding of the etiology of muscle atrophy and hypcnrophy. The conceptual models of interdependence of selected variables within muscle fibers shown in Figs. 1 and 2 do not incorporate the additional level of complexity of intcrnuclear coordination that apparently exists. Although the problem of shared responsibilities among myonuclei of a fiber has intrigued biologists for some time, it has been only recently that the molecular techniques needed to study gene expression of individual myonuclei have been developed. The simplest model of the concept of nuclear d?mains (Cheek, 1985; Hall and Ralston. 1989) would be for each myonuclcus within a fiber to express the same kinds and amounts of protein and to maintain the cytoplasm in the immediate vicinity [Fig. 3(a)] (Pavlath et al., 1989; Ralston and Hall, 1989). The other extreme (most complex) model of a nuclear domain would be for each myonuclcus to express unique proteins for the entire fiber. In spite of the obvious capability ofdiffenntial expression of proteins along the length of a fiber (Merlie and Sanes. 1985; Tidball et al., 1986; Rotundo, 1990; Dix and Eisenberg. 1990). the extreme asymmetry in the physical dimensions (i.e.. length and CSA) of muscle fibers and the known limitations of half-lives, diffusion rates and distances of mRNA and proteins (Lewis er al.. 1984; Merlie and Sanes. 1985; Gauthier. 1990). significant differential

Fig. 3.Thc tbcorcticalcxtremesofthemanncrinwhichmyonuclci could control protein expression in the cytoplasm (nuclcor domain) of o fiber ore shown. In “0” the myonuclci would control only the cytoplasmic omo in the immediate vicinity. In 3” there would be o unique expressionof proteinsby diffenznt myonuclci ond thesemyonuclci would sharethe control of the somevolume of cytoplasm.

expression among myonuclei throughout the length of the fiber seems unlikely. It is more likely that then is some spatial limitation for each myonucleus with respect to the distance and/or volume of cytoplasm that a myonucleus can influence (Ralston and Hall, 1989; Pavlath er uf.. 1989). In either case. the concept of a nuclear domain seems useful if one accepts the possibility that there arc spatial limitations of the influence of a myonucleus. These limitations, however, do not preclude shared rcsponsibilities among neighboring myonuclei (Fmir and Peterson, 1983); [Fig. 3(b)]. especially with respect to the more soluble cytoplasmic proteins. e.g., the glycolytic enzymes. Another level of coordination of gene expression is required between the DNA in myonuclei and mitochondria. since some mitochondrial proteins are encoded by mitochondrial DNA (Grivell. 1988). The adaptation of fiber size and shape readily occurs in response to chronic modulation of neuromuscular function. Assuming some utility of the concept of nuclear domains, the extent to which the volume of cytoplasm can adapt may be limited by the number and kind of myonuclei. This limitation may, in turn. reflect the extent to which satellite cells can be a resource for the genetic control of a given amount of cytoplasm. The extent to which the modulation of myonuclei number and kind reflects the basic cellular stmtegies for muscle fiber hypcrtmphy and atrophy are unknown. A simple model forthe regulation of fiber CSA is shown in Fig. 4. If the CSA of a muscle fiber

126

V. R. EKN.XR~N and R. R. ROY

Hypaflrophy

Normal

laon

Atrophy Fig. 4. A Schematicshowingsix hypoktic;ll proccsscsthat could occur when the size of r muscle fiber changes.A normal muscle fibercould hypertrophywith the numberofmyonuclei dcczasing (a). remaining rhc same (b) or incnasing (c). Similarly, a normal muscle Wbcr could atrophy wilh the number of myonuclci dccreasing (d). rcmining the WTIC (0 or increasing (g). Euch of ksc options is discussedin the 1~x1.

incrcascs. the cytoplasm to nucleus ratio within the libcr can dccrcase [Fig. 4(a)]. remain the same [Fig. 4(b)] or increase [Fig. 4(c)].Thcrc is somccvidcncc that myonuclcar number incrcascs in response to compensatory hypcrtrophy (Schiaflino ef al.. 1976; unpublished observations. Tseng. Kaspcr and Edgerton). Similar scenarios could occur when a muscle fiber atrophies. As in most adaptation models. the principal adjustment occurs in fiber CSA with further adaptations possible in fiber length. if a fiber becomes smaller in size. the number of myonuclei may decrease at a rate proportional to the loss in cytoplasm, thus maintaining a normal cytoplasm to nucleus ratio [Fig. 4(d)]. If the myonuclei number remains constant while fiber size decreases, then the ratio would decrease [Fig. 4(e)]. Hypothetically, the number of myonuclei could increase in atrophying muscle as shown in Fig. 4(f). With respect to nuclear death, however, the frequency of this process among myonuclei remains largely unknown. The sourceof new myonuclei is the satellitecell (Moss, 1968: Moss and LeBlond. 1971; Snow, 1981; Darr and Schultz, 1989; Bischoff. 1989).Thesecellsarelocatedjust outside the plasma membrane and underneath the basal laminn of a fiber. Unlike those myonuclei inside the fiber, the satellite cells can undergo mitosis and usually one of the daughter cells becomes a true myonucleus (Schmalbruch, 1986). A variety of perturbations, e.g., compensatory hypcrtrophy (Schiaffino ef al.. 1976) and denervation

(Schmalbruch.

1986). can induce satellite

cells to divide with at least one of the daughter cells entering into the cytoplasm to become a new source of

protein regulation. It has been estimated that less than 5% of the nuclei (myonuclei plus satellite cells) associated with a fiber are satellite cells. It is not known how often the

satellite cells can divide in viva. Myoblasts in culture. however, can divide as many as 35-45 times (Blau et ~1.. 1983). Therefore. even though relatively few of the nuclei arc satellite cells. successive mitotic divisions of satellite cells can double the number of myonuclci in a single fiber within 4 days, at least in culture iBischoff, 1989): An additional cellular adaptation that could affect muscle function is related to cellmembrane rcorganization. There is a sentiment, however, toward accepting the view that cellular division (hyperpiasia) OECUIBrelatively rarezly in hypettrophicd muscles of mammals. Similarly, during muscle atrophy the myonuclei number could CCmain constant while the cell membrane reorganizes to form smaller cells (i.e., hyperplasia). There is evidence that. at least in some experimental models. when a muscle atrophies the myonuclear number decreases [Fig. 4(e)]. These data, however. are based on an absence of a normally occurring increase in myonuclei during develop ment (Darr and Schultz, 1989).

PHYSIOLOGICAL

CONSEQUENCES

OF VOLUME

ADAPTATION

As illustrated in Fig. 4. fibers can hypertrophy or atrophy in such a way that the cytoplasm to myonucleus ratio incrcascs. decreases or remains constant. If there is a limited capacity to the amount of a given protein that can bc sustained homcostatically by a single myonucleus. then that protein may be maintained at a lower concentration than normal after hypcrtrophy [Fig. 5(a)]. Consequently. the functional properties of the fiber can be affected. For example. if the volume of the fiber increases as a result of an elevation in the net synthesis of contractile proteins and this increase is disproportionate to an increase in mitochondrial protein, then the fiber’s capacity to metabolically support the fiber is diminished. In this case. the hypertrophicd fiber will have a higher capacity to perform work, but a reduced capacity to suppon the work. The relative fatigue nsistancc of the fiber will have been compromised because the ratio of its metabolic oxidative potential to myosin ATpast? activity will have been reduced. It is also possible for the mitochondrial protein concentration to incnasc as well as the contractile protein content, resulting in a stronger than normal fiber that is fatigue resistant [Fig. 5(b)]. This concept is relevant to atrophied as well as hypcrtrophicd fibers [Fig. 5(c), (d)]. There have been repeated examples of severely atrophied muscles maintaining their normal resistance to fatigue and mitochondrial enzyme concentrations as shown in Fig. 5(d). e.g., after 4 weeks of hindlimb suspension in rats, after 6 months of complete low thoracic spinal transection or spinal isolation, etc. (see Roy ef al.. 1991 for a recent review). Although a number of similar scenarios could be used as other examples of the regulation of protein composition relative to cell volume, the basic concept is the same, i.e.. at any given point in the adaptation process. the physiological consequences of protein modulation must be viewed relative to modulations in cell volume. Further, the

Controlof skeletal muscle fiber size and shape

I?7

models which induce adaptationsin proteins of muscle tibers. Each chronic pemrbation. e.g.. exercise, functional overload,hormonalregulation,immobilization. spinal Wansection,starvation,etc., will be somewhat unique in the specific proteinsprincipally affected and tbe degtee to and rate at which each proteinis a&c&d. In spiteof the large volume of data reported on the topic of muscle adaptations.the lack of understandingor considerationof thecriticalphysiological.biochemicalandmolecularevents associated with any given penurbation in any of these experimental models is quite evident (Fig. 6). One of the most commonly adopted views is that the amount of neuromuscular activity, i.e., the amount of

b

--.I.--. ..-..-.. -CzI

Physiological-Blochemmkal

m

Events

Force (g) Time (set)

Fig. 5. A schematicshowing tk intcrrclationshipbc~wccnf&r size. mitochondrial content (*), thus oxidativc potential. and

fatigueIrsistancc.Thesizeandmitochondrial concentration of a normalfiber is illustmtcda the lop left. A series of tctnnic contmctions illustratingthe relative fatigability of the fiber is shown OIthe top tight. In option (n) the fiber hypcrtrophieswhile maintsiningitsnorrnal mitocbondrialcontent.thusdccrcasingits mitochondrial conctnlmlion. Thcrcforc, tk libcr in (n) can

PROTEIN A (e.g., slow myosin)

producemom force, but is mom fatigabk than normal. In option (b) tk fiber hypertrophies and tk mitochondrial content is increased in propoclion to tk increase in fiber size. Thus, the

Physlologlcal-Biochemical

PROTEIN B (e.g., fast myosln)

Events

fikr in (b) can produce molt force and sustain the force at tk normal level relative to its maximum force cnpability. In option (c). the fiber atrophies and has a decrease in mitochondrial content. This fikr is werk and fatigable. In option (d) the fiber atrophies while maintaining its normal mitochondrial content. thus increasingits mitochondrialconcentration.Tk fiber in (d). tkrcforc, produces less tension than normal. but is relatively nonfatigable.

scenariospresentedare not meant to imply that mitochondrial concentrationis the only or even the principal factor whichdictates fatigability. Certainly. fibersize.capillarity density,typtofmyosinandmanyotherintn-andintertiber elements are responsible for the fatigue properties of muscles. The fact remains, however, that in any given muscle the fibers with the lowest mitochondriai densities are likely to be the most fatigable.

PHYSZOLOCZCAL StCNAZS THAT RECIJLATR PROTEIN EXPRESSION

The theoretical possibilities for the modification of cytoplasmic volume and myonuclei number per muscle fiber in response to hypertrophy or atrophy as shown in Fig. 4 are equally relevant to a range of experimental

PROTEIN A (e.g., slow myosln)

PROTEIN B (e.g., fast myosln)

Fig. 6. II is clear that tk activafionor the absenceofactivation of a muscle has a regulatory effect on its physiological and biochemical properties.Two scenariosam presented:1) some of the physiologicaleventsthat triggertkexpmsion of proteinsA and B may k common or shared;and 2) there PIEcompletely unique physiologicalevents which induce proteinsA and B. Scenario I seems mom likely than 2. For example. tk gencmtion of action potentials and tk release of calcium from the sarcoplasmic reticulum occur in every contractionand are likely to play some role in initiating thosebiochemicaleventswhicheventually lead IOa given combinationof proteinsking expressed.

I3

V. R. Enoeitro~ and R. R. ROY

electrical activation. is the primary physiological signal which initiates a cascade of mechanical-biochemical responses (Fig. 6) that eventually modulates gene expnssion an&or other post-transcriptional mechanisms which may alter protein expression. This viewpoint has been evident for many years (see Jolesz and Sreter. 1981 and Pette and Vrbova. 1985 for reviews). Interestingly, the limitationofthis viewpoint is best illustrated by theexperiments focusing on the adaptation of skeletal muscle to chronic electricat stimulation. For example, chronic electrical stimulation without concern for the loading properties on the muscle invariably results in atrophy of the stimulated muscles (Eisenberg crnl.. 1984). It also has been proposed that the quantity of electrical stimulation is the physiological signal which dictates the type of myosin expressed and maintained in a fiber. It is clear, however, that there are elements other than the number of impulses reaching a muscle that affect the type of myosin expressed (Butler-Browne er nl.. 1982; Miller and Stockdale. 1986; Hoh and Hughes. 1988; Schiaftino er al.. 1988; Roy et al., 1991). Although increased neuromuscular activity can result in muscle fiber hypertrophy and decreased activity can result in atrophy.other factors also affect fibersize. Simple anccdotalobsetvations thatendurance-trainedathletesshow littlc muscle liber hypettrophy while weightlifters. whose durations of activity are much shorter. show a hypertrophic rcsponsc. indicate that the relationship between the amount of activity and fiber size is more complex. Direct evidcncc ofthe complexity of the relationship between the quantity of activity and fiber size is the response to chronic electrical stimulation (Salmons and Henriksson. 1981; Eisenbcrg er ~1.. 1984). Finally. even in normal muscles. those fibers which are the most active arc generally the smallest, since low threshold motor units are the most active and high threshold units are the lcast active (Burke and Edgerton. 1975; Henneman and Mendell. 1981). Other experimental data which support the view that the level of activation is not directly related to fiber size have been obtained from chronic EMG recordings of hindlimb muscles from I) rats having theirkneeand ankle joints immobilized (Foumierrr al.. 1983); 2) cats spinally transected at a low thoracic level (Alaimo er al.. 1984); 3) rats having their hindlimbs unloaded (Alford et& 1987); and 4) cats having their lumbar spinal cord functionally isolated, i.e., silencing the hindlimb muscles by eliminating the supraspinal and peripheral synaptic input to the motoneurons (Pierotti er al.. In Press). The relationship between fiber size and chronic activity levels in each of these models relative to normal is plotted in Fig. 7. These data clearly indicate that the amount of electrical activity cannot be the only signal that dictates fiber size. An alternative and perhaps additional view is that the active and/or passive force triggered by the electrochemical and/or mechanical events in the sarcolemmaand cytoplasm is the physiologically event that translates into an adjustment in protein metabolism, and therefore, the size of a fiber. There are numerous studies which provide evidence forthisviewpoint(Vandenburgher al., 1989;see Royer al., 1991 forreview). Adirect.quantitative illustra-

tion of the interactive effects of electro-chemical and mechanical events that occur during muscle contraction in wivoremains undefined (Fig. 6). Variations in chronic levels and kinds of neuromuscular activity result in the modulation of muscle proteins which often are manifested as physiological adaptations. For example. chronic increases in activation may increase the concentration of mitochondrial and associated proteins and. in turn, will result in a greater resistance to fatigue. The potent effect of increasing the quantity of neuromuscular activity of a muscle fiber which often can stimulate mitochondrial protein synthesis is usually interpreted to demonstrate that mitochondrial proteins are controlled by neuromuscular activity levels alone. Based on this interpretation, all muscle fibers would have the same mitochondrial densities in the absenceof neuromuscularactivity.Theimplicationisthat the fiberheterogeneitycommonly referred to as FG. FOG and SO fiber types reflects differential activation levels of motor unit types. i.e., fast fatigable, fast fatigue resistant and slow units (Burke and Edgerton. 1975). The evidence that this is not the case is rather convincing. For example, neither marked downward modulation of neuromuscular activity by chronic spinalization (Roy et al.. 1984; Baldwin er al., 1989; Jiang erd., 1990). the absence of activity in motoneurons devoid of synaptic input from supraspinal and peripheral sources (Graham cl al., In Press; Pierotti er 01.. In Press), nor by the presumed reversal of activity patterns by crossreinnervation of slow and fast muscles (Edgerton er al., 1980;Chaner al.. 1982)resultinthechanges that wouldbe predicted based on an activity-dependent phenomenon. The results from each these experiments have demonstrated a marked differential sensitivity-responsiveness to perturbedchronicactivity pattemsandlor levels (see Fig. 7; also see Roy er al., 1991 for a review). The implication of the differential responses of mitochondrial proteins to activity perturbations is that muscle fibers have unique potentials in expressing a given family ofproteins.Anexampleofproteinexpression independent of neurally-controlled activity patterns is shown by the expression of specific myosin types in myoblasts and myofiberspriortoinnetvationduringdevelopment(Miller andstockdale, 1986;TaylorandBandman. 1989;Gauthier. 1990). An intrinsic (non-neural) control, as opposed to motoneuronal control via activity patterns. is also suggested by the absence of conversion of myosin type and the apparent absence of a down modulation of mitochondrial proteinofsomeslow fibenin thesoleusthat arereinnervated by fast fatigable motoneurons (Edgenon eral.. 1980; Chart er 01.. 1982). The concept that is consistent with the experimental data from a variety of models of neuromuscular plasticity is that there is a high probability that one of three profiles of families of proteins will be expressed in muscle fibers of mammalian limb muscles (manifested as FG. FOG or SO) under practically all physiological states. For example. there are relatively few mammalian muscles that are comprised solely of one fiber type, i.e., homogeneous (the soleus of the cat is one exception). Even with an extreme perturbation such as a chronic absence of

Contmi of skeletal muscle fiber size and shape

I

200-

,cl

0

CSA

0

SW

lf

129

.O

I I I I I I : I

lb

I

E

l

5 0 6

100 ._____

e

l a

3 L a

+ @te I - ~_____-01__-_--_____---I

C

_ _______________

I I

I

oe

Old

Ob

I I

‘oa

OB

&J I I I I 1

I 0

,I I.

100 Actlvlty (percent of control)

1

200

Pig.7. Theintcrrciationshiprmong neuromuscularoctivity level(i.e., totaldaily EMGoctivity). ~bersuccinotedchydrogcnast(SDH) nctivity and tibcr cross-scctionnlarea (CSA) of the .sokusmuscle(from Graham et ul.. In Press).Control values for each variable an rcprcscntcdas lW%. (a) 6 months ufter spinal isolation in adultCPIS(GmhsmCIal., In Pmss:Piemttiet ~1..In Pmss):(b) 6 months uftcr spinal transectionin cats PI 2 weeks of age (Alaimo et a/., 1984: Hoffmann c/ ul.. 1990): (c) 6 monthsafter spinal transectionin adult cats(assumingneuromuscularactivity lcvcls similar to that reportedin Alaimo cr ul.. 1984) (Jiang cr al.. 1990); (d) after 7 days ofspaccflight in adult tills (assumingsimilarncummuscularactivity kvcls to that reportedin Alford cr al., 1987)(Martincr al.. 1988); (c) after 28 days of hindlimb suspensionin adult rats (Alford cr al., 1987: Hauschka cr al.. 1987: Graham cr d. 1989); (f) after cndumncctraining in rats (awming 1h01the mtal daily neuromuscularactivity is incrcrucdby the daily treadmill cxcrcisc) (Holloszy and Booth. 1976: Picmtti et al.. 1989): (g) alter 28 days of chronic clcctrical stimulation(assumingthat the total daily neuromuscular activity is incrcascd by the electrical stimulation paradigm) (Joksz and Srcter. 1981:Salmonsnnd Hcnrilcsson. 1981;Pcncond Vrbova. 1985).

neuromuscular activity such as that associated with spinal isolation (i.c., complete transection of the spinal cord at two sites followed by a bilateral dorsal rhizotomy to functionally isolate a portion of the cord), the affected muscles do not acquire a homogeneous fiber type composition (Roy er oi.. 1991). Further. it is evident that this probability of expression of a given family of proteins can be influenced, although usually nor controlled.by changes in chronically imposed activity-related events (Pette and Staron. 1990). Thus, to define the role of neuromuscular activity in regulating muscle tiber properties, the cascade of physiological and biochemical events associated with muscle function that induce modifications in the exptession of muscle proteins must be demonstmted.

EFFECT OF FIBER AND ,MUSCLE SHAPE ON FUNCTION IN HUMANS

Given the heterogeneity of muscle fibers within a muscle and the inherent limitations associated with studying humans, the theoretical concepts of muscle adaptation presented may be viewed ashaving little practical utility in studying muscle plasticity in humans. However, this may BM 24Suppl. I-l

not be the cast. For example, if one takes into account the apparent general biological strategies of adaptation at the subccllular, interccllularand organ levels. then the critical and pertinent data reflecting the level, kind and source of neuromuscular adaptation that occurs in humans can be acquired. For example. given the consistencies in the architecturn) designs that are unique to each muscle (Wickiewicz er al., 1983; Friederich and Brand, 1990). it may be possible to noninvasively estimate the potential of a specific muscle or muscle group to genemte force and torque. By optically sectioning the full length of limb segments using magnetic resonance imaging (MRI) techniques. muscles as well as the location of the tendons and their attachments (thus their moment arms) can be visualized. Using this approach, individual musclescan be&fined from the MRI scan and an accumte muscle volume and length subsequently measuredunder very well controlled conditions, e.g., position of limb and joints, time of day relative to activity levels. etc. (Shellock. 1989). The physiological CSA of each muscleand, therefore, of musck groupsthen canbe reasonablyestimatedbasedonmusclevolumeJt%er length ratios (Wickiewicz eraf.. 1983. 1984 Fukuttaga et al.. Submitted), assuminga constantdensityof muscle

V. R. Effiaarm and R. R. ROY

130

LO

L,,.. POP

FHL 0 FDL

0 . . . . . ..-......-...............*

EDL 0 I.-...__ PL 0

--..--

PB 0

5

10

15 Anatomlcal

20

25

30

35

40

45

cm loeatlon of MRI scan

Fig. 8. Anrnomical cross-sectionalareas(ACSA)of musclesalong the length of the leg in one humansubjectusingmagneticresonance imaging techniques(Shellock. 1989: FukunogaCI af.. Submitted). The ACSA was determinedat I cm intervals with slice II identifud by the proximal edge of Ihe patella. Note thrurhe maximum ACSA is PIadiffereru level for each muscleand that no singk crosssection of the leg adequatelympre.sentsthe maximum CSA of each musclewithin any functionalmusclegroup (also see EdgertonCI 01.. 1986). These Dadaemphasize the importanceofdetermining the physiologicnlCSA as describedin the 1~x1.Muscle abbrevkionsam: medial gasrrocnemius (MG). herd gastrocncmius (LG). soleus (Sol), popliteus (Pop). tibidis posterior (TP). flexor haliucis iongus (FBL). flexordigitorum longus(PDL). tibialis anterior (TA). exfensordighorum longus(EDL to include the extensorhallucis lonps andthe peroneustertius). pcroneuslongus (PL). peroneusbrevis (PB).

Comml of skeletal muscle tibcr sin and shape (Mendez and Keys, 1960) and ignoring the angle of pinnation of the fibers (see below). Fortunately, the fiber Icngth:musclc length ratios in adult humans of varying sizes appear to be relatively consistent for any given muscle (Wickiewicr er al.. 1983; Friederich and Brand. 1990)and thus the tibcr lengthscan be estimated from the measund musclelengths.Although the angle of pinnation of the fibers. a factor that affects the force-velocity properties of a muscle (Spector er ul.. 1980; Gans. 1982). cannot be determined using MRI. this is not thoughtto be a major limitation. Based on a simple two dimensional estimation of fiber angle in chemically fixed muscles,the anglesofpinnationmeasuredinthemajorhindlimbmuscles of the cat (Sacks and Roy, 1982) or guinea pig (Powell et al.. 1984) and the lower limbs of humans(Wickiewicz et al.. 1983; Friederich and Brand, 1990) are lessthan 20”. correspondingto a maximum of a 6% loss in the force measuredatthe tendon.Thusassumingthat fiberangle has a relatively small effect on the force potential, it seems quite feasible to use MRI techniques to estimate the changes in the force capability of a muscle group in humansin responseto pezturbations,e.g..before and after prolonged spaceflight. Some whole muscle volumes and shapes have been obtained from normal human subjectsusing the imaging approach dcscribcd above (Fukunaga Edal.. Submitted) (see Fig. 8). From thcsc images. it is clear that a single crosssection at any point along the Icg cannot bc used to cstimatc adcquatcly the volume of a muscle or muscle group. Although the anatomical CSA of a muscle or muscle group can bc measured repcatcdly at any well dclined proximo-distal location, one cannot assumethat a muscle atrophiesor hypertrophiesuniformly nor that all musclesatrophy similarly. Both are known not to be the case in animal experimental models of muscle plasticity (Roy er al.. 1991). Thus becauseof the changesin muscle cross-sectionalarea along the lengthof a muscle,a marked over or underestimationof changesin muscle function is likely when conclusionsare drawn from a single anatomical CSA of a muscle group. For example, Fig. 8 clearly shows that the maximum CSA of the triceps surae group can not be accurately determined from any single cross section, i.e., the largestCSA of the soleusis located more distally than the largestCSA of either headof the gastrocnemius. A similar conclusion was reached by Edgenon er al. (1986) for the elbow flexors. If, however, the physiolo&cal CSA is accurately determined under conditions not complicated by shifts in fluid within the muscles, it would appearthat the force potential of normal, atrophied or hypertrophiedmusclescan be estimated. An inevitable limitation with any measureof function in humanswill be that the net effect of musclegroupsand not individual muscles nor portions of muscles can be tested.On the other hand, modeling of human movement requires an estimation of the output of muscle groups. Thus, if the morphological featuresof a musclegroup and its moment arm are well defined. biomechanical and neuromuscularmodelingcan provideconsiderableinsight into important physiological events such as motor unit recruitment patterns.

131

SUMMARY

This paper presentsa perspective that extends from factors that affect gene expression to the physiological propertiesof human musclegmups. Although addmsing the issue of neuromuscular adaptation with this broad perspectiveseemsformidable, perhapsit is not.The dcvelopment of more sensitivetechniquesand tools for detecting messenger RNAs, for localizing trace amounts of specific proteins, for quantifying enzyme activities in single fibers, for studyingthe physiological propertiesof single fibers, motor units and functional muscle groups and for the noninvasiveimaging of the morphologicaland biochemical propertiesof muscles,have made thesewide range integrative studiesmore feasible.

Acknowledge~nlarrs-TheauthorsthankBrianTseng.Dave Pierotti and Dr. Gordon Chalmers for their critical review of the manuscript. This work was supported by NIH Grant NS I6333 and NASA Grants NCA- IR390-502 and

NCCZ-535.

REFERENCES

Alaimo. M. A., Smith. J. L.. Roy, R. R. and Edgenon.V. R. ( 1984) EMG activityof slowandfastankleex~cnsors following spinal cord transection.1. Appl. Physiol. 56. 1603-1613. Alford. E. K.. Roy. R. R.. Hodgson. J. A. and Edgerton. V. R. (1987)Elcctmmyographyofratsokus.mcdialga.stm~ncmius and tibialis anteriorduring hindlimbsuspcnsion.Erp. Ncurol. 96.635-649. Armstrong. R. 8. and Phelps. R. 0. (1984) Muscle fiber type compositionof the rat hindlimb. Am. 1. Anut. 171.259-272. Baldwin. K. M.. Roy, R. R.. Edgerton. V. R. and Hcrrkk. R. E. (I 989) IntcracGonof nerve activity and skektal muscle mcchanical activity in regulating isomyosinexpression.In: Advances in Myockmisfry, (Edited by Bcnzi. G.). pp 83-92, John Libbey. Eumrext Ltd. 6ardccn.C. R.(l903)Variations in the inurnalarchitcctunofthc M. obliquus abdominis cxtemus in certain mammals. Anur. Anr. 23.24 l-249. Barren. 6. (I%?) The length and mode of termination of individual muscle fibrcs in the human sartorius and posterior femoral muscks. Acra Ana:. 48.242-257. Bischoff, R. (1989) Analysis of muscleregenerationusingsingle fibers in culture. Med. SC{. Spans &rerc. 21 (Suppl. 5). Sl64-s172. Blau, H. M.. Webster. C. and Pavlath. G. K. (1983) Defective myoblastsi&nGficd in Duchcnnemuscular dystrophy.Pmt. Nod. Acad. Sci. 800.48S6-4860. Bodinc. S. C.. Roy, R. R.. Meadows. D.A.. Zemickc. R. F.. Sacks, R. D.. Foumicr. M. and Edgcrton.V. R. (1982) Archikctuml. histochcmical. and contractile chamcteristicsof a unique biarticular muscle: The cat scmitendinosus. 1. Neurophysiol. 48,192.201. V. R. (1975) Motor unit properties and selective involvement in moverncn(. &rem. Span Sci. Rev. 3.31-81. Burleigh. LG. (1977) Observations on the number of nucki Burke. R. E. and Edpnon.

within tkfib~ofsome~dandwhitemus~l~~. 269.284.

3. CelISci.23.

I32

V.R.Eooaaro~andR.RRo~

Butkr-Brownc. G. S.. Bugaisky. L B.. Cucnoud, S.. Schwartz K. and Whalen. R. G. (1982) Dcncrvation of ncwbom tat musclesdoesnot block the appcaranccof adult myosin heavy chain. Natun 299.830833. Chart. A. K.. Edgcrton. V. R.. Goslow. Jr., G. E.. Kurata. H.. Rasmussctt,S. A. and Spector. S. A. (1982) Histochcmical and physiologicalpropcrticsof cat motor units after self- and cross-rcinncrvation.1. Physbi. (Lend.) 33% 343-361. Chak, D. B. (1985) The control of cell massand replication. The DNA unit - a persona) 20-year study. Early Hum. Dev. 12. 21 l-239. Clamann. H. P. and Schclhom. T. B. (1988) Nonlinear force addition of newly recruited motor units in the cat hindlimb. Muscle & Ncwc 11.1079-1089. Darr. K. C. and Schultz, E. (1989) Hindlimb suspensionsupprcsscsmusclegrowth and satellite cell proliferation. 1. Appi. -Physioi. 67. I 827- 1834. Dix. D. J. and Eiscnbcrg.B. R. (1990) Myosin mRNA accumulation and myotibrillogcncsis at the myotendinousjunction of stretchedmuscle libcrs. 1. Ceil Bioi. 111. 1885-1894. Edgerton. V. R.. Bodinc. S. C. and Roy, R. R. (1987) Muscle architectureand pcrformancc:Stressand stnin relationships in a muscle with two compamncnts arranged in scrics. In: MedicineondSporr Scbtces: Muscular Function in Exercise and Training. (Edited by Marconnct. P. and Komi. P.V.). 26: 12-23. Karger. Bascl. Edgerton. V. R.. Roy, R. R. and Apor. P. ( 1986) Specific tension of human elbow flexor muscles. In: /n~emu~ionaiSeries on Sport Sciences: Biochcmiswy of Erercise VI. (Edited by Saltin. B.). pp487-500, Human Kinetics Publishers. Inc.. Champaign, Illinois. Edgerton.V. R..Goslow. Jr., G. E.. Rasmussen.S. A.and Spector. S. A. (1980) Is resistance to fatigue controlled by its motoncuroncs?Narnre 285.589-590. Edgerton,V. R.. Roy, R. R.. Bodinc-Fowkr. S. C.. Pierotti. D. J., Ungucz, G. A., Manin. T. P.. Jiang. 8. and Chalmers. G. R. (1990) Motoneurons- muscle tibcr connectivity and interdcpcndencc.In: The DynumicState ofhfuscie Fibers. (Edited by Pctte. D.). pp 2 17-23 I, Walter dc Gruytcr & Co., Berlin. Eiinbcrg. B. R.. Brown, J. M. C. and Salmons, S. (1984) Rcstoration of fast muscle characteristics following cessation of chronic stimulation: the ultmstructurcof slow-to-fast tritnsformation. Ceil Tissue Rrs. 238,22 I-230. Eiinbcrg. B. R., Kennedy. J. M.. Wcndcroth. M. P. and Dix. D. J. (1989) Satellite cells. isomyosin switching and muscle growth. In: CeiiuiarandUolecuiar Biologyof Muscle Drveiopmenr. pp 451-460. Alan R. Liss. Inc.. New York. Emonct-Denand. F., Laportc. Y.. and Proskc. U. (1990) Summation of tensionin motor units of the solcus muscle of the cat. Neurosci. Lrrr. 116. 112-l 17. Epstein. H. F. and Fischman. D. A. (1991) Molecular analysis of protein assembly in muscle dcvclopmcnt. Science 251. 1039-1044. Foumicr. M.. Roy, R. R.. Pcrham, H..Simard. C. P.and Edgcnon. V. R. (1983) Is limb immobiliwion a modci of muscle disuse?Erp. Neuroi. 80.147-I 56. Fmir. P. M. and Pctcrson.A. C. (1983) The nuclcarcytoplasmic relationship in “mosaic” skclctal muscle fibcn from mouse chimacras. Erp. Ceil Res. 145. I67- 178. Fricdcrich. J. A. and Brand, R. A. (1990) Muscle fiber architcctutc in the human lower limb. /. Biomechanlcs 23, 91.95. Fukunaga. T.. Roy, R. R.. Shcllock. F. G.. Hodgson. J. A., Lee. P. L.. Kwong-Fu, H. and Edgcrton. V. R. (Submitted) Physiological cross-sectionalama of human leg musclesbased on magnetic resonanceimaging. 1. &f/top. Res.

Gans. C. (1982) Ftbcr architcctum and muscle function. &em. Sport Sci. Rev. 10.160-207. Gauthii.G. F.(l99O)DiffcrcntiaJdistributionofmyosinisoforms among tbc myofibrils of individual dcvcloping musclefibers. J. Ceil Eioi. 110.693-701. Graham.S.C..Roy.R. R..Wcst.S. P..Thomason,D.andBaJdwin. K. M. (1989) Excrcisc cffccts on the sire and metabolic propcrticsof solcusfibers in hindlimbsuspcndcd rats.Aviar. Space Environ. Med. 60.226-234. Graham. S. C.. Roy, R. R.. Navarro, C, Jig. B.. Picrotti. D.. Bodinc-Fowlcr. S. and Edgerton.V. R. (In Press)Enzyme and size profilcs in chronically inactive cat solcus musck fibcn. Muscle & Nerve. Grivcll. L. A. (1988) Protein import into mitochondria.Inr. Rev. cyroi. 111.107-141. Hall. 2. W. and Ralston, E. (1989) Nuclear domains in muscle cells. Cell 59.77 I-772. Hauschka. E 0.. Roy. R. R. and Edgcnon. V. R. (1987) Sia and metabolic propcrticsof single muscleBbcrsin rat sokus after hindlimb suspension.1. Appi. Physiol. 62.2338-2347. Hcnncman. E. and Mendell. L.M. (1981) Functional organixation of motoncuron pool and its input. In: Handbook of Physiology. Section 1. The Nervous System Vol. Ii. Motor Control. Port 1. (Edited by Brookhart, B. ct al.). pp. 423-507. Amcricon Physiological Society. Bcthcsda.Mnrylnnd. Hoh. J. F. Y. and Hughes, S. (1988) Myogcneic and ncurogcnic regulation of myosin gene expression in cat jaw-closing musclesngcncrating in fast and slow musclekds. J. Muscle Res. Cell Motii. 9.59-72. Hoffmann. S. J.. Roy, R. R.. Blanco, C. E. and Edgcrton. V. R. (1990) Enzyme protilcs of single tibcrs never exposed to normal neuromuscular activity. J. Appi. Physiol. 69. 1150-l158. Holloszy. J. 0. and Booth. F. W. (1976) Biochemical adaptations to cndumnce cxercisc in mu&. Ann. Rev. Physiol. 38. 273-291. Jinng. 8.. Roy. R. R. and Edgcrton, V. R. (1990) Expressionofa fast fiber enzyme profile in the cat sokus after spinalization. Muscle & Newe. 13. 1037-1049. Joksz. F. and Sretcr, F. A. (1981) Dcvclopmcnt. innervation.and activity-pattern inducedchangesin skeletal musck.Ann. Rev. Physiol. 43.531-552. Landing, B. H., Dixon, L. G. and Wells. T. R. (1974) Studies on isolated human skclctal muscle Ebcn: Including a propoad pattern of nuclear distribution and a conceptof nuclear tcrritories. Hum. Parhoi. 5.44 I-46 I. Lewis. S. E. M.. Kelly. F. and Goldspink. D. F. (1984) Pm- and post-natal growth and protein tumovcr in smooth musck. heart and slow- and fast-twitch skcktal muscles of the rat. Biochem 1.217.517326. Loch. G. E.. Pratt, C. A., Chanaud, C. M. and Richmond. F. J. R. (1987) Distribution and innervation of short, intcrdigitatcd musck tibcrs in pamllcl-fibcmd musclesof the cat hindlimb. 1. Morphoi. 191. I-IS. Mattin. T. P.. Edgcnon. V. R. and Grindeland. R. E (1988) lntlucncc of spa&tight on mt skcktal muscle. 1. Appi. Physiol. 65.23 18-2325. Martin, T. P.. Veilas. A. C.. Durivogc. J. 8.. Edgcnon. V. R. and Castleman. K. R. (1985) Quantitative histochcmicaldctcrmination of muscle enzymes: Biochemical verification. J. Hls~ochcm.Cyrochem.33. 1053-1059. Mcndcx. J. and Keys. A. (1960) Density and composition of mammalian murlc. Meksbolism 9, IS4-188. Mcrlic. 1. P. and Sams. J. R. (1985) Concentrationof acctykh* line receptor mRNA in synaptic regions of adult muscle fibers. Nature 317.66-68.

Controlof skeletal muscle fiber size and shape Miller. J. 8. and Stockdale. F. E. (1986) Developmental qulation of tha multipk myogcnic cell lineages of tbc avian embryo. 1. Cell Biol. 103.2 197-2208. Moss. F. P. (1968)Thcmlationshipktwccnthcdimtnsionsofthc fibrus and the number of nuclei during normal growth of skeletal muscle in the domestic fowl. Am 1. Anot. 122. SSSJ64. Moss. F. P. aud LeBlond. C. P. (197 I) Satellite cells as the source of nuclei in muscla of growing mts. Anof. Rec. 170.42 I-436. Nemcth. P. M. and Wilkinson, R. S. (1990) Metabolic uniformity of the motor unit. In: The Dynamic State of Muscle Fibers. (Edited by Pctte, D.), pp 233-245. Walter de Gruyter & Co.. Berlin. Ounjian. M.. Roy. R. R.. Eldred. E. Garfinkel. A.. Payne. J.. Armstrong. A.. Toga. A. and Edgerton. V. R. (1991) Physiological and developmental implications of motor unit anatomy. J. Nettrobiol. 22.547-559. Pavlath. G. K.. Rich, K.. Webster. S. G. and Blau. H. M. (1989) Localization of muscle gene products in nuclear domains. Nature 337,570-573. Peter. J. B.. Barnard. R. J.. Edgenon. V. R.. Gillespie. C. A. and Stempcl. K. E. (1972) Metabolic profiles of three fiber types ofskeletal musclein guinea pigsand rabbits.Biochemisq 11. 2627-2633. Petit. J.. Filippi, G. M.. Emonct-Dcnnnd. F.. Hunt. C. C. and Laporte. Y. (1990) Changes in muscle stiffnessproducedby motor units of different types in pcmncus longus muscle of cat. 1. Nettropbysiol. 63. 190-197. Pctte. D. and Staron. R. S. ( 1990) Cellular and molecular divcrsities of mammalian skclctnl muscle fibers. Rev. Ph,vsiol. Biochem. Pharmucol. 116. I-76. Pette. D. nnd Vrbovn. G. (1985) Neural control of phcnotypic cxprcssion in mammalian muscle fibrcs. Muscle 8 Nerve 8, 676-689. Pctte. D.. Wimmcr. M. and Ncmcth, P. (1980) Do enzyme nctivities vary along muscle fibrcs? Histochemistty 67, 225-23 I. Picmtti. D. J.. Roy, R. R.. Gregor. R. J. and Ed8crton. V. R. (1989)Ekctmmyographicactivityofcat hindlimb flcxonond extensors during locomotion nt varying speeds and inclines. Brain Res. 481.5766. Pierotti. D. J.. Roy, R. R.. Bcdinc-Fowlcr. S. C.. Hodgson. J. A. and Edgerton. V. R. (In Press) Mcchonical and morphologic properties of chronically inactive cat tibialis anterior motor units. 1. Physiol. (Lontf.) Powell. P. L.. Roy. R. R.. Kanim. P.. Bcllo. M. A. and Edgcrton. V. R. (1984) Predictability of skclctal muscle tension from nrchitcctuml&terminations in guinea pig hindlimbs.1. Appl. Physiol. 57. 1715-1721. Ralston. E. and Hall. 2. W. (1989) Transfer of 4 pmtcin encoded byasinglcnuckustonearbynuckiinmultinuckatcd myotuks. -Science 244. 1066-1069. Richmond. F. 1. R.and Armstmng. 1. B. (1988) Fibcrarchitecturc and histochcmistty in the cat neck muscle. bivcntcr ccrvicis. 1. Nettrophysiof.60.4659. Rotundo.R. L. (1990) Nucleus-specifictranslationand assembly ofacctylcholincstcrascin multinuckotcd musclecells. 1. Cell Biol. 110.715-719. Roy, R. R.. Baldwin. K. M. and Edgerton. V. R. (1991) The plasticity of skclctrl muscle: Effects of neuromuscularactivity. &rerc. Sports Sci. Rev. 19.269-312.

133

Roy. R. R. aud E&ton. V. R. (In Press) SkcktaJ musclearchitecture and performaace. In: Encylopedia o/Sports Me&

tine: The Olynpic

Book of Strength and Power in Sportr.

(Edited by Komi. P.V.). Blackwell Scientific Publications, Oxford. Roy. R. R.. Bcllo, M. A.. Bouissou.P. and Edgerton.V. R. ( 1987) Size and metabolic pmpcrtics of fibers in rat fast-twitch muscles after hindlimb suspension.1. Appl. Phyriol. 62. 2348-2357.

Roy. R. R.. Sacks,R. D.. Baldwin. K. M..Sbort. M. and Edgenon. V. R.( 19&)1nterrclationshipsofcontructiontimc.Vmaxand my&n ATPasc after spinal transection.J. Appl. Physiol. 56. 1594-1601. Sacks. R. D. and Roy, R. R. (1982) Architectum of the hind limb muscles of cats: Functional significance. 1. Motphol. 173, 185-195.

Salmons. S. and Henriksson. J. (1981) The adaptive response of skeletal muscle to increased use. Muscle & Nerve 4. 94-105. Schiaffino. S.. Picrobon Bormioli. S. and Aloisi. M. (1976) The fate of newly formed satellite cells during compensatory muscle hypcnmphy. Virchows Arch IS ]2l.I13-I IS. Schiaffino. S.. Gorza. L.. Pitton. G.. Saggin. L.. Ausoni. S.. Sanore. S. and Lotno. T. (1988) Embryonic and neonatal myosin heavy chain in dcncrvatedand paralyxcd rat skeletal muscle. Dev. Biol. 127. l-l I. Schmalbruch. H. (1986) Muscle regeneration:fetal myogcncsis in a new setting. Bibl. Anttf. 29. 126-153. Shcllock. F. G. (1989) Biological effects and safety aspectsof magnetic rcsonanccimaging. Magn. Reson. Q. 5.243-261. Snow, M. (1981) Satcllitc cell distribution within the solcus muscle of the adult mouse.Anar. Rec. 201.463469. Spector. S. A.. Gardiner. P. F.. Zernickc, R. F.. Roy, R. R. and Edgerton. V. R. (1980) Muscle architecture and forccvelocity characteristicsof cnt sokus and medial gastrocnemius: Implications for motor control. 1. Nettrophyslol. 44. 9s l-960. Taylor, L D. and Bandman, E. (1989) Distribution of fast myosin hcavychainisoformsin thickfilamcntsofdcvelopingchickcn pectoral muscle. 1. Cell Mol. 108,X%3-542. Tidball, 1. G., O’Halioran, T. and Burridgc, K. (1986) Talin at myotcndinousjunctions. 1. Cell Biol. 103. 1465-1472. Trotter. J. A. (1990) lntcrtibcr tension transmission in scricstibcmd muscles of the cat hindlimb. J. Motphol. 206, 3Sl361. Trotter. 1. A. (1991) Dynamic shape of tapered skclctal muscle fibers. 1. Mot-phol. 207.21 l-223. Tscng. B. S.. Kaspcr. C. E. nnd Edgcrton. V. R. (1990) Nuclear density in isolated single rat skclctal muscle ftbcrs with respectto fiber type. Blophys. 1.57.5514. Vandcnburgh, H. H.. Hatfaludy. S.. Karlisch, P. and Shansky. J. (1989) Skclctol muscle growth stimulated by intermittent stretch-relaxation in tissue culture. Am. 1. Physiol. 256. C674-C682. Wickicwicr T. L., Roy. R. R.. Powell, P. L. and Edgerton. V. R. (1983) Musck architecture of the human lower limb. C/in. Orrhop. 179,275-283. Wickiewicz. T. L. Roy. R. R.. Powell, P. L.. Pet-tine.J. J. and Edgcrton. V. R. (1984) Muscle architecture and forcevelocity relationships in humans. J. Appl. Physiol. 57. 435-443.