Energetics of Muscular Exercise PDF
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Guido Ferretti Energetics of Muscular Exercise Energetics of Muscular Exercise Guido Ferretti Energetics of Muscular Exercise 123 GuidoFerretti Dipartimentodi ScienzeCliniche e SperimentaliHealth&Wealth@unibs UniversitàdiBrescia Brescia Italy and Départementsde Anesthésiologie, PharmacologieetSoins Intensifs etde NeurosciencesFondamentales UniversitédeGenève Geneva Switzerland ISBN 978-3-319-05635-7 ISBN 978-3-319-05636-4 (eBook) DOI 10.1007/978-3-319-05636-4 LibraryofCongressControlNumber:2015933155 SpringerChamHeidelbergNewYorkDordrechtLondon ©SpringerInternationalPublishingSwitzerland2015 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilarmethodologynowknownorhereafterdeveloped. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexempt fromtherelevantprotectivelawsandregulationsandthereforefreeforgeneraluse. Thepublisher,theauthorsandtheeditorsaresafetoassumethattheadviceandinformationinthis book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained hereinorforanyerrorsoromissionsthatmayhavebeenmade. Printedonacid-freepaper SpringerInternationalPublishingAGSwitzerlandispartofSpringerScience+BusinessMedia (www.springer.com) This book is dedicated to my four great loves: my wife Silvia and my children Lavinia, Livio and Lapo Foreword Pietro di Prampero’s review of 1981 is the landmark that set the state of the art of our knowledge in the field of the energetics of muscular exercise in those times. It resumed what can now be considered as classical studies under the perspective of the general theory of the energetics of muscular exercise developed within the School of Milano created by Rodolfo Margaria, which di Prampero is an eminent member of. The study of the energetics of muscular exercise dives back in history muchmorethanwemayimagineatafirstsight,owingalottoclassicalchemistry, morphology and biochemistry. Yet the first key concepts of it were essentially outlined byMargariain1933,andprogressivelyrefined,firstbyMargariahimself, thenbyhispupils,aslongastheexperimentalevidencegrew.Thaterawasfinally framed by di Prampero in 1981. The general theory of the energetics of muscular exercise has been the cultural reference of this book. Other physiological schools played crucial roles in that story as well. In par- ticular, I would like to mention the Scandinavian school, whose most eminent representatives in those times had been Erich Höhwu Christensen in the 1930s, Erling Asmussen in the 1950s and Per-Olaf Åstrand in the 1960s. That school, however, was mainly characterised by an empirical approach to science: those physiologists generated huge, fundamental amounts of original experimental data, but were scarcely attracted by theoreticalthinking, which conversely was the most important treat of Margaria and co-workers. Great theoretical advances were obtained in Rochester and Buffalo within the school led by Wallace Fenn and Hermann Rahn, and in Göttingen by Johannes Piiper: those advances, however, were mostly in the field of gas exchange rather than in that of exercise energetics, althoughtheheirsofMargariaowealotoftheirwayofthinkingalsotoBuffaloand Göttingen, especially Paolo Cerretelli and Pietro Enrico di Prampero. Similar exchanges can be perceived also between the general theory of the energetics of muscular exercise and the cross-bridge theory of muscle contraction, the link being represented by the role of ATP as main energy source for muscular contraction. The former concerns theway chemicalenergy is generated and stored in ATP, the latter has to do with the way the chemical energy stored in ATP is transformed into mechanical energy in the cross-bridge cycle. These exchanges, vii viii Foreword however, were less direct, as long as there had been no collaboration between Margaria and Andrew Huxley. When Margaria spent some time with Archibald Vivian Hill and John Barcroft, Huxley was a child. Thirty-fouryearsoffurtherworkandthinking,summarizedinthesixchaptersof this book, have elapsed since the publication of di Prampero’s review. The ques- tions now are: How did the general theory of the energetics of muscular exercise resist thedeterioration of time andthe impact of new knowledge? What isits state of health? What are the most important advances of the last 30 years? Concerningthelastquestion,Iwouldsaythatseveralnewconceptswerecreated and took shape since 1981. I do not include in this list George Brooks’ lactate shuttle, a concept which di Prampero was already aware of in 1981, although it receivedtheoreticalsystematisationwithinthegeneraltheoryonlymorerecently:di Prampero’s concepts of hyperaerobic and hypoaerobic muscle fibres make use of the lactate shuttle, which is therefore included in the general theory. Most important novel contributions appear, to my eyes, the multifactorial models of maximal oxygen uptake limitation, the critical power models, the identification of the slow component (although its occurrence had already been suggested in 1981),althoughitstheoreticalsystematisationisstilltobeperfected,andthedouble exponential model of oxygen uptake kinetics. All the above-listed concepts challenged the general theory seriously; most of them could be conveniently accommodated in it; some required refinements of the general theory. Yet I would concludethatoverall thegeneral theoryhasresistedprettywellandisstillingood health, despite the modifications that had to be introduced. Inmyvision,themostimportantthreattothegeneraltheorycamefromthetwo- sitemodelofthekineticsofoxygenuptakeuponexerciseonset,whichundermined theautomaticlinkbetweenoxygendeficitandenergydelivery.Significantchanges inthegeneraltheorywithrespecttodiPrampero’sreviewhadtobeintroducedfor this reason. Thanks essentially to the excellent work carried out by the British schoolledbyBrianWhipp,itisgenerallyacceptednowadays,evenbythosewithin the school of Milano who were opposed to that vision in the past, that the first exponentialofthetwo-sitemodel(phaseI)isunrelatedtoanenergeticmechanism. Theneedfortightlycouplingoxygendeliveryandoxygenconsumptionimpliedthe emergence of other mechanisms, related to cardio-respiratory control, in the regulation of oxygen delivery. The concept that what is measured at the mouth reflects what occurs from the energetic viewpoint in contracting muscles has weakened.Thecurrentformulationofthegeneraltheoryadmitsdifferentregulation of oxygen delivery and of oxygen consumption. The former includes specific features(phaseI)relatedtocardio-respiratorycontrolandhasaprimarycomponent whose time constant increases with the exercise intensity. The latter is dictated by thekineticsofactivationofglycolysisandischaracterisedbyatimeconstantwhich is invariant and independent of exercise intensity. Therefore, as long as the former time constant is faster than the latter, oxygen delivery copes with oxygen demand, and the time constant of oxygen consumption corresponds to that of muscle phosphocreatine hydrolysis and of the primary components of pulmonary oxygen uptake:insum,tothatofglycolysisactivation.Conversely,whenthetimeconstant Foreword ix of the primary component of oxygen delivery becomes slower than that of gly- colysisactivation, oxygendeliverybecomesinadequateandthekineticsofoxygen consumptionslowsdown.Asaconsequence,earlylactateisaccumulated,asmuch as is necessary to cover the missing energy, compatibly with an energy equivalent of blood lactate accumulation of 3 ml mmol−1 kg−1. Another important point concerns the slow component, for which the situation appears more ambiguous. The appearance of the slow component implies a non- linear relationship between oxygen uptake and power and the lack of a clearly visiblesteadystateforoxygenuptakeduringintenseexercise.Thisinducedseveral scientiststorejecttheconceptofmaximalaerobicpowerasthepowerrequiringan oxygenuptakeequivalenttothemaximum.Ikindlydisagreewiththisview,aslong as we have no demonstration that the extra-oxygen consumption pertaining to the slow component is necessary to replace the chemical energy that is converted into mechanical work in the cross-bridge cycle. To my mind, the classical relationship betweenoxygenuptakeandpowerconcernsthesteadystateattainedbytheprimary component (phase 2 in the double exponential model) of the oxygen uptake kinetics,ontheassumptionthattheslowcomponentreflectsotherphenomenathan energy transformation in the cross-bridge cycle. However, the mechanistic under- standing of the slow component is still poor, and some evidence suggests that it may be related to phenomena occurring within the active muscle mass. Thus I cannot exclude that in the near future our view of the slow component may still change, with consequences on the formulation of the general theory. Thecriticalpowerhasbecomecrucialconceptinexercisephysiology.Although theconceptstillexistedin1981,itwasconsideredminorandwasnotevenmentioned in di Prampero’s review. Its importance grew in more recent years, thanks to the experimentalworkoftheBritishschool,especiallyDavidPooleandAndyJones.Yet weowethetheoreticalsystematisationofthecriticalpowerconcepttoHughMorton, whodevelopedthetwo-parameterandthethree-parametermodelsofcriticalpower andformulatedthetheoryapplyingthecriticalpowerconcepttorampexercise.His theoreticalwork,ofgreatestimportanceinthefield,fitswellinthegeneraltheoryas long as we admit a univocal relation, with precisely known values of mechanical efficiency,betweenmechanicalenergy(work)andmetabolicenergy. Lastbutnotleast,themultifactorialmodelsofmaximaloxygenuptakelimitation revolutionised our understanding of this issue. The resumption, by Ewald Weibel andDickTaylor,oftheoxygenconductanceequationleddiPramperotocreatethe concept, to the formulation of which I also contributed. In parallel, Peter Wagner elaboratedaniceanalysisoftheinteractionbetweencardiovascularoxygendelivery andmuscleoxygendiffusion,whichdesignedanovelintegrativeperspectiveofthe subject. It is curious to note that the two multifactorial models were considered in competition for long. Only very recently the demonstration came, that the two models say the same things in a different language, and lead to the same conclu- sions. Starting from this notion, I expect a generalisation of the multifactorial models to include quantitatively the entire respiratory system in all experimental conditions: this however requires a sound theoretical, rather than empirical, mathematical solution of the oxygen equilibrium curve. x Foreword Thepanoramaofactivityinthefieldhasremarkablychangedinthelast40years, gettingopentoalargervarietyofcontributions,whethertheoreticalorexperimental, whichrepresentaremarkableintellectualenrichmentwithrespecttothepast.Several individuals, I think especially of Peter Wagner and Brian Whipp, developed new majorschoolsofexercisephysiology,whichgeneratedimportantthinking,produced remarkable experimental results, and disseminated a large number of important scientists,aboveall,forhisstrongacquaintancewithbothschools,DavidPoole.The Scandinavian school has not disappeared; it rather further developed, under the forcefulexampleofBengtSaltin,whoimposedhimselfasactualschoolleader.Under his example, the Scandinavian school expanded its interests to new experimental contextsandmuchinformationacquiredinthosecontextsprovidedusefulpiecesof evidence completing empty slots of the general theory, think as an example of the lactate-protonco-transporter.Yetthatschoolcontinuedtogenerateessentiallyhuge amountsofexperimentaldata,poorlycontributingtotheoreticalthinking.Theschool of Milano is still in good shape, with new generations of scientists continuing its tradition.Thetechnicaldevelopments havedramatically enlargedthepossibilityof obtainingandtreatingcomplexphysiologicaldata.Severalindividuals,outsidethose schools, have produced original contributions, also in theoretical analysis (Hugh Mortonaboveall).Exercisetherapyandexercisephysiopathologyareopeningnew perspectives in the field. The picture generates an optimistic view of the future, althoughthefieldofexercisephysiologyisnotascentralasitwas50yearsagointhe considerationoffundingagencies. Whatshouldweexpectnext?Idonotknow,Idonotdaretosay.Multifactorial models, slow component, critical power are concepts that are well present in the current debate of the energetics of muscular exercise, although they could not be predictedwhendiPramperowrotehisreview.Itispreposteroustoindicatetheway research should take. What will come next depends on the freedom, fantasy and originality in thinking that scientists in the field will show in next years. I never- theless put forward two predictions: that the general theory will survive, after several readjustments under the pressure of new experimental evidence, and that new people will continue the task of elaborating new concepts and ideas. In this book, I have re-elaborated the current knowledge of the energetics of muscular exercise under the perspective of the general theory and from the viewpoint of the schoolofMilano,wheremywayofthinkingwasforged.Iwillnotbesurprised,ifI willhavethechanceofgettingold,tolearnthatsomebodyofanewgenerationwill have donethesame job within 30years from now, re-analysing thegeneral theory andadaptingittothenewconceptsthatwillhavebeencreatedandthenewfindings that will have been obtained meanwhile. The picture of the general theory of the energetics of muscular exercise, which will then be reported, will be significantly different form the one emerging from this book, pretty much the same way this book reports a significantly different picture from that outlined by di Prampero in 1981. Acknowledgments Iwouldliketoexpressmygratitudetoallthosewhotrainedmeandforgedmyway of thinking, above all Paolo Cerretelli and Pietro Enrico di Prampero, whom I had the longest acquaintance and the most intense exchanges with, but also Dave Pendergast, Don Rennie, and Arsenio Veicsteinas. A special thanks goes to my friends and colleagues Carlo Capelli, Fabio Esposito, Hans Hoppeler, and Dag Linnarsson,aswellastoallthosewhohappenedtocollaboratewithmeatdifferent times,forthepleasureandenrichmentthatthefruitfuldiscussionswhichwehadso many times provided to me. A special thanks goes to Giovanni Vinetti for careful re-reading of the entire text. xi
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