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Alveolar gas exchange kinetics in humans during moderate and supra-maximal intensity exercise.
No full textQuesta tesi si apre con l’introduzione della definizione di cinetica del consumo di ossigeno (V’O2) e dei suoi determinanti (Capitolo I.2), a cui segue una dettagliata descrizione dei cambiamenti che tale cinetica subisce a differenti intensità di esercizio (Capitolo I.3). Il quarto capitolo è, invece, dedicato all’analisi dei principali risultati presenti in letteratura inerenti alla risposta della gittata cardiaca e dell’estrazione muscolare periferica in corso d’esercizio, allo scopo di dare una visione il più possibile completa ed integrata dei meccanismi centrali e periferici che sottendono al trasporto e all’utilizzazione dell’ossigeno quale fonte energetica per il sostegno della contrazione muscolare.
Dopo questa introduzione di carattere generale, il Capitolo II presenta i risultati di uno studio condotto su un gruppo di giovani adulti a seguito di una campagna di bed rest (head down -6°). Lo scopo di questa indagine sperimentale è stato quello di capire se il sistema di trasporto dell’ossigeno sia o meno un fattore limitante della cinetica del consumo di ossigeno, durante esercizi di moderata intensità. A seguito del decondizionamento cardiovascolare indotto dall’allettamento, i risultati ottenuti hanno evidenziato un rallentamento della risposta del sistema di trasporto dell’O2, non accompagnata da modificazioni a carico della cinetica del V’O2. Questo dato suggerisce e conferma l’idea secondo la quale la velocità di utilizzo (o consumo) d’ossigeno a livello muscolare sia controllata principalmente da meccanismi periferici cellulari o microvascolari responsabili dell’utilizzo e/o del trasporto dell’ossigeno, piuttosto che dal trasporto sistemico di ossigeno.
Infine, la terza parte della tesi propone analiticamente i risultati ottenuti dalla valutazione in parallelo della cinetica: i) del consumo di ossigeno (V’O2); ii) della gittata cardiaca (Q’), quale surrogato della cinetica del trasporto di ossigeno da parte del sistema cardiovascolare; iii) dell’estrazione di ossigeno a livello muscolare periferico (o deossigenazione muscolare, [HHb]), indagata tramite NIRS. Quattordici soggetti sono stati valutati durante l’esecuzione di esercizi ad onda quadra a carico costante, al cicloergometro, condotti ad intensità moderata (80%LT) e sovra-massimale (o severa, 120%wmax). La somministrazione di esercizi di così elevata intensità, come quella sovra-massimale, è stata volutamente scelta per creare delle condizioni sperimentali “estreme”, per merito delle quali fosse possibile approfondire la comprensione dei meccanismi che regolano e condizionano il consumo di ossigeno sistemico e muscolare. In particolare, si è tentato di valutare in queste condizioni estreme il peso rivestito dai fattori centrali e periferici nella limitazione del metabolismo ossidativo.
I risultati ottenuti da questo studio hanno messo in evidenza un significativo accorciamento della Fase Principale della cinetica del consumo di ossigeno durante l’esercizio severo rispetto al moderato, nel caso i dati fossero interpolati con metodo non-lineare. Risultati contrastanti sono stati ottenuti quando i dati di consumo di ossigeno sono stati interpolati linearmente dopo trasformazione semilogaritmica. In questo caso, la costante di tempo della Fase 2 ha messo in evidenza un significativo rallentamento della cinetica del V’O2 durante esercizio sovra-massimale rispetto al moderato. Questi risultati sembrano non confermare l’ipotesi secondo la quale la cinetica del V’O2, all’inizio di un esercizio di intensità sovra-massimale, sia accelerata in funzione dell’aumento della richiesta metabolica. Questo implica inoltre che il sistema di regolazione del consumo di ossigeno non si comporti in maniera lineare nel dominio degli esercizi di intensità severa.
Per quello che concerne, invece, il sistema di trasporto dell’O2, i dati sperimentali hanno riportato una velocizzazione di Q’ all’inizio dell’esercizio sovra-massimale rispetto all’esercizio moderato. Inoltre la riposta di Q’ è risultata essere più rapida di quella di V’O2 nelle medesime condizioni di esercizio severo. A livello microvascolare questo scenario si è modificato. Infatti, il tempo di ritardo della cinetica della deoxy-emoglobina all’inizio dell’esercizio è risultato essere significativamente più breve durante l’esercizio severo rispetto al moderato. La diminuzione di questo parametro indica che il rapporto tra delivery ed uptake locali di ossigeno si rompa precocemente durante esercizio sovra-massimale e suggerisce una situazione più compromessa a carico della perfusione capillare del muscolo.In this thesis the characteristics and the physiological determinants of gas exchange kinetics in humans (Chapters I.2) during exercise performed at different intensities are illustrated and discussed on the basis of the recent data presented in the literature (Chapters I.3), including the findings regarding the dynamic responses of cardiac output and muscle oxygen extraction at the onset of exercise (Chapter I.4).
After this introduction, the results obtained by investigating the possible role of systemic oxygen delivery in limiting gas exchange kinetics are summarized. To this aim, the model of the head down tilt bed rest and its consequences on cardiovascular deconditioning were exploited (Chapter II). Cardiovascular deconditioning slowed oxygen delivery response, without affecting gas exchange kinetics at the onset of exercise. The invariance of oxygen uptake kinetics after HDTBR suggests and confirms the conclusion that muscular O2 uptake during moderate intensity exercise is mainly controlled by peripheral mechanisms, rather than by the bulk O2 delivery to the muscles.
Subsequently, the results obtained by evaluating in parallel pulmonary O2 uptake, cardiovascular O2 delivery, as measured with cardiac output (Q’), and muscular deoxygenation kinetics, as assessed by means of NIRS, during supra-maximal intensity and moderate intensity cycling exercises are illustrated (Chapter III). Supra-maximal exercise has been selected as a valuable tool, in these extreme exercise conditions, for examining the dynamic coupling between oxygen delivery and oxygen uptake and for inferring on the relative role of central and peripheral limitations of oxidative metabolism as a function of the exercise intensity.
This study showed that the time constants of the Primary Phase of V’O2 kinetics was significantly shorter during supra-maximal than during moderate-intensity exercise when the data were fitted with a non-linear curve-fitting procedure. In contrast, the time constants of the Primary Phase of V’O2 kinetics resulted significantly larger during supra-maximal than at moderate-intensity exercise when a semilogarithmic transformation was applied to the data.
The findings seem to disprove the hypothesis that Q’ kinetics at the onset of supra-maximal exercise accelerates as the metabolic power request increases. This would also imply that the system controlling O2 uptake does not behave linearly in the supra-maximal intensity domain. Concerning systemic O2 delivery, Q’ response at the beginning of supra-maximal exercise was faster than at the onset of moderate exercise and it was also faster than V’O2 kinetics at supra-maximal intensity. At the microvascular level, this scenario changed, as the delay occurring at the onset of the exercise before the increase in HHb was significantly shorter during supra-maximal than during moderate exercise. After this delay, the kinetics of muscle deoxygenation was rapid. These findings suggest that the muscular O2 consumption-to-perfusion ratio was characterised by a greater rate of change during the 120%Wmax tasks than during 80%LT exercise trials
Oxygen deficit during supramaximal cycling exercise in humans: a new estimation method.
No full textWe propose a method to estimate O2 deficit (DefO2) during supramaximal exercise based on the analysis of gas exchanges.
We determined in 14 male subjects breath-by-breath O2 at the onset of step exercise of moderate and supramaximal intensities. O2 response during moderate-intensity exercise was fitted as a bi-exponential model. Third, we modelled a theoretical O2 kinetics during supramaximal exercise assuming that it conformed to the O2 kinetics measured during moderate-intensity exercise and tended towards a O2 equal to that hypothetically elicited by the workload. The difference between the O2 volume that would have been consumed in the hypothetical condition and that actually taken up during exercise yielded the lactacid fraction of DefO2. Finally, the difference between the estimated total energy requirement and the theoretical O2 volume consumed by the subject yielded in the alactacid fraction of total DefO2.
Total DefO2 was 77.6 ml O2 kg-1; its anaerobic alactic and lactic fractions amounted to 38.5 and 39.1 ml O2 kg-1, respectively. The latter did not differ significantly from the value calculated independently from the peak increase in lactate concentration during supramaximal exercise times the energy equivalent of lactate.
These results suggest that supramaximal DefO2 and its components may be estimated from an analysis of gas exchange kinetics. In addition, they are consistent with the view that the mechanism regulating muscular O2 uptake is regulated by the muscular concentration of putative metabolites
Élite swimmers’ hydrodynamic drag: evaluation of the coefficient of simple passive drag and the propulsion of the legs
No full textThe Q’-V’O2 diagram: An analytical interpretation of oxygen transport in arterial blood during exercise in humans.
No full textA new analysis of the relationship between cardiac output (Q ̇) and oxygen consumption V ̇O2 is presented (Q ̇-V ̇O2 diagram). Data from different sources in the literature have been used for validation in three conditions: exercise and rest in normoxia, and exercise in hypoxia. The effects of changes in arterial oxygen concentration CaO2 on Q ̇ are discussed, as well as the effects of predominant sympathetic or vagal stimulation. Differences appear depending on whether CaO2 is varied by means of changes in blood haemoglobin concentration or changes in arterial oxygen saturation. The present Q ̇-V ̇O2 diagram allows comprehensive description of oxygen transport in exercising humans; it expands applicability of the historical Q ̇-V ̇O2 relationship to include CaO2 variations; it opens new pathways for understanding underlying mechanisms; it allows computation of Q ̇ from CaO2 and V ̇O2 measurements, when Q ̇ cannot be measured
O2 uptake and cardiac output kinetics during moderate and supra-maximal intensity exercise in humans.
No full textEffects of step duration in incremental ramp protocols on peak power and maximal oxygen consumption
No full textPurpose:
Morton (J Sport Sci 29:307–309, 2011) proposed a model of the peak power attained in ramp protocol ( w ̇peak ) that included critical power (CP) and anaerobic capacity as constants, and mean ramp slope (S) as variable. Our hypothesis is that w ̇peak depends only on S, so that Morton’s model should be applicable in all types of ramps. The aim of this study was to test this hypothesis by validating Morton’s model using stepwise ramp tests with invariant step increment and increasing step duration.
Methods:
Sixteen men performed six ramp tests with 25 W increments. Step duration was: 15, 30, 60, 90, 120 and 180 s. Maximal oxygen consumption ( V ̇O2max ) and w ̇peak were identified as the highest values reached during each test. An Åstrand-type test was also performed. We measured oxygen consumption and ventilatory variables, together with lactate and heart rate.
Results:
V ̇O2max was the same in all tests; w ̇peak was significantly lower the longer the step duration, and all values differed from the maximal power of the Åstrand-type test ( w ̇max ). Morton’s model yielded an excellent fitting, with mean CP equal to 198.08 ± 37.46 W and anaerobic capacity equal to 16.82 ± 5.69 kJ.
Conclusions:
Morton’s model is a good descriptor of the mechanics of ramp tests. Further developments of Morton’s model demonstrated that, whereas w ̇peak is a protocol-dependent variable, the difference between w ̇max and CP is a constant, so that their values do not depend on the protocol applied
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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