non-invasive methods in paediatric exercise physiology

SYMPOSIUM / SYMPOSIUM

Non-invasive methods in paediatric exercisephysiology

Neil Armstrong and Samantha G. Fawkner

Abstract: Oded Bar-Or’s hypothesis that children may be ‘‘metabolic non-specialists’’, even when engaging in specializedsports, has stimulated the study of paediatric exercise metabolism since the publication of his classic text Pediatric sportsmedicine for the practitioner in 1983. Evidence drawn from several methodologies indicates an interplay of anaerobic andaerobic exercise metabolism in which children have a relatively higher metabolic contribution from oxidative energy path-ways than adolescents or adults, whereas there is a progressive increase in glycolytic support of exercise with age, at leastinto adolescence and possibly into young adulthood. The picture is generally consistent but incomplete, as research withyoung people has been limited by both ethical and methodological constraints. The recent rigorous introduction of non-in-vasive techniques such as breath-by-breath respiratory gas analysis and magnetic resonance spectroscopy into paediatricexercise physiology promises to open up new avenues of research and generate unique insights into the metabolism of theexercising muscle during growth and maturation. It therefore appears that we might have available the tools necessary toanswer some of the elegant questions raised by Professor Bar-Or over 25 years ago.

Key words: oxygen uptake kinetics, magnetic resonance spectroscopy, children, metabolism.

Resume : L’hypothese du Pr Bar-Or selon laquelle les enfants n’auraient pas l’aptitude a ajuster leur metabolisme energe-tique en fonction du sport choisi et qui a ete elaboree en 1983 dans un manuscrit intitule « Pediatric sports medicine forthe practitioner » a amene de nombreux chercheurs a etudier le metabolisme de l’effort en pediatrie. D’apres des observa-tions faites dans des etudes methodologiques, on remarque que les enfants puisent davantage d’energie dans le metabo-lisme aerobie que ne le font les adolescents et les adultes quand les exigences de l’effort demande s’adressent aux deuxformes de fourniture d’energie, en aerobiose et en anaerobiose ; avec l’age, on note un plus grand recours au metabolismeanaerobie, du moins durant la puberte et au debut de l’age adulte. Malgre le consensus a propos de cette these, les etudesdemeurent incompletes notamment a cause des limites de nature ethique et des contraintes methodologiques qu’imposentl’experimentation chez des enfants. Avec l’introduction recente en pediatrie de l’effort de methodes rigoureuses et non ef-fractives telles l’analyse des gaz expires a chaque respiration et la spectroscopie par resonance magnetique, on peut envisa-ger de nouveaux champs de recherche qui jetteront une lumiere particuliere sur le metabolisme du muscle a l’effort durantla croissance et le developpement de l’enfant. Il semble que nous ayons maintenant les outils pour repondre a quelques-unes des questions pertinentes posees par le Proffeseur Bar-Or il y a plus de 25 ans.

Mots-cles : cinetique de la consommation d’oxygene, spectroscopie par resonance magnetique, enfants, metabolisme.

[Traduit par la Redaction]

Introduction

In his seminal text Pediatric sports medicine for the prac-titioner, Oded Bar-Or (1983) noted that the child who is thesprinting star of his class is often also above average inlong-distance running and successful in a variety of teamsports. Data collected in Bar-Or’s laboratory in the 1970s

suggested that children who did well in the Wingate anaero-bic test also had a high peak oxygen uptake ( _VO2 peak). Pro-fessor Bar-Or therefore hypothesized that children,particularly prepubertal children, might be ‘‘metabolic non-specialists’’, even when engaging in specialized sports.

Since the 1970s, the assessment and interpretation ofchildren’s performances on aerobic and anaerobic exercisetests has become more sophisticated (Armstrong and Wels-man 1994, 2000; Welsman and Armstrong 2006) and subse-quent research has questioned the validity of the concept ofchildren’s ‘‘metabolic non-specialization’’ (see Rowland2002 and Blimkie 2006 for critical comment). It can be ar-gued that prepubertal and early pubertal children are un-likely to demonstrate the degree of metabolic specializationseen in adults’ exercise performances simply because oftheir immaturity. As children enter puberty, biological

Received 10 April 2007. Accepted 21 June 2007. Published onthe NRC Research Press Web site at apnm.nrc.ca on 15 March2008.

N. Armstrong1 and S.G. Fawkner.2 Children’s Health andExercise Research Centre, University of Exeter, Exeter, UK.

1Corresponding author (e-mail: [email protected]).2Present address: Heriot-Watt University, Riccarton, Edinburgh,EH14 4AS, UK.

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clocks running at rates specific to the individual drivechanges in body size, body shape, muscularity, neuromuscu-lar coordination, biomechanics of movement, and receptive-ness to training, all of which undoubtedly influencemeasures of aerobic and anaerobic performance duringgrowth and maturation. The complexity of the interactionsof these factors that can influence aerobic and anaerobicperformance make it difficult to interpret changes in per-formance in relation to metabolic non-specialization, andthe hypothesis remains, as Blimkie (2006) commented, in-teresting but unproven. However, Bar-Or’s elegant hypothe-sis has stimulated and underpinned the study of paediatricexercise metabolism for 25 years.

Comparative anaerobic and aerobic data are available in asingle mixed longitudinal study where the peak power out-put and the _VO2 peak of the same participants were measuredat 12, 13, and 17 years. Peak power increased by 120% inboys and 66% in girls, whereas the corresponding increasesin _VO2 peak were somewhat less at 70% and 50% for theboys and girls, respectively (Armstrong and Welsman 2001;Armstrong et al. 2001). These data indicate that there areage and sex-related changes in anaerobic and aerobic powerthat are not synchronous, and that both boys and girls expe-rience a more marked increase in anaerobic metabolism thanaerobic metabolism as they move through adolescence.However, metabolic profiles derived from anaerobic andaerobic exercise tests require confirmation at the musclelevel and do not provide the quality and specificity of datarequired to tease out changes in exercise metabolism duringgrowth and maturation.

Paediatric exercise physiologists are normally limited toblood and respiratory gas markers of exercise metabolism.Although studies of substrate utilization during exercise(Boisseau and Delmarche 2000; Armstrong and Welsman2006), hormonal responses to exercise (Berg and Keul1998), recovery from high-intensity exercise (Ratel et al.2003), and exercise blood lactates (Pfitzinger and Freedson1997) have indirectly enhanced our knowledge, ethical con-siderations have restricted more informative studies at thelevel of the muscle cell. A limited number of muscle biopsyinvestigations of children’s fibre types (Jansson 1996), en-ergy stores and utilization (Eriksson 1980), and enzyme ac-tivity (Eriksson 1980; Haralambie 1982; Berg and Keul1998) have contributed to our understanding of the interplayof anaerobic and aerobic metabolism, but the data are diffi-cult to interpret and clouded by small sample sizes andmethodological difficulties such as having to be collected atrest.

Evidence is generally consistent and suggests that duringexercise children have relatively higher rates of oxidativemetabolism than adolescents or adults and that there is aprogressive increase in glycolytic activity with age, at leastuntil adolescence and possibly into adulthood (Armstrongand Welsman 2006). Nevertheless, understanding of paediat-ric exercise metabolism is far from complete and non-invasive methods capable of interrogating muscle duringexercise are required to progress knowledge. In this paper,we will examine two non-invasive techniques — namely,breath-by-breath respiratory gas analysis and magnetic res-onance spectroscopy (MRS) — which have recently beenused in paediatric exercise physiology research. We will

discuss their contribution to our knowledge of paediatricexercise metabolism and comment specifically on method-ology appropriate for use with young children.

Oxygen uptake kineticsThe availability of on-line breath-by-breath respiratory

gas analysis systems has enabled detailed study of the _VO2kinetic response at the onset of exercise of different inten-sities. However, breath-by-breath _VO2 responses are inher-ently ‘‘noisy’’ with a large variance from one breath to thenext, and the trace of a typical subject during even steady-state exercise can appear to be extremely erratic. In situa-tions where the signal-to-noise ratio is low (as is often thecase with children), it is possible to reduce the magnitudeof the noise by averaging the response profiles of a numberof identical transitions. This is achieved by interpolating thebreath-by-breath signal to a given time frame (usually 1 s),time aligning the transitions, and averaging the data pointsat each time interval. The technique of averaging multipletransitions to provide a single _VO2 profile for analysis istime consuming but essential for rigorous interpretation ofthe response. The _VO2 response must then be quantified.This is achieved using non-linear regression and iterative fit-ting techniques that fit a specified model to the availabledata as best as possible by choosing the line of best fit thatreduces the residual error. It is imperative to compute 95%confidence intervals for the response parameters and to re-port them. We have analysed and justified the use of appro-priate modelling techniques for paediatric data andcommented in detail on confidence intervals elsewhere(Fawkner and Armstrong 2002a, 2003a, 2004a).

The assessment and interpretation of _VO2 kinetic re-sponses to exercise is complex, but with the application ofsuitable methodology and appropriate modelling techniquesthe data have been shown to provide a useful non-invasivewindow into metabolic activity at the muscular level(Rossiter et al. 1999). Before discussing _VO2 kinetics inchildren, we will outline the characteristics of the _VO2 ki-netic response to provide a framework in which to discusspaediatric data — interested readers are referred to detailedreviews published elsewhere (Fawkner and Armstrong2003a, 2006).

Characteristics of the oxygen uptake kinetic responseThe characteristics of the _VO2 kinetic response can be de-

fined in relation to identifiable exercise intensity domainsthat have been called moderate, heavy, very heavy, and se-vere and set in relation to clear demarcation thresholds.Moderate-intensity exercise does not involve a sustainedanaerobic contribution to adenosine triphosphate (ATP) re-synthesis and the upper marker of this domain is thereforethe anaerobic threshold or a suitable derivative such as thelactate threshold or, more usually with children, the non-in-vasive ventilatory threshold (VT) (Fawkner et al. 2002a).Anaerobic glycolysis makes a larger contribution to energyrequirements during heavy exercise than during moderateexercise; however, blood lactate accumulation stabilizesover time, reflecting a balance between the rate of appear-ance and rate of removal. The heavy exercise domain there-fore lies above the VT with the maximal lactate steady state

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(MLSS) or the critical power (CP) as an upper marker. Thedetermination of MLSS requires multiple blood samples, soCP is the preferred variable for use with children and itoccurs in the region of 70%–80% _VO2 peak (Fawkner andArmstrong 2002b). The determination of CP is, however,very time consuming and, in practice, 40% of the differencebetween VT and _VO2 peak is normally used as the upperthreshold of the heavy exercise domain. Fawkner and Arm-strong (2003b) have showed that this threshold is appropri-ate, as it falls below CP in most children. Exerciseintensities that involve the eventual achievement of_VO2 peak are termed very heavy and exercise that requires atheoretical _VO2 above _VO2 peak is classified as severe. Tointerpret adequately _VO2 kinetic data the exercise domainmust be clearly defined and strictly adhered to. Exercise do-mains are illustrated in Fig. 1.

_VO2 kinetics are normally studied in the laboratory via astep transition where a period of rest or low-level exercise(e.g., unloaded pedalling on a cycle ergometer) is followedby a sudden increase in exercise intensity to a prescribedlevel. At the onset of the step transition in exercise there isan almost immediate increase in _VO2 at the mouth. This car-diodynamic phase (phase 1), which lasts about 15 s, isclosely associated with the increase in cardiac output thatoccurs prior to the arrival at the lungs of venous blood fromthe exercising muscles. Phase 1 is therefore independent ofoxygen uptake at the muscles and is predominantly a reflec-tion of the increase in pulmonary blood flow with exercise.During phase 2, which follows the cardiodynamic phase, hy-poxic and hypercapnic blood arising from the exercisingmuscles arrives at the lung. Phase 2 _VO2 has been shown tobe closely representative of oxygen uptake at the muscle de-spite some disassociation due to muscle utilization of oxy-gen stores and differences in blood flow at the muscle andlung (Rossiter et al. 1999).

During moderate-intensity exercise, pulmonary _VO2 risesin an exponential manner towards a steady state (phase 3)that is proportional to the exercise intensity. The speed ofthe phase 2 (primary) response is described by the time con-stant (�), which represents the time taken to achieve 63% ofthe change in _VO2. In phases 1 and 2, ATP resynthesis can-not be fully supported by oxidative phosphorylation and theadditional energy requirements of the exercise are met pri-marily by the breakdown of phosphocreatine (PCr) with aminor contribution from anaerobic glycolysis and oxygenstores. The oxygen equivalent of these energy sources istermed the oxygen deficit and the faster the � , the smallerthe oxygen deficit. In adults, _VO2 increases to a steady state(phase 3) within 2–3 min with an oxygen cost per unit ofwork (or ‘‘gain’’) of about 10 mL�min–1�W–1 above thatfound during unloaded pedalling. Empirical evidence fromwell-designed studies is equivocal; however, in their review,Barstow and Scheuermann (2006) conclude that the phase 2oxygen cost per unit of work is ‘‘substantially elevated’’ inchildren performing cycle ergometry, thus inferring an age-related difference in the contribution of oxidative phos-phorylation.

During heavy-intensity exercise, the primary phase 2 gainis similar to that observed during moderate exercise, but theoxygen cost of exercise becomes elevated over time. A slow

component of _VO2 kinetics is superimposed upon the pri-mary _VO2 response and the achievement of a steady statemight be delayed by 10–15 min. The gain of the steady statein adults might be as high as 13 mL�min–1�W–1, but rela-tively little is known about the magnitude of the slow com-ponent in children. The mechanisms underlying the slowcomponent remain speculative, but the most plausible theoryproposes a combined influence of muscle fibre distribution,motor unit recruitment, and the matching of oxygen deliveryto active muscle fibres.

In the very heavy-intensity exercise domain a steady stateis not achieved, the slow component causes _VO2 to rise toits peak level, and lactate increases until exercise is termi-nated by exhaustion. During severe-intensity exercise, wherethe projected _VO2 is greater than _VO2 peak, the response istruncated with the rapid attainment of _VO2 peak. The phasesof the kinetic rise in _VO2 in response to exercise in differentdomains are illustrated in Fig. 1.

Oxygen uptake kinetics in childrenAs indicated earlier, a high degree of rigour is required to

elucidate _VO2 kinetics in children. Small _VO2 amplitudesand large breath-by-breath variations are inherent to child-ren’s response profiles and reduce the confidence withwhich kinetic parameters can be estimated. Consequently,confidence intervals might be beyond accepted limits unlesssufficient identical transitions are aligned and averaged toimprove the signal-to-noise ratio. Data from young peopleare sparse and the literature is clouded by studies lackingsuitable methodological rigour. Fawkner and Armstrong(2003a, 2006) and Barstow and Scheuermann (2006) havecritically reviewed paediatric _VO2 kinetic studies and herewe will focus on those studies that have used breath-by-breath analysis, reduced the noise by averaging multipletransitions, used appropriate modelling techniques, exhibitedstrict adherence to well-defined exercise domains, and havereported the 95% confidence intervals of their data.

Exercise below the ventilatory thresholdSeveral studies using less than ideal methodology have

addressed the dependence of the phase 2 _VO2 response onage. Results are equivocal, but the data trend is towards afaster � in children than in adults (Fawkner and Armstrong2003a). The only study to satisfy fully the aforementionedcriteria is that of Fawkner et al. (2002b), who evaluated thephase 2 _VO2 kinetics of 11- to 12-year-old children andyoung adults during exercise below 80% VT. Up to 10 tran-sitions were analysed for each participant and those inwhom the confidence intervals exceeded 5 s were removedfrom the study. This left 12 boys, 11 girls, 13 men, and 12women for analysis, and it was demonstrated that the phase2 _VO2 time constant was significantly faster in boys than inmen (19.0 vs. 27.9 s, p < 0.01) and in girls than in women(21.0 vs. 26.0 s, p < 0.05). _VO2 amplitude, oxygen deficit,and oxygen deficit relative to _VO2 amplitude were all signif-icantly higher in adults than in children (males, p < 0.01;females, p < 0.05). No sex differences in � were observedin either children or adults.

Children’s faster increase in _VO2 to a new steady state,and therefore lower contribution to ATP resynthesis from

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anaerobic sources during phase 2, may be due to a more ef-ficient oxygen-delivery system, a greater relative capacityfor oxygen utilization at the muscle, or both. There is nostrong evidence to suggest that delivery of oxygen to the mi-tochondria is enhanced in children compared with adults orthat increased availability of oxygen to the working musclesspeeds _VO2 kinetics during exercise below VT. The faster �and lower relative oxygen deficit are therefore likely to re-flect children’s enhanced capacity for oxidative phosphory-lation during moderate-intensity exercise (for detaileddiscussion see Fawkner and Armstrong 2006; Barstow andScheuermann 2006).

To provide further insights into paediatric exercise metab-olism, more research using suitable techniques is required toclarify age-related differences in phase 2 _VO2 kinetics in themoderate exercise domain and, in particular, the issue of apotential independent effect of maturation on _VO2 kineticshas not been addressed.

Exercise above the ventilatory thresholdElucidating _VO2 kinetic responses above VT requires pre-

cisely relating the response to a specific exercise domain. Asthe absolute work rate equivalents of the exercise domainsare smaller in children than in adults, this sets a formidablechallenge. Several studies have not discriminated betweenthe heavy and very heavy exercise domains and used an ar-bitrary point between VT and _VO2 peak or a simple percent-age of _VO2 peak to set the required exercise intensity.

Despite concerns with aspects of the methodologyadopted, both published studies that have investigated theresponses of children and adults to exercise in the heavy –very heavy exercise domains have observed significantlyfaster phase 2 time constants and greater oxygen gains dur-ing the primary component in children (>90% boys) than inmen (Armon et al. 1991; Williams et al. 2001; see Fawknerand Armstrong 2003a and Barstow and Scheuermann 2006for a critique of methodology). Both studies also reportedthat phase 3 children’s responses could be modelled with asingle exponential, implying that children did not display

the slow component of _VO2 identified in adults, an observa-tion in conflict with that of Obert et al. (2000) who reportedthe presence of a _VO2 slow component in 14 boys and 9girls aged 10–13 years, and exercising at 90% _VO2 peak.

In the first paediatric study to report the 95% confidenceintervals of the primary time constants (within 5 s), Fawknerand Armstrong (2004b) carefully investigated changes inprepubertal children’s responses to heavy exercise over a2 year period. On the first test occasion the children exhib-ited a significantly faster � (boys, 16.8 vs. 21.7 s; girls, 21.1vs. 26.4 s; p < 0.01) and a significantly (p < 0.05) greateroxygen gain during the primary component than on the sub-sequent test. A slow component of _VO2 was apparent in 13boys and 9 girls, aged 10 years, and it contributed about10% of the final _VO2 after 9 min of well-defined, heavy ex-ercise, increasing to about 15% 2 years later. The oxygencost by the end of the exercise period on both test occasionswas equal, suggesting that the phosphate turnover requiredto maintain heavy exercise was in fact independent of ageand that, in the older children, a lesser proportion of the re-quired oxygen for the given exercise intensity was achievedin the primary phase. The same authors confirmed empiri-cally elsewhere that a slow component does exist in childrenand stipulated that paediatric data from heavy exerciseshould not be modelled as a single exponential process(Fawkner and Armstrong 2004a).

In a subsequent study using identical methodology, Fawk-ner and Armstrong (2004c) demonstrated sex differences inthe responses to heavy exercise of 48 prepubertal children(25 boys, 23 girls), aged 10 years. The primary time con-stant was significantly (p < 0.05) faster in boys (17.6 vs.21.9s) and the slow component contribution to the totalchange in amplitude was significantly (p < 0.05) greater ingirls (11.8% vs. 8.9%).

Explanations for age-related changes in response to heavyexercise are difficult to substantiate with confidence, butseveral hypotheses do exist. It has been suggested that oxy-gen delivery might limit the phase 2 response to heavy exer-cise, but there is little evidence to suggest that oxygen

Fig. 1. The three phases of the kinetic rise in pulmonary oxygen uptake in response to a step change in exercise in four different exerciseintensity domains (reprinted with permission from Fawkner and Armstrong 2003a).


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delivery during exercise is either impaired in healthy sub-jects or decreases with age. A more plausible proposal isthat changes in the magnitude of the oxygen cost of the pri-mary component (mL�min–1�W–1), the speed of the primary� , and the size of the slow component of _VO2 during exer-cise above VT indicate the presence of a developmental in-fluence on the mitochondrial oxygen utilization potentialthat supports an enhanced oxidative function during child-hood. These directional responses are also characteristic ofsubjects with a high ratio of type I to type II muscle fibres,and although the evidence supporting age-related changes inthe proportion of fibre types is equivocal, children mighthave a higher population of type I fibres than adults(Jansson 1996).

Why there are sex differences in _VO2 kinetics above butnot below VT is not readily apparent. Studies with adultshave reported negative relationships between percentage oftype I fibres and the primary � and slow component inheavy exercise, but no relationship between � and percent-age of type I fibres in moderate exercise (Jones et al. 2006).If, as has been reported in at least one study, boys have agreater percentage of type I fibres than girls (Jansson 1996),this would be consistent with the observed sex differences in_VO2 kinetics.

31P magnetic resonance spectroscopy

Magnetic resonance spectroscopy (MRS) is a non-inva-sive, safe technique appropriate for use with children, whichprovides a window through which muscle can be interro-gated during exercise both in real time and in vivo. MRShas the potential to reveal sensitive changes in muscle me-tabolism and therefore unique insights into exercise metabo-lism during growth and maturation (for further detail onMRS and its applications see Kent-Braun et al. 1995).

In a typical exercise test in an MR scanner, the subjectlies either prone or supine, the magnetic field is activated,and, following a period of rest, single leg calf or quadricepsmuscle exercise using a non-magnetic ergometer is initiatedand monitored. The nuclei of atoms align with the magneticfield, then an oscillating magnetic field is applied and thesubsequent nuclear transitions allow spectral analysis of theinterrogated muscles. Different molecules produce their ownspectra and once the muscles have been identified anychanges in the spectral lines can be interpreted. The nucleusmost extensively used for metabolic studies is 31P, the natu-rally occurring phosphorus nucleus. 31P MRS enables themonitoring of metabolites that play a central role in bioener-getics, namely ATP, PCr, and inorganic phosphate (Pi).Typical 31P MRS spectra obtained during rest, exercise, andrecovery are illustrated in Fig. 2 (from left to right, thepeaks representing Pi, the single phosphorus of PCr, and thethree phosphate nuclei of ATP can be identified). The de-cline in PCr and corresponding rise in Pi with incrementalexercise are clearly evident. The ratio of Pi to PCr can bedetermined from the respective Pi and PCr spectral areasduring curve quantification, and is commonly used in inter-pretation of 31P MRS spectra. Intracellular pH can be calcu-lated from the shift in the Pi spectral peak relative to thePCr peak. The pH reflects the acidification of the muscle

and therefore muscle glycolytic activity, but it is not a directmeasure of glycolysis.

Progressive incremental exercise tests to exhaustion in anMR scanner exhibit non-linear changes in both the ratio ofPi to PCr and pH. An initial shallow slope is followed by asecond steeper slope with increasing exercise intensity andthe transition point is known as the intracellular threshold(IT). ITs occur at a relative exercise intensity similar to theVT during cycle ergometer exercise. For example, in ourCentre we have observed the MRS determined Pi:PCr IT tooccur at 59% of maximal intensity exercise and the cycle er-gometer VT to occur at 58% _VO2 peak in the same 9–11 yearolds (Barker et al. 2008). ITs have proven valuable indica-tors of children’s metabolism during incremental exerciseand they can be determined as illustrated in Fig. 3 by plotsof the ratio of Pi to PCr or pH against power output.

Experimentally, paediatric MRS studies are constrainedby the need to exercise within a small-bore tube, which canbe very challenging for children. Furthermore, the require-ment that the acquisition of data be synchronized with therate of muscle contraction is often problematic with youngpeople. Few studies with children have been published andappropriate habituation to the exercise task has seldom beenaddressed.

Comparisons of existing paediatric MRS data are difficultthrough inter-study differences in exercise protocol,muscle(s) interrogated, subject characteristics, and data nor-malization. Some studies (e.g., Zanconato et al. 1993; Tay-lor et al. 1997) have used a treadle ergometer to exercisethe calf muscles and this technique has limitations when ap-plied to children. The small size of the calf muscles in chil-dren leads to a lower signal-to-noise ratio and hence moredifficulty in curve-fitting the spectra. In combination withthe small size, the known heterogeneous metabolic composi-tion of the gastrocnemius and soleus muscles represents apotential source of error in any studies comparing individu-als of different size. The soleus is composed mainly of typeI and the gastrocnemius mainly of type II fibres; if there arevarying amounts of soleus and gastrocnemius in the volumeof interest of muscle being interrogated, this might bias theinterpretation of metabolic responses. Published results ofpaediatric MRS studies therefore need to be interpreted withrespect to the muscles interrogated, the use of small, mixed-age and (or) sex participant groups, and the rigour of habit-uation, data collection, data analysis, and interpretation.

The first 31P MRS exercise study to include children wascarried out by Zanconato et al. (1993) who reported datafrom 8 boys and 2 girls, aged 7–10 years, and 5 men and 3women, aged 20–40 years, who underwent a supine, pro-gressive treadle exercise to voluntary exhaustion. Zanconatoet al. (1993) observed a slow phase and a fast phase ofPi:PCr increase and pH decrease with increasing exercise in-tensity in both children and adults and detected an IT in75% of the adults and 50% of the children. The characteris-tics of the initial linear slopes in Pi:PCr and pH were similarin children and adults, but following the ITs the incline inPi:PCr and decline in pH were both steeper in adults thanin children, which suggested to Zanconato et al. (1993)growth-related differences in energy metabolism in thehigh-intensity exercise range. The final pH observed in


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adults was significantly lower than in children, whose end-exercise Pi:PCr was, on average, only 27% of adult values.

The observations of Zanconato et al. (1993) were subse-quently supported by a study of 14 trained and 23 untrained12- to 15-year-old boys and 6 adults with an average age of25 years (Kuno et al. 1995). MRS spectra were collectedfrom the quadriceps during supine exercise to exhaustionand lower values of intracellular pH and the ratio of PCr to(PCr+Pi) were noted in the adults at exhaustion. No signifi-cant differences were observed between the trained and un-trained boys.

In a study of ageing effects on skeletal muscle, Taylor etal. (1997) compared the 31P MRS spectra at rest, duringmaximal calf muscle exercise, and during recovery from ex-ercise in 20 adults aged 20 to 29 years , as well as 15 youthsaged6 to 12 years; both groups consisted of unspecifiednumbers of males and females. The children had a higherpH during exercise, indicating a lower glycolytic contribu-tion to metabolism and a faster resynthesis of PCr during re-covery compared with the adults. Taken together, the

findings of these three studies consistently indicate lowerglycolytic activity during childhood than during adulthood.Although the incremental tests to exhaustion imply no age-related difference in the rate of mitochondrial oxidative me-tabolism during low-intensity exercise, the investigators’ re-covery data suggest a higher oxidative capacity in childrenthan in adults (Taylor et al. 1997). Potential sex differencesin exercise metabolism during childhood and adolescencewere not addressed.

Peterson et al. (1998) examined the effects of maturationon exercise metabolism by evaluating the responses of nine10-year-old prepubertal and nine 15-year-old pubertaltrained girl swimmers to 2 min of plantar flexion exerciseat light (40% maximal work capacity) and supramaximal(140% maximal work capacity) exercise. Puberty was self-assessed using the stages of maturation described by Tanner(1962). At the end of the exercise, intracellular pH (6.66 vs.6.76) was lower and the Pi:PCr (2.18 vs. 1.31) was higher inthe pubertal girls, but the differences were not statisticallydifferent. Peterson et al. (1998) concluded that, in conflict

Fig. 2. 31P magnetic resonance spectra obtained from a child during rest, exercise, and recovery. From left to right, the peaks representphosphate (Pi), phosphocreatine (PCr), and the three phosphorus nuclei of adenosine triphosphate (ATP).

Fig. 3. A child’s pH and the ratio of Pi to PCr in relation to power output determined during a progressive incremental knee-extensionexercise to exhaustion in an MR scanner. The intracellular thresholds (ITs) are indicated.


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with earlier MRS studies, glycolytic metabolism in physi-cally active children is not maturity dependent. However,they commented that this conclusion should be drawn withsome caution. Scrutiny of the magnitude of the pubertal/prepubertal difference (66%) in the Pi:PCr, the high varia-bility, and the small sample sizes suggest that the observeddifference between the two groups might have biologicalmeaning and deserves further study.

The few published MRS studies have promoted knowl-edge of paediatric exercise metabolism, but the potential ofthe technique to provide unique insights is relatively un-tapped and more research is required. In the following sec-tions, we will focus on a series of recent exploratory studiesin our Centre that have, as yet, only been published in part(Barker et al. 2006a, 2006b, 2008). We have emphasised ap-propriate habituation of children before they exercise in theMR scanner, evaluation of the reliability of children’sPi:PCr and pH responses (including ITs) to knee extensorexercise, and explored sex differences in responses to mod-erate- and heavy-intensity exercise.

To overcome any fears of exercising within a confinedtube and to allow comprehensive habituation at low cost wehave constructed a to-scale replica of our MR scanner. Ontheir first visit to the Centre and prior to any experimentalwork within the MR scanner, the children practice exercis-ing within the replica scanner. Using VelcroTM, the child’sfoot is fastened securely to a padded foot brace connectedto a non-magnetic ergometer load basket via a pulley sys-tem that provides a variable resistance against which uni-lateral knee extensions can be performed while lyingprone. To standardize the exercise protocol required in theMR scanner the children practice following an image of avertical metronomic cursor projected onto a visual displayin front of them, using a second vertical cursor under theircontrol. The frequency of the metronomic cursor is set to40 pulses�min–1 to ensure that the children practice kneeextensions at a cadence in unison with the magnetic pulsesequence used in the MR scanner. It is only when the chil-dren are fully habituated to the exercise in the replicascanner and capable of maintaining the required knee ex-tension cadence that they transfer to the MR scannerwhere, to prevent displacement of the quadriceps muscles’volume of interest relative to the MRS coil and to mini-mize adjacent muscles contributing to the exercise task,the child’s position is secured with Velcro straps over thelegs, hips, and lower back.

Incremental exercise tests to exhaustion follow a 2 minresting baseline measurement period and start with an initialergometer basket load of 0.5 kg. The basket load is in-creased in steps of 0.5 kg each minute using brass weightsuntil the child can no longer comply with the knee extensorrate. Increments of 0.5 kg are used, as pilot work with chil-dren has demonstrated that this protocol ensures exhaustionwithin 7–12 min. Following time alignment of the ergome-try data to the start of the exercise test, work done is inter-polated second by second and averaged every 30 s tocoincide with the resolution used for metabolite acquisition.Power output is derived from each data bin by dividing thework done by time. 31P spectra are obtained every 1.5 s and20 measurements are performed leading to spectra being ac-quired every 30 s. The changes in spectral areas in Pi:PCr

and pH during rest and exercise are quantified and each var-iable plotted as a function of power output as shown inFig. 3. Using this methodology we have demonstrated a de-tection rate of 93% and 81% for the ITs of Pi:PCr and pH,respectively, and good reliability with typical errors acrossthree trials 1 week apart of about 10% for both Pi:PCr andpH ITs (Barker et al. 2006a).

Why Zanconato et al. (1993) had such a low IT detectionrate compared with Barker et al. (2006a) is not readily appa-rent; however, in addition to differences in the muscles in-terrogated (calf vs. quadriceps), explanations might includelimited habituation leading to less motivation to exercise toexhaustion and (or) a lack of compliance to the exercise pro-tocol promoting test termination before reaching the ITs. Insupport of this, it is worthy to note that Barker et al. (2006a)observed end Pi:PCr of over 2.0, compared with the child-ren’s end Pi:PCr of 0.54 reported by Zanconato et al.(1993).

Sex differences in exercise metabolism were investigatedin 18 girls and 15 boys, mean age 10.7 years, who com-pleted a single-leg, knee-extensor, incremental exercise testto exhaustion. Power output at the Pi:PCr IT and pH IT dis-played no significant sex differences, but the girls demon-strated significantly greater (p < 0.05) changes in Pi:PCrand pH as a function of power output following the ITs.Peak power at maximal exercise was not significantly differ-ent, but the boys exhibited a significantly lower increase inPi, decrease in PCr, and increase in Pi:PCr compared withgirls. At end exercise pH tended to be lower in girls (p =0.058) than in boys. These results indicate that during incre-mental exercise there are sex differences in the quadricepsmuscles’ metabolic response to heavy exercise (above theIT), but not to moderate exercise (below the IT). However,it should be noted that although the children were all aged10 years, stage of maturation was not determined and furtherwork is required to investigate sex differences in relation tomaturation.

To examine further potential sex differences in exerciseresponses PCr kinetics at the onset of moderate exercisewere investigated in prepubertal and early pubertal chil-dren (9 boys and 11 girls, mean age 9.9 years). The studywas designed in a similar manner to those described in thesection on _VO2 kinetics and each child was required to visitthe Research Centre on 7–10 occasions. Following habitua-tion to the required exercise protocol in the replica scanner,Pi:PCr ITs were determined for each child and the constant-work exercise was set at 80% of the individual’s IT to en-sure that the exercise was of moderate intensity. 31P spectrawere obtained every 1.5 s and 4 measurements were madeleading to spectra being acquired every 6 s to improve thesignal-to-noise ratio of the profile while providing high-density data for kinetic parameter estimation. To estimatethe PCr time constant with 95% confidence limits of <6 sthe children completed, on average, 6 constant-work exer-cise transitions that were averaged for determination of thePCr response dynamics. In all cases, a single exponentialmodel with no time delay provided an appropriate fit tothe PCr response and there was no significant sex differ-ence in the time constants (boys: 21.4 s vs. girls: 23.6 s).The findings of these two MRS studies are in accord withthe oxygen uptake kinetics data described earlier and indi-


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cate no sex differences in either PCr or _VO2 kinetics dur-ing moderate-intensity exercise, but during heavy exercisethe mechanisms relating oxygen delivery to and (or)utilization by the muscle might be compromised in younggirls compared with boys or, as hypothesized in the sectionon _VO2 kinetics, the explanation might lie in the distribu-tion of muscle fibre types.

Studies with adult subjects have demonstrated that withthe implementation of appropriate modelling techniques thephase 2 pulmonary _VO2 response provides a close relation-ship with PCr kinetics at the onset of exercise both when_VO2 kinetics are pre-determined on a cycle ergometer(Koga et al. 2005) and when they are determined simultane-ously with PCr kinetics using knee-extensor exercise in anMR scanner (Rossiter et al. 1999). To explore the strengthof this association in children who characteristically displaygreater signal noise coupled with a lower signal amplitudecompared with adults, 6 boys and 6 girls, mean age9.9 years, underwent incremental exercise tests to exhaus-tion on a cycle ergometer and in an MR scanner. Duringthese tests the VT and IT, respectively, were determinedand intensities for the constant-work exercise tests were in-dividually set at 80% of these domain markers to ensure ex-ercise was of moderate intensity. On average, the childrencompleted 6 or 7 repeat constant-work transitions for deter-mination of the _VO2 and PCr response kinetics, respectively.In all cases, a single exponential model provided an appro-priate fit for the response from the onset of exercise for PCrand, following phase 1 and including a time delay for phase2, _VO2 kinetics and the group average 95% confidence in-tervals for all time constants was 5 s. A significant correla-tion (r = 0.5) between the PCr and _VO2 time constants wasobserved and there was no significant difference in the esti-mated time constants for PCr and _VO2. The 95% confidenceintervals spanning the estimated PCr and _VO2 time con-stants failed to overlap in only one child. It was thereforeconcluded that characterization of the phase 2 _VO2 kineticsat the onset of cycle ergometer exercise closely resemblethose of PCr kinetics at the onset of knee-extension exercise.

The analysis of both breath-by-breath respiratory gas andMR spectra during exercise of varying intensities hasprovided new insights into paediatric exercise metabolism.The high cost of obtaining appropriate MR spectra, thetime-consuming habituation of children to a rigorous exer-cise protocol within a tube, and the restricted availability ofMR scanners for research with healthy children has limitedthe development and application of the technique to the ex-ercising child. More research using MRS is urgently re-quired; however, in the absence of MRS data, the closerelationship between PCr and _VO2 kinetics encourages theuse of more child-friendly and less-expensive _VO2 kineticsas an additional and valuable non-invasive window intomuscle metabolism during growth and maturation.

ConclusionThe appropriate assessment and interpretation of _VO2 and

PCr kinetics data derived from exercise studies using breath-by-breath respiratory gas and magnetic resonance spectro-scopy has enhanced our knowledge of children’s exercisemetabolism, but well-designed and -executed studies using

these techniques have a huge untapped potential to providefurther insights into the underlying physiology of the re-sponses to exercise of children and adolescents.

One of the many legacies of Professor Bar-Or is that hisoriginal hypothesis of children’s metabolic non-specializationinspired a generation of paediatric exercise scientists to re-search the interplay between aerobic and anaerobic exercisemetabolism during growth and maturation. With the develop-ment and rigorous application of new non-invasive techni-ques to the exercising child we might now have the tools toprovide some of the answers to the elegant questions raisedby Oded Bar-Or over 25 years ago.

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