ORIGINAL ARTICLE

Muscle phosphocreatine and pulmonary oxygen uptake kineticsin children at the onset and offset of moderate intensity exercise

Alan R. Barker Æ Joanne R. Welsman ÆJonathan Fulford Æ Deborah Welford ÆCraig A. Williams Æ Neil Armstrong

Accepted: 4 December 2007 / Published online: 3 January 2008

� Springer-Verlag 2007

Abstract To further understand the mechanism(s)

explaining the faster pulmonary oxygen uptake ðp _VO2Þkinetics found in children compared to adults, this study

examined whether the phase II p _VO2 kinetics in children

are mechanistically linked to the dynamics of intramus-

cular PCr, which is known to play a principal role in

controlling mitochondrial oxidative phosphorylation dur-

ing metabolic transitions. On separate days, 18 children

completed repeated bouts of moderate intensity constant

work-rate exercise for determination of (1) PCr changes

every 6 s during prone quadriceps exercise using 31P-

magnetic resonance spectroscopy, and (2) breath by

breath changes in p _VO2 during upright cycle ergometry.

Only subjects (n = 12) with 95% confidence intervals

B±7 s for all estimated time constants were considered

for analysis. No differences were found between the PCr

and phase II p _VO2 time constants at the onset (PCr 23 ±

5 vs. p _VO2 23� 4 s;P ¼ 1:000Þ or offset (PCr 28 ± 5

vs. p _VO2 29� 5 s; P ¼ 1:000Þ of exercise. The average

difference between the PCr and phase II p _VO2 time

constants was 4 ± 4 s for the onset and offset responses.

Pooling of the exercise onset and offset responses

revealed a significant correlation between the PCr and

p _VO2 time constants (r = 0.459, P = 0.024). The close

kinetic coupling between the p _VO2 and PCr responses at

the onset and offset of exercise in children is consistent

with our current understanding of metabolic control and

suggests that an age-related modulation of the putative

phosphate linked controller(s) of mitochondrial oxidative

phosphorylation may explain the faster p _VO2 kinetics

found in children compared to adults.

Key words 31P-MRS � Developmental � Metabolism �Muscle oxygen consumption

Introduction

At the onset of a ‘‘step’’ transition from rest to a higher

metabolic rate, the rise in muscle O2 utilization ðm _VO2Þexhibits considerable delay before the rate of oxidative

ATP supply and ATP hydrolysis in the myocyte are

suitably matched. Debate exists as to whether this

delayed rise in m _VO2 is limited by the provision of

oxygen to the contracting muscle, or through an intrinsic

ability of the muscle to utilize O2 (Grassi 2005; Hughson

2005). While an interaction between the above factors is

likely to determine the adaptation of m _VO2 (Tschakovsky

and Hughson 1999), there is strong evidence supporting

the theory that the rise in m _VO2 is principally controlled

by one (or a combination) of the reactant(s) and/or

product(s) involved in the creatine kinase splitting of

muscle PCr (Grassi 2005; Meyer 1988; Rossiter et al.

2005).

Using 31P-magnetic resonance spectroscopy (31P-MRS),

Rossiter et al. (1999) established a close kinetic coupling

between the respective fall and rise in PCr and phase II

A. R. Barker � J. R. Welsman � C. A. Williams �N. Armstrong (&)

Children’s Health and Exercise Research Centre,

University of Exeter, St Luke’s Campus, Exeter EX1 2LU, UK

e-mail: N.Armstrong@exeter.ac.uk

J. Fulford

Peninsula Medical School, University of Exeter, Exeter, UK

D. Welford

Cardiff School of Sport, University of Wales Institute,

Cardiff, UK

123

Eur J Appl Physiol (2008) 102:727–738

DOI 10.1007/s00421-007-0650-1

pulmonary O2 uptake ðp _VO2; which provides a close

reflection of m _VO2; Grassi et al. 1996) at the onset of

prone knee-extensor exercise. Moreover, the acute inhibi-

tion of the creatine kinase reaction resulted in a pronounced

speeding (*50%) in the fall in PO2 (which reflects m _VO2

in this model) at the onset of isometric contractions in frog

muscle (Kindig et al. 2005). Therefore, following a ‘‘step’’

change in external work-rate, muscle PCr provides an

immediate temporal buffer for ATP, thereby delaying the

rise in the putative metabolic feedback controllers to signal

an increased rate of mitochondrial oxidative phosphoryla-

tion. Specifically, changes in cellular PCr/Cr may modulate

the sensitivity of the mitochondria to ADP via creatine

kinase isoforms in the cytoplasmic and mitochondrial

spaces (Walsh et al. 2001).

Investigations designed to quantify the adjustment of

p _VO2 at the onset of exercise in children have provided

interesting insights into the kinetic response parameters.

A consistent finding is that younger children display a

faster phase II rise in p _VO2 at the onset of moderate and

heavy intensity exercise compared to older children or

adults (Fawkner and Armstrong 2004; Fawkner et al.

2002b; Williams et al. 2001). In a longitudinal study,

Fawkner and Armstrong (2004) found the phase II p _VO2

time constant during heavy intensity cycling exercise

increased (i.e. slower kinetics) by 25 and 29% in

11–13 year-old girls and boys, respectively. In combina-

tion with the results obtained from an earlier study for

moderate cycling exercise (Fawkner et al. 2002b), the

authors proposed an age-related decline in the muscles’

potential to utilize O2, perhaps via a slower activation of

metabolic enzymes and/or the build-up of putative meta-

bolic controllers may account for these findings. This,

however, has yet to be investigated.

Considering the prominent role muscle PCr plays in

controlling the adaptation of m _VO2 during metabolic

transitions, we were interested in examining the kinetic

association between the phase II p _VO2 and muscle PCr

responses in children in an attempt to further understand

the mechanisms underlying the faster p _VO2 kinetics

found in children compared to adults. In contrast to

experimental work conducted in adults whereby the

kinetics of muscle PCr and p _VO2 were determined

simultaneously during knee-extensor exercise (Rossiter

et al. 1999), the p _VO2 signal arising from the quadriceps

muscle in children is insufficient to accurately quantify

the response characteristics. We therefore based our

experimental model upon the work by Barstow et al.

(1994). The kinetics of p _VO2 were measured during

upright cycle ergometry and compared to the kinetics of

muscle PCr determined during prone quadriceps exercise

using 31P-MRS. Although cycling in the supine position

results in a slowing of the exercise onset p _VO2 kinetics

compared to the upright position (Hughson et al. 1993),

exercise modalities that recruit less muscle mass, i.e.

plantar-flexors or knee-extensors, in the supine/prone

body position demonstrate no differences in the kinetics

of PCr or p _VO2 when compared to upright cycling p _VO2

kinetics (Barstow et al. 1994; Rossiter 2000). Therefore,

in comparison to p _VO2 kinetics determined during upright

cycling, these data indicate that body position does not

exert a limiting influence on the kinetics of muscle PCr

and p _VO2 providing that the exercise modality in the

prone/supine body position is restricted to a small muscle

mass.

Therefore, the purpose of the present study was to test

the hypothesis that the phase II p _VO2 kinetics in children

are mechanistically linked to the putative metabolic con-

troller muscle PCr, both at the onset and offset of moderate

intensity exercise.

Materials and methods

Subjects

Eighteen 9–11 year-old children (eight boys and ten

girls) recruited from a local primary school were inclu-

ded in the current study. After written and verbal

explanation of the study’s aims, risks and procedures, all

children and their parent(s)/guardian(s) provided

informed consent to partake in the project that was

approved by the institutional ethics committee. Pre-

experimental questionnaires identified that all subjects

were healthy and showed no contraindications to exer-

cising inside the MR scanner.

Each subject made between six and ten visits to the

Research Centre. The purpose of the first visit was to

ensure that the subjects were fully familiarised with the

laboratory setting and exercise procedures. In particular, all

children were habituated to exercising on a knee extension/

flexion ergometer at the required cadence inside a purpose

built, to scale model of the MR scanner. All subjects

completed a number of repeat trials on the ergometer at a

range of work intensities, identical to the actual protocols

employed in the study. All subjects were well habituated to

the test procedures.

Descriptive characteristics

Subjects’ body mass and stature were measured using a

calibrated balance beam scale (Avery, Birmingham, UK)

and a stadiometer (Holtain, Crymych, Dyfed, UK),

respectively. Subjects’ age was calculated as the difference

between the date of birth and the date of the first visit.

728 Eur J Appl Physiol (2008) 102:727–738

123

Quadriceps exercise inside the MR scanner

Ergometer description

The quadriceps exercise consisted of performing dynamic

knee extensions and flexions with the right leg on a non-

magnetic ergometer whilst lying prone inside the MR

scanner. The right foot was fastened to a padded foot

brace, which was connected to the ergometer load basket

using a rope and pulley system. This provided resistance

against which continuous concentric and eccentric

quadriceps contractions could be performed inside the

MR scanner over a distance of *0.22 m. To ensure

interrogation of the quadriceps muscles for metabolite

changes occurred in the same volume of interest (VOI)

and to standardise the exercise protocol both within and

between subjects, the quadriceps exercise was performed

at a cadence set in unison with the magnetic pulse

sequence (40 repetitions min-1). Alignment of the knee

extensions/flexions with the pulse sequence was guided

using a projected image of a vertical moving cursor set

to the frequency of 40 pulses min-1. The subjects were

required to follow the metronomic cursor using a second

vertical cursor under voluntary control of the subject. To

prevent displacement of the quadriceps VOI relative to

the surface coil and minimise adjacent muscles contrib-

uting to the exercise task, nylon straps were fastened

over the subject’s legs, hips and lower back. Power

output (W) was calculated continuously during the

quadriceps exercise as described previously (Barker et al.

2006).

Step-incremental test

Each subject completed an incremental test to exhaustion

inside the MR scanner for determination of the intra-

cellular threshold (IT) between the ratio of Pi and PCr

(ITPi/PCr). Following a 2 min baseline measurement per-

iod and starting with an initial basket load of 0.5 kg, an

incremental test was undertaken whereby the basket load

was increased in ‘‘steps’’ of 0.5 kg min-1 until subject

exhaustion occurred. This was typically within 7–12 min.

Using a plot of Pi/PCr versus power output at a sample

resolution of 30 s, each subject’s ITPi/PCr was identified

by two investigators as previously described (Barker et

al. 2006). The ITPi/PCr was defined as the power output

at which the first sudden and sustained increase in Pi/PCr

from the initial linear slope occurred (Marsh et al. 1991).

Using this technique with children, the ITPi/PCr typical

error over three repeat tests within our laboratory

resulted in a coefficient of variation of 10% (Barker

et al. 2006).

Constant work-rate exercise

Subsequently, each subject completed repeated constant-

load exercise transitions inside the MR scanner with the

work rate set to 80% of ITPi/PCr. The exercise protocol

consisted of a 2 min rest period for baseline measures, then

6 min constant work-rate quadriceps exercise followed by

6 min rest for assessment of the recovery dynamics.

Between two to four repeat constant work-rate exercise

transitions were performed on a given day, with at least

15 min rest given between each test. In total between three

and nine repeat transitions were performed by each subject.

31P-MRS measurement and quantification

A 1.5 T whole body MR scanner (Philips Gyroscan Intera)

was used to monitor the changes in quadriceps muscle

energetics. A 6 cm 31P transmit/receive surface coil was

fastened securely to the scanner bed and positioned under

the subject’s right quadriceps muscle at the midpoint

between the hip and knee joints. Gradient echo images

were initially acquired to ensure the quadriceps muscle was

positioned correctly relative to the coil. An automatic

shimming protocol using the 1H signal was undertaken

within a volume that defined the quadriceps muscle in

order to optimise the homogeneity of the local magnetic

field (with a typical line width of *14 Hz at FWHM),

thereby leading to maximum signal collection. Tuning and

matching of the coil was then performed to maximise

energy transfer between the coil and the muscle. 31P

spectra were obtained using an adiabatic pulse every 1.5 s,

with a spectral width of 1,500 Hz and 512 data points.

Phase cycling with four phase cycles was employed and

four measurements were performed, leading to spectra

acquired every 6 s to improve the signal to noise of the

profile yet provide a high PCr sampling resolution for

kinetic analysis. As the signal intensities for 31P spectra

were significantly saturated during the test protocol, T1

correction factors were determined during the rest phase

using a pulse interval of 20 s and applied to all peak

intensities. The assumption that the T1 relaxation time for

muscle PCr does not change from rest to steady-state

exercise has recently been confirmed during calf exercise

below the intracellular threshold (Cettolo et al. 2006).

The 31P spectra areas were quantified using a non-linear

least squares peak fitting software package (jMRUI Soft-

ware, version 2.0) employing the AMARES fitting

algorithm (Naressi et al. 2001; Vanhamme et al. 1997).

Spectral areas were fitted assuming prior knowledge of the

following peaks: Pi, phosphodiester, PCr, a-ATP (two

peaks, amplitude ratio 1:1), c-ATP (two peaks, amplitude

ratio 1:1) and b-ATP (three peaks, amplitude ratio 1:2:1).

Eur J Appl Physiol (2008) 102:727–738 729

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Changes in PCr were expressed as a percentage change

from baseline, set to 100%, using the mean PCr concen-

tration obtained during the 2 min rest period. Intracellular

pH was determined using the chemical shift of the Pi

spectral peak relative to the PCr peak:

pH ¼ 6:75þ logðr� 3:27Þ=ð5:96� rÞ

where r represents the chemical shift in parts per million

between the Pi and PCr resonance peaks (Taylor et al.

1983).

Upright cycling exercise

Peak oxygen uptake

Each child exercised to volitional exhaustion on a

mechanically braked cycle ergometer (Lode, Groningen,

Netherlands) in a temperature controlled laboratory (19-

22�C) for determination of the ventilatory threshold (VT).

Appropriate adjustments were made to the ergometer seat,

handlebar and pedal cranks for each subject. Peak p _VO2

was determined using a ramp incremental test initiated

from baseline pedalling with work rate increasing 10 W

min-1 until subject exhaustion occurred (typically within

8–12 min). Cycle cadence was maintained between 70 ±

5 rpm during the test. Heart rate was measured continu-

ously during the test using telemetry (Polar, Accurex Plus,

Kemple, Finland). In addition to subjective signs of

sweating, hyperpnea and intense effort, the attainment of a

peak p _VO2 was supported using the following criteria: (1) a

respiratory exchange ratio (RER) C 1.00, and/or (2)

attainment of a maximum heart rate C 90% of the age

predicted maximum. In all cases, each subject satisfied at

least one of these criteria. Peak p _VO2 was recorded as the

highest 10 s stationary average attained during the test. The

p _VO2 at which the VT occurred was established using a

combination of methods: (1) a plot of carbon dioxide

production ð _VCO2Þ against p _VO2 and (2) plots of the

ventilatory equivalents for p _VO2 and _VCO2 against time.

The reliability of these techniques with children in our

laboratory has previously being reported (Fawkner et al.

2002a).

Constant work-rate exercise

On subsequent visits, each child completed moderate

intensity constant work-rate exercise transitions on the

cycle ergometer. The protocol consisted of 6 min baseline

pedalling (*15 W) followed by 6 min constant-work

exercise at the subject’s target intensity (80% VT), and then

6 min baseline pedalling for recovery measurements.

Between two to three repeat transitions were performed on a

given day, with at least 30 min rest given between each test.

In total the children repeated between three and ten exercise

transitions for determination of the p _VO2 kinetic responses.

Measurement of breath by breath gas exchange

Breath by breath changes in gas exchange and ventilation

were measured using a standard algorithm (Beaver et al.

1973) and displayed using an on-line computer system. Gas

fractions of oxygen and carbon dioxide were drawn con-

tinuously from a low dead space (90 mL) mouthpiece-

turbine assembly and determined by mass spectroscopy

following calibration with gases of known concentration

(EX671, Morgan Medical, Kent, UK). Expired volume was

measured by a turbine flow meter (VMM-401, Interface

Associates, CA, USA), which was manually calibrated

over a range of flow rates using a 3-L syringe (Hans

Rudolph, Kansas City, MO, USA). Appropriate adjust-

ments for the capillary line gas transport and analyser

response time were made before gas concentrations and

volumes were aligned for calculation of breath by breath

changes in p _VO2 and _VCO2: All calibration procedures

were performed prior to each exercise test.

Kinetic parameter estimation

All PCr and _VO2 kinetic parameters were estimated using

an iterative least-squares non-linear regression procedure

along with their corresponding 95% confidence intervals

(95% CIs) to establish the precision of the point estimate

(GraphPad Prism, GraphPad Software, San Diego, CA,

USA).

Muscle PCr kinetic analysis

Each separate constant-work exercise transition was ini-

tially checked for PCr sample fluctuations that were greater

than ±4 standard deviations (SD) from a moving local

mean (Whipp and Rossiter 2005). Such large PCr fluctua-

tions are considered unrelated to the underlying

physiological profile and are likely to arise due to the low

signal to noise properties of the 31P-MRS technique and/or

the acute mistiming of a quadriceps contraction relative to

the pulse sequence during exercise. To enhance the under-

lying features of the PCr response profile for kinetic

parameter estimation, each subject’s repeat constant work-

rate exercise transitions were time aligned to the onset of

exercise (t = 0 s) and averaged, yielding a single PCr

response profile with a 6 s sample resolution (Rossiter et al.

730 Eur J Appl Physiol (2008) 102:727–738

123

2000). The resultant averaged PCr response profile for each

subject was normalised relative to the previous steady-state

baseline, using the average PCr value during the 2 min rest

or exercise period for determination of the onset and offset

kinetics, respectively.

The PCr exercise onset and offset responses were ini-

tially modelled using a single-exponential function

including a delay term:

PCrðtÞ ¼ DPCrss � ð1� e�ðt�TDÞ=sÞ

where PCrðtÞ;DPCrss; TD and s are the value of PCr at a

given time (t), the amplitude change in PCr from the

control baseline to a new steady-state at the onset or offset

of exercise, time delay, and the time constant of the

response, respectively. However, preliminary analyses

revealed the model delay term was not significantly

(P [ 0.05) greater than 0 s both at the onset and offset of

exercise. This indicates that the breakdown and resynthesis

of muscle PCr occurred with no detectable delay at both the

onset and offset of exercise. A single exponential model

with no delay term was therefore employed for all

analyses:

PCrðtÞ ¼ DPCrss � ð1� e�t=sÞ

Pulmonary _VO2 kinetic analysis

Each breath by breath constant work-rate profile was

interpolated on a second by second basis to provide a

uniform p _VO2 sample resolution. Following time align-

ment to the onset of exercise (t = 0 s), subjects’ repeat

constant work-rate exercise transitions were superimposed

and averaged to enhance the signal to noise ratio and

thereby the underlying features of the p _VO2 response

(Lamarra et al. 1987). The averaged p _VO2 profile for each

subject was subsequently normalised relative to the previ-

ous steady-state baseline as described above for the PCr

data analysis. As the accurate identification of the p _VO2

phase I-II transition using the RER, and end tidal (PCO2 and

PO2) dynamics was difficult due to the noise present in the

children’s response profile, the duration of phase I was

taken as 15 s for all subjects both at the onset and offset of

exercise (Hebestreit et al. 1998; Ozyener et al. 2001).

Characterisation of the phase II p _VO2 exercise onset and

offset responses was achieved by fitting a single-expo-

nential model including a delay term following the initial

15 s (to exclude phase I) of the exercise or recovery

regions (Whipp et al. 1982):

p _VO2ðtÞ ¼ Dp _VO2ss � ð1� e�ðt�TDÞ=sÞ

where p _VO2ðtÞ;Dp _VO2ss; TD and s represent the value of

p _VO2 at a given time (t), the amplitude change in p _VO2

from baseline to a new steady-state amplitude, time delay

and the time constant of the response, respectively.

Statistical analyses

To enhance the statistical confidence and sensitivity in

comparing the PCr and phase II p _VO2 kinetic time constants

either within or between subjects, only subjects with 95%

CIs equal to or less than ±7 s for all estimated time con-

stants were considered for analysis. Potential mean

differences in the estimated time constants were examined

using a two-way repeated measures ANOVA with response

variable (PCr vs. p _VO2Þ and exercise transition (onset vs.

offset) as the model factors. Changes in pH during the rest,

exercise and recovery regions of the constant work-rate

exercise tests were examined using a one-way repeated

measures ANOVA. The assumption of sphericity was ver-

ified using Mauchly’s test for all repeated measures

ANOVA models. Significant differences were followed-up

using planned pairwise comparisons employing the Bon-

ferroni alpha adjustment procedure. Single linear regression

was employed to investigate the association between the

PCr and phase II p _VO2 estimated time constants. Results

are presented as mean ± SD, with rejection of the null

hypothesis accepted at an alpha level of P = 0.05. Analyses

were performed using SPSS (version 11.0).

Results

Descriptive and exhaustive test responses

Out of the 18 subjects recruited, 12 children (six boys and

six girls) satisfied the time constant 95% confidence

interval criterion. The descriptive characteristics of these

12 subjects’ were: age (9.9 ± 0.3 years), body mass (33.8

± 6.7 kg) and stature (1.38 ± 0.06 m). The subjects’

average exercise responses during the incremental tests to

exhaustion for determination of the ITPi/PCr and VT are

presented in Table 1.

Constant work-rate exercise tests

The average work rate and pH responses during the quad-

riceps constant work-rate exercise are presented in Fig. 1. It

is clear from the power profile in Fig. 1a that work rate

increased and decreased instantaneously at the onset and

offset of exercise, thus highlighting the subjects’ high

compliance with the quadriceps ergometer. This resulted in

the attainment of an average power output of 7 ± 1 W,

which corresponds to 83 ± 13% of the ITPi/PCr. The

Eur J Appl Physiol (2008) 102:727–738 731

123

averaged pH response in Fig. 1b demonstrates that the

quadriceps constant work-rate exercise did not result in a

metabolic acidosis during the exercise tests. Compared to

rest (7.05 ± 0.03) a small but significant increase in quad-

riceps pH was noted during the last 2 min of constant-work

exercise (7.08 ±0.02, P = 0.033). pH during the final 2 min

of recovery (7.02 ± 0.02) was significantly lower than both

rest (P = 0.018) and exercise (P = 0.000). During the cycle

ergometry constant work-rate exercise, the children

achieved a p _VO2 amplitude of 0.90 ± 0.13 l min-1

(equivalent to 87 ± 11% of the p _VO2 at VT). In combi-

nation, these observations indicate that the imposed work

rates for both exercise modalities were below the metabolic

ITs (ITPi/PCr and VT) and considered to reflect ‘‘moderate’’

exercise.

On average, the children completed 6 ± 2 and 7 ± 2

repeat constant work-rate exercise transitions for determi-

nation of the PCr and p _VO2 kinetics responses

respectively. A typical PCr and p _VO2 response profile for a

child subject at the onset and offset of exercise is shown in

Fig. 2, along with the fitted non-linear regression model. In

all cases the single-exponential model provided an appro-

priate fit of the PCr and p _VO2 dynamics, as indicated by

the unsystematic and random residual profiles.

The estimated time constants and 95% CIs for the

subjects’ PCr and phase II p _VO2 responses at the onset

and offset of exercise are shown in Table 2. The two-

way repeated measures ANOVA revealed a significant

main effect for exercise transition (P = 0.001), with the

average PCr and phase II p _VO2 offset time constant (28

± 5 s) being slower than those at the onset (23 ± 4 s).

However, the follow-up pairwise comparisons revealed

no significant difference between the PCr onset and

offset kinetics (23 ± 5 vs. 28 ± 5 s, P = 0.064),

whereas the phase II p _VO2 onset kinetics were signifi-

cantly faster than those at the offset (23 ± 4 vs. 29

±5 s, P = 0.015). Comparison of the kinetic responses

between the PCr and phase II p _VO2 time constants

revealed similar values at the onset (PCr 23 ± 5 vs.

p _VO2 23� 4 s; P ¼ 1:000Þ and offset (PCr 28 ± 6 vs.

p _VO2 29� 7 s; P ¼ 1:000) of exercise. The 95% CIs for

the time constants at the onset and offset of exercise

were: PCr 5 ± 1 s (range 3–7 s), 5 ± 2 s (range 2–7 s);

p _VO2 5� 1 s (range 3–7 s), 5 ± 2 s (range 2–7 s),

respectively.

Within a given child’s response, the average difference

between the PCr and phase II p _VO2 time constants was 4 ±

4 s for the onset and offset kinetics profiles. Out of the 24

kinetic response profiles, the 95% CIs spanning the esti-

mated PCr and phase II p _VO2 time constants failed to

overlap in only two subjects (see footnote a in Table 2). A

line of identity plot showing the agreement between the

PCr and phase II p _VO2 time constants at the onset and

offset of exercise is presented in Fig. 3. Linear regression

analysis revealed no significant relationship between the

PCr and phase II p _VO2 time constants at the onset

(r = 0.225, P = 0.482) or offset (r = 0.298, P = 0.347) of

exercise. However, after pooling of the exercise onset and

Table 1 Subjects’ incremental test responses

Variable Group (n = 12)

Quadriceps ergometry

Peak power (W) 15 ± 3

ITPi/PCr (W) 9 ± 2

ITPi/PCr (% PP) 59 ± 9

Cycle ergometry

Peak p _VO2 (l min-1) 1.81 ± 0.23

p _VO2 at VT (l min-1) 1.05 ± 0.19

VT peak p _VO2 (%) 58 ± 8

Data are presented as mean ± SD

W, watts; IT, intracellular threshold; % PP, IT work rate expressed as

a % peak power; p _VO2; pulmonary oxygen uptake; VT, ventilatory

threshold

-120 0 120 240 360 480 600 720

6.95

7.00

7.05

7.10

7.15b

Rest RecoveryExercise

Time (s)

pHi

0

2

4

6

8

10a

Wor

k ra

te (

W)

Fig. 1 Average work-rate (a) and pH (b) responses during quadri-

ceps constant work-rate exercise. Dotted lines signify the onset and

offset of exercise. The solid horizontal line in a represents the group

average power output at the ITPi/PCr

732 Eur J Appl Physiol (2008) 102:727–738

123

offset responses to form a single data set, a significant

relationship between the PCr and phase II p _VO2 time

constants was observed (r = 0.459, P = 0.024). Moreover,

when the two responses that failed to demonstrate an

overlap in the 95% CIs for the PCr and phase II p _VO2 time

constants were removed from the analysis (see asterisk in

Fig. 3), a stronger correlation was observed (r = 0.711,

P = 0.000).

Discussion

This is the first study to investigate the kinetic association

between muscle PCr, a putative metabolic feedback con-

troller of m _VO2; and the phase II p _VO2 response at the

onset and offset of moderate intensity exercise in children.

The main finding of this study is that a close kinetic

coupling was evident between the quadriceps PCr and

cycling phase II p _VO2 responses both at the onset and

offset of exercise. This finding is supported by three lines

of reasoning. Firstly, at both the onset and offset of exer-

cise, the group average PCr and phase II p _VO2 time

constants maintained a striking equivalence despite the

recovery kinetics demonstrating a longer time constant

compared to the onset responses. Secondly, we observed an

overlapping between the PCr and phase II p _VO2 time

constants 95% CIs in *92% of the kinetic responses,

suggesting that within the sensitivity of the experimental

measures employed, the physiological responses are

mechanistically linked. Thirdly, a significant correlation

was observed between the PCr and phase II p _VO2 time

constants.

Collectively, these results are consistent with the control

of m _VO2 being functionally linked to the kinetics of PCr in

-10

-5

0

5

10

PCr onset kinetics

slaudiseR

-10

-5

0

5

10

PCr offset kinetics

slaudise

R

-120 -60 0 60 120 180 240 300 360-30

-20

-10

0

10

Time (s)

egnahc rC

P %

-0.2

-0.1

0.0

0.1

0.2

VO2 onset kinetics

slaudiseR

-0.2

-0.1

0.0

0.1

0.2

VO 2 offset kinetics

sla

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-120 -60 0 60 120 180 240 300 360-0.2

0.0

0.2

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Time (s)

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-120 -60 0 60 120 180 240 300 360-0.8

-0.6

-0.4

-0.2

-0.0

0.2

Time (s)

OV

2)ni

m/l(

-120 -60 0 60 120 180 240 300 360-10

0

10

20

30

Time (s)

egnahc rC

P %

Fig. 2 PCr and p _VO2 kinetics

at the onset and offset of

exercise in a child subject. An

example PCr and p _VO2 taken

from subject number 3. The

kinetic parameters are shown in

Table 2. The data presented are

the result of time-aligning and

averaging six repeat exercise

transitions for determination of

the PCr kinetics, and ten for the

p _VO2 kinetics. The continuousline represents the fitted single-

exponential function, with the

resulting residuals displayed

above. Vertical dotted linessignify either the onset or offset

of exercise

Eur J Appl Physiol (2008) 102:727–738 733

123

children, as previously demonstrated in adults (Barstow

et al. 1994; Rossiter et al. 1999), and implies that an age-

related modulation of the dynamics of the creatine kinase

reaction and/or build-up of metabolic feedback controllers,

may explain the faster phase II p _VO2 found in children

compared to adults (Fawkner and Armstrong 2004;

Fawkner et al. 2002b; Williams et al. 2001).

At the onset of moderate intensity exercise the rise of

m _VO2 occurs immediately, i.e. without delay, and is well

characterised by a single exponential function (Behnke et

al. 2002). The modelling simulations by Barstow et al.

(1990) predict that for a variety of muscle blood flow and

venous volume parameters, the underlying m _VO2 response

is expressed to within ±10% through the phase II p _VO2

time constant at the onset of moderate exercise. This asso-

ciation has been experimentally confirmed using the direct

Fick technique to determine m _VO2 across the contracting

thigh muscle during upright cycle ergometry in humans

(Grassi et al. 1996). In contrast, a recent computerised

simulation found a significant dissociation between the

mean response time for m _VO2 and p _VO2 at the onset of

moderate (13 vs. 65 s), heavy (13 vs. 100 s) and very heavy

(15 vs. 82 s) cycling exercise in adolescent boys (Lai et al.

2006). However, these results must be interpreted with

caution as no attempt was made to characterise the phase II

kinetics from the p _VO2 response profile, which clearly

resulted in the development of the p _VO2 slow component

during heavy and very heavy exercise. In addition, no slow

component was evident for the simulated m _VO2 response

during heavy or very heavy exercise, which is in contra-

diction with human studies (Koga et al. 2005).

The first-order model of metabolic control predicts an

inverse proportional relationship between the dynamics of

muscle PCr and m _VO2 at the onset and offset of submaxi-

mal exercise (Mahler 1985; Meyer 1988). One would

therefore expect a similar kinetic coupling (±10%) between

PCr and phase II p _VO2 to be evident if the kinetics of PCr

was the principal controller of m _VO2: In support of this

model, Rossiter et al. (1999, 2002) established that the fall

in muscle PCr and rise in phase II p _VO2 were in agreement

to within ±10% at the onset and offset of moderate intensity

knee-extensor exercise in adult men. However, the associ-

ation between the muscle PCr and phase II p _VO2 kinetic

responses at the onset and offset of exercise in the present

study demonstrates a lower level of agreement in children

(±18%, see Fig. 3). This, however, appears to be a conse-

quence of the inherently ‘fast’ PCr kinetics in children

(*25 s) compared to adults (*35 s, Rossiter et al. 1999),

as the PCr time constant for a given subject was on average

to within ±4 s to that of the phase II p _VO2 response both at

the onset and offset of exercise. Such an association is in

direct agreement with the results obtained by Rossiter et al.

(1999, 2002). In contrast, McCreary et al. (1996) found an

Table 2 Subjects’ PCr and _VO2 kinetic responses at the onset and

offset of exercise

Subject Onset kinetics Offset kinetics

sPCr CIs sp _VO2 CIs sPCr CIs sp _VO2 CIs

2 25 6 21 5 24 7 26 3

3 32 6 27 5 33 6 37 4

4 15a 5 28a 5 31 7 33 7

5 22 5 19 4 22 4 22 5

9 22 5 24 5 35a 7 20a 2

10 28 5 25 7 26 4 27 6

13 27 6 25 5 25 5 31 5

14 19 5 24 5 31 6 31 6

15 25 5 19 4 31 6 33 7

16 19 4 17 4 25 5 33 5

17 22 3 27 3 28 2 28 3

18 14 3 20 5 21 5 25 4

Mean 23 5 23 5 28 5 29 5

SD 5 1 4 1 5 2 5 2

The time constant (s) and 95% confidence intervals (CIs, ±s) values

are reported in seconds. Only subjects with 95 % confidence intervals

equal to or less than ±7s are shown

PCr, phosphocreatine; p _VO2; pulmonary oxygen uptakea Subject’s PCr and p _VO2 profile where the 95% CIs around the time

constant failed to overlap

0 10 20 30 40 500

10

20

30

40

50

20%

20%

10%

10%

0%

*

*

r=0.459, P=0.024

VO2 time constant (s)

)s( tnatsn

oc emit r

CP

Fig. 3 PCr and p _VO2 line of identity plot. PCr and p _VO2 time

constants at the onset (open circle) and offset (closed circle) of

moderate intensity exercise for all 12 subjects. When the two

responses where the PCr and p _VO2 time constant’s confidence

intervals failed to overlap were removed from the analysis (asterisk),

the Pearson correlation increased to r = 0.711, P = 0.000

734 Eur J Appl Physiol (2008) 102:727–738

123

average dissociation of approximately ±16 s (±40%)

between the phase II p _VO2 and PCr time constants during

the onset and offset of submaximal plantar-flexor exercise.

However, a limitation of this study was the poor level of

confidence in which the phase II p _VO2 time constant was

obtained (95% CIs ±10 s), which confines interpretation of

these data with respect to the role muscle PCr may have in

controlling m _VO2:

Collectively, the results from the present study and those

previously published in adults (Barstow et al. 1994;

Rossiter et al. 1999, 2002) highlight the prominent role

muscle PCr plays in modulating the dynamics of m _VO2

during a ‘‘step’’ change in metabolic rate. These results are

consistent with the first-order model of metabolic control

proposed by Meyer (1988), an appreciation of which, may

provide greater insight into the mechanisms underlying the

faster p _VO2 kinetics found in children compared to adults

(Fawkner and Armstrong 2004; Fawkner et al. 2002b;

Williams et al. 2001). The model predicts that the kinetics

of muscle PCr will be faster in muscle with a higher

mitochondrial density and/or content of mitochondrial

enzymes (‘‘resistor’’), and slower in muscle with a higher

PCr content (‘‘capacitance’’, Meyer 1988).

There is some evidence supporting a decrease in the

activity of oxidative enzymes in 13–15 year-old children

through to adulthood (Haralambie 1982), although the data

available are equivocal (Berg et al. 1986; Kaczor et al.

2005). Moreover, the relative density of mitochondria in 6-

year old boys’ and girls’ vastus lateralis muscle is not

appreciably different from that recorded in sedentary adults

(Bell et al. 1980). There are limited data showing a pro-

gressive increase in the rectus femoris muscle PCr stores

(equivalent to the model ‘‘capacitance’’) in a small group of

boys between the ages of 11 and 16 years (Eriksson and

Saltin 1974), which may account for the faster phase II

p _VO2 kinetics found in children compared to adults.

However, adding further complexity are studies showing

children to have faster (Taylor et al. 1997) or similar

(Kuno et al. 1995) recovery of muscle PCr compared to

adults following a progressive maximal exercise test.

Clearly, further research is needed to examine the muscle

PCr kinetic responses between children and adults in var-

ious exercise intensity domains before any firm conclusions

can be drawn with regard to the mechanisms underlying the

child–adult differences in p _VO2 kinetics.

An interesting finding in the present study was the lack

of symmetry between the kinetic responses at the onset and

offset of exercise, which was consistent across both exer-

cise modalities and response variables. While the muscle

PCr and p _VO2 dynamics retained single exponential

properties, the recovery time constants for both variables

were *25% longer than the onset kinetics. Interestingly,

Rossiter et al. (2002) also demonstrated a close identity

between the muscle PCr and phase II p _VO2 kinetics but an

even greater slowing of the time constants during the offset

(*50 s) compared to the onset (*35 s) of moderate

intensity knee-extensor exercise. The presence of asym-

metrical time constants implies that the mechanism(s)

controlling the dynamics of m _VO2 differ at the onset and

offset of exercise. Candidate mechanisms may include a

greater sensitivity of metabolic control to O2 delivery

during recovery compared to the onset of exercise (Haseler

et al. 1999, 2004), a pH dependent effect on the creatine

kinase equilibrium favouring PCr breakdown thereby

slowing the recovery of PCr (McMahon and Jenkins 2002),

and/or a direct effect of acidosis limiting the rate of

mitochondrial oxidative phosphorylation and thereby the

resynthesis of muscle PCr (Jubrias et al. 2003). In contrast,

using a theoretical model of energy balance, Kushermick

(1998) attributed the asymmetric behaviour of the muscle

PCr kinetics to changes in the flux of creatine kinase at the

onset (forward flux) and offset (backward flux) of exercise.

However, the magnitude of difference between the onset

and offset time constants in the present study (*5 s) lies

within the boundaries of confidence surrounding the time

constants (95% CIs *±5 s). This reduces the certainty

with which the muscle PCr and phase II p _VO2 kinetic

responses can be described as asymmetrical in children and

warrants further investigation.

Methodological considerations

The results of the current study provide support for the

hypothesis that muscle PCr, or some related function,

controls the respective rise and fall in m _VO2 at the onset

and offset of exercise in children. However, a number of

methodological considerations require discussion. While it

has been established that the phase II p _VO2 response pro-

vides a close reflection of the rise in m _VO2 during

moderate intensity work-rates in adults (Grassi et al. 1996),

this association has yet to be verified in children, largely

due to ethical and methodological constraints. In particular,

factors such as the cardiac output response dynamics,

muscle-lung transit time and the utilisation of body O2

stores during the non-steady state, all have the potential to

dissociate the phase II p _VO2 response from the kinetics of

m _VO2 during the non steady-state (Barstow et al. 1990;

Francescato et al. 2003). However, given the close kinetic

coupling between phase II p _VO2 and muscle PCr in *92%

of the responses in the present study, the latter of which is

routinely used as surrogate of m _VO2 (e.g. Barstow et al.

1994; McCreary et al. 1996), the assumption that phase II

p _VO2 provides a reflection of the dynamics of m _VO2

appears appropriate in children and is within acceptable

limits.

Eur J Appl Physiol (2008) 102:727–738 735

123

The kinetics of phase II p _VO2 in the present study were

determined on a breath by breath basis using a conven-

tional algorithm which has been criticized for its failure to

account for changes in alveolar O2 stores (Cautero et al.

2002). However, despite intense investigation, the most

appropriate algorithm to determine ‘true’ alveolar p _VO2

during exercise remains controversial, largely due to the

various assumptions imposed by each technique. Indeed, a

recent study highlighted the considerable variability

(*10 s or *25%) across the proposed algorithms for

determination of the phase II time constant within a group

of adults (Cautero et al. 2002). Although a technique that

measures breath-by-breath p _VO2 at the alveolar level has

been developed (Aliverti et al. 2004), its comparison

against the traditionally used algorithms in humans, and in

particular children, has yet to be investigated. Therefore,

the degree to which the phase II p _VO2 time constant in the

present study represents that of ‘‘true’’ alveolar p _VO2

dynamics is unknown.

The use of different exercise modalities to investigate

the kinetic association between p _VO2 and muscle PCr is a

limitation of the present study, as inter-modality differ-

ences in body posture, muscle recruitment patterns and

muscle contraction type, and muscle mass may result in

fundamentally different p _VO2 (and presumably muscle

PCr) kinetics (Jones and Burnley 2005). Although the

simultaneous measurement of p _VO2 alongside PCr during

the quadriceps exercise would allay such concerns (Whipp

et al. 1999), this technique has limited application in

children. In particular, the p _VO2 amplitude during single-

legged quadriceps exercise is insufficient to achieve an

adequate signal to noise ratio to determine the p _VO2

kinetic response parameters to within an acceptable level of

precision in children. As a consequence, the measurement

of p _VO2 during upright cycling was necessary in order to

determine the phase II p _VO2 time constant to within an

acceptable level of confidence in order to infer control

mechanisms in relation to the PCr dynamics, i.e. 95% CIs

±5 s on average.

At the onset of moderate intensity upright cycling, the

phase II p _VO2 kinetics have been demonstrated to be

similar to that determined during moderate knee-extensor

exercise both in the upright (Koga et al. 2005) and prone

body positions (Rossiter et al. 2000). As highlighted in the

introduction, this latter finding is in conflict with the slower

p _VO2 kinetics found during moderate intensity cycling in

the supine compared to the upright body position (Hughson

et al. 1993). Moreover, Barstow et al. (1994) found no

difference between the kinetics of muscle PCr determined

during plantar-flexor exercise when compared to the

kinetics of _VO2 measured during upright cycling. Collec-

tively, these data therefore support the notion that body

position is not modulating the kinetics of PCr or _VO2

determined during prone quadriceps exercise in the current

study.

As upright cycling predominantly involves concentric

muscle contractions, an important consideration is the

degree to which the concentric and eccentric components

of the quadriceps exercise may have influenced the kinetics

of muscle PCr in the present study. While the steady-state

PCr cost of muscle contraction is twofold higher during

5 min of concentric exercise at 30% maximum voluntary

contraction when compared to eccentric exercise (Ryschon

et al. 1997), the recovery of muscle PCr has been shown to

be independent of muscle contraction type (Combs et al.

1999). Whether this is also the case for PCr kinetics during

concentric and eccentric exercise at the onset of exercise is

currently unknown. However, Perrey et al. (2001) observed

no significant differences in the phase II p _VO2 time con-

stant between high intensity eccentric and moderate

intensity concentric cycling exercise. This indirectly sug-

gests that the kinetics of muscle PCr may be similar

between concentric and eccentric exercise, although this

requires clarification from future studies. In addition, the

extrapolation of results obtained from studies using ‘‘iso-

lated’’ concentric and eccentric muscle contraction regimes

to the ‘‘mixed’’ concentric and eccentric exercise in the

current study is difficult.

Given the reasoning above, it appears that the PCr and

phase II p _VO2 kinetics during prone quadriceps exercise

and upright cycling are similar regardless of inter-modality

differences in body posture, muscle recruitment and con-

traction regimes, and muscle mass. For these reasons, the

current experimental design appears appropriate to inves-

tigate the kinetic association between quadriceps muscle

PCr and upright cycling p _VO2 dynamics during moderate

exercise and importantly, with high statistical confidence in

the kinetic parameters.

Practical implications

The recovery kinetics of muscle PCr (determined using31P-MRS) is routinely employed as a non-invasive and

valid measure of the muscle oxidative capacity (McCully

et al. 1993; Paganini et al. 1997). However, this technique

is very expensive, time consuming and often inaccessible

to paediatric physiology research groups. The close kinetic

coupling between the p _VO2 and PCr kinetic profiles at the

onset and offset of exercise in the present study show that

in majority of cases (*92%), similar information can be

provided by characterising the phase II p _VO2 recovery

time constant following moderate upright cycling exercise

in young people. These results therefore support the use of

the phase II p _VO2 time constant as a proxy measure of the

muscle PCr kinetics, which can for example, be used to

736 Eur J Appl Physiol (2008) 102:727–738

123

investigate the influence of maturity and physical training

on the muscles’ O2 capacity in young people.

Conclusion

This is the first study to quantify the kinetics of quadri-

ceps muscle PCr and cycling p _VO2 during moderate

intensity exercise in children. The kinetic changes in

muscle PCr were closely coupled (to within ±4 s) with

the phase II p _VO2 response both at the onset and offset of

exercise and are therefore consistent with m _VO2 being

controlled either directly or indirectly by the dynamics of

muscle PCr during the non-steady-state in children. These

data support the theory that an age-related modulation of

the putative phosphate linked controller(s) of m _VO2 may

explain the faster _VO2 kinetics found in children com-

pared to adults.

Acknowledgments We would like to express our gratitude to the

children and staff from Wynstream Primary School for participation

in this project. The technical expertise provided by David Childs in

designing the quadriceps ergometer and analysis software was most

appreciated. Grants: this project was funded by the Darlington Trust.

References

Aliverti A, Kayser B, Macklem PT (2004) Breath-by-breath assess-

ment of alveolar gas stores and exchange. J Appl Physiol

96:1464–1469

Barker A, Welsman J, Welford D, Fulford J, Williams C, Armstrong

N (2006) Reliability of 31P-magnetic resonance spectroscopy

during an exhaustive incremental exercise test in children. Eur J

Appl Physiol 98:556–565

Barstow TJ, Lamarra N, Whipp BJ (1990) Modulation of muscle and

pulmonary O2 uptakes by circulatory dynamics during exercise.

J Appl Physiol 68:979–989

Barstow TJ, Buchthal SD, Zanconato S, Cooper DM (1994) Muscle

energetics and pulmonary oxygen uptake kinetics during mod-

erate exercise. J Appl Physiol 77:1742–1749

Beaver WL, Wasserman K, Whipp BJ (1973) On-line computer

analysis and breath-by-breath graphical display of exercise

function tests. J Appl Physiol 34:128–132

Behnke BJ, Barstow TJ, Kindig CA, McDonough P, Musch TI, Poole

DC (2002) Dynamics of oxygen uptake following exercise onset

in rat skeletal muscle. Respir Physiol Neurobiol 133:229–239

Bell RD, MacDougall JD, Billeter R, Howard H (1980) Muscle fiber

types and morphometric analysis of skeletal muscle in 6-year-old

children. Med Sci Sports Exerc 12:28–31

Berg A, Kim SS, Keul J (1986) Skeletal muscle enzyme activities in

healthy young subjects. Int J Sport Med 7:236–239

Cautero M, Beltrami AP, di Prampero PE, Capelli C (2002) Breath-

by-breath alveolar oxygen transfer at the onset of step exercise in

humans: methodological implications. Eur J Appl Physiol

88:203–213

Cettolo V, Piorico C, Francescato MP (2006) T(1) measurement of

(31)P metabolites at rest and during steady-state dynamic

exercise using a clinical nuclear magnetic resonance scanner.

Magn Reson Med 55:498–505

Combs CA, Aletras AH, Balaban RS (1999) Effect of muscle action

and metabolic strain on oxidative metabolic responses in human

skeletal muscle. J Appl Physiol 87:1768–1775

Eriksson BO, Saltin B (1974) Muscle metabolism during exercise in

boys aged 11–16 years compared to adults. Acta Paediatrica

Belgica 28:257–265

Fawkner SG, Armstrong N (2004) Longitudinal changes in the kinetic

response to heavy-intensity exercise in children. J Appl Physiol

97:460–466

Fawkner SG, Armstrong N, Childs DJ, Welsman JR (2002a)

Reliability of the visually identified ventilatory threshold and

v-slope in children. Pediatr Exerc Sci 14:181–192

Fawkner SG, Armstrong N, Potter CR, Welsman JR (2002b) Oxygen

uptake kinetics in children and adults after the onset of

moderate-intensity exercise. J Sports Sci 20:319–326

Francescato MP, Cettolo V, Di Prampero PE (2003) Relationships

between mechanical power, O(2) consumption, O(2) deficit and

high-energy phosphates during calf exercise in humans. Pflugers

Arch 445:622–628

Grassi B (2005) Delayed metabolic activation of oxidative phosphor-

ylation in skeletal muscle at exercise onset. Med Sci Sports

Exerc 37:1567–1573

Grassi B, Poole DC, Richardson RS, Knight DR, Erickson BK,

Wagner PD (1996) Muscle O2 uptake kinetics in humans:

implications for metabolic control. J Appl Physiol 80:988–998

Haralambie G (1982) Enzyme activities in skeletal muscle of 13–

15 years old adolescents. Bull Europ Physiopath Resp 18:65–74

Haseler LJ, Hogan MC, Richardson RS (1999) Skeletal muscle

phosphocreatine recovery in exercise-trained humans is depen-

dent on O2 availability. J Appl Physiol 86:2013–2018

Haseler LJ, Kindig CA, Richardson RS, Hogan MC (2004) The role

of oxygen in determining phosphocreatine onset kinetics in

exercising humans. J Physiol 558:985–992

Hebestreit H, Kriemler S, Hughson RL, Bar-Or O (1998) Kinetics of

oxygen uptake at the onset of exercise in boys and men. J Appl

Physiol 85:1833–1841

Hughson RL (2005) Regulation of VO2 on-kinetcs by O2 delivery. In:

Jones AM, Poole DC (eds) Oxygen uptake kinetics in sport,

exercise and medicine. Routledge, London, pp 185–211

Hughson RL, Cochrane JE, Butler GC (1993) Faster O2 uptake

kinetics at onset of supine exercise with than without lower body

negative pressure. J Appl Physiol 75:1962–1967

Jones AM, Burnley M (2005) Effect of exercise modality on VO2

kinetics. In: Jones AM, Poole DC (eds) Oxygen uptake kinetics

in sport, exercise and medicine. Routledge, London, pp 95–114

Jubrias SA, Crowther GJ, Shankland EG, Gronka RK, Conley KE

(2003) Acidosis inhibits oxidative phosphorylation in contract-

ing human skeletal muscle in vivo. J Physiol 553:589–599

Kaczor JJ, Ziolkowski W, Popinigis J, Tarnopolsky MA (2005)

Anaerobic and aerobic enzyme activities in human skeletal

muscle from children and adults. Pediatr Res 57:331–335

Kindig CA, Howlett RA, Stary CM, Walsh B, Hogan MC (2005)

Effects of acute creatine kinase inhibition on metabolism and

tension development in isolated single myocytes. J Appl Physiol

98:541–549

Koga S, Poole DC, Shiojiri T, Kondo N, Fukuba Y, Miura A, Barstow

TJ (2005) Comparison of oxygen uptake kinetics during knee

extension and cycle exercise. Am J Physiol Regul Integr Comp

Physiol 288:R212–R220

Kuno S, Takahashi H, Fujimoto K, Akima H, Miyamaru M, Nemoto

I, Itai Y, Katsuta S (1995) Muscle metabolism during exercise

using phosphorus-31 nuclear magnetic resonance spectroscopy

in adolescents. Eur J Appl Physiol Occup Physiol 70:301–304

Kushmerick MJ (1998) Energy balance in muscle activity: simula-

tions of ATPase coupled to oxidative phosphorylation and to

Eur J Appl Physiol (2008) 102:727–738 737

123

creatine kinase. Comp Biochem Physiol B Biochem Mol Biol

120:109–123

Lai N, Dash RK, Nasca MM, Saidel GM, Cabrera ME (2006)

Relating pulmonary oxygen uptake to muscle oxygen consump-

tion at exercise onset: in vivo and in silico studies. Eur J Appl

Physiol 97:380–394

Lamarra N, Whipp BJ, Ward SA, Wasserman K (1987) Effect of

interbreath fluctuations on characterising exercise gas exchange

kinetics. J Appl Physiol 62:2003–2012

Mahler M (1985) First-order kinetics of muscle oxygen consumption,

and an equivalent proportionality between QO2 and phosphoryl-

creatine level. Implications for control of respiration. J Gen

Physiol 86:135–165

Marsh GD, Paterson DH, Thompson RT, Driedger AA (1991)

Coincident thresholds in intracellular phosphorylation potential

and pH during progressive exercise. J Appl Physiol 71:1076–

1081

McCreary CR, Chilibeck GD, Marsh GD, Paterson DH, Cunningham

DA, Thompson CH (1996) Kinetics of pulmonary oxygen uptake

and muscle phosphates during moderate-intensity calf exercise. J

Appl Physiol 81:1331–1338

McCully KK, Fielding RA, Evans WJ, Leigh Jr JS, Posner JD (1993)

Relationships between in vivo and in vitro measurements of

metabolism in young and old human calf muscles. J Appl

Physiol 75:813–819

McMahon S, Jenkins D (2002) Factors affecting the rate of

phosphocreatine resynthesis following intense exercise. Sports

Med 32:761–784

Meyer RA (1988) A linear model of muscle respiration explains

monoexponential phosphocreatine changes. Am J Physiol Cell

Physiol 254:C548–C553

Naressi A, Couturier C, Devos JM, Janssen M, Mangeat C, de Beer R,

Graveron-Demilly D (2001) Java-based graphical user interface

for the MRUI quantitation package. MAGMA 12:141–152

Ozyener F, Rossiter HB, Ward SA, Whipp BJ (2001) Influence of

exercise intensity on the on- and off-transient kinetics of

pulmonary oxygen uptake in humans. J Physiol 533:891–902

Paganini AT, Foley JM, Meyer RA (1997) Linear dependence of

muscle phosphocreatine kinetics on oxidative capacity. Am J

Physiol Cell Physiol 272:C501–C510

Perrey S, Betik A, Candau R, Rouillon JD, Hughson RL (2001)

Comparison of oxygen uptake kinetics during concentric and

eccentric cycle exercise. J Appl Physiol 91:2135–2142

Rossiter HB (2000) Dynamic inter-relationships between intramus-

cular high-energy phosphate metabolism and pulmonary oxygen

uptake during exercise in humans, PhD Thesis, University of

London

Rossiter HB, Ward SA, Doyle VL, Howe FA, Griffiths JR, Whipp BJ

(1999) Inferences from pulmonary O2 uptake with respect to

intramuscular [phosphocreatine] kinetics during moderate exer-

cise in humans. J Physiol 518:921–932

Rossiter HB, Howe FA, Ward SA, Kowalchuk JM, Griffiths JR,

Whipp BJ (2000) Intersample fluctuations in phosphocreatine

concentration determined by 31P-magnetic resonance spectros-

copy and parameter estimation of metabolic responses to

exercise in humans. J Physiol 528:359–369

Rossiter HB, Ward SA, Kowalchuk JM, Howe FA, Griffiths JR,

Whipp BJ (2002) Dynamic asymmetry of phosphocreatine

concentration and O(2) uptake between the on- and off-transients

of moderate- and high-intensity exercise in humans. J Physiol

541:991–1002

Rossiter HB, Howe FA, Ward SA (2005) Intramuscular phosphate

and pulmonary VO2 kinetics during exercise: implications for

control of skeletal muscle oxygen consumption. In: Jones AM,

Poole DC (eds) Oxygen uptake kinetics in sport, exercise and

medicine. Routledge, London, pp 154–184

Ryschon TW, Fowler MD, Wysong RE, Anthony A, Balaban RS

(1997) Efficiency of human skeletal muscle in vivo: comparison

of isometric, concentric, and eccentric muscle action. J Appl

Physiol 83:867–874

Taylor DJ, Bore PJ, Styles P, Gadian DG, Radda GK (1983)

Bioenergetics of intact human muscle: a 31P nuclear magnetic

resonance study. Mol Biol Med 1:77–94

Taylor DJ, Kemp GJ, Thompson CH, Radda GK (1997) Ageing:

effects on oxidative function of skeletal muscle in vivo. Mol Cell

Biochem 174: 321–324

Tschakovsky ME, Hughson RL (1999) Interaction of factors deter-

mining oxygen uptake at the onset of exercise. J Appl Physiol

86:1101–1113

Vanhamme L, van den Boogaart A, Van Huffel S (1997) Improved

method for accurate and efficient quantification of MRS data

with use of prior knowledge. J Magn Reson 129:35–43

Walsh B, Tonkonogi M, Soderlund K, Hultman E, Saks V, Sahlin K

(2001) The role of phosphorylcreatine and creatine in the

regulation of mitochondrial respiration in human skeletal

muscle. J Physiol 537:971–978

Whipp BJ, Rossiter HB (2005) The kinetics of oxygen uptake:

physiological inferences from the parameters. In: Jones AM,

Poole DC (eds) Oxygen uptake kinetics in sport, exercise and

medicine. Routledge, London, pp 62–94

Whipp BJ, Ward SA, Lamarra N, Davis JA, Wasserman K (1982)

Parameters of ventilatory and gas exchange dynamics during

exercise. J Appl Physiol 52:1506–1513

Whipp BJ, Rossiter HB, Ward SA, Avery D, Doyle VL, Howe FA,

Griffiths JR (1999) Simultaneous determination of muscle 31P

and O2 uptake kinetics during whole body NMR spectroscopy. J

Appl Physiol 86:742–747

Williams CA, Carter H, Jones AM, Doust JH (2001) Oxygen uptake

kinetics during treadmill running in boys and men. J Appl

Physiol 90:1700–1706

738 Eur J Appl Physiol (2008) 102:727–738

123