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Human Performance Laboratory, Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, Texas 78712
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
ACKNOWLEDGEMENTS
FOOTNOTES
REFERENCES
ABSTRACT 
Turley, Kenneth R., and Jack H. Wilmore. Cardiovascular
responses to treadmill and cycle ergometer exercise in children and
adults. J. Appl. Physiol. 83(3):
948-957, 1997.
This study was conducted to determine whether
submaximal cardiovascular responses at a given rate of work are
different in children and adults, and, if different, what mechanisms
are involved and whether the differences are exercise-modality
dependent. A total of 24 children, 7 to 9 yr old, and 24 adults, 18 to
26 yr old (12 males and 12 females in each group), participated in both
submaximal and maximal exercise tests on both the treadmill and cycle
ergometer. With the use of regression analysis, it was determined that
cardiac output (
) was significantly lower
(P 
0.05) at a given
O2 consumption level
(
O2, l/min) in boys vs. men
and in girls vs. women on both the treadmill and cycle ergometer. The
lower in the children was compensated for by a
significantly higher (P 0.05)
arterial-mixed venous O2
difference to achieve the same or similar
O2. Furthermore, heart rate
and total peripheral resistance were higher and stroke volume was lower
in the children vs. in the adult groups on both exercise modalities.
Stroke volume at a given rate of work was closely related to left
ventricular mass, with correlation coefficients ranging from
r = 0.89-0.92 and
r = 0.88-0.93 in the males and females, respectively. It was concluded that submaximal cardiovascular responses are different in children and adults and that these differences are related to smaller hearts and a smaller absolute amount
of muscle doing a given rate of work in the children. The differences
were not exercise-modality dependent.
submaximal exercise; cardiac output; blood pressure; left
ventricular mass
INTRODUCTION IT HAS BEEN REPORTED that both boys (2, 16, 30, 32, 42,
52) and girls (30, 32, 42, 52) have a lower cardiac output
( A close examination reveals a number of weaknesses in these studies
that have reported comparisons of cardiovascular responses to
submaximal exercise in children vs. adults. First, many of the studies did not collect data on adults but instead have compared data on children with adult values reported elsewhere (2, 16-18, 30).
In addition, of all the studies that have compared submaximal cardiovascular responses with exercise in children and adults, only one
used children with a mean age younger than 10 yr, and that study used
only three boys with a mean age of 9.7 yr (19). Furthermore, all of
these studies have used the cycle ergometer as the exercise modality.
Whether cardiovascular response differences between children and adults
are attenuated, exacerbated, or the same on the treadmill is unknown.
Finally, none of these studies has attempted to determine the mechanism
behind the differences that are generally reported between children and
adults. Only one study measured submaximal blood pressure (BP) (16),
and none attempted to estimate heart size to see whether differences may be related to either peripheral resistance or differing heart sizes
of the subjects.
The present study investigated cardiovascular responses to submaximal
exercise using both the cycle ergometer and treadmill with 7- to
9-yr-old children and young adults to determine the differences, if
any, in responses between boys vs. men and girls vs. women and to
determine whether these relationships are affected by the exercise
modality. Left ventricular mass (LVM) and submaximal BP were measured
to define the mechanism for any differences that may exist.
METHODS
) than adults at a given absolute submaximal rate of work or O2 uptake
(O2). This lower
at a given submaximal rate of work is attributed to
a lower stroke volume (SV), which is only partially compensated for by
a higher heart rate (HR). A higher arterial-mixed venous
O2 difference
(a-
)O2
in children then compensates for their lower to
achieve the same or similar O2 (2, 15, 32, 42).
Although the majority of studies have reported a lower
in children vs. adults at a given submaximal rate of
work, others have reported similar values
for adults and children (17-19).
O2 max) was
determined(random draw of either treadmill or cycle ergometry),
anthropometric measurements were obtained, and a 10-min accommodation
period on the treadmill [5 min at both 3.0 and 5.0 miles/h
(mph)] was provided. On the second visit for children, a
submaximal steady-state 4.0-mph walk on the treadmill and a second
maximal test on the ergometer (whichever was not used in the first
test) were conducted, with the tests being separated by 20-30
min. On the second visit for adults, only the second maximal test was conducted. On each of the next four visits, one of
four randomized submaximal steady-state exercise tests was conducted (2 cycle and 2 treadmill tests). Both children and adults exercised at
three different submaximal rates of work on each ergometer so that
accurate regression lines could be developed through as wide a range of
O2 values as possible.
On the last visit, M-mode echocardiography was used to determine the
subject's left ventricular dimensions at rest, just before exercise,
and a blood sample was drawn immediately after exercise (optional for
the children) to determine hemoglobin concentration ([Hb])
for use in the calculation of . Testing for each
subject was conducted within a 2- to 4-wk period, with a minimum of 24 h between tests.
Maximal-exercise tests.
Before the commencement of the maximal tests on the motor-driven
treadmill (Quinton Q65), both children and adults practiced walking and
running for 3-5 min, after which the protocol began. For children,
the protocol began with walking at 3.0 mph at 0% grade for 1 min, with
a 2.5% increase in grade at the beginning of both
minute 2 and minute
3. At the start of minute
4, the speed was increased to 5.0 mph, with an
additional increase to 5.5 mph at the beginning of
minute 5. Grade was then increased
2.5% at the start of both minutes 6 and 7, after which the speed was
increased 0.5 mph each minute until exhaustion. For safety purposes, a
spotter was positioned behind the children during maximal treadmill
testing. The adult protocol began with walking at 3.0 mph for 1 min,
with a subsequent 0.5 mph increase in speed at the beginning of every minute up to 6.5-7.5 mph, dependent on the fitness level of the subject. Once maximal speed was reached, grade was subsequently increased 2.5% at the start of every minute until exhaustion.
Maximal tests on the electronically braked cycle ergometer (Ergoline
800S, SensorMedics) used different continuous incremental protocols for
children and adults. The children performed unloaded cycling at
65-75 revolutions/min (rpm) for the first minute, after which the
work rate was increased to 20 W for the second minute, and then
increased by 15-W increments at the start of every minute until
exhaustion. The adults pedaled at 65-75 rpm at 50 W for the first
minute, after which the work rate was increased by 25 W at the
beginning of every minute until exhaustion. The cycle ergometer was
calibrated daily.
During maximal tests, the subjects exercised to volitional fatigue.
Tests were considered maximal in children when at least two of the
following criteria were achieved: 1)
failure to maintain the work rate,
2) respiratory exchange ratio
(RER) 
1.00, and 3) maximal HR
95% of age-predicted maximum. Because it has recently been reported
that a plateau in O2 is
seldom achieved in children (45, 46) or adolescents (41), attainment of
a plateau was not used as a criterion for
O2 max in children.
For adults, a test was considered a valid maximal test when at least
two of the following criteria were achieved:
1) failure to maintain the work
rate, 2) RER 1.10,
3) maximal HR age-predicted
maximum, and 4) an increase in work
rate with no further increase in
O2. If two or more of these
criteria were not achieved, a second maximal test was performed.
Submaximal exercise tests.
Submaximal exercise tests on the treadmill were at 3.0, 4.0, and 5.0 mph for children and at 3.0 and 5.0 mph and ~60% of
O2 max for the
adults. A relative work rate was used in adults so that a similar
physiological stress was experienced by both the children and adults,
thus also allowing comparison of cardiovascular responses at similar
relative rates of work and increasing the range of O2 values from which
regression lines were developed. From the results of pilot
work, we found that children were just able to complete
steady-state cardiovascular measurements on the treadmill when they
exercised at both 3.0 and 5.0 mph on the same day. Hence, the children
completed the 4.0-mph portion of their submaximal treadmill test on
their second visit, as described above. Children completed the 4.0-mph
work rate only once. One boy did not do the 4.0-mph work rate.
On the cycle, children exercised at 20, 40, and 60 W, whereas adults
exercised at 40 and 60 W and at ~60% of
O2 max, both groups
cycling at 65 ± 5 rpm. One girl was not able to
complete the 60-W work rate. As with the treadmill, a relative work
rate was used in adults to increase the range of
O2 values from which regression lines were developed.
Before the commencement of both the treadmill and cycle ergometer
submaximal tests, the children and adults were allowed a 3-5 min
warm-up period. The treadmill was calibrated at each speed during each
submaximal test to ensure that the appropriate speed was maintained,
and the cycle ergometer was calibrated daily. Although the
electronically braked cycle ergometer used in this study maintained the
work rate independent of rpm, rpm during submaximal testing was the
same for both children and adults (65 ± 5) to eliminate the
possible effects that different pedaling speeds might have on their
metabolic and cardiovascular responses (49). During all submaximal
tests, subjects were allowed to drink water ad libitum, and a fan was
used for cooling at an airflow rate of ~2.0-3.0 m/s.
Submaximal cardiovascular and metabolic data were collected during
steady-state exercise. Steady state for all subjects was defined as a
HR response within ±5 beats/min, and three consecutive 20-s values
for both O2 and
CO2 production
(CO2) within ±10%. On
average, both children and adults exercised for ~3-6 min before steady state was achieved, and each steady-state work rate lasted ~14-18 min. Once steady state had been achieved,
, HR,
O2, and BP measurements
were obtained in duplicate. For children, a 3- to 5-min rest period was
allowed between each work rate and between duplicate measurements at
the highest work rate when necessary. For adults, a 3- to 5-min rest
period was given between the two highest work rates.
Before the start of the submaximal treadmill and cycle tests, subjects
were connected to a Colin model STBP-780 semiautomated BP-measurement
device (Colin Medical Instruments, San Antonio, TX). The BP cuff was
selected so that the cuff bladder width was ~40% of the subject's
upper arm circumference measured at midbicep (20, 26).
Once steady state was achieved, a BP measurement was taken, followed by
measurements of HR, CO2,
and O2. The HR and
O2 (collected as the
rolling average of 3 consecutive 20-s intervals) used as steady-state
values were a 1-min average taken just before the
CO2-rebreathing maneuver. Once
these steady-state values had been attained, end-tidal
CO2 pressure
(PETCO2) values were
collected and a CO2-rebreathing
measurement was performed.
Measurements.
Height, weight, and skinfold thicknesses were measured during the
subject's first visit to the laboratory. Relative body fat in children
was estimated from triceps and subscapular skinfold thicknesses by
using the equations of Slaughter et al. (see Ref. 25). For adult men,
abdominal, chest, and thigh skinfolds were used to estimate body
density via the Jackson and Pollock equation (23), and the Brozek et
al. equation (6) was used to estimate relative body fat from body
density. Body density of the women was estimated from the Jackson,
Pollock, and Ward equation (24) by using the triceps, abdominal,
suprailium, and thigh skinfolds. The Lohman equation (37) was then used
to determine relative fat in women. Fat mass (FM) was determined by
relative body fat (%) times body weight (kg) divided by 100. Fat-free
mass (FFM) was determined by body weight (kg) minus FM. Relative body
fat was estimated on two separate occasions in six subjects of each group (boys, girls, men, and women) to determine reliability of the
measurement. Body surface area (BSA,
m2) was calculated by using the
Haycock et al. formula (21). Body mass index (BMI) was calculated by
dividing body weight (kg) by stature squared
(m2).
Expired gases during both the maximal and submaximal tests were
collected and analyzed by using a SensorMedics 2900 metabolic cart
(Yorba Linda, CA), which utilizes a zirconium oxide cell for fractional
percentage of expired O2
determination, an infrared absorption analyzer for fractional percent
of expired CO2
(FECO2) determination, and a mass flowmeter for measuring minute ventilation. Both before and after each test, the gas analyzers were calibrated with
gases of known concentration and the flowmeter was calibrated with a
known volume of air.
O2 max and maximal
RER were the average of the two highest consecutive 20-s values.
HR was monitored and recorded with a Polar HR monitor at 5-s intervals
throughout the treadmill and cycle maximal tests and at 15-s intervals
during the submaximal tests.
was measured indirectly by using the
CO2-rebreathing equilibration
method (7). The CO2-rebreathing
apparatus was modified for the children. A 2600 series Hans Rudolph
two-way valve (48-ml dead space) was connected to an 8200 series Hans
Rudolph rebreathing switching system for the children, whereas a 2700 series Hans Rudolph two-way valve (108-ml dead space) was attached to
the rebreathing switch for the adults. In addition, a 3-liter bag was
used for CO2-rebreathing in
children and small adults, and a 5-liter bag was used in larger adults.
Gas concentrations used during rebreathing for children ranged from 8.0 to 10.0% whereas values for adults ranged from 9.0 to 13.5%.
Of the variables used to calculate ,
CO2 was obtained by
averaging three consecutive 20-s
CO2 values during
steady-state exercise. The partial pressure of
CO2 in arterial blood
(PaCO2) was estimated from a 20-s
average of PETCO2. The
partial pressure of CO2 in mixed
venous blood
(
CO2)
was estimated from the equilibration method as described by Jones (28,
29). The downstream correction was applied to the partial pressure of
equilibrium CO2 to adjust
CO2
for the alveolar-to-blood
PCO2 difference (27,
40).
CO2
and PaCO2 were converted to content of
CO2 in the venous
(
CO2)
and arterial blood
(CaCO2),
respectively, through an equation derived from the
CO2 dissociation curve as
described by Jones and Campbell (27) and as adapted from McHardy (39). This content was then corrected for the effect of [Hb] on
CO2-carrying capacity of the blood
(27, 39). The [Hb] used for the children who did not
consent to having their blood drawn was the average value obtained in
this study for their gender.
SV (ml) was calculated as (ml)/HR (beats/min). MBP
(mmHg) was determined as [systolic BP + (2 · diastolic BP)]/3. Total peripheral
resistance (TPR, units) was calculated as MBP/.
O2 was divided by
to calculate
(a-)O2
(ml/100 ml).
Left ventricular dimensions were measured from standard M-mode
echocardiography obtained with a Johnson & Johnson Ultrasound Imaging
System with the use of a 3.0-mHz transducer. The subjects were
measured in the left lateral decubitus position. M-mode tracings were
obtained at the level just above the papillary muscle and recorded onto
a standard videocassette recorder tape for off-line measurements.
Left ventricle dimensions were measured off-line with a digital
imaging-acquisition system (Freeland Prism) according to the American
Society of Echocardiography (47) and using leading edge-to-leading edge
methodology. The intraventricular septal wall thickness, left
ventricular internal diameter
(LVIDd and LVIDs),
and left ventricular posterior wall thickness were measured in both
diastole (d) and systole (s), respectively. Each dimension was measured
from three different cardiac cycles, and the average value was used.
All M-mode tracings and dimensions were measured by the same
technologist. M-mode tracings of 17 subjects were read twice, once on
each of two separate days to assess intraobserver reliability
(R = 0.97). Interobserver reliability
(R = 0.98) was assessed on six
subjects by comparing the measurements of the primary technologist with
those of a registered cardiac sonographer.
Shortening fraction (SF, %) was calculated as
[(LVIDd 
LVIDs)/LVIDd]
×100. LVM was calculated using the formula proposed by the
American Society of Echocardiography (47), as described by Devereux et
al. (13), which has been validated in adult necrospy studies (12, 13),
a child autopsy study (11), and in a study of animals with heart sizes
that were similar to those of the children involved in this study (9).
Immediately after the last submaximal exercise test in adults and
children, blood was collected for [Hb] assessment. A
venipuncture in the antecubital vein was performed while subjects were
in the sitting position. [Hb] was measured in
quadruplicate by the cyanmethemoglobin method. An average of the four
measurements was used as the [Hb]. To most accurately
adjust for the change in [Hb] during
exercise, blood was drawn immediately after exercise.
Analysis.
The differences in submaximal cardiovascular responses
to exercise between the boys and girls have been reported elsewhere (51). Because the primary focus of this paper is child vs. adult differences in submaximal cardiovascular responses to exercise, the
differences between the men and women are presented but not discussed,
and only the differences between the boys vs. men and girls vs. women
are presented in detail. Differences in descriptive variables between
groups were analyzed with an analysis of variance (ANOVA). Test-retest
reliability of submaximal metabolic and cardiovascular variables for
each modality was determined by intraclass correlation R calculated from a one-way ANOVA
model and is reported as the reliability of the mean of the test scores
(MS) for each subject [(MSb MSw)/MSb],
while reliability for relative fat and LVM is reported as the
reliability of scores collected on 1 day
(MSb MSw)/(MSb + MSw) (3), where
MSb is mean square between and MSw is mean square within.
For the adult vs. child comparisons of the cardiovascular variables
measured during submaximal exercise, the mean of the values collected
on the first day and second day for both the treadmill and cycle
ergometer was used. Differences between adults and children in
metabolic and cardiovascular variables at a given rate of work were
determined with a one-way ANOVA. Child vs. adult cardiovascular response differences at a given
O2 were determined by
regression analysis. Analysis of covariance of heterogeneous regression
lines (SPSS) was used to determine whether significant differences
existed between the slopes and intercepts of the cardiovascular
variables on O2 (l/min) for
children vs. adults. All significant differences are at the
P 0.05 level unless stated
otherwise.
RESULTS
The physical characteristics of the subjects are presented in Table 1. All values for the subjects' physical characteristics were significantly lower in the children (boys and girls) than in both the men and women except for relative fat and SF. Relative fat was significantly higher in the women than in both the children (boys and girls) and men. Furthermore, relative fat was significantly higher in the girls than men, but it was not significantly different between the boys and men. The reliability coefficients for relative fat estimates in this study for 12 children and 12 adults were R = 0.96 and R = 0.98, respectively. SF was not different between any of the groups.
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Table 2 presents the
O2 max data from both
the treadmill and cycle ergometer for all groups.
O2 max (l/min) was
significantly higher in the men vs. women, boys, and girls and in the
women vs. the boys and girls.
O2 max relative to body
mass
(ml · kg
1 · min1)
was not different between the children (boys and girls) and men, but it
was significantly lower in the women than in the boys, girls, and men
on both the treadmill and cycle ergometer. Maximal HR in the women was
significantly lower than in the boys and girls on both modalities.
Maximal RER was significantly higher on both modalities in both the men
and women vs. both the boys and girls.
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Day-to-day reliability for each of the steady-state submaximal
cardiovascular variables was moderately high for both modalities in all
groups (Table 3).
O2 (l/min) day-to-day
reliability was also high on both the cycle ergometer and treadmill in
all groups (Table 3).
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The cycle ergometer and treadmill submaximal metabolic and
cardiovascular data for the boys vs. men and girls vs. women are summarized in Tables 4 and
5, respectively. There were significant differences in O2 (l/min) at
equivalent rates of work between both groups on the treadmill, and
between the boys and men on the cycle ergometer. Thus, to determine
whether there were significant differences in cardiovascular responses
between the groups at equivalent
O2 (l/min)
levels, the regression lines of each of the cardiovascu- lar
variables on O2 (l/min) were
statistically compared.
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Figures 1-5 present the mean cardiovascular data on
O2 (l/min) for each work rate
for all groups on both the cycle ergometer (A) and treadmill
(B). The
insets in each of these figures
present the regression equation for the specific cardiovascular
variable and O2 (l/min), and
the legends give the statistical significance of differences, if any,
in slopes and intercepts of the regression lines between the groups.
Figure 1,
A and
B, demonstrates that
at a given
O2 was lower in boys vs. men
and girls vs. women for both the cycle ergometer and treadmill,
respectively. When the regression lines of on O2 were compared, the
intercept for the boys vs. men and girls vs. women was significantly
lower on both modalities. The slopes of these relationships were not
different between the groups.
) on
O2 consumption (vo2) for cycle
ergometer (A) and treadmill
(B).
Insets: regression equations used in statistical analysis. * Intercept significantly different from corresponding adult value; P 0.05. 
Intercept significantly different from women;
P 0.05.
The mean HR responses for all groups for both the cycle ergometer and
treadmill can be seen in Fig. 2,
A and
B, respectively. There were no
significant differences in the intercepts of the regression lines of HR
on O2 between the boys vs.
men or girls vs. women. The slope of the
HR-O2 relationship in the
boys vs. men and girls vs. women was significantly greater on both
modalities. The significantly steeper slopes of this relationship in
children explain why the intercepts of the
HR-O2 relationship in the children vs. adults are not significantly higher. However, Fig. 2,
A and
B, shows that HR in the children was
higher than HR in the adults at a given
O2 on both the cycle
ergometer and treadmill.
O2 for
cycle ergometer (A) and treadmill
(B).
Insets: regression equations used in
statistical analysis. 
Slope significantly different from
corresponding adult value; P 0.05.
Slope significantly different from women;
P 0.05.
The comparison of SV-O2
between the groups is presented in Fig. 3,
A and
B, for both the cycle ergometer and
treadmill, respectively. The intercept of SV on
O2 for the boys vs. men and
girls vs. women was significantly lower on both modalities, although
the slopes of these relationships were not different between the
groups.
O2 for
cycle ergometer (A) and treadmill
(B).
Insets: regression equations used in
statistical analysis. * Intercept significantly different from
corresponding adult value; P 0.05. Intercept significantly different from women;
P 0.05.
Figure 4,
A and
B, present the
(a-)O2-O2
relationship for all groups on both the cycle ergometer and
treadmill, respectively. The slope of this relationship was
significantly greater in the boys vs. men and girls vs. women on both
the cycle ergometer and treadmill. The reason that the
(a-)O2-O2
intercept was not significantly higher in the children is likely due to
the phenomenon discussed above for the
HR-O2 relationship. Figure 4,
A and B, demonstrates that, at a
given O2, the
(a-)O2
is higher in the boys vs. men and girls vs. women on both the cycle
ergometer and treadmill, respectively.
)O2]
on O2 for cycle ergometer
(A) and treadmill
(B).
Insets: regression equations used in
statistical analysis. Slope significantly different from
corresponding adult value; P 0.05.
Slope significantly different from women;
P 0.05.
The TPR-O2 relationship for
each group is presented in Fig. 5,
A and
B, for the cycle ergometer and
treadmill, respectively. The intercept of this relationship was
significantly higher, and the slope was significantly greater, in the
boys vs. men and girls vs. women on both modalities. Although the slope
differences may be primarily responsible for the significant
differences in intercepts between the children vs. adults, Fig. 5
demonstrates that TPR was higher in the boys and girls at a given
O2.
O2 for cycle ergometer
(A) and treadmill
(B).
Insets: regression equations used in statistical analysis. * Intercept significantly different from corresponding adult value; P 0.05. Slope significantly different from corresponding adult
value; P 0.05. Intercept
significantly different from women; P 0.05. Slope significantly different from women;
P 0.05.
The relationship between resting LVM and SV on the cycle ergometer at
40 W is presented in Fig. 6 for both the
males and females. LVM was closely related to SV on both the cycle
ergometer at 40 W (r = 0.89 and 0.90)
and 60 W (r = 0.91 and 0.88) and on
the treadmill at 3 mph (r = 0.92 and
0.93) and 5 mph (r = 0.92 and 0.90) in
males and females, respectively.
DISCUSSION
The results of this study indicate that
in 7- to 9-yr-old boys and girls is significantly
lower at a given O2
compared with adult men and women on both the cycle ergometer and
treadmill. The lower is the result of a lower SV in
the children that is only partially compensated for by a higher HR at a
given absolute O2. The
lower SV is related to the smaller heart size in the children. The
lower in children at a given
O2 is compensated for by a
higher
(a-)O2.
Several methodological factors must be considered when these data are
interpreted. Both shorter (10) and longer (52) recirculation times have
been reported in children compared with adults. Differing recirculation times could affect estimations
when the CO2-rebreathing method is
used. However, we did not see an effect, as the rebreathing curves for
both the children and adults showed good equilibration and no signs of
recirculation. Furthermore, the accuracy of using a single
postexertional hemoglobin (Hb) measurement for
calculation is not ideal. However, to best account for the changes that
occur in Hb levels during exercise, while still limiting the
invasiveness of the procedures (which is necessary when working with
young children), a single postexertional measurement is most
appropriate.
In our study, at a
O2 of 1.0 l/min was 2.5 and
1.5 l/min lower on the cycle ergometer and was 2.9 and 1.3 l/min lower on the treadmill in boys vs. men and in girls vs. women, respectively. A lower of a similar magnitude on the cycle
ergometer in children vs. adults at a given rate of work is supported
by research literature (2, 16, 30, 32, 42, 52) but is in conflict with
others who have reported on the cycle ergometer to
be the same (17-19). The studies that have reported lower
in children vs. adults have reported similar
absolute values and age-related differences.
Of the studies that have reported no difference between children and
adults, Gadhoke and Jones (17) determined in two groups of 20 boys (mean ages of 11.1 and 13.5 yr) during cycling exercise at 200 and 400 kilopond m/min. They did not find
a difference in compared with values for adults
reported by Higgs et al. (22). Godfrey et al. (18) tested 117 boys and
girls, ages 6-16 yr, on the cycle ergometer at one-third and
two-thirds of their
O2 max. When they
compared their results to those of Bevegard et al. (5), they concluded
that there was no difference in at a given
O2. Also, Godfrey et al. (19)
measured during cycle ergometer exercise at 40 and
80 W in three children (mean age, 9.7 yr) and three adults (mean age,
23.3 yr). Although they did not compare their child vs. adult
differences statistically, by comparing the
-O2
relationship in their six subjects, combined with the data on children
from Godfrey et al. (18) and the adult data from Rowell (43), they
concluded that the child and adult values were not
different. The reason for the discrepancies between our findings and
the findings of these earlier studies (17-19) is uncertain.
Combining our results with the data from the remaining research
literature, it can be concluded that, during exercise,
at a given
O2 (l/min) is ~1.0 to 2.9 l/min lower in young children than adults on both the cycle ergometer
and treadmill. Additionally, our results indicate (Figs. 2 and 3), and
other studies agree (2, 19, 32, 42), that HR is higher and SV is lower
at a given O2 in children vs.
adults. The lower SV in children is generally attributed to their
smaller body size. We found that the significantly lower SV in children
vs. adults was eliminated or greatly reduced when SV was scaled to BSA.
We also found that resting estimated LVM was closely correlated with
both SV (see RESULTS) and HR in the
males (r = 0.85, 0.90,
0.73, 0.64) and females
(r = 0.80, 0.82,
0.60, 0.60) at 40 W, 60 W, 3 mph, and 5 mph,
respectively. Because the adults did not do the 20-W work rate and the
children did not do the 60% work rate, these rates of work were not
included in this analysis.
It has been suggested that the lower
-O2
relationship in children is an indicator of depressed myocardial
functional reserve or inability to generate SV (1). This may not
necessarily be the case. The increase in with
increasing O2 is met by
changes in both SV and HR. SV is related to body size (thus, it is
higher in adults) and increases very little beyond light work rates. Thus, to complete the Fick equation, HR and
(a-)O2
increase more in children, so that they have a
O2 similar to
adults. Thus the cardiovascular response of children
seems to be normal for their size. Our similar
-O2 slopes
in children and adults (Fig. 1) further support this normal
cardiovascular response of children to exercise that is relative to
their body size. Also, there were no significant differences in LVM/FFM
between boys vs. men (3.21 ± 0.49 vs. 2.92 ± 0.29) or girls vs.
women (2.81 ± 0.36 vs. 2.89 ± 0.50), suggesting that the heart
mass available to generate SV during exericse is not different.
Other factors may contribute to the lower SV in children. It has been
suggested that SV contributions to are higher with larger muscle mass activity (14). In the following section, we suggest
that children use a smaller muscle mass to do a given amount of work.
The smaller muscle mass in children could result in an attenuated
venous return (preload) and thus contribute to their lower SV.
Additionally, the higher whole body TPR (Fig. 5) may represent a higher
afterload, one of three primary factors that affect SV (14), and thus
contribute to a lower SV in children.
As previously mentioned, we suggest that for children to do the
same rate of work (O2) as
adults, the children most likely use a smaller absolute amount of
muscle mass. A smaller absolute muscle mass is thus stressed to a
relatively greater extent, resulting in a greater buildup of
metabolites, thus providing more feedback to the medulla to increase HR
(8). The assumption of a smaller absolute muscle mass in children vs.
adults, during exercise using the same muscle group, is based on the
fact that adults have a significantly larger body mass relative to
muscle mass. Not only are children smaller, but the percentage of their
body weight that is muscle is less. The total muscle mass of a 7- to
9-yr-old boy represents ~44% of his body weight, whereas that of a
20- to 29-yr-old man is ~52% (38). This trend is not found in girls and women, likely due to their greater increase in FM with growth. The
higher HR and lower SV response associated with a smaller muscle mass
has been demonstrated in studies in adults comparing large and small
muscle masses during exercise (4, 36, 50).
The possibility that children use a smaller absolute amount of muscle
mass than adults use to exercise at the same rate of work has been
proposed by others (30, 32) as a possible mechanism for the higher
(a-)O2
in children, as found in the present study and others (2, 15, 32, 42).
As stated above, the smaller muscle mass would be stressed to a
relatively greater extent and thus would likely generate a larger
amount of metabolic by-products (36) and heat per unit of muscle, which
would then 1) increase O2 liberated from Hb by decreasing
its affinity for O2 at the muscle;
2) increase vasodilatation of the
arteries entering the muscle (36), thus increasing muscle blood flow
(33, 34); and 3) increase feedback
to the medulla via group III and IV afferents to increase HR (8), as
discussed earlier.
Furthermore, the higher
(a-)O2
in children may be due to more of their blood flow going to active
muscle. This is supported by the work of Koch (33, 34), who reported
higher muscle blood flow in 12- to 14-yr-old boys compared with
25-yr-old adults after ischemic work and during maximal cycle ergometer
exercise. The possibility of higher muscle blood flow in children is
indirectly supported by work of Saltin et al. (48) in adults. They
reported that leg blood flow was inversely related to [Hb].
However, Kurdak et al. (35) report that, in maximally working canine
muscle in situ, lower [Hb] did not significantly change
blood flow distribution.
The [Hb] of the children in the present study was
significantly lower (P 0.05) than
that of the adults (see Table 1). When we scaled our
(l/min) and TPR (units) to FFM (kg) we found that
· FFM1 · min1
was higher and TPR/FFM was lower in the children vs.
adults at a given O2 on both
the cycle ergometer and treadmill. Because muscle receives up to
80-85% of the , or blood flow, during exercise
(44), these results suggest that for a given muscle mass (FFM) children
have a larger or blood flow and a lower TPR (higher
conductance to accept the higher at the muscle) than
adults have at a given rate of work, indirectly indicating higher
muscle blood flow in children. Higher muscle blood flow combined with a
lower at a given
O2 seems incongruent but may
be explained by a smaller muscle mass performing a given rate of work
in children vs. adults, as discussed above.
Finally, it has been reported that maximal
(a-)O2
in boys is not different from that in men (16). Furthermore, the
(a-)O2 of children is higher at a given submaximal
O2 (l/min). Thus the
(a-)O2
of children at a given absolute submaximal
O2 (l/min) is nearer their
maximal value. This may partially explain the higher
(a-)O2
values at a given submaximal
O2 (l/min) in
children compared with adu
