Division of Physiology and Metabolism, Department of Nutrition Sciences, University of Alabama at Birmingham, Birmingham, Alabama 35294-3360
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ABSTRACT |
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The purpose of this study was to examine
differences in resting, submaximal, and maximal
(O2 max) oxygen
consumption (
O2) in African-American (n = 44) and
Caucasian (n = 31) prepubertal children aged 5-10 yr. Resting
O2 was measured via
indirect calorimetry in the fasted state. Submaximal
O2 and
O2 max were
determined during an all out, progressive treadmill exercise test
appropriate for children. Dual-energy X-ray absorptiometry was used to
determine total fat mass (FM), soft lean tissue mass (LTM), and leg
soft LTM. Doubly labeled water was used to determine total energy
expenditure (TEE) and activity energy expenditure (AEE). A significant
effect of ethnicity (P < 0.01) was
found for
O2 max but
not resting or submaximal
O2, with African-American children having absolute
O2 max ~15% lower
than Caucasian children (1.21 ± 0.032 vs. 1.43 ± 0.031 l/min,
respectively). The lower
O2 max persisted in
African-American children after adjustment for soft LTM (1.23 ± 0.025 vs. 1.39 ± 0.031 l/min; P < 0.01), leg soft LTM (1.20 ± 0.031 vs. 1.43 ± 0.042 l/min;
P < 0.01), and soft LTM and FM (1.23 ± 0.025 vs. 1.39 ± 0.031 l/min;
P < 0.01). The lower
O2 max persisted also
after adjustment for TEE (1.20 ± 0.02 vs. 1.38 ± 0.0028 l/min P < 0.001) and AEE
(1.20 ± 0.024 vs. 1.38 ± 0.028 l/min;
P < 0.001). In conclusion, our data
indicate that African-American and Caucasian children have similar
rates of
O2 at rest and
during submaximal exercise, but
O2 max is ~15%
lower in African-American children, independent of soft LTM, FM, leg
LTM, TEE, and AEE.
oxygen consumption; fitness; energy expenditure; ethnicity
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INTRODUCTION |
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IT IS ESTIMATED that 22-27% of all children in the United States are obese (25), and in certain ethnic populations this value may be greater (30). Cornelius (10) reported that African-American children in the United States were three times as likely to be overweight, compared with Caucasian children of the same age.
Most studies that have measured maximal aerobic capacity
(O2 max) have examined
only Caucasian adolescents (20, 23, 24, 29), and only a few have
examined adolescents of different ethnic groups (15, 28). Pivarnik et
al. (28) reported that a group of adolescent African-American females
(mean age = 13.4 yr) had
O2 max values that were
14% lower than values reported for Caucasian adolescents of the same
age in several other countries (2, 18, 19) and were ~14% lower (43.4 vs. 37.3 ml · kg
1 · min
1)
than those of female Caucasian adolescents (mean age = 13.6 yr) in five
prior US studies. Whereas some studies have found lower aerobic
capacity in African-Americans (14, 27, 28), the data were not adjusted
for soft lean tissue mass or leg soft lean tissue mass. It is also
unclear whether the lower
O2 max that has
previously been shown in African-Americans can be explained by lower
resting oxygen consumption
(
O2), or by the "net"
O2 at maximal effort during
exercise. It is also unknown how habitual physical activity energy
expenditure in African-American and Caucasian children affects aerobic
capacity. By using the doubly labeled water technique, we were able to
measure total energy expenditure (TEE) and calculate activity energy
expenditure (AEE) to examine whether the energy cost of daily physical
activity might contribute to ethnic differences in aerobic capacity.
The objective of this study was therefore to examine in
African-American and Caucasian male and female children (aged 5-10 yr) resting, submaximal, and maximal
O2. By using dual-energy X-ray absorptiometry (DEXA) to assess body composition and doubly labeled water to measure free-living energy expenditure, we were able
to examine whether observed differences in
O2 were explained by differences in soft lean tissue mass, leg soft lean tissue mass,
total fat mass, TEE, or AEE.
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METHODS |
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Protocol.
Children were admitted to the General Clinical Research Center (GCRC)
in the late afternoon for an overnight visit. On arrival, anthropometric measurements were obtained, and dinner was served at
~1700. An evening snack was allowed as long as it was consumed before
2000. After 2000, only water and noncaloric, noncaffeinated beverages
were allowed until after the morning testing was completed. The
following morning, resting energy expenditure (REE) was measured for 30 min, starting between 0600 and 0730. After testing was completed, the
children were fed breakfast and allowed to leave. Two weeks later, the
children arrived at the Energy Metabolism Research Unit at 0700 in the
fasted state. Submaximal and maximal O2 were determined by an
all out, progressive treadmill test using a protocol appropriate for
children. Body composition was determined by DEXA.
Subjects. A total of 94 healthy African-American (n = 60) and Caucasian (n = 34) children 5-10 yr of age completed the protocol. Children were recruited from the Birmingham, Alabama area by use of radio advertisements, flyers, and word of mouth. Children who were taking medications known to affect body composition or physical activity were excluded from the study, as were children diagnosed with Cushing's syndrome, Down's syndrome, type I diabetes, or hypothyroidism. The study was approved by the University of Alabama at Birmingham (UAB) Institutional Review Board for human use, and informed consent was obtained from all subjects before testing.
REE.
REE was measured in all subjects in the early morning in the fasted
state after subjects had spent the night at the GCRC. A Deltatrac
Metabolic Monitor (Sensormedics), which was calibrated before each test
against standard gases, was used for each REE measurement. During
testing, all subjects were instructed to lie as still as possible. An
adult-size canopy hood was used to collect the expired air for 20 min
after a 10-min equilibrium period, and
O2 and carbon dioxide
production (
CO2) were
measured continuously during this time. Energy expenditure was
calculated using the equation of de Weir (11).
Exercise testing. Subjects reported to the Energy Metabolism Research Unit at 0700 in the fasted state. After becoming familiar with the testing equipment, such as the mouthpiece and headgear, the children were allowed to practice walking on the motorized treadmill until they were able to walk without holding on to the railings. Subjects followed an all out, progressive, continuous treadmill protocol appropriate for children (3). The children walked for 4 min at 0% grade and 4 km/h, after which the treadmill grade was raised to 10%. Each ensuing work level lasted 2 min, during which the grade was increased by 2.5%. The speed remained constant until a 22.5% grade was reached, at which time the speed was increased by 0.6 km/h until the subject reached exhaustion.
Measurement of body composition. DEXA was used to measure total and regional body composition (Lunar DPX-L densitometer, Lunar Radiation, Madison, WI). The total dose of radiation for a scan is less than several hours of background exposure (0.02 mrem). The following information was obtained from the DEXA scan: fat, lean, and bone mineral mass (in grams). Soft lean tissue mass is defined as fat-free mass plus essential lipids. Scans were analyzed using the pediatric medium or large mode (n = 73) (Pediatric Software, Version 1.5e) or the adult fast mode (n = 2) (DPX-L Version 1.3z), depending on the weight of the child. Limb soft lean tissue mass was used as an index of appendicular skeletal muscle mass (16).
Measurement of total and physical activity-related energy
expenditure.
TEE was measured over 14 days under free-living conditions with the
doubly labeled water technique, using a protocol that has a theoretical
precision of <5% for assessment of
CO2, as previously
described (13). Briefly, four timed urine samples were collected after
oral dosing with doubly labeled water, two the morning after dosing,
and two in the morning 14 days later with a loading dose of 0.15 g
H218O and
0.12 g of
2H2O/kg
body mass. Samples were analyzed in triplicate for
H218O and
2H2O
by isotope ratio mass spectrometry at The Energy Metabolism Research
Unit in the Department of Nutrition Sciences at the UAB. The facility
at UAB consists of a Fisons Optima isotope ratio mass spectrometer, and
samples are prepared and analyzed in a similar fashion to that
previously described (13), except that carbon dioxide is analyzed for
oxygen-18 content by continuous flow isotope ratio mass spectrometry.
The intra-assay standard deviation for triplicate analysis of samples
at the laboratory is ~4
and 0.20
for deuterium
and oxygen-18, respectively. Complete doubly labeled water was obtained
for 62 of the 75 children.
Statistical analysis.
All statistical analyses were performed using SAS (SAS Institute, Cary,
NC; SAS for Microsoft Windows; Release 6.10). A two-way analysis of
covariance (ANCOVA) design was used to test for the main effects of
gender and ethnicity, as well as for the interactive effect of
ethnicity by gender. Because gender did not affect the major outcome
variables, all subsequent analyses were combined into two groups
(African-American and Caucasian). The main outcome variables were
resting O2, submaximal
O2,
and
O2 max,
with soft lean tissue mass as the covariate. After adjustment for soft lean tissue mass, fat mass and leg soft lean tissue mass were entered
into the model to determine if either total body fat or regional soft
lean tissue mass distribution explained differences in
O2 between the two ethnic
groups (African-American and Caucasian).
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RESULTS |
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Descriptive statistics for the four subgroups (African-American and
Caucasian males and females) are shown in Table
1. All groups were similar in age. A
significant gender difference was found for both total body weight
(P < 0.05) and total fat mass (P < 0.001), with females being
heavier and having greater fat mass than males. However, soft lean
tissue mass was similar among all four subgroups. Data for
O2 in absolute terms are
summarized in Table 2. No significant
influence of gender or ethnicity was observed for resting
O2 or submaximal
O2, but a significant effect
of ethnicity was found for
O2 max
(P < 0.01). African-American children had absolute peak
O2
values that were 15% lower than those of the Caucasian children. The
lower
O2 max
was seen in both males and females. There were no significant effects
of gender or ethnicity on maximum respiratory exchange ratio or maximum heart rate (Table 2). Submaximal heart rate was significantly higher
(P < 0.05) in females, and
submaximal respiratory exchange ratio was significantly greater
(P < 0.05) in African-Americans (Table 2). Caucasian males had significantly longer treadmill times
than their African-American counterparts
(P < 0.01), and the same was true
for females (P < 0.01; Table 2).
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Resting O2, submaximal
O2, and
O2 max were related to
soft lean tissue mass in both African-American and Caucasian children. Resting
O2, submaximal
O2, and
O2 max are plotted as a
function of soft lean tissue mass in Fig.
1. For resting
O2 adjusted for soft lean
tissue mass, there were no significant differences in either slopes or
intercepts between African-American and Caucasian subjects (Fig.
1A). For submaximal
O2 adjusted for soft lean tissue mass, there were also no significant differences in either slopes or intercepts between African-Americans and Caucasians (Fig.
1B). For
O2 max, the regression
slopes adjusted for soft lean tissue were not significantly different,
but the intercepts were significantly different
(P < 0.05), with the
African-American children demonstrating lower adjusted
O2 max (Fig.
1C). Similarly, the lower
O2 max persisted in
African-American children after data were adjusted for leg soft lean
tissue mass (Fig. 2). The lower
O2 max in
African-American children could not be explained by differences in
total body fatness. When total fat mass was entered into the model, in
addition to soft lean tissue mass, the lower adjusted
O2 max values persisted
in the African-American group (1.23 ± 0.025 vs. 1.39 ± 0.031 l/min; P < 0.01; Table
3).
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Although we were confident that we reached a true
O2 max in the
children who achieved two of the three criteria, it could be argued
that children who reached three criteria may be more motivated in
determining
O2 max.
Therefore, a subset of subjects (n = 39) who achieved all three of the physiological criteria for
O2 max were
analyzed. When the relationship between leg soft lean tissue mass and
O2 max in this subset
was plotted, the lower
O2 max in the
African-American children persisted (Fig.
3). When the values were adjusted for leg
soft lean tissue mass, the resulting
O2 max values of the
African-American subjects remained significantly lower than
those of the Caucasian children (1.54 ± 0.040 vs.
1.27 ± 0.044 l/min, respectively,
P < 0.01).
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Adjusted means of TEE, REE, and AEE are shown in Table
4. There were no significant differences
for TEE, REE, or AEE between the four groups. No significant effect of
ethnicity was found for any of the dependent variables. To determine if
TEE or AEE could explain the observed difference in
O2 max between the two
groups of children, both variables were entered into the ANCOVA model
separately in addition to soft lean tissue mass and fat mass. When TEE
was entered into the model, the lower
O2 max persisted in the
African-American children (1.20 ± 0.024 vs. 1.38 ± 0.028 l/min;
P < 0.001). When AEE was entered
into the model in addition to soft lean tissue mass and fat mass, the
lower
O2 max also
persisted in the African-American children (1.20 ± 0.024 vs. 1.38 ± 0.028 l/min, P < 0.001, in
African-American and Caucasian children, respectively).
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DISCUSSION |
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The main conclusion from this study is that
O2 max was 15% lower
in the African-American vs. Caucasian prepubertal children. The lower
aerobic capacity was also associated with decreased exercise endurance
in the African-American children, as measured by treadmill time to
exhaustion. Another indication of the lower fitness in general in the
African-American children was that 25% of tested children, compared
with 12% of Caucasian children, did not meet the criteria for a
successful
O2 max test,
possibly reflecting a reduced motivation for maximal exercise effort.
The significantly lower O2 in
African-American children was observed only at maximal exercise effort
and was independent of soft lean tissue mass, total fat mass, and leg
soft lean tissue mass. In other words, none of the independent
variables we examined explained the ethnic difference in
O2 max, and, in
additional regression analysis, ethnicity remained a significant and
independent predictor of
O2 max. The difference
in aerobic capacity also was independent of physiological criteria for
reaching
O2 max (heart
rate >195 beats/min, respiratory exchange ratio >1.0 and/or plateauing of
O2), indicating
that the ethnic difference was not due to differences in motivation
during the treadmill test. Moreover, the ethnic difference in
O2 max was independent
of habitual free-living physical activity-related energy expenditure.
Other body composition variables may play a role in the observed
differences in aerobic capacity seen between the two groups. Previous
studies have shown that the contribution of bone mass to fat-free mass
may be greater in African-American than in Caucasian adults (26). One
could speculate that the lower
O2 max found in the
African-American children in our sample was due to the fat-free mass of
the African-Americans containing more bone mass and less skeletal
muscle compared with that of the Caucasian children. However, our
analyses were done using soft lean tissue mass (bone excluded).
Furthermore, the African-American and Caucasian children had similar
amounts of skeletal muscle. Thus differences in bone mineral content of
the African-American children and the Caucasian children were not a
factor in the observed lower
O2 max in the African-American children.
Habitual physical activity and exercise patterns may have a significant
influence on aerobic capacity. Although there are many social and
behavioral factors that determine physical activity habits, some
studies have implicated ethnicity as a determinant of exercise
patterns, with African-American and other ethnic minorities being less
active than Caucasians (7, 8, 12). According to the 1990 Youth Risk
Behavior Survey, female African-American students (grades 9-12)
were the least likely to be vigorously active three or more times per
week (9). We do not have any descriptive data regarding the physical
activity patterns of the children, but we do have the daily AEE for the
children derived from the doubly labeled water data. It is important to
note that the AEE value represents only the average daily AEE and does
not give any information about the intensity or duration of the
activities performed by the children. There were no significant
differences between the groups with regard to TEE or AEE. However, in
additional analysis, we found that the relationship between
O2 max and AEE was
significant in the Caucasian children
(r2 = 0.18;
P < 0.05) but was not significant
(r2 = 0.066;
P = 0.143) in African-American
children. In addition, African-American and Caucasian children had
similar AEE values, but
O2 max was lower in
African- American children. Although we are not able to make a
conclusive statement regarding this finding, these data suggest that
the Caucasian children participated in activities at higher
intensities. One limitation of the doubly labeled water technique is
that, although AEE can be calculated, it gives no information regarding
the type or intensity level of the activities, and thus further studies
using more qualitative assessment of physical activity pattern are
warranted.
There are several factors that we did not examine that could possibly
explain the lower
O2 max in the
African-American children. Although the lower aerobic capacity in
African-American children could not be explained by leg soft lean
tissue mass (Fig. 2), it could have been due in part to differences in
muscle fiber type in these two groups. African-American adult males
have been found to have a greater percentage of type II, anaerobic
fibers, and lower percentage of type I, aerobic fibers, compared with Caucasian males (1). Because fiber type and peak
O2 have been shown to be
significantly correlated in adults (4, 17), it is possible the lower
proportion of type I fibers in African-Americans may limit the ability
to perform continuous, endurance-type activities that require a steady
rate of aerobic energy transfer (21). If these differences in fiber
type also occurred in this sample of young children, they may have been
responsible for all or part of the difference in reduced
O2 max and treadmill
time in the African-American children. This hypothesis could not be
examined because muscle tissue from these children was not available.
Another factor that might explain our findings is ethnic differences in hemoglobin concentrations ([Hb]). When [Hb] levels are low, there is a decrease in the blood's oxygen carrying capacity and, consequently, a corresponding decrease in ability to perform even mild aerobic exercise (21). Pivarnik et al. (27) found that, in a group of African-American and Caucasian adolescent females (age = 13.5 yr), the African-Americans had [Hb] levels that were significantly lower than those of the Caucasian girls (13.0 ± 1.1 vs. 13.8 ± 0.9 g/dl; P < 0.01). Whereas the values were within normal physiological limits, it is unknown whether the lower [Hb] concentration contributed to a lower O2 extraction during exercise (27).
The implications and clinical significance of the difference in the
O2 max between
African-American and Caucasian children cannot fully be defined, and it
is difficult to generalize our findings to the general population.
However, current epidemiological data indicate that low fitness is a
powerful precursor of mortality in adults. Moderate levels of physical
fitness exhibit a protective effect against the influence of such
mortality predictors as smoking, hypertension, and hypercholesterolemia
(5). It is unknown whether aerobic capacity or physical activity
patterns in children would affect long-term adult health outcomes.
However, it has been postulated that physical activity and/or
fitness during childhood serves as the foundation for a lifetime of
regular physical activity (6). Further research is warranted both to
determine habitual physical activity patterns of children of different
ethnic and cultural groups and to find appropriate ways to educate and
motivate children to adopt regular physical activity patterns. The
long-term relationship between aerobic fitness and risk of obesity and
other chronic diseases has yet to be determined in different ethnic groups.
In conclusion, O2 max
was significantly lower in African-American compared with Caucasian
prepubertal boys and girls. This difference could not be explained by
differences in body composition, including fat mass, total soft lean
tissue mass, leg soft lean tissue mass, TEE, or AEE. The difference in
O2 was not apparent during
rest or submaximal exercise and was only observed at maximal effort of
exercise.
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ACKNOWLEDGEMENTS |
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We thank all of the children who participated in the study as well as Tena Hilario for the wonderful job of recruiting subjects.
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FOOTNOTES |
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This study was supported by grants to M. I. Goran (United States Department of Agriculture Grant 95-37200-1643 and the National Institute of Child Health and Human Development Grants R29-32668 and RO1 HD/HL-33064) and in part by a grant from the General Clinical Research Centers at the University of Alabama at Birmingham (M01-RR-00032).
Address for reprint requests: M. I. Goran, Division of Physiology and Metabolism, Dept. of Nutrition Sciences, Univ. of Alabama at Birmingham, Birmingham, AL 35294-3360.
Received 28 April 1997; accepted in final form 3 July 1997.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Ama, P. F. M.,
J. A. Simoneau,
M. R. Boulay,
O. Serresse,
G. Theriault,
and
C. Bouchard.
Skeletal muscle characteristics in sedentary Black and Caucasian Americans.
J. Appl. Physiol.
61:
1758-1761,
1986
2.
Armstrong, N.,
J. Balding,
P. Gentle,
J. Williams,
and
B. Kirby.
Peak oxygen uptake and physical activity in 11-to-16-year olds.
Pediatr. Exerc. Sci.
2:
349-358,
1990.
3.
Astrand, P. O.
Exercise Studies of the Physical Walking Capacity in Relation to Sex and Age. Copenhagen: Enjar Munksgaard, 1952.
4.
Barstow, T. J.,
A. M. Jones,
P. H. Nguyen,
and
R. Casaburi.
Influence of muscle fiber type and pedal frequency on oxygen uptake kinetics of heavy exercise.
J. Appl. Physiol.
81:
1642-1650,
1996
5.
Blair, S. N.,
J. B. Kampert,
H. W. Kohl III,
C. E. Barlow,
C. A. Macera,
R. S. Paffenbarger, Jr.,
and
L. W. Gibbons.
Influences of cardiorespiratory fitness and other precursors on cardiovascular disease and all-cause mortality in men and women.
JAMA
276:
205-210,
1996[Abstract].
6.
Blair, S. N.,
H. W. Kohl III,
R. S. Paffenbarger, Jr.,
D. G. Clark,
K. H. Cooper,
and
L. W. Gibbons.
Physical fitness and all-cause mortality: a prospective study of healthy men and women.
JAMA
262:
2395-2401,
1989[Abstract].
7.
Caspersen, C. J.,
G. M. Christensen,
and
R. A. Pollard.
The status of the 1990 Physical Fitness Objectivesevidence from NHIS 1985.
Public Health Rep.
101:
587-592,
1986[Medline].
8.
Caspersen, C. J.,
and
R. K. Merritt.
Trends in physical activity patterns among older adults: the behavioral risk factor survey system, 1986-1990 (Abstract).
Med. Sci. Sports Exerc.
24:
S26,
1992.
9.
Centers for Disease Control and Prevention.
Physical activity among high school studentsUnited States, 1990.
Morb. Mortal. Wkly. Rep.
41:
33-35,
1992[Medline].
10.
Cornelius, L. J.
Health habits of school-age children.
J. Health Care
2:
374-395,
1991.
11.
De Weir, J. B.
New methods for calculating metabolic rate with special reference to protein metabolism.
J. Physiol. (Lond.)
109:
1-9,
1949.
12.
Dipietro, L.,
and
C. J. Caspersen.
National estimates of physical activity among white and black Americans. (Abstract).
Med. Sci. Sports Exerc.
23:
S105,
1991.
13.
Goran, M. I.,
W. H. Carpenter,
A. McGloin,
R. Johnson,
J. M. Hardin,
and
R. L. Weinsier.
Energy expenditure in children of lean and obese parents.
Am. J. Physiol.
268 (Endocrinol. Metab. 31):
E917-E924,
1995
14.
Gutin, B.,
S. Islam,
T. Manos,
N. Cucuzzo,
C. Smith,
and
M. E. Stachura.
Relation of percentage of body fat and maximal aerobic capacity to risk factors for atherosclerosis and diabetes in black and white seven- to eleven-year-old children.
J. Pediatr.
125:
847-852,
1994[Medline].
15.
Gutin, B.,
A. Trinidad,
C. Norton,
E. Giles,
A. Giles,
and
K. Stewart.
Morphological and physiological factors related to endurance performance of 11- to 12-year-old girls.
Res. Q. Exerc. Sport
49:
44-52,
1978.
16.
Heymsfield, S. B.,
R. Smith,
M. Aulet,
B. Bensen,
S. Lichtman,
J. Wang,
and
R. N. Pierson, Jr.
Appendicular skeletal muscle mass: measurement by dual-photon absorptiometry.
Am. J. Clin. Nutr.
52:
214-218,
1990[Abstract].
17.
Ivy, J. L.,
D. L. Costill,
and
B. D. Maxwell.
Skeletal muscle determinants of maximum aerobic power in men.
Eur. J. Appl. Physiol. and Occup. Physiol.
44:
1-8,
1980[Medline].
18.
Kemper, H. C. G.,
and
R. Verschuur.
Maximal aerobic power in 13-14 year-old teenagers in relation to biologic age.
Int. J. Sports Med.
2:
97-100,
1981[Medline].
19.
MacDougall, J. D.,
P. D. Roche,
O. Bar-Or,
and
J. R. Moroz.
Maximal aerobic capacity of Canadian school-children: prediction based on age-related oxygen of running.
Int. J. Sports Med.
4:
194-198,
1983[Medline].
20.
Maffeis, C.,
F. Schena,
M. Zaffanello,
L. Zoccante,
Y. Schutz,
and
L. Pinelli.
Maximal aerobic power during running and cycling in obese and non-obese children.
Acta Paediatr.
83:
113-116,
1994[Medline].
21.
McArdle, W. D.,
F. I. Katch,
and
V. I. Katch.
Energy transfer in exercise.
In: Exercise Physiology: Exercise, Nutrition and Human Performance. Philadelphia, PA: Lea & Febiger, 1991, p. 123-144.
22.
McArdle, W. D.,
F. I. Katch,
and
V. I. Katch.
Gas exchange and transport.
In: Exercise Physiology: Exercise, Nutrition and Human Performance. Philadelphia, PA: Lea & Febiger, 1991, p. 254-269.
23.
McCormack, W. P.,
K. J. Cureton,
T. A. Bullock,
and
P. G. Weyand.
Metabolic determinants of 1-mile run/walk performance in children.
Med. Sci. Sports Exerc.
23:
611-617,
1991[Medline].
24.
Nagle, F. J.,
J. Hagberg,
and
S. Kamei.
Maximal O2 uptake of boys and girls ages 14-17.
Eur. J. Appl. Physiol. Occup. Physiol.
36:
75-80,
1977.[Medline]
25.
National Health and Nutrition Examination Survey.
Prevalence of overweight among adolescentsUnited States, 1988-1991.
Morb. Mortal. Wkly. Rep.
43:
818-821,
1994[Medline].
26.
Ortiz, O.,
M. Russell,
T. L. Daley,
R. N. Baumgartner,
M. Waki,
S. Lichtman,
J. Wang,
R. N. Pierson, Jr.,
and
S. B. Heymsfield.
Differences in skeletal muscle and bone mineral mass between black and white females and their relevance to estimates of body composition.
Am. J. Clin. Nutr.
55:
8-13,
1992[Abstract].
27.
Pivarnik, J. M.,
M. S. Bray,
A. C. Hergenroeder,
R. B. Hill,
and
W. W. Wong.
Ethnicity affects aerobic fitness in U. S. adolescent girls.
Med. Sci. Sports Exerc.
27:
1635-1638,
1995[Medline].
28.
Pivarnik, J. M.,
J. E. Fulton,
W. C. Taylor,
and
S. A. Snider.
Aerobic capacity in black adolescent girls.
Res. Q. Exerc. Sport
64:
202-207,
1993[Medline].
29.
Walker, J. L.,
T. D. Murray,
C. M. Johnson,
D. L. Rainey,
and
W. G. Squires, Jr.
The oxygen cost of a 20-minute steady-state jog for high school boys and girls.
Pediatr. Exerc. Sci.
2:
272-280,
1990.
30.
Webber, L. S., W. A. Wattigney, S. R. Srinivasan and G. S. Berenson. Obesity studies in
Bogalusa. Am. J. Med. Sci. 310, Suppl.
1: S53-S61, 1995.