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Research Reports |
B Drinkard, PT, MSPT, CCS, is Senior Physical Therapist, Rehabilitation Medicine Department, Warren Grant Magnuson Clinical Center, National Institutes of Health (NIH), Bethesda, MD 20892 (USA) (bart_drinkard{at}nih.gov). Address all correspondence to Mr Drinkard
J McDuffie, PhD, is Postdoctoral Fellow, Unit on Growth and Obesity, Developmental Endocrinology Branch, National Institute of Child Health and Human Development (NICHHD), NIH
S McCann, BA, is Research Trainee, Unit on Growth and Obesity, Developmental Endocrinology Branch, and Division of Digestive Diseases and Nutrition, National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), NIH
G Uwaifo, MD, is Clinical Fellow, Unit on Growth and Obesity, Developmental Endocrinology Branch
J Nicholson, MD, is Clinical Research Trainee, Unit on Growth and Obesity, Developmental Endocrinology Branch
JA Yanovski, MD, PhD, is Chief, Unit on Growth and Obesity, Developmental Endocrinology Branch
Dr Yanovski and Mr Drinkard are commissioned officers in the US Public Health Service
Mr Drinkard, Dr McDuffie, and Dr Yanovski provided concept/research design. Mr Drinkard, Dr McDuffie, Dr Uwaifo, and Dr Yanovski provided writing. Mr Drinkard, Dr McDuffie, Dr Nicholson, and Dr Yanovski provided data collection, and Dr Yanovski, Dr McDuffie, and Ms McCann provided data analysis. Dr Yanovski, Ms McCann, and Dr Uwaifo provided project management. Dr Yanovski provided fund procurement, subjects, and institutional liaisons. Mr Drinkard provided facilities/equipment. The authors thank Susan Mihans, PT, for her assistance in conducting the study
Submitted October 31, 2000;
Accepted March 27, 2001
 | Abstract |
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O2peak), anaerobic threshold (AT), peak power (Wpeak), and power at the anaerobic threshold (WAT) were determined. Body composition was determined by air displacement plethysmography. Results. The mean percentage of body fat was 48.6% (SD=5.3%, range=40.3%–60.4%). Percentage of body fat and BMI were each inversely related to D9, D12, and O2peak (all P<.005). Peak oxygen uptake (r=.72, P=.0001), O2peak/kg lean body mass (r=.60, P<.005), Wpeak (r=.88, P<.0001), and WAT (r=.72, P=.0007) were all related to D12, with greater r values than for D9. If D9–12was included in regression analyses, D9 did not account for additional variance in any of the cycle ergometry variables. Discussion and Conclusion. These results suggest that an easily obtained measurement of physical performance (distance traveled during a 12-minute walk/run test) is related to cardiorespiratory fitness and to body composition in adolescents who are overweight. The 12-minute walk/run distance is more predictive of cycle ergometry test results than the 9-minute distance.
Key Words: Adolescence • Body adiposity • Exercise testing • Obesity • Physical fitness • Walking
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Decreased physical activity and a sedentary lifestyle have been implicated as factors in the development of pediatric obesity.8–10 Aerobic exercise is widely used in the management of obesity.11,12 However, there is little information regarding how clinicians should evaluate cardiorespiratory fitness in children and adolescents who are overweight. We believe there is a need for a practical method of measuring fitness, without the requirement of specialized equipment or expertise, in adolescents who are overweight.
Walk/run tests, in which time or distance is the outcome measure, are generally accepted methods of reflecting cardiorespiratory fitness.13–18 One-mile walk/run test performance is related to peak oxygen uptake (O2peak) in children and adolescents who are not overweight.16,17 Time-based walk/run tests of 9 and 12 minutes have been found to yield reliable measurements (with test-retest reliability coefficients of .92–.94)13,19 and to have concurrent validity with measurements of O2peak in children and adolescents who are not overweight.13,14,19 Some investigators have speculated that the 12-minute walk/run test may better measure exercise tolerance among people than tests of shorter duration.20 Other investigators21,22 have reported that walk/run tests of longer duration (eg, 12–15 minutes) versus shorter duration (eg, 5–9 minutes) are more reflective of O2peak. However, Jackson and Coleman14 have reported that there are no differences between the 9- and 12-minute tests' relationship with O2peak. To our knowledge, there have been no systematic investigations of walk/run performance in adolescents who are overweight.
We examined the relationships among walk/run performance, cardiorespiratory fitness, and body composition in adolescents who were obese. We hypothesized that the distance traveled during a 12-minute walk/run test would be highly correlated with oxygen uptake (O2), peak power (Wpeak), and power at the anaerobic threshold (WAT) measured during cycle ergometry. In addition, we hypothesized that the walk/run distance at 12 minutes would be more closely related to the oxygen uptake, Wpeak, and WAT than the distance at 9 minutes.
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95th percentile for age, sex, and race). Subjects were included only if they were in good general health, with the exception of a predefined list of obesity-related comorbid conditions such as hyperlipidemia and hyperinsulinemia. Subjects were excluded if they had used a suppressant medication within the past 6 months; were pregnant; had major pulmonary, hepatic, or cardiac disorders; or had recently lost weight. Before exercise testing commenced, a medical history and physical examination was done for each subject, and it included a cardiopulmonary examination and a 12-lead electrocardiogram. All subjects were free of musculoskeletal injury as determined by a physician, and American Heart Association guidelines for exercise testing were observed.23 The subjects' parents provided signed consent statements (and children gave their assent) for all studies.
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Walk/Run Testing Procedures
A single 12-minute walk/run test was performed. Before exercise, resting blood pressure and heart rate were measured. Each subject was encouraged to give his or her best effort—to walk and/or run to cover as much distance as possible in 12 minutes. A hallway, 47 m in length and 1.8 m in width, was used as the course.
Heart rate was measured and recorded at 3, 6, 9, and 12 minutes into the walk/run test using a heart rate telemeter with a wristwatch receiver (Polar Vantage NV*). The absolute highest heart rate during the walk/run test was defined as the peak walk/run heart rate. The average heart rate for the 12-minute test also was calculated. Total distance (in meters) traveled at 9 minutes (D9) and at 12 minutes (D12) was measured using a measuring wheel (model MM34
). From these 2 measurements, distance traveled between 9 and 12 minutes (D9–12) was determined.
Cycle Ergometry Testing Procedures
Subjects were studied in the afternoon after they ate. Following familiarization with cycle ergometer (Ergoline 800
) testing procedures, resting heart rate and blood pressure were measured. Subjects were then encouraged to exercise to the limit of their self-determined tolerance and were instructed to maintain pedaling cadence between 60 and 70 revolutions per minute. Exercise began with a 4-minute warm-up period with no additional resistance applied to the pedals, followed by progressively increasing workloads of 15 to 20 W/min, until the subject was unable to continue (signaled by the subject with a "thumbs down" sign) or could no longer maintain the prescribed pedaling cadence. The rate of workload increase for each subject was selected based on predicted maximal power.24
Expired gas exchange was measured breath by breath during exercise using a metabolic cart (Sensormedics Vmax). Peak oxygen uptake, which is the highest O2 achieved during exercise, and respiratory exchange ratio were calculated as average values during the last 20 seconds of exercise. Anaerobic threshold (AT) was determined using the V-slope method.25 In order to scale O2peak and AT among subjects, those measures were expressed relative to body weight as mL O2/kg/min for data analysis. Power at AT (WAT) and at peak exercise (Wpeak) were also determined and recorded in watts per kilogram. Heart rate was measured by a 12-lead electrocardiogram (Sensormedics Max-1) during exercise. Blood pressure was measured every 3 minutes during exercise. Peak exercise rating of perceived exertion was measured within the first minute of exercise recovery using the 20-point Borg Rating of Perceived Exertion Scale.
Body Composition Assessment by Air Displacement Plethysmography
For determination of body composition, subjects were studied in the morning, after an overnight fast. They were instructed to void before measurements were obtained. Body density was assessed using an air displacement plethysmography body composition system (Bodpod
) according to the manufacturer's directions using the procedures previously described.26
Measurements were taken with all subjects wearing minimal clothing (either underwear or a tight-fitting bathing suit) and wearing a swim cap. Thoracic gas volume was measured during tidal breathing and during exhalation against a mechanical obstruction. The procedure has been described in detail elsewhere.26 Percentage of body fat was determined from body density using the standard 2-compartment model.27 Measurements of percentage of body fat were converted into kilograms of body fat before analysis by multiplying percentage of body fat and total body weight together. We did not examine the reliability of these measurements.
Statistical Analysis
Data were analyzed on a Macintosh PowerPC using SuperAnova 1.11 and StatView 4.5 software.|| Simple linear regression was used to determine the relationships of D9, D12, and D9–12 with each other, with body composition, with heart rate during the walk/run test, and with measurements derived from the cycle ergometer test (Tab. 2). To determine whether D12 was superior as a predictor of cycle ergometry performance compared with D9, 2 prediction models for O2peak, O2peak/kg lean body mass, AT, Wpeak, WAT, and peak respiratory exchange ratio were constructed—one containing D9 and the other containing both D9 and D9–12. For each variable of interest, we compared the amount of variance accounted for by these 2 models using the F test for analysis of concomitant variables.28 This test determines whether inclusion of a factor, even one that may be correlated with other variables in the model, adds information and improves the ability of the model to predict the dependent variable.
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O2peak and AT. The O2peak and AT values were predicted first from a previously published equation that included height and age29 and second from a previously published equation that included weight and age.30 | Results |
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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Results from the 18 subjects who completed both the walk/run test and the cycle ergometer test are given in Table 2. Average (±SD) 9- and 12-minute walk/run distances were 883.3±162.2 m (D9) and 1,174.4±208.1 m (D12), respectively. Mean heart rate rose between the initial pre-exercise level and 3 minutes of the walk/run test (from 83 bpm [SD=9.2, range=68–102] to 165 bpm [SD=18.1, range=118–200], P<.001), and remained stable thereafter (Fig. 1). Although there was some change in mean heart rate between 9 and 12 minutes of the walk/run test (from 165.0 bpm [SD=18.0, range=127–199] to 171 bpm [SD=13.0, range=140–192]), this increase was not statistically significant. The D9, D12, and D9–12 measurements were not different between the male and female subjects and were not correlated with age. Both D12 (P<.004) and D9–12 (P<.008), but not D9, were significantly correlated with height. The D9, D12, and D9–12 measurements were each negatively correlated with total body weight, BMI, and body fat mass (Tab. 2, all r values between –.51 and –.82, P<.005). The D9, D12, and D9–12 measurements were positively correlated with each other (Tab. 2, all r values between .64 and .97, P<.005). Average walk/run heart rate was related to D9, and D12, and peak walk/run heart rate was related to D12 (Tab. 2, P<.05).
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O2peak, and AT were 53%±21% and 60%±25% of predicted values, respectively (both P<.001). In contrast, there were no differences between measured and predicted O2peak and AT when predicted values were determined using height and age.
Walk/run and cycle ergometer test results were found to be related to one another (Tab. 2). Measurements of O2peak, O2peak/kilograms of lean body mass, AT, Wpeak, and WAT were all positively related to D9, D12, and D9–12 (Tab. 2). The strongest relationships were found between walk/run distance and either Wpeak (Tab. 2; all r.82, P<.005) or O2peak (Figs. 2A–2C; all r.63, P<.005). Twelve-minute walk/run distance explained a greater percentage of variance than 9-minute distance for the following cycle ergometer variables: O2peak, O2peak/kilograms of lean body mass, AT, Wpeak, peak respiratory exchange ratio (Tab. 2). When D9–12 (the distance from minute 9 to minute 12) was included in a multiple regression analysis, the distance traveled during the first 9 minutes of the test did not supply additional explanatory power for any of these variables (all P>.2; F test for analysis of concomitant variables).
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| Discussion |
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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O2peak, AT, Wpeak, and WAT, and were negatively correlated with measurements of body adiposity, including body weight, BMI, and body fat mass. These findings appear to support and extend previous reports of the relationships between walk/run performance and cardiorespiratory fitness in children and adolescents who were not overweight13,14,16,17,19,21 and the relationships between walk/run performance and body composition in children who were not overweight.15,31
We found walk/run distances to be substantially less than those reported for people who were not obese. Other researchers14,15,19,32 have reported 12-minute walk/run distances ranging from approximately 1,800 m to 2,500 m in children and adolescents. The mean 12-minute walk/run distance we found was 1,174 m, and no subject exceeded 1,500 m. Some authors33,34 have concluded that cardiorespiratory fitness is decreased during exercise in adults and children who are obese compared with people who are not obese. Yet, Cooper et al35 reported that the majority of children who were obese in their study had a normal O2peak compared with predicted norms. Other researchers36,37 have concluded that cardiorespiratory fitness is not decreased in children and adolescents who are obese but that walk/run performance may be decreased as a consequence of the increased energy cost of work associated with the load caused by adipose tissues.
Although subjects in our study had an average O2peak that was less than 60% of values predicted from age and body weight, O2peak was 92% of values predicted from age and height. We believe that we studied an insufficient number of subjects to determine the possibility of decreased cardiorespiratory fitness in the larger population. Given that a large proportion (
50%) of the variance in walk/run performance could be explained by variation in body mass, however, our results are consistent with suggestions that differences in walk/run performance among children who were obese and children who are not obese are largely a function of the load caused by adipose tissue.
Epstein et al38 compared the O2 associated with carrying 2 different backpack loads during prolonged treadmill walking at a constant speed. They found that the oxygen cost of carrying a backpack was constant over time for a 25-kg load. Carrying a 40-kg backpack induced a continual increase in O2 over time, which became greater than the O2 for the 25-kg backpack during the latter stages of exercise. Epstein et al hypothesized that large loads, eliciting greater than 50% O2peak, alter locomotion biomechanics and lead to an increase in energy costs over time and subsequent physical fatigue. The adolescents we examined who were greatly overweight all carried more than 30 kg of adipose tissue, and 70% had body fat mass in excess of 40 kg. We believe that our subjects have altered biomechanics during locomotion. This may, at least in part, account for their poor performance on the walk/run test.
Oxygen uptake varies only with work rates below the AT, whereas it varies with both work rate and time at exercise intensities above the AT.39 Probably not coincidentally, Epstein et al38 reported an increase in the oxygen cost of work over time at exercise intensities greater than 50% of O2peak. This is the exercise intensity at which AT usually occurs.24 In addition, some authors39 believe the increase in O2 over time for a given work rate is greater in people who are less fit. Although the precise cause of this phenomenon is unknown, it may have direct implications for tests such as the walk/run test. Walk/run tests of longer duration may better discriminate cardiorespiratory fitness among people.
Butland et al20 found greater variability in 12-minute walk/run distance compared with 2- and 6-minute test results for the same subjects. They speculated that the increased variability in 12-minute test results represented greater discrimination of exercise tolerance among subjects. Because they did not measure O2, they could not offer strong conclusions in this regard. Other researchers21,22 have found that walk/run tests of longer duration are more strongly related to O2peak. In our study, we found that differences in O2peak were better distinguished using the 12-minute distance than the 9-minute distance (Tab. 2).
When the 9-minute distance (D9) was included in a regression model with D9–12, D9 did not explain any further variance in cardiorespiratory variables. We hypothesize that the improved ability of the 12-minute test to predict cardiorespiratory fitness may be due to the increased relative aerobic requirement associated with the longer test. We found evidence that most of our subjects performed the latter portion of the walk/run test at intensities above AT where an increase in O2 over time is likely to occur. Therefore, as a result of having less aerobic reserve, subjects who were less fit could be predisposed to decrease the distance traveled during the latter portion of the 12-minute test. We, however, did not perform a separate 9-minute walk/run test, and such testing might yield a different result.
We believe that it is unlikely that the total energy cost of work performed during the walk/run test would be reflected by O2 alone. Most subjects exceeded their heart rate at the anaerobic threshold (determined from the cycle test) during the walk/run test, indicating an anaerobic energy contribution to walk/run performance. Accordingly, Wpeak and WAT explained greater variance in walk/run distance than O2peak and AT. Peak power has been shown to be a good predictor of endurance in adults.40 Our findings suggest that power expressed relative to body mass may be a better predictor of endurance performance than O2in adolescents who are overweight. One possible explanation for our results may be that both aerobic and anaerobic energy expenditure are represented by the Wpeak and WAT measurements.
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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O2 are not available. Adolescents who are overweight may exceed their anaerobic thresholds during the walk/run test. In our opinion, the effect of an increase in O2 over time is likely to be an important contributor to the relationship between walk/run performance and cardiorespiratory fitness. Additional investigation is needed with direct measures of O2 during the walk/run test to confirm this hypothesis. Further investigations, using separate walk/run tests of varying duration, are needed to determine more exact relationships between walk/run test duration and cardiorespiratory fitness.
| Footnotes |
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This work was supported by a grant from the National Institute of Child Health and Human Development (Z-01-HD-04-00641) to Dr Yanovski.
This work was presented at the October 2000 meeting of the North American Association for the Study of Obesity, Long Beach, Calif.
The opinions expressed in this article are those of the authors and not necessarily those of the US Public Health Service or the National Institutes of Health.
* Polar Electro Inc, 370 Crossways Park Dr, Woodbury, NY 11797. 
Rolotape Corp, 2701 N Van Marter Dr, Spokane, WA 99206.
Sensormedics Corp, 22705 Savi Ranch Pkwy, Yorba Linda, CA 92687.
Life Measurement Instruments, 1980 Olivera Rd, Ste C, Concord, CA 94520.
|| Abacus Concepts Inc, 1984 Bonita Ave, Berkeley, CA 94704.
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