There has been substantial evidence to support the importance of regular physical activity (PA) in maintaining good health and avoiding chronic disease, including coronary heart disease, hypertension, diabetes and some cancers (4,6,9,12,14–18). Public health recommendations have evolved to emphasize that, in addition to vigorous intensity exercise or sports, moderate-intensity lifestyle activities can provide health benefits for everyone (25). Therefore, public health goals have been established for both moderate and vigorous activity levels (24). Specifically, Healthy People (HP) 2010 objective 22-2 recommends that U.S. adults engage in at least moderate-intensity PA on at least 5 d·wk−1 for at least 30 min·d−1, or vigorous intensity PA three or more days per week for 20 or more minutes per day. Objective 22-3 recommends engagement in vigorous intensity PA three or more days per week for 20 or more minutes per day. To determine whether these goals are being met, national PA surveillance is conducted through two mechanisms, the National Health Interview Survey (NHIS), and the Behavioral Risk Factor Surveillance System (BRFSS).
The BRFSS is a random-digit dialed telephone survey of noninstitutionalized adults aged 18 and over. The survey asks about different health risk factors and is conducted by state public health departments and the Centers for Disease Control and Prevention. BRFSS prevalence data are used by state governments to design public health programs and make policy decisions. The BRFSS contains a PA module that is surveyed biennially. The current PA module, used for the first time in 2001, asks about moderate- and vigorous-intensity leisure-time, household, and transportation activities. For example, the moderate activity questions begin with “Now thinking about the physical activities that you do when you are not working, in a usual week, do you do moderate activities for at least 10 min at a time, such as brisk walking, bicycling, vacuuming, gardening, or anything else that causes some increase in breathing and heart rate?” Questions about frequency and duration follow for respondents who report doing moderate PA (questions taken from the BRFSS PA module and used in this study are included in Table 1).
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TABLE 1: BRFSS 2001 physical activity module questions utilized in this study.
Accurate PA estimates from surveillance systems are necessary in order to effectively track progress toward public health goals for PA and establish valid national PA prevalence data. Limited data exist on the accuracy of surveys used for tracking PA prevalence (1,27), and to date, the BRFSS 2001 PA module questions have not been evaluated. A multitude of different methods can be used to examine the accuracy of information recalled from PA surveys, such as doubly labeled water, motion sensors, heart rate (HR) monitoring, and PA records/logs. Although each one of these methods can provide an estimate of PA, there are inherent disadvantages to their individual use, which have been discussed elsewhere (2,10,13,20). Research has recently shown that the simultaneous use of HR and motion sensors is more accurate in quantifying energy expenditure (EE) than using either HR or motion sensors in isolation (20). The simultaneous HR-motion sensor technique (HR+M) has been shown to provide accurate predictions of EE and time spent in light (<3 METs), moderate (3–6 METs), and vigorous (≥6 METs) activity in comparison with indirect calorimetry (21). The HR+M served as the criterion measure for this investigation.
The primary purpose of this study was to compare moderate and vigorous PA reported from the BRFSS 2001 PA telephone module versus moderate and vigorous PA measured by the HR+M. A secondary objective was to compare BRFSS and HR+M classifications of participants who did or did not meet national PA recommendations.
METHODS
Participants
Volunteers participating in this study were recruited through word of mouth and from posted announcements in the local community. Twenty-five participants (12 men, 13 women) were enrolled and completed the study. Each participant read and signed an informed consent form approved by the University’s Institutional Review Board before participation.
Protocol
After completion of the informed consent, participants completed a health history questionnaire, were measured for height and weight, performed a submaximal arm crank test and a submaximal treadmill test, and received instructions on wearing the HR monitor and two motion sensors. Height and weight were measured by a stadiometer (Seca Corp., Columbia, MD) and a calibrated physician scale (Health-O-Meter, Bridgeview, IL), respectively. Body mass index was calculated using the formula body mass (kg) divided by height squared (m2). Submaximal arm and leg exercise tests were performed to develop individualized HR-oxygen uptake (V̇O2) calibration curves. The day immediately after the completion of both submaximal exercise tests, participants wore the HR monitor and two motion sensors (HR+M technique) for seven consecutive days to assess regular daily PA during all waking hours. The day after objective monitoring, participants completed questions taken from the BRFSS 2001 PA module questionnaire by telephone.
Submaximal Exercise Testing
Submaximal treadmill test.
Each participant completed a continuous, incremental walking treadmill (Quinton Instrument Co., Q65, Bothell, WA) protocol consisting of 3-min stages. Participants began walking at 67 m·min−1 and 0% grade, were increased to 94 m·min−1 and 0% grade during the second stage, and maintained a constant speed of 94 m·min−1 while the grade was increased by 2% each subsequent stage thereafter. The test was terminated once the participant reached 80–85% of age-predicted maximal HR or they requested to stop. During this time, HR was measured continuously by a Polar Vantage HR watch and transmitter band (Polar NV, Polar Oy, Kempele, Finland), and V̇O2 was measured by the TrueMax 2400 computerized metabolic measurement system (ParvoMedics, Salt Lake City, UT). The validity of the TrueMax 2400 system has previously been demonstrated in our laboratory (3). Minute-by-minute HR data and gas exchange data were imported into a Windows-based program for the development of individualized leg HR-V̇O2 regression equations.
Submaximal arm ergometer test.
Participants performed successive 3-min stages on a Monark arm ergometer (Monark 881E, Varberg, Sweden). The initial cadence was set at 50 rpm, and initial resistance at 0 kp. Thereafter, cadence remained constant at 50 rpm and resistance increased by 0.25 kp each stage. The test was terminated once the participant reached 80–85% of age-predicted maximal HR or they requested to stop. Heart rate and V̇O2 were measured using the same methods used during the treadmill test. Minute-by-minute HR data and gas exchange data from the arm ergometer test were used for the development of individualized arm HR-V̇O2 regression equations.
7-Day Field Test
The Polar Vantage NV heart watch used in laboratory testing was also used for the 7-d field assessment. This device is capable of storing 134 h of HR information in 60-s intervals. All HR data were immediately downloaded after the 7-d field test and imported into a digital file. Polar HR technology has been shown to be valid in comparison with electrocardiographic measurements in both laboratory and field settings (8,11,23).
The accelerometer (MTI Model AM7164, Manufacturing Technology Inc., Ventura, CA) was used to monitor motion during the 7-d of free-living activity during all waking hours. One MTI device was attached using a Velcro strap to the posterior aspect of the dominant arm, over the center line of the wrist. A second MTI accelerometer was attached with an elastic bandage to the mid-axillary line of the dominant thigh, orientated vertically along the femur. The specifications of this accelerometer have been reported elsewhere (21). The MTI monitors were initialized the day before the 7-d monitoring began, were both synchronized to the HR recording time, and were programmed to record data in 60-s epochs. The MTI data were downloaded after the 7-d period and imported into a digital file. Calibration of the MTI accelerometers took place at the beginning and end of the study and the monitors were found to meet the manufacturer’s specifications (± 5% of reference value).
Estimation of Energy Expenditure during Daily Activity
Data from the HR recording device and the arm and leg motion sensors were analyzed to derive minute-by-minute measures of EE. Minute-by-minute EE values were derived from V̇O2 values established from individualized laboratory generated HR-V̇O2 regression equations, using the HR+M technique. Minute-by-minute V̇O2 values in milliliters per kilograms per minute obtained from the HR+M were divided by 3.5 to derive MET per minute values (1 MET is equal to 3.5 mL·kg−1·min−1). The HR+M technique utilizes MTI motion sensors placed on the arm and leg to determine whether the activities performed were primarily upper- or lower-body activities. We have previously shown that a threshold of 500 counts·min−1 on the arm and leg motion sensors reflects a demarcation between rest and light activity (21). Once above the threshold of activity/inactivity (500 counts·min−1), we have also shown that a ratio of 25:1 between arm and leg motion sensor counts can be used to distinguish whether predominantly arm, leg, or combined arm and leg activity is taking place. Once this distinction has been made, corresponding individualized regression equation can be used to predict PA and EE. A more detailed description of this procedure is provided elsewhere (20,21).
Computation of Time Spent in Different Activity Intensities and Physical Activity Classifications
Minute-by-minute EE values over the 7 d of activity monitoring using the HR+M were used to derive individual patterns of EE. In this study, the pattern of EE is reported as time accumulated each day in moderate (3–6 METs) and/or vigorous (≥6 METs) activities. The number of days per week that participants engaged in moderate and/or vigorous activities were also established. Accumulated time and days per week of moderate and vigorous activity obtained from the BRFSS were then compared with values obtained from the HR+M. The BRFSS only asks about PA that lasted for 10 min or longer. For this purpose, HR+M estimates of PA were only included for analysis if an activity lasted at least 10 min in duration. Moderate and/or vigorous activity lasting 10 min or longer were then summed to give a total daily value of time spent (min) in moderate and/or vigorous activity.
Using information generated from individual patterns of EE, we determined whether the sample group performed either moderate PA 5 or more days per week for at least 30 min·d−1, or vigorous PA at least 3 d·wk−1 for 20 or more minutes per day (Healthy People 2010, objective 22-2), or met vigorous recommendations only (Healthy People 2010, objective 22-3).
Data Analysis
The BRFSS survey data in this study was scored at the Centers for Disease Control and Prevention, Atlanta, GA. Means and standard deviations were calculated for all measures of PA. Paired t-tests were performed to compare both days per week and minutes per day between the BRFSS and HR+M. Bland-Altman plots (5) were used to illustrate the difference scores (HR+M minus the BRFSS) for minutes per day of moderate and vigorous activities, and relationships of error were analyzed using correlation analysis. Spearman-rank order correlations were used to examine minutes of moderate and/or vigorous activity per day reported from the BRFSS and those measured by the HR+M. Percentage agreement between the HR+M and the BRFSS was calculated and analyzed using chi-square. Cohen’s kappa was used to provide evidence of the consistency of classification between measures, expressed as a percentage of agreement. All analyses were performed using SPSS version 10.0.7 (Chicago, IL) with the alpha level set at 0.05.
RESULTS
During the 7-d field test, the HR data transmitted between the chest strap and the watch-receiver was sometimes subject to interference, usually from certain types of electronic equipment near the participant, such as hairdryers and some radios. Such interference is typically indicated by a HR greater than 220 beats·min−1. In addition, a loose contact between the individual and the chest strap occasionally resulted in readings of 0 beats·min−1. Some participants also had readings of 0 beats·min−1 when traveling in an automobile. Aberrant readings were replaced by the average of the previous and subsequent values; however, if more than five aberrant readings occurred in succession, the data were not used in the analysis. A total of 22 ± 7 min·d−1 per participant were not used in data analyses for such reasons.
Participant demographic characteristics are listed in Table 2. On average, 14:00 ± 0:59 h·d−1 of data for men and 13:31 ± 0:40 h·d−1 of data for women were available for analysis. All analyses are reported for males and females combined, as no gender differences were found (data not shown).
TABLE 2: Participant characteristics (mean ± SD).
According to data from the HR+M participants spent an average of 34 min·d−1 in moderate activities, 24 min·d−1 in vigorous activities, and 58 min·d−1 in combined moderate and vigorous activities. According to the BRFSS participants spent an average of 30 min·d−1 in walking activities, 31 min·d−1 in moderate activities, 27 min·d−1 in vigorous activities, and 58 min·d−1 in combined moderate and vigorous activities. There were no significant differences between time spent in moderate or vigorous activities estimated by the BRFSS and the HR+M. The number of reported days per week of moderate or vigorous activities from the BRFSS were not significantly different than the HR+M method (see Table 3).
TABLE 3: Days per week and minutes per day of physical activity (mean ± SD, N = 25).
Spearman rank-order correlation coefficients between the estimates of moderate and/or vigorous activities are shown in Table 4. Significant correlations were found for vigorous activities between the BRFSS and the HR+M (r = 0.54, P = 0.005), and for combined moderate and vigorous activities from the BRFSS in comparison with vigorous activities from the HR+M (r = 0.46, P = 0.021). Individual differences between the BRFSS and the HR+M are shown in Bland-Altman plots (Fig. 1, A and B) for time spent in moderate and vigorous activities. Differences ranged from −70 to 77 min·d−1 for moderate activities (mean = 3.32 min·d−1, SE ± 7.36 min·d−1) and −39 to 33 min·d−1 for vigorous activities (mean = −2.76 min·d−1, SE ± 3.57 min·d−1). Correlation analysis showed that the individual differences were linearly related to the criterion standard for moderate activities (r = 0.54, P = 0.005), but not for vigorous activities (r = 0.12, P = 0.566).
TABLE 4: Spearman rank-order correlation coefficients between time spent in physical activity: BRFSS versus HR + M (N = 25).
FIGURE 1: Participant difference scores for (A) accumulated minutes per day of moderate physical activity in 10-min bouts or longer and (B) accumulated minutes per day of vigorous physical activity in 10-min bouts or longer. The horizontal solid line represents the mean differences, and the dashed lines represent the 95% confidence interval. The solid correlation line represents the line of best fit for the difference scores (N = 25); *P < 0.05.
The classification of participants PA is provided in Table 5. There was 80% agreement between the BRFSS and HR+M methods of classifying participants who met either vigorous activity recommendations or moderate and/or vigorous PA recommendations (χ2 = 7.350, K = 0.61, and χ2 = 6.625, K = 0.58, respectively). Sensitivity and specificity were calculated respectively as the proportion of those meeting moderate, vigorous, or moderate and/or vigorous activity recommendations and not meeting moderate, vigorous, or moderate and/or vigorous recommendations correctly identified. For moderate activity, vigorous activity and meeting the recommendations either moderately and/or vigorously respectively, the sensitivity was 0.83, 0.88, and 0.91, and specificity was 0.68, 0.77, and 0.71. Both sensitivity and specificity were lower for moderate activity classifications than they were for vigorous, or meeting the recommendations either moderately and/or vigorously.
TABLE 5: Intra-individual agreement between physical activity classifications by the BRFSS and HR + M (N = 25).
DISCUSSION
The BRFSS 2001 PA module is used to collect data for estimating PA prevalence at the state and national level, to evaluate compliance with national PA recommendations, and to address Healthy People 2010 objectives. Previously, the findings from an earlier version of the BRFSS PA module have been compared with objective monitoring techniques (1). Ainsworth et al. (1) reported that a waist-mounted CSA accelerometer, using three different published regression formulas relating counts per minute to gross EE, showed only weak relationships with a similar PA telephone survey for moderate activity (r = −0.01 to 0.03), and modest relationships for nonoccupational walking and moderate (r = 0.11–0.26) and vigorous (r = 0.31–0.33) activity. Employing the use of a waist-mounted accelerometer to evaluate different patterns of PA is, however, subject to several limitations. Investigations examining the accuracy of waist-mounted accelerometers to predict daily activity have shown wide limits of agreement when attempting to predict EE (2,7,10,20,22,28). The wide limits of agreement stem from the drawbacks of accelerometry. A motion sensor positioned at the waist cannot accurately quantify upper body PA or activities such as inclined walking or stair climbing. Accelerometers positioned on the hip, therefore, underestimate EE when a significant amount of upper bodywork or external work is performed. This represents a significant limitation to the use of accelerometry in quantifying different PA patterns, as these types of activities can represent a large portion of activity encountered during the course of a day. Therefore, the purpose of this study was to compare the current 2001 BRFSS PA module to an objective PA assessment technique, the HR+M, which has been shown to accurately quantify different intensity subcategories of EE and time spent in different PA classifications (20,21).
Estimates of time spent in various activity intensities.
This study showed that BRFSS group estimates of moderate and/or vigorous PA did not differ from group estimates derived from the HR+M. However, when individual error scores were examined, larger differences were apparent for moderate activities (SE ± 7.36 min·d−1; range −70 to 77 min·d−1) than for vigorous activities (SE ± 3.57 min·d−1; range −39 to 33 min·d−1). Correlation analysis of the moderate activity difference scores showed that compared with the criterion method, less active people overestimated their activity on the BRFSS whereas more active people underestimated their activity (r = 0.54, P = 0.005, Fig. 1A). Interestingly, 80% of the participants in this study reported moderate activity levels of between 15 and 45 min (range 0–90 min), irrespective of their actual measured moderate activity level (data not shown). The phenomenon of under- and overreporting was not apparent for vigorous activities performed (r = 0.12, P = 0.556, Fig. 1B). The under- and overreporting of moderate activities cancelled each other out so that no mean group differences were found between values obtained from the BRFSS and the HR+M. Other studies examining self-report measures of total PA have also demonstrated strong trends of less active individuals overreporting activity levels and more active individuals under-reporting activity levels (10,19). Further insight, with larger samples, into the behavioral aspects associated with self-reported PA would be beneficial to understand the bias of under- and overreporting.
We found weak linear relationships between reported and measured time spent in walking, moderate, and combined moderate and vigorous activities (r = 0.08, −0.10, and 0.10, respectively). Walking and moderate PA are not mutually exclusive on the BRFSS, and walking activity is not used to determine if activity recommendations are being met. A significant relationship was found between reported and measured vigorous activities (r = 0.54, P = 0.005). These results, in addition with the moderate and vigorous difference plots, are consistent with suggestions that individuals have greater ability to recall structured, vigorous PA than ubiquitous, moderate activities performed throughout the day (13,26). The greater ability to recall vigorous activities is likely due to the planned nature of such behaviors.
We were also interested in comparing the average number of accumulated minutes for moderate activity from the HR+M, including minutes of PA lasting less than 10 minutes in duration (data not shown), with those reported from the BRFSS (lasting greater than/equal to 10 min in duration). Participants spent an average of 92 min·d−1 in moderate activities as assessed by the HR+M method, whereas on the BRFSS they reported an average of 31 min·d−1. Ainsworth and colleagues (1) also reported that a previous BRFSS PA module underestimated total accumulated minutes in moderate activities over a 21-d period in comparison with a 48-item PA log (35 vs 113 min·d−1, respectively). Collectively, these findings are a function of the design of the BRFSS PA module only asking for PA that lasts for 10 min or greater. Current national PA recommendations state that PA can be accumulated in shorter bouts (8–10 min) throughout the day to total 30 min or more per day of moderate activity (25). The BRFSS only asks for activity in 10 min bouts or longer to track compliance with national PA recommendations and Healthy People objectives. The fact that individuals could be accumulating more minutes of moderate activity than previously thought could have important implications for health and weight balance. In accordance with the design of the survey, however, the BRFSS provides reasonable group mean estimates of accumulated daily minutes spent in moderate, vigorous, and combined moderate and vigorous PA.
Physical activity classifications.
There was 80% (kappa = 0.61, P = 0.002) agreement between the BRFSS and HR+M methods of classifying those who either met the recommendations (either moderate and/or vigorous) or did not meet the recommendations. Although both sensitivity and specificity were lower for moderate-activity classifications compared with vigorous or combined moderate and/or vigorous classifications, these results demonstrate that the BRFSS was able to distinguish between individuals who met PA recommendations and those who did not.
This study provides a comparison of questions taken from the BRFSS 2001 PA module with an objective measure able to accurately evaluate different dimensions of PA—but is subject to methodological limitations. The complexity and the labor intensity of the HR+M limited this study to a small nonrandom sample. The sample used in this investigation was more lean, fit, and active than the general population. For example, over 40% of sample participants met vigorous activity recommendations. Thus, this sample is not representative of the general population. Another limitation to this study is that the HR+M was not able to distinguish between different types of activities performed within individual intensity classifications (i.e., walking vs other moderate activities). Furthermore, it is possible that wearing the activity monitors could have sensitized our study sample to PA in the preceding week. This may have enhanced their ability to recall PA over the monitoring period, thus increasing the accuracy of survey responses; however, individuals were unaware of the specific nature of the questions to be asked.
In summary, the BRFSS 2001 PA module and the HR+M method gave similar group estimates of time spent in moderate and vigorous PA, but individual differences between the two methods were greater for moderate than for vigorous activity. BRFSS estimations of group compliance with national PA recommendations were similar to those of the HR+M. Additional studies with larger more heterogeneous samples would prove useful to further evaluate this particular PA questionnaire.
The authors would like to acknowledge the helpful comments given by Drs. Harold W. Kohl III and Caroline R. Richardson in preparing this manuscript.
REFERENCES
1. Ainsworth, B. E., D. R. Bassett, Jr., S. J. Strath, et al. Comparison of three methods for measuring the time spent in physical activity. Med. Sci. Sports Exerc. 32: S457–S464, 2000.
2. Bassett, Jr., D. R., B. E. Ainsworth, A. M. Swartz, et al. Validity of four motion sensors in measuring moderate intensity physical activity. Med. Sci. Sports Exerc. 32: S471–S480, 2000.
3. Bassett, Jr., D. R., E. T. Howley, D. L. Thompson, et al. Validity of inspiratory and expiratory methods of measuring gas exchange with a computerized system. J. Appl. Physiol. 91: 218–224, 2001.
4. Blair, S. N. Physical activity, physical fitness, and health. Res. Q. Exerc. Sport 64: 365–376, 1993.
5. Bland, J. M., and D. G. Altman. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 8:1: 307–310, 1986.
6. Helmrich, S. P., D. R. Ragland, R. W. Leung, and R. S. Paffenbarger, Jr. Physical activity and reduced occurrence of non-insulin dependent diabetes mellitus. N. Engl. J. Med. 325: 147–152, 1991.
7. Hendelman, D., K. Miller, C. Bagget, E. Debold, and P. Freedson. Validity of accelerometry for the assessment of moderate intensity physical activity in the field. Med. Sci. Sports Exerc. 32: S442–S449, 2000.
8. Karvonen, J., J. Chwalbinska-Moneta, and S. Saynajakangas. Comparison of heart rates measured by ECG and microcomputer. Physician Sportsmed. 12: 65–69, 1984.
9. Lee, I.–M. Physical activity, fitness and cancer, In: Physical Activity, Fitness, and Health, C. Bouchard, R. J. Shepard, and T. Stephens (Eds.). Champaign, IL: Human Kinetics, Inc., 1994, pp. 814–831.
10. Leenders, N. Y. J. M., W. M. Sherman, H. N. Nagaraja, and C. L. Kien. Evaluation of methods to assess physical activity in free-living conditions. Med. Sci. Sports Exerc. 33: 1233–1240, 2001.
11. Leger, L., and M. Thivierge. Heart rate monitors: validity, stability, and functionality. Physician Sportsmed. 16: 143–151, 1988.
12. Manson, J. E., D. M. Nathan, A. S. Krolewski, M. J. Stampfer, W. C. Willett, and C. H. Hennekens. A prospective study of exercise and incidence of diabetes among U.S. male physicians. JAMA 268: 63–67, 1992.
13. Montoye, H. J., H. C. G. Kemper, W. H. M. Saris, and R. A. Washburn. Measuring Physical Activity and Energy Expenditure. Champaign, IL: Human Kinetics, Inc., 1996, p. 103.
14. Morris, J. N., D. G. Clayton, M. G. Everitt, A. M. Semmence, and E. H. Burgess. Exercise in leisure time: coronary attack and death rates. Br. Heart J. 63: 325–334, 1990.
15. Paffenbarger, Jr., R. S., A. L. Wing, R. T. Hyde, and D. L. Jung. Physical activity and incidence of hypertension in college alumni. Am. J. Epidemiol. 117: 245–257, 1983.
16. Paffenbarger, Jr., R. S., R. Hyde, A. Wing, and C. Hsieh. Physical activity, all-cause mortality, and longevity of college alumni. N. Engl. J. Med. 314: 605–613, 1986.
17. Shaper, A. G., and G. Wannamethee. Physical activity and ischaemic heart disease in middle-aged British men. Br. Heart J. 66: 384–394, 1991.
18. Shaper, A. G., G. Wannamethee, and M. Walker. Physical activity, hypertension and risk of heart attack in men without evidence of ischaemic heart disease. J. Hum. Hyperten. 8: 3–10, 1994.
19. Starling, R. D., D. E. Matthews, P. A. Ades, and E. T. Poehlman. Assessment of physical activity in older individuals: a doubly labeled water study. J. Appl. Physiol. 86: 2090–2096, 1999.
20. Strath, S. J., D. R. Bassett, Jr., A.M. Swartz, L. Thompson. Simultaneous heart rate-motion sensor technique to estimate energy expenditure. Med. Sci. Sports Exerc. 33: 2118–2123, 2001.
21. Strath, S. J., D. R. Bassett, Jr. D.L. Thompson, and A. M. Swartz. Validity of the simultaneous heart rate motion sensor technique to estimate energy expenditure. Med. Sci. Sports Exerc. 34: 888–894, 2002.
22. Swartz, A. M., S. J. Strath, D. R. Bassett, Jr., W. L. O’Brien, G.A. King, and B. E. Ainsworth. Estimation of energy expenditure using CSA accelerometers at the hip and wrist sites. Med. Sci. Sports Exerc. 32: S450–S456, 2000.
23. Treiber, F. A., L. Musante, S. Hartdagan, H. Davis, M. Levy, and W. B. Strong. Validation of a heart rate monitor with children in laboratory and field settings. Med. Sci. Sports Exerc. 21: 338–342, 1989.
24. U.S. Department of Health and Human Services. Healthy People 2010, Vol. II, Conference Edition. Washington, DC: U.S. Department of Health and Human Services, 2000, p. 72.
25. U.S. Department of Health and Human Services. Physical Activity and Health: A Report of the Surgeon General. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, 1996, pp. 6–7.
26. Wareham, N. J., and K. L. Rennie. The assessment of physical activity in individuals and populations: why try to be more precise about how physical activity is assessed? Int. J. Obes. 22: S30–S38, 1998.
27. Weiss, T. W., C. H. Slater, L. W. Green, V. C. Kennedy, D. L. Albright, and C. Wun. The validity of a single item, self-assessment questions as measures of adult physical activity. J. Clin. Epidemiol. 43: 1123–1129, 1990.
28. Welk, G. J., S. N. Blair, K. Wood, S. Jones, and R. Thompson. A comparative evaluation of three accelerometry-based physical activity monitors. Med. Sci. Sports Exerc. 33: S489–S497, 2000.
