- Email: [email protected]
Meeting the 2008 Physical Activity Guidelines for Americans Among U.S. Youth
Meeting the 2008 Physical Activity Guidelines for Americans Among U.S. Youth MinKyoung Song, PhD, RN, FNP-BC, Dianna D. Carroll, PhD, Janet E. Fulton, PhD This activity is available for CME credit. See page A3 for information.
Background: Participation in physical activity brings health benefıts for adolescents. However, limited data are available on the percentage of U.S. adolescents who engage in levels of aerobic and muscle-strengthening activities recommended in the 2008 Physical Activity Guidelines for Americans (2008 Guidelines).
Purpose: To examine the prevalence at which U.S. adolescents aged 12–17 years meet the 2008 Guidelines, and whether demographic and BMI variables influence that prevalence. Methods: Using data from an interviewer-administered self-report questionnaire in the 1999 –2006 National Health and Nutrition Examination Survey (analyzed in 2011), estimates were made of the percentage of adolescents who engaged in recommended levels of aerobic and muscle-strengthening activities (ⱖ60 minutes of aerobic activity/day and participation in muscle-strengthening activities ⱖ3 days/week).
Results: Among 6547 U.S. adolescents aged 12–17 years, 16.3% (95% CI⫽14.9%, 17.9%) met both aerobic and muscle-strengthening guidelines; 14.7% (13.3%, 16.2%) met the aerobic guideline only, 21.3% (19.4%, 23.3%) met the muscle-strengthening guideline only, and 47.8% (45.4%, 50.1%) met neither guideline. Adjusted for covariates, odds of meeting either the aerobic or muscle-strengthening guideline only or both guidelines versus meeting neither guideline were (p⬍0.05) higher among boys than girls. The odds of meeting the aerobic guideline only were higher among underweight/normalweight adolescents than among obese adolescents. No clear pattern was observed by family groups according to poverty-to-income ratio. Conclusions: Less than 20% of adolescents reported engaging in recommended levels of both aerobic and muscle-strengthening activities. (Am J Prev Med 2013;44(3):216 –222) Published by Elsevier Inc. on behalf of American Journal of Preventive Medicine
Introduction
P
hysical activity has numerous health benefıts for youth including increased cardiorespiratory fıtness and muscular strength, reduced body fat, improved cardiovascular and metabolic health biomarkers, enhanced bone health, and reduced symptoms of depression and/or anxiety.1–3 Accordingly, the DHHS detailed physical activity guidelines for youth aged 6 –17 years in the 2008 Physical Activity Guidelines for Americans (2008 Guidelines).4 These guidelines recommend that youth participate in a minimum of 60 minutes of physical activFrom the Epidemic Intelligence Service (Song), the Division of Nutrition, Physical Activity, and Obesity (Song, Carroll, Fulton), CDC, and the Commissioned Corps, U.S. Public Health Service (Carroll), Atlanta, Georgia Address correspondence to: MinKyoung Song, PhD, RN, FNP-BC University of Michigan School of Nursing, 400 North Ingalls Building, Ann Arbor MI 48109-5482. E-mail: [email protected]. 0749-3797/$36.00 http://dx.doi.org/10.1016/j.amepre.2012.11.016
216 Am J Prev Med 2013;44(3):216 –222
ity daily and engage in muscle-strengthening activity at least 3 days per week. However, there is a gap in existing knowledge about how many U.S. youth engage in physical activity at levels suffıcient to meet those guidelines. Although a few studies have described, separately, levels of aerobic5–7 and muscle-strengthening activity8 in youth, only one, a CDC study9 published in 2010, has reported levels of aerobic and muscle-strengthening activities together. That CDC study9 assessed aerobic and muscle-strengthening activity levels among U.S. high school students. Self-administered questionnaires, with one question to assess aerobic activity and one to assess muscle-strengthening activity, were used in classrooms, and showed that only one in ten students met the recommended levels of aerobic and muscle-strengthening activities. To augment those CDC fındings,9 the current study assessed data collected through face-to-face interviews using questions on a broad range of activities.
Published by Elsevier Inc. on behalf of American Journal of Preventive Medicine
Song et al / Am J Prev Med 2013;44(3):216 –222
The primary objectives were to use more-detailed data to examine the prevalence of U.S. adolescents meeting the 2008 Guidelines and to determine whether demographic and BMI variables are associated with meeting the guidelines. Four exclusive assessment criteria based on the 2008 Guidelines were used: meeting the aerobic guideline only, meeting the muscle-strengthening guideline only, meeting both aerobic and muscle-strengthening guidelines, and meeting neither guideline. Assessing levels of physical activity from participation in specifıc activities will strengthen understanding in this sparsely researched area.
Methods Survey Description The National Health and Nutrition Examination Survey (NHANES) is a stratifıed and multistage probability cluster sample survey designed to represent the U.S. non-institutionalized civilian population.10 Conducted in 2-year cycles, NHANES collects data from participants (of all ages) through household interviews followed by physical examinations, including measurement of height and weight, at a mobile examination center. Physical activity data were collected in household interviews for youth aged 16 –17 years and in interviews at such centers for those aged 12–15 years. The interview questions were the same, regardless of the interview location. Physical activity questions were the same across all four of the 2-year cycles (1999 –2006). Overall response rates for household interviews (for all participants) ranged from 79% (2003–2004) to 84% (2001–2002); among the target group of youth aged 12–17 years, the response rates for household interviews and the response rate for the mobile examination center interviews ranged from 83% to 88%.10
Sample Although NHANES is conducted in 2-year cycles, for the purposes of the current study, 1999 –2006 data were pooled to obtain an adequate sample size of youth aged 12–17 years. During the 2-year cycles from that period, sample sizes ranged from 12,160 (1999 – 2000) to 13,156 (2001–2002), resulting in a combined sample of 41,474 for the entire 8-year span. From this total sample, 6968 participants aged 12–17 years were identifıed, after accounting for the sample weights. After excluding participants with missing data on physical activity, BMI, or demographic variables, this analysis included 6547 U.S. adolescents aged 12–17 years. The four waves of data were more or less equally represented in the fınal analytic sample (n⫽6547) (1999 –2000: 22.0%, 2001–2002: 27.5%, 2003– 2004: 24.7%, and 2005–2006: 25.7%).
217
slight to moderate increase in breathing or heart rate? Some examples are brisk walking, bicycling for pleasure, golf, or dancing. Participants who answered yes to either of these questions were asked to name each activity they engaged in, provide details about the frequency of their participation in the past 30 days, and report the average duration of each session. The NHANES includes information on 48 physical activities, of which 42 aerobic activities (excluding activities categorized as “other” in the questionnaire and fıve muscle-strengthening activities: push-ups, sit-ups, stretching, weight-lifting, and yoga) were included. The weekly frequency was calculated for each participant, for each reported activity by dividing the 30-day frequency by 4.29. Next, the weekly frequencies were summed for all moderateand vigorous-intensity-level activities reported by each individual to yield a total weekly frequency of aerobic activity. Youth with a total weekly frequency of seven or more times met the frequency recommended by the 2008 Guidelines. To estimate the total daily volume of aerobic activity, fırst the reported duration of each activity session was multiplied by the weekly frequency of that activity to obtain a weekly volume for each activity. If a participant reported engaging in an activity at both a moderate- and vigorous-intensity level, a weekly volume for each intensity was calculated. Then the weekly moderate- and vigorous-intensity volume was summed for each activity. Next, the total weekly volume of aerobic activity was calculated by summing the volume of all reported activities. Last, the daily time spent in aerobic activity was estimated by dividing the total weekly volume of activities by 7. Participants who reported participating in ⱖ60 minutes of aerobic activity per day, 7 days per week, met the recommendation for aerobic activity according to the 2008 Guidelines.
Muscle-strengthening activity. To assess muscle-strengthening, participants were asked: Over the past 30 days, did you do any physical activities specifıcally designed to strengthen your muscles such as lifting weights, push-ups, or sit-ups? Participants who answered yes to this question were asked to provide the frequency of their participation. The monthly frequency of activities participants engaged in was divided by 4.29 to obtain weekly estimates of muscle-strengthening activity. Participants who reported engaging in muscle-strengthening activity three or more times/week met the frequency recommended by the 2008 Guidelines. Meeting the 2008 Guidelines. Study participants were di-
Measures
vided into four mutually exclusive subgroups: those who met the aerobic guideline only (ⱖ60 minutes of aerobic activity/day, 7 days/ week, but did not meet the muscle-strengthening guideline); those who met the muscle-strengthening guideline only (muscle-strengthening activities three or more times/week but did not meet the aerobic guideline); those who met both guidelines (ⱖ60 minutes of aerobic activity/day, 7 days/week, and muscle-strengthening activities three or more times/week); and those who met neither guideline.
Aerobic activity. To assess aerobic activity, participants were asked about vigorous- and moderate-intensity activities engaged in at school or in leisure time over the previous 30 days. To assess vigorousintensity aerobic activity, participants were asked, Did you do any vigorous activities for at least 10 minutes that caused heavy sweating, or large increases in breathing or heart rate? Some examples are running, lap swimming, aerobics classes, or fast bicycling. To assess moderateintensity aerobic activity, participants were asked: Did you do moderate activities for at least 10 minutes that cause only light sweating or a
Demographic characteristics and BMI. Participants were categorized into two age groups (12–15 years and 16 –17 years) using self-reported age. Race/ethnicity was categorized into four groups (non-Hispanic white, non-Hispanic black, Mexican Americans, and other) using a variable derived by combining responses to questions on race and Hispanic origin. Multiracial adolescents were classifıed as other. Family poverty-to-income ratio is a variable representing the ratio of household income to the DHHS’s poverty threshold based
March 2013
218
Song et al / Am J Prev Med 2013;44(3):216 –222
on family size and adjusted for annual updates of inflation. The ratio was categorized into four groups (⬍100%, 100%–199%, 200%–299%, and ⱖ300%), where ⬍100% is below the poverty threshold. BMI was computed by standard formula, and categorized into three groups (underweight/normal-weight [⬍85th percentile]; overweight [85th– ⬍95th percentile]; and obese [ⱖ95th percentile]) according to the gender- and age-specifıc reference data from the Year-2000 CDC growth charts.11
Data Analysis First, the prevalence and 95% CI was estimated of U.S. adolescents aged 12–17 years who met the aerobic guideline only, met the musclestrengthening guideline only, met both guidelines, or met neither guideline. Differences by categories of gender, age, race/ethnicity, family poverty-to-income ratio, and BMI were assessed overall and using t-tests for pairwise subgroup comparisons if any overall difference existed within the group. Second, using multinomial logistic regression, the association of demographics, BMI, and meeting the guidelines in unadjusted and adjusted models was examined, controlling for gender, age, race/ethnicity, family poverty-to-income ratio, and BMI. The SAS-callable SUDAAN, version 9.0.1, software was used to account for the complex survey design. To produce nationally representative estimates, sample weights were used to account for differential probabilities of selection, nonresponse, and noncoverage. SEs were estimated with SUDAAN using Taylor-series linearization. Signifıcance was determined at p ⬍0.05, and Bonferroni corrections were used for multiple comparisons among different characteristic groups (e.g., underweight/normal-weight versus overweight versus obese). Sensitivity analysis was performed to determine whether the excluded participants differed from the participants in the fınal sample in terms of demographics and BMI. Linear and quadratic trends of meeting the guidelines over the four waves of data were also tested. The analysis was conducted in 2011.
Results Nationally representative estimates from the current study were based on a sample of 6547 participants. Selected characteristics of the weighted sample are presented in Table 1. Based on 2008 Guidelines criteria, 14.7% of U.S. adolescents met the aerobic guideline only, and 21.3% met the muscle-strengthening guideline only (Table 2). The percentage of boys who met the aerobic guideline only or the muscle-strengthening guideline only was higher than that for girls. The percentage meeting the aerobic guideline only was higher among nonHispanic white adolescents than Mexican-American and other adolescents; and among underweight/normalweight adolescents than obese adolescents. Less than 20% of U.S. adolescents (16.3%) met both the aerobic and muscle-strengthening guidelines. The percentage of adolescents meeting both guidelines was higher among boys than girls; among non-Hispanic white adolescents than Mexican-American adolescents; and among those in the ⱖ300% poverty-to-
Table 1. Weighteda sample characteristics of adolescents, National Health and Nutrition Examination Survey, 1999 –2006 Characteristic
n
% (95% CI)
Male
3307
51.1 (49.4, 52.7)
Female
3240
48.9 (47.3, 50.6)
12–15
4288
65.4 (63.7, 67.1)
16–17
2259
34.6 (32.9, 36.3)
Non-Hispanic white
1681
61.8 (58.1, 65.3)
Non-Hispanic black
2088
14.4 (12.1, 17.0)
524
12.8 (10.5, 15.4)
Gender
Age group, years
Race/ethnicity
Other Mexican-American
2254
11.1 (9.2, 13.2)
Family poverty-to-income ratio, % ⱖ300
1717
39.6 (36.8, 42.5)
200–299
954
16.2 (14.7, 17.8)
100–199
1605
20.5 (18.8, 22.2)
⬍100
2271
23.7 (21.9, 25.7)
Underweight or normal-weight (⬍85)
4135
67.2 (65.2, 69.1)
Overweight (85–⬍95)
1068
15.6 (14.2, 17.2)
Obese (ⱖ95)
1344
17.2 (15.7, 18.8)
b
BMI (percentile)
Note: Percentages may not add up to 100.0 because of rounding. a Estimates are weighted to account for the complex survey design (including oversampling); survey nonresponse; and post-stratification. b BMI estimates were calculated by standard formula and classified based on gender- and age-specific reference data from the Year 2000 CDC growth charts.
income ratio level than those in the 100%–199% level or those in the ⬍100% level (Table 2). Slightly less than half of U.S. adolescents (47.8%) met neither the aerobic nor the muscle-strengthening guideline. The percentage of adolescents meeting neither guideline was higher among girls than boys; among those in the ⱖ300% poverty-to-income ratio level than those in the 100%–199% level or those in the ⬍100% level; and among obese adolescents than underweight/normalweight adolescents (Table 2).
Demographics/Body Mass Index and Meeting Physical Activity Guidelines The unadjusted and adjusted odds of meeting either or both of the guidelines (aerobic guideline only, musclewww.ajpmonline.org
Song et al / Am J Prev Med 2013;44(3):216 –222 a
219 b
Table 2. Weighted prevalence of U.S. adolescents who met the 2008 Guidelines by selected characteristics, NHANES, 1999 –2006, (% [95% CI])
Characteristic Total
Aerobic guideline only
Musclestrengthening guideline only
Both guidelines
Neither guideline
14.7 (13.3, 16.2)
21.3 (19.4, 23.3)
16.3 (14.9, 17.9)
47.8 (45.4, 50.1)
Male
17.0c (14.9, 19.3)
23.0c (20.8, 25.4)
20.1c (18.1, 22.2)
40.0c (37.5, 42.5)
Female
12.3d (10.8, 13.9)
19.5d (17.3, 21.9)
12.4d (10.8, 14.2)
55.9d (52.8, 58.9)
12–15
15.2c (13.5, 17.1)
21.8c (19.4, 24.3)
15.4c (13.7, 17.2)
47.7c (45.0, 50.3)
16–17
13.7c (11.7, 16.0)
20.3c (18.1, 22.7)
18.0c (15.8, 20.5)
47.9c (44.4, 51.4)
Non-Hispanic white
16.0c (14.2, 18.0)
20.7c (18.1, 23.6)
17.0c (15.0, 19.2)
46.3c (43.4, 49.2)
Non-Hispanic black
13.7c,d (12.1, 15.4)
21.8c (19.7, 24.1)
15.7c,d (13.7, 17.9)
48.8c (46.0, 51.6)
Other
11.2d (8.3, 15.1)
20.7c (15.8, 26.6)
16.6c,d (13.3, 20.4)
51.6c (46.1, 57.0)
Mexican- American
12.5d (10.9, 14.4)
24.4c (22.2, 26.8)
13.0d (11.2, 15.0)
50.0c (47.1, 53.0)
ⱖ300
15.9c (13.7, 18.3)
21.4c (18.9, 24.1)
19.9c (17.9, 22.0)
42.9c (40.0, 45.8)
200–299
16.2c (13.0, 20.0)
22.3c (18.5, 26.5)
14.8c,d (11.9, 18.2)
46.8c,d (42.6, 51.0)
100–199
12.1c (9.5, 15.3)
19.8c (16.6, 23.4)
12.6d (10.0, 15.8)
55.5d (50.5, 60.4)
⬍100
13.7c (11.8, 15.9)
21.8c (19.0, 24.8)
14.6d (12.1, 17.5)
49.9d (46.2, 53.6)
16.1c (14.3, 18.1)
21.2c (19.0, 23.5)
16.2c (14.5, 18.2)
46.5c (43.5, 49.5)
13.6c,d (10.4, 17.6)
22.6c (19.2, 26.4)
15.4c (12.5, 18.9)
48.4c,d (44.2, 52.6)
9.9d (7.4, 13.2)
20.6c (17.7, 23.8)
17.4c (14.5, 20.7)
52.2d (48.2, 56.1)
Gender
Age group, years
Race/ethnicity
Family poverty-to-income ratio level, %
BMIe (percentile) Underweight or normal-weight (⬍85) Overweight (85–⬍95) Obese (ⱖ95)
Note: Percentages may not add up to 100.0 because of rounding and these columns are exclusive to each other. a Estimates are weighted to account for the complex survey design (including oversampling), survey nonresponse, and post-stratification. b (1) Met aerobic guideline only: defined as performing at least 1 hour of aerobic activity daily and did not meet the muscle-strengthening guideline; (2) met muscle-strengthening guideline only: defined as performing muscle-strengthening activities ⱖ3 days per week and did not meet the aerobic guideline; (3) met both guidelines: defined as performing at least 1 hour of aerobic activity daily and performing muscle-strengthening activities ⱖ3 days/week; (4) met neither guideline: defined as not meeting either aerobic or muscle-strengthening guideline. c,d Different footnote letters represent different estimates at p⬍0.05. For example, the percentage of boys who met the aerobic guideline only, muscle-strengthening guideline only, both guidelines, or neither guideline was different from the percentage of girls e BMI estimates were calculated by standard formula and classified based on gender- and age- specific reference data from the 2000 CDC growth charts. NHANES, National Health and Nutrition Examination Survey
strengthening guideline only, or both guidelines) versus meeting neither of the guidelines are presented in Table 3. Adjusting for covariates, the odds of meeting the aerobic guideline only, the muscle-strengthening guideline only, and both guidelines were higher in boys than girls. The odds of meeting the aerobic guideline only were higher in underweight/normal-weight adolescents. The odds of meeting both guidelines were higher in the ⱖ300% poverty-to-income ratio level compared with the ⬍100% level. Trend analysis over the four waves of the March 2013
data showed no differences in prevalence of meeting the guidelines.
Discussion The current study found that in a nationally representative sample of 6547 U.S. adolescents in 1999 –2006, less than two of ten (⬍20%) reported meeting both aerobic and muscle-strengthening guidelines, and approximately one half reported meeting neither the aerobic nor the
220
Song et al / Am J Prev Med 2013;44(3):216 –222
Table 3. ORs (95% CIs) for meeting the 2008 Guidelines,a National Health and Nutrition Examination Survey, 1999 –2006 Met aerobic guideline only vs met neither guideline Characteristic
Unadjusted
Adjustedb
Met muscle-strengthening guideline only vs met neither guideline Unadjusted
Adjustedb
Met both guidelines vs neither guideline Unadjusted
Adjustedb
Gender Male Female
1.94 (1.57, 2.38) 1.96 (1.59, 2.41) 1.65 (1.43, 1.90) 1.65 (1.43, 1.91) 2.27 (1.87, 2.76) 2.28 (1.87, 2.77) 1.00
1.00
1.00
1.00
1.00
1.00
Age group, years 12–15 16–17
1.11 (0.88, 1.41) 1.10 (0.87, 1.39) 1.08 (0.90, 1.28) 1.07 (0.89, 1.28) 0.86 (0.69, 1.06) 0.85 (0.68, 1.06) 1.00
1.00
1.00
1.00
1.00
1.00
Race/ethnicity Non-Hispanic white
1.38 (1.12, 1.70) 1.20 (0.96, 1.49) 0.92 (0.75, 1.12) 0.84 (0.67, 1.06) 1.42 (1.12, 1.78) 1.18 (0.89, 1.57)
Non-Hispanic black
1.12 (0.90, 1.38) 1.10 (0.88, 1.37) 0.91 (0.76, 1.10) 0.91 (0.76, 1.09) 1.24 (0.96, 1.60) 1.22 (0.94, 1.59)
Other
0.87 (0.59, 1.28) 0.84 (0.58, 1.22) 0.82 (0.59, 1.14) 0.82 (0.58, 1.14) 1.24 (0.92, 1.67) 1.20 (0.86, 1.66)
Mexican-American
1.00
1.00
1.00
1.00
1.00
1.00
Family poverty-to-income ratio level, % ⱖ300
1.35 (1.04, 1.75) 1.21 (0.91, 1.61) 1.14 (0.92, 1.42) 1.16 (0.93, 1.46) 1.58 (1.24, 2.03) 1.55 (1.17, 2.06)
200–299
1.26 (0.91, 1.74) 1.21 (0.86, 1.70) 1.09 (0.82, 1.45) 1.12 (0.84, 1.49) 1.08 (0.76, 1.54) 1.08 (0.74, 1.58)
100–199
0.79 (0.58, 1.08) 0.77 (0.57, 1.05) 0.82 (0.65, 1.04) 0.82 (0.65, 1.04) 0.78 (0.58, 1.05) 0.77 (0.57, 1.04)
⬍100
1.00
1.00
1.00
1.00
1.00
1.00
BMIc (percentile) Underweight or normal-weight (⬍85)
1.83 (1.28, 2.61) 1.84 (1.28, 2.66) 1.15 (0.95, 1.40) 1.18 (0.96, 1.45) 1.05 (0.83, 1.33) 1.05 (0.82, 1.34)
Overweight (85–⬍95)
1.48 (0.94, 2.35) 1.50 (0.94, 2.40) 1.18 (0.88, 1.60) 1.21 (0.90, 1.63) 0.96 (0.70, 1.32) 0.98 (0.72, 1.32)
Obese (ⱖ95)
1.00
1.00
1.00
1.00
1.00
1.00
Note: Percentages may not add up to 100.0 because of rounding. a (1) Met aerobic guideline only: defined as performing ⱖ1 hour of aerobic activity daily and did not meet the muscle-strengthening guideline; (2) met muscle-strengthening guideline only: defined as performing muscle-strengthening activities ⱖ3 days per week and did not meet the aerobic guideline; (3) met both guidelines: defined as performing ⱖ1 hour of aerobic activity daily and performing muscle-strengthening activities ⱖ3 days per week; (4) met neither guideline: defined as not meeting either aerobic or muscle-strengthening guideline. b Adjusted for demographic (gender, age group, race/ethnicity, family poverty-to-income ratio level) and BMI variables c BMI estimates were calculated by standard formula and classified based on gender- and age-specific reference data from the 2000 CDC growth charts.
muscle-strengthening guideline. Efforts to increase levels of youth activity currently receive federal support through state, local, and community organizations (www.letsmove.gov; www.saferoutesinfo.org) but the fındings of the present study suggest that more work is needed. Analyzing NHANES 1999 –2006 data for activity levels that meet the 2008 Guidelines may help establish a criterion for evaluating the effectiveness of existing efforts. Additionally, trend analysis over the four waves of data used in this study showed no trends; thus, these fındings can be used as a comparison in evaluating any trends in prevalence over subsequent periods of time. To date, only one previous study9 has examined the prevalence of adolescents who met guidelines for both aerobic and muscle-strengthening activities. In that
study, investigators observed a slightly lower prevalence than reported in the current study using data from NHANES.9 Several factors may explain why the estimates in the current study from the NHANES (16.3%) are slightly higher than those from the previous study that used the 2010 National Youth Physical Activity and Nutrition Survey (NYPANS; 12.2%). The NHANES included questions to identify a range of activities (42 activities) of moderate and vigorous intensities, whereas the NYPANS included one overall physical activity question about aerobic activity. Accordingly, the higher estimates of this study might be a result of more choices on the questionnaire. Second, the NHANES questionnaire was administered via face-to-face interviews and provided opportunities for follow-up probing or clarifıcation, which may inwww.ajpmonline.org
Song et al / Am J Prev Med 2013;44(3):216 –222
crease recall. In the NYPANS, the questionnaire was administered via a self-administered paper-and-pencil questionnaire. Third, participants in NHANES were slightly younger (aged 12–17 years) than those in NYPANS (Grades 9 –12, aged 14 –18 years), and physical activity levels are often higher in younger adolescents.12 Finally, the period of data collection was different. The NHANES is conducted continuously throughout the year, whereas the NYPANS was conducted during the spring, with most data having been collected during the colder months of February and March when physical activity participation is generally lower.13,14 It is important to assess the prevalence of youth who meet both guidelines, as both aerobic and muscle-strengthening activities bring health benefıts. Aerobic activities primarily improve cardiorespiratory fıtness,4 and muscle-strengthening activities also improve cardiovascular fıtness as well as bone mineral density and muscular strength.15–17 It is additionally important to examine aerobic and muscle-strengthening activity separately, as that can help public health offıcials identify demographic or BMI subgroups, for which prevalence of meeting the guidelines may be particularly low for one type of activity, and therefore appropriately target strategies for the needs of particular subgroups. In fact, from adjusted models, the results of the current study suggest that girls and obese adolescents would benefıt more, relative to youth in other groups, from strategies that target aerobic activity, and adolescent girls might benefıt from a focus on musclestrengthening activity. Also, other studies suggest that interventions to increase physical activity self-effıcacy may help girls overcome barriers to physical activity.18,19 Interesting contrasts and similarities were found between this study and previous studies, by subgroup. Findings of this study are consistent with other studies showing boys to be more active than girls,5,6,9,20 and adolescents that are under or at normal BMI levels to be more active aerobically than obese adolescents.5–7,9 By family income levels, the fındings of this study were inconsistent with other studies. No clear pattern was observed in prevalence of meeting guidelines by family income levels, whereas some previous studies5,6 have shown higher levels of aerobic activity among students in lower-income families.5,6 These studies5,6 used accelerometers to estimate physical activity. However, it is unclear if the difference in measurement methodologies would fully explain the differences in fındings by levels of family income.
Limitations First, the reported prevalence of youth who met the guidelines may have been influenced by recall bias due to the diffıculty of recalling the frequency and duration of March 2013
221
specifıc physical activities over a 30-day period, and/or social-desirability bias, because data were collected through face-to-face interviews. It is often noted that self-report data have a potential of overestimation. Two studies21,22 using accelerometer data from NHANES 2003–2006 reported a lower prevalence of meeting the aerobic guideline than the current study, but it is not certain that the relatively higher prevalence in the current fındings was from using self-report data as compared to accelerometer data. Another study that compared accelerometer data to self-report physical activity questions showed an acceptable level of correlation (0.45– 0.51) between accelerometer and self-report questions.23 Second, the data used in the present study were collected using a 30-day-period questionnaire; thus, the estimates of meeting the 2008 Guidelines are not based on actual daily/weekly volume/frequency values as listed in the 2008 Guidelines criteria, but on averaged values. However, these estimates are relatively similar to a previous study (reported during the past 7 days)9 that used the 2008 Guidelines criteria. Third, the muscle-strengthening questions in the NHANES data might not be the most accurate and comprehensive question to capture activities of all the major muscle groups (e.g., legs, hips, and back). Fourth, as noted above, the authors acknowledge that challenges existed in determining the specifıc criteria to use in determining meeting the 2008 Guidelines with the NHANES data. The criteria used in the present study were consistent with those used in previous CDC studies9,24 of physical activity in youth, and the authors think the current paper provides baseline information for meeting the 2008 Guidelines. Lastly, although the determination of having met the 2008 Guidelines was based on data that were collected prior to the establishment of that guideline, the authors believe this presents no substantive problem with the current analysis, as levels of physical activity among U.S. adolescents were quite consistent during the last decade.25
Conclusion The fınding that few U.S. adolescents meet the levels of physical activity recommended in the 2008 Guidelines has important public health implications. Broadly, those implications can be aligned along two prominent settings: school-based or non-school-based. Several strategies and action plans have already been initiated to address each of these settings. Physical activity in schoolbased settings is the focus of the School Health Guidelines to Promote Healthy Eating and Physical Activity,26 and its recommendations include using a coordinated approach to develop, implement, and evaluate school policies and practices. With respect to addressing non-school-based
222
Song et al / Am J Prev Med 2013;44(3):216 –222
settings, the Guide to Community Preventive Services27 recommends evidence-based strategies to increase physical activity such as creating, enhancing access to, and increasing youth awareness of recreational facilities, school playgrounds, or other places for physical activity. Further, the State Indicator Report on Physical Activity28 provides information on urban design, land-use, environmental, and transportation policies that will help increase physical activity among youth, as well as suggestions for developing and maintaining a public health workforce competent in physical activity. In addressing both settings, the National Physical Activity Plan (www. physicalactivityplan.org) identifıes the importance of using a multisector approach, and sustained efforts in multisectoral settings that involve a variety of stakeholders will help to increase the levels of both aerobic and musclestrengthening activities among U.S. youth. The current study examined the prevalence of U.S. adolescents meeting the 2008 Physical Activity Guidelines for Americans. The fındings that less than 20% of adolescents reported meeting both guidelines and nearly 50% reported meeting neither guideline have substantial public health implications. To increase physical activity among youth, there is a compelling need to implement effective programs and/or policies at home, in schools, in communities, and on various healthcare levels. No fınancial disclosures were reported by the authors of this paper.
References 1. Strong WB, Malina RM, Blimkie CJ, et al. Evidence based physical activity for school-age youth. J Pediatr 2005;146(6):732–7. 2. Janssen I, Leblanc AG. Systematic review of the health benefıts of physical activity and fıtness in school-aged children and youth. Int J Behav Nutr Phys Act 2010;7:40. 3. Physical Activity Guidelines Advisory Committee. Physical Activity Guidelines Advisory Committee report, 2008. In Washington DC, 2008. 4. DHHS. 2008 physical activity guidelines for Americans. Washington DC: DHHS, 2008. 5. Nader PR, Bradley RH, Houts RM, McRitchie SL, O’Brien M. Moderateto-vigorous physical activity from ages 9 to 15 years. JAMA 2008; 300(3):295–305. 6. Belcher BR, Berrigan D, Dodd KW, Emken BA, Chou CP, Spuijt-Metz D. Physical activity in U.S. youth: effect of race/ethnicity, age, gender, & weight status. Med Sci Sports Exerc 2010;42(2):2211–21.
7. Eaton DK, Kann L, Kinchen S, et al. Youth risk behavior surveillance, U.S., 2009. MMWR Surveill Summ 2010;59(5):1–142. 8. Grunbaum JA, Kann L, Kinchen S, et al. Youth risk behavior surveillance, U.S., 2003. MMWR Surveill Summ 2004;53(2):1–96. 9. CDC. Physical activity levels of high school students, U.S., 2010. MMWR Morb Mortal Wkly Rep 2011;60(23):773–7. 10. CDC. National Health and Nutrition Examination Surveys (NHANES). www.cdc.gov/nchs/nhanes.htm. 11. Kuczmarski RJ, Ogden CL, Guo SS, et al. 2000 CDC growth charts for the U.S.: methods and development. Vital Health Stat 11 2002; (246):1–190. 12. Wall MI, Carlson SA, Stein AD, Lee SM, Fulton JE. Trends by age in youth physical activity: Youth Media Campaign longitudinal survey. Med Sci Sports Exerc 2011;43(11):2140 –7. 13. Belanger M, Gray-Donald K, O’Loughlin J, Paradis G, Hanley J. Influence of weather conditions and season on physical activity in adolescents. Ann Epidemiol 2009;19(3):180 – 6. 14. Tucker P, Gilliland J. The effect of season and weather on physical activity: a systematic review. Public Health 2007;121(12):909 –22. 15. Benson AC, Torode ME, Fiatarone Singh MA. A rationale and method for high-intensity progressive resistance training with children and adolescents. Contemp Clin Trials 2007;28(4):442–50. 16. Lukacs A, Mayer K, Juhasz E, Varga B, Fodor B, Barkai L. Reduced physical fıtness in children and adolescents with type 1 diabetes. Pediatr Diabetes 2012;13(5):432–7. 17. McCambridge TM, Stricker PR. Strength training by children and adolescents. Pediatrics 2008;121(4):835– 40. 18. Trost SG, Pate RR, Dowda M, Saunders R, Ward DS, Felton G. Gender differences in physical activity and determinants of physical activity in rural fıfth grade children. J Sch Health 1996;66(4):145–50. 19. Lee LL, Kuo YC, Fanaw D, Perng SJ, Juang IF. The effect of an intervention combining self-effıcacy theory and pedometers on promoting physical activity among adolescents. J Clin Nurs 2012;21(7– 8):914 –22. 20. Troiano RP, Berrigan D, Dodd KW, et al. Physical activity in the U.S. measured by accelerometer. Med Sci Sports Exerc 2008;40(1):181– 8. 21. Chung AE, Skinner AC, Steiner MJ, Perrin EM. Physical activity and BMI in a nationally representative sample of children and adolescents. Clin Pediatr 2012;51(2):122–9. 22. LeBlanc AG, Janssen I. Difference between self-reported and accelerometer measured moderate-to-vigorous physical activity in youth. Pediatr Exerc Sci 2010;22(4):523–34. 23. Weston AT, Petosa R, Pate RR. Validation of an instrument for measurement of physical activity in youth. Med Sci Sports Exerc 1997;29(1):138 – 43. 24. Eaton DK, Kann L, Kinchen S, et al. Youth risk behavior surveillance, U.S., 2011. MMWR Surveill Summ 2012;61(4):1–162. 25. CDC. Trends in the prevalence of physical activity. National YRBS: 1991–2009. 26. CDC. School health guidelines to promote healthy eating and physical activity. MMWR Recomm Rep 2011;60(RR–5):1–76. 27. CDC. The guide to community preventive services. www. thecommunityguide.org/pa. 28. CDC. State indicator report on physical activity, 2010. Atlanta GA: DHHS, 2010.
www.ajpmonline.org
Background: Participation in physical activity brings health benefıts for adolescents. However, limited data are available on the percentage of U.S. adolescents who engage in levels of aerobic and muscle-strengthening activities recommended in the 2008 Physical Activity Guidelines for Americans (2008 Guidelines).
Purpose: To examine the prevalence at which U.S. adolescents aged 12–17 years meet the 2008 Guidelines, and whether demographic and BMI variables influence that prevalence. Methods: Using data from an interviewer-administered self-report questionnaire in the 1999 –2006 National Health and Nutrition Examination Survey (analyzed in 2011), estimates were made of the percentage of adolescents who engaged in recommended levels of aerobic and muscle-strengthening activities (ⱖ60 minutes of aerobic activity/day and participation in muscle-strengthening activities ⱖ3 days/week).
Results: Among 6547 U.S. adolescents aged 12–17 years, 16.3% (95% CI⫽14.9%, 17.9%) met both aerobic and muscle-strengthening guidelines; 14.7% (13.3%, 16.2%) met the aerobic guideline only, 21.3% (19.4%, 23.3%) met the muscle-strengthening guideline only, and 47.8% (45.4%, 50.1%) met neither guideline. Adjusted for covariates, odds of meeting either the aerobic or muscle-strengthening guideline only or both guidelines versus meeting neither guideline were (p⬍0.05) higher among boys than girls. The odds of meeting the aerobic guideline only were higher among underweight/normalweight adolescents than among obese adolescents. No clear pattern was observed by family groups according to poverty-to-income ratio. Conclusions: Less than 20% of adolescents reported engaging in recommended levels of both aerobic and muscle-strengthening activities. (Am J Prev Med 2013;44(3):216 –222) Published by Elsevier Inc. on behalf of American Journal of Preventive Medicine
Introduction
P
hysical activity has numerous health benefıts for youth including increased cardiorespiratory fıtness and muscular strength, reduced body fat, improved cardiovascular and metabolic health biomarkers, enhanced bone health, and reduced symptoms of depression and/or anxiety.1–3 Accordingly, the DHHS detailed physical activity guidelines for youth aged 6 –17 years in the 2008 Physical Activity Guidelines for Americans (2008 Guidelines).4 These guidelines recommend that youth participate in a minimum of 60 minutes of physical activFrom the Epidemic Intelligence Service (Song), the Division of Nutrition, Physical Activity, and Obesity (Song, Carroll, Fulton), CDC, and the Commissioned Corps, U.S. Public Health Service (Carroll), Atlanta, Georgia Address correspondence to: MinKyoung Song, PhD, RN, FNP-BC University of Michigan School of Nursing, 400 North Ingalls Building, Ann Arbor MI 48109-5482. E-mail: [email protected]. 0749-3797/$36.00 http://dx.doi.org/10.1016/j.amepre.2012.11.016
216 Am J Prev Med 2013;44(3):216 –222
ity daily and engage in muscle-strengthening activity at least 3 days per week. However, there is a gap in existing knowledge about how many U.S. youth engage in physical activity at levels suffıcient to meet those guidelines. Although a few studies have described, separately, levels of aerobic5–7 and muscle-strengthening activity8 in youth, only one, a CDC study9 published in 2010, has reported levels of aerobic and muscle-strengthening activities together. That CDC study9 assessed aerobic and muscle-strengthening activity levels among U.S. high school students. Self-administered questionnaires, with one question to assess aerobic activity and one to assess muscle-strengthening activity, were used in classrooms, and showed that only one in ten students met the recommended levels of aerobic and muscle-strengthening activities. To augment those CDC fındings,9 the current study assessed data collected through face-to-face interviews using questions on a broad range of activities.
Published by Elsevier Inc. on behalf of American Journal of Preventive Medicine
Song et al / Am J Prev Med 2013;44(3):216 –222
The primary objectives were to use more-detailed data to examine the prevalence of U.S. adolescents meeting the 2008 Guidelines and to determine whether demographic and BMI variables are associated with meeting the guidelines. Four exclusive assessment criteria based on the 2008 Guidelines were used: meeting the aerobic guideline only, meeting the muscle-strengthening guideline only, meeting both aerobic and muscle-strengthening guidelines, and meeting neither guideline. Assessing levels of physical activity from participation in specifıc activities will strengthen understanding in this sparsely researched area.
Methods Survey Description The National Health and Nutrition Examination Survey (NHANES) is a stratifıed and multistage probability cluster sample survey designed to represent the U.S. non-institutionalized civilian population.10 Conducted in 2-year cycles, NHANES collects data from participants (of all ages) through household interviews followed by physical examinations, including measurement of height and weight, at a mobile examination center. Physical activity data were collected in household interviews for youth aged 16 –17 years and in interviews at such centers for those aged 12–15 years. The interview questions were the same, regardless of the interview location. Physical activity questions were the same across all four of the 2-year cycles (1999 –2006). Overall response rates for household interviews (for all participants) ranged from 79% (2003–2004) to 84% (2001–2002); among the target group of youth aged 12–17 years, the response rates for household interviews and the response rate for the mobile examination center interviews ranged from 83% to 88%.10
Sample Although NHANES is conducted in 2-year cycles, for the purposes of the current study, 1999 –2006 data were pooled to obtain an adequate sample size of youth aged 12–17 years. During the 2-year cycles from that period, sample sizes ranged from 12,160 (1999 – 2000) to 13,156 (2001–2002), resulting in a combined sample of 41,474 for the entire 8-year span. From this total sample, 6968 participants aged 12–17 years were identifıed, after accounting for the sample weights. After excluding participants with missing data on physical activity, BMI, or demographic variables, this analysis included 6547 U.S. adolescents aged 12–17 years. The four waves of data were more or less equally represented in the fınal analytic sample (n⫽6547) (1999 –2000: 22.0%, 2001–2002: 27.5%, 2003– 2004: 24.7%, and 2005–2006: 25.7%).
217
slight to moderate increase in breathing or heart rate? Some examples are brisk walking, bicycling for pleasure, golf, or dancing. Participants who answered yes to either of these questions were asked to name each activity they engaged in, provide details about the frequency of their participation in the past 30 days, and report the average duration of each session. The NHANES includes information on 48 physical activities, of which 42 aerobic activities (excluding activities categorized as “other” in the questionnaire and fıve muscle-strengthening activities: push-ups, sit-ups, stretching, weight-lifting, and yoga) were included. The weekly frequency was calculated for each participant, for each reported activity by dividing the 30-day frequency by 4.29. Next, the weekly frequencies were summed for all moderateand vigorous-intensity-level activities reported by each individual to yield a total weekly frequency of aerobic activity. Youth with a total weekly frequency of seven or more times met the frequency recommended by the 2008 Guidelines. To estimate the total daily volume of aerobic activity, fırst the reported duration of each activity session was multiplied by the weekly frequency of that activity to obtain a weekly volume for each activity. If a participant reported engaging in an activity at both a moderate- and vigorous-intensity level, a weekly volume for each intensity was calculated. Then the weekly moderate- and vigorous-intensity volume was summed for each activity. Next, the total weekly volume of aerobic activity was calculated by summing the volume of all reported activities. Last, the daily time spent in aerobic activity was estimated by dividing the total weekly volume of activities by 7. Participants who reported participating in ⱖ60 minutes of aerobic activity per day, 7 days per week, met the recommendation for aerobic activity according to the 2008 Guidelines.
Muscle-strengthening activity. To assess muscle-strengthening, participants were asked: Over the past 30 days, did you do any physical activities specifıcally designed to strengthen your muscles such as lifting weights, push-ups, or sit-ups? Participants who answered yes to this question were asked to provide the frequency of their participation. The monthly frequency of activities participants engaged in was divided by 4.29 to obtain weekly estimates of muscle-strengthening activity. Participants who reported engaging in muscle-strengthening activity three or more times/week met the frequency recommended by the 2008 Guidelines. Meeting the 2008 Guidelines. Study participants were di-
Measures
vided into four mutually exclusive subgroups: those who met the aerobic guideline only (ⱖ60 minutes of aerobic activity/day, 7 days/ week, but did not meet the muscle-strengthening guideline); those who met the muscle-strengthening guideline only (muscle-strengthening activities three or more times/week but did not meet the aerobic guideline); those who met both guidelines (ⱖ60 minutes of aerobic activity/day, 7 days/week, and muscle-strengthening activities three or more times/week); and those who met neither guideline.
Aerobic activity. To assess aerobic activity, participants were asked about vigorous- and moderate-intensity activities engaged in at school or in leisure time over the previous 30 days. To assess vigorousintensity aerobic activity, participants were asked, Did you do any vigorous activities for at least 10 minutes that caused heavy sweating, or large increases in breathing or heart rate? Some examples are running, lap swimming, aerobics classes, or fast bicycling. To assess moderateintensity aerobic activity, participants were asked: Did you do moderate activities for at least 10 minutes that cause only light sweating or a
Demographic characteristics and BMI. Participants were categorized into two age groups (12–15 years and 16 –17 years) using self-reported age. Race/ethnicity was categorized into four groups (non-Hispanic white, non-Hispanic black, Mexican Americans, and other) using a variable derived by combining responses to questions on race and Hispanic origin. Multiracial adolescents were classifıed as other. Family poverty-to-income ratio is a variable representing the ratio of household income to the DHHS’s poverty threshold based
March 2013
218
Song et al / Am J Prev Med 2013;44(3):216 –222
on family size and adjusted for annual updates of inflation. The ratio was categorized into four groups (⬍100%, 100%–199%, 200%–299%, and ⱖ300%), where ⬍100% is below the poverty threshold. BMI was computed by standard formula, and categorized into three groups (underweight/normal-weight [⬍85th percentile]; overweight [85th– ⬍95th percentile]; and obese [ⱖ95th percentile]) according to the gender- and age-specifıc reference data from the Year-2000 CDC growth charts.11
Data Analysis First, the prevalence and 95% CI was estimated of U.S. adolescents aged 12–17 years who met the aerobic guideline only, met the musclestrengthening guideline only, met both guidelines, or met neither guideline. Differences by categories of gender, age, race/ethnicity, family poverty-to-income ratio, and BMI were assessed overall and using t-tests for pairwise subgroup comparisons if any overall difference existed within the group. Second, using multinomial logistic regression, the association of demographics, BMI, and meeting the guidelines in unadjusted and adjusted models was examined, controlling for gender, age, race/ethnicity, family poverty-to-income ratio, and BMI. The SAS-callable SUDAAN, version 9.0.1, software was used to account for the complex survey design. To produce nationally representative estimates, sample weights were used to account for differential probabilities of selection, nonresponse, and noncoverage. SEs were estimated with SUDAAN using Taylor-series linearization. Signifıcance was determined at p ⬍0.05, and Bonferroni corrections were used for multiple comparisons among different characteristic groups (e.g., underweight/normal-weight versus overweight versus obese). Sensitivity analysis was performed to determine whether the excluded participants differed from the participants in the fınal sample in terms of demographics and BMI. Linear and quadratic trends of meeting the guidelines over the four waves of data were also tested. The analysis was conducted in 2011.
Results Nationally representative estimates from the current study were based on a sample of 6547 participants. Selected characteristics of the weighted sample are presented in Table 1. Based on 2008 Guidelines criteria, 14.7% of U.S. adolescents met the aerobic guideline only, and 21.3% met the muscle-strengthening guideline only (Table 2). The percentage of boys who met the aerobic guideline only or the muscle-strengthening guideline only was higher than that for girls. The percentage meeting the aerobic guideline only was higher among nonHispanic white adolescents than Mexican-American and other adolescents; and among underweight/normalweight adolescents than obese adolescents. Less than 20% of U.S. adolescents (16.3%) met both the aerobic and muscle-strengthening guidelines. The percentage of adolescents meeting both guidelines was higher among boys than girls; among non-Hispanic white adolescents than Mexican-American adolescents; and among those in the ⱖ300% poverty-to-
Table 1. Weighteda sample characteristics of adolescents, National Health and Nutrition Examination Survey, 1999 –2006 Characteristic
n
% (95% CI)
Male
3307
51.1 (49.4, 52.7)
Female
3240
48.9 (47.3, 50.6)
12–15
4288
65.4 (63.7, 67.1)
16–17
2259
34.6 (32.9, 36.3)
Non-Hispanic white
1681
61.8 (58.1, 65.3)
Non-Hispanic black
2088
14.4 (12.1, 17.0)
524
12.8 (10.5, 15.4)
Gender
Age group, years
Race/ethnicity
Other Mexican-American
2254
11.1 (9.2, 13.2)
Family poverty-to-income ratio, % ⱖ300
1717
39.6 (36.8, 42.5)
200–299
954
16.2 (14.7, 17.8)
100–199
1605
20.5 (18.8, 22.2)
⬍100
2271
23.7 (21.9, 25.7)
Underweight or normal-weight (⬍85)
4135
67.2 (65.2, 69.1)
Overweight (85–⬍95)
1068
15.6 (14.2, 17.2)
Obese (ⱖ95)
1344
17.2 (15.7, 18.8)
b
BMI (percentile)
Note: Percentages may not add up to 100.0 because of rounding. a Estimates are weighted to account for the complex survey design (including oversampling); survey nonresponse; and post-stratification. b BMI estimates were calculated by standard formula and classified based on gender- and age-specific reference data from the Year 2000 CDC growth charts.
income ratio level than those in the 100%–199% level or those in the ⬍100% level (Table 2). Slightly less than half of U.S. adolescents (47.8%) met neither the aerobic nor the muscle-strengthening guideline. The percentage of adolescents meeting neither guideline was higher among girls than boys; among those in the ⱖ300% poverty-to-income ratio level than those in the 100%–199% level or those in the ⬍100% level; and among obese adolescents than underweight/normalweight adolescents (Table 2).
Demographics/Body Mass Index and Meeting Physical Activity Guidelines The unadjusted and adjusted odds of meeting either or both of the guidelines (aerobic guideline only, musclewww.ajpmonline.org
Song et al / Am J Prev Med 2013;44(3):216 –222 a
219 b
Table 2. Weighted prevalence of U.S. adolescents who met the 2008 Guidelines by selected characteristics, NHANES, 1999 –2006, (% [95% CI])
Characteristic Total
Aerobic guideline only
Musclestrengthening guideline only
Both guidelines
Neither guideline
14.7 (13.3, 16.2)
21.3 (19.4, 23.3)
16.3 (14.9, 17.9)
47.8 (45.4, 50.1)
Male
17.0c (14.9, 19.3)
23.0c (20.8, 25.4)
20.1c (18.1, 22.2)
40.0c (37.5, 42.5)
Female
12.3d (10.8, 13.9)
19.5d (17.3, 21.9)
12.4d (10.8, 14.2)
55.9d (52.8, 58.9)
12–15
15.2c (13.5, 17.1)
21.8c (19.4, 24.3)
15.4c (13.7, 17.2)
47.7c (45.0, 50.3)
16–17
13.7c (11.7, 16.0)
20.3c (18.1, 22.7)
18.0c (15.8, 20.5)
47.9c (44.4, 51.4)
Non-Hispanic white
16.0c (14.2, 18.0)
20.7c (18.1, 23.6)
17.0c (15.0, 19.2)
46.3c (43.4, 49.2)
Non-Hispanic black
13.7c,d (12.1, 15.4)
21.8c (19.7, 24.1)
15.7c,d (13.7, 17.9)
48.8c (46.0, 51.6)
Other
11.2d (8.3, 15.1)
20.7c (15.8, 26.6)
16.6c,d (13.3, 20.4)
51.6c (46.1, 57.0)
Mexican- American
12.5d (10.9, 14.4)
24.4c (22.2, 26.8)
13.0d (11.2, 15.0)
50.0c (47.1, 53.0)
ⱖ300
15.9c (13.7, 18.3)
21.4c (18.9, 24.1)
19.9c (17.9, 22.0)
42.9c (40.0, 45.8)
200–299
16.2c (13.0, 20.0)
22.3c (18.5, 26.5)
14.8c,d (11.9, 18.2)
46.8c,d (42.6, 51.0)
100–199
12.1c (9.5, 15.3)
19.8c (16.6, 23.4)
12.6d (10.0, 15.8)
55.5d (50.5, 60.4)
⬍100
13.7c (11.8, 15.9)
21.8c (19.0, 24.8)
14.6d (12.1, 17.5)
49.9d (46.2, 53.6)
16.1c (14.3, 18.1)
21.2c (19.0, 23.5)
16.2c (14.5, 18.2)
46.5c (43.5, 49.5)
13.6c,d (10.4, 17.6)
22.6c (19.2, 26.4)
15.4c (12.5, 18.9)
48.4c,d (44.2, 52.6)
9.9d (7.4, 13.2)
20.6c (17.7, 23.8)
17.4c (14.5, 20.7)
52.2d (48.2, 56.1)
Gender
Age group, years
Race/ethnicity
Family poverty-to-income ratio level, %
BMIe (percentile) Underweight or normal-weight (⬍85) Overweight (85–⬍95) Obese (ⱖ95)
Note: Percentages may not add up to 100.0 because of rounding and these columns are exclusive to each other. a Estimates are weighted to account for the complex survey design (including oversampling), survey nonresponse, and post-stratification. b (1) Met aerobic guideline only: defined as performing at least 1 hour of aerobic activity daily and did not meet the muscle-strengthening guideline; (2) met muscle-strengthening guideline only: defined as performing muscle-strengthening activities ⱖ3 days per week and did not meet the aerobic guideline; (3) met both guidelines: defined as performing at least 1 hour of aerobic activity daily and performing muscle-strengthening activities ⱖ3 days/week; (4) met neither guideline: defined as not meeting either aerobic or muscle-strengthening guideline. c,d Different footnote letters represent different estimates at p⬍0.05. For example, the percentage of boys who met the aerobic guideline only, muscle-strengthening guideline only, both guidelines, or neither guideline was different from the percentage of girls e BMI estimates were calculated by standard formula and classified based on gender- and age- specific reference data from the 2000 CDC growth charts. NHANES, National Health and Nutrition Examination Survey
strengthening guideline only, or both guidelines) versus meeting neither of the guidelines are presented in Table 3. Adjusting for covariates, the odds of meeting the aerobic guideline only, the muscle-strengthening guideline only, and both guidelines were higher in boys than girls. The odds of meeting the aerobic guideline only were higher in underweight/normal-weight adolescents. The odds of meeting both guidelines were higher in the ⱖ300% poverty-to-income ratio level compared with the ⬍100% level. Trend analysis over the four waves of the March 2013
data showed no differences in prevalence of meeting the guidelines.
Discussion The current study found that in a nationally representative sample of 6547 U.S. adolescents in 1999 –2006, less than two of ten (⬍20%) reported meeting both aerobic and muscle-strengthening guidelines, and approximately one half reported meeting neither the aerobic nor the
220
Song et al / Am J Prev Med 2013;44(3):216 –222
Table 3. ORs (95% CIs) for meeting the 2008 Guidelines,a National Health and Nutrition Examination Survey, 1999 –2006 Met aerobic guideline only vs met neither guideline Characteristic
Unadjusted
Adjustedb
Met muscle-strengthening guideline only vs met neither guideline Unadjusted
Adjustedb
Met both guidelines vs neither guideline Unadjusted
Adjustedb
Gender Male Female
1.94 (1.57, 2.38) 1.96 (1.59, 2.41) 1.65 (1.43, 1.90) 1.65 (1.43, 1.91) 2.27 (1.87, 2.76) 2.28 (1.87, 2.77) 1.00
1.00
1.00
1.00
1.00
1.00
Age group, years 12–15 16–17
1.11 (0.88, 1.41) 1.10 (0.87, 1.39) 1.08 (0.90, 1.28) 1.07 (0.89, 1.28) 0.86 (0.69, 1.06) 0.85 (0.68, 1.06) 1.00
1.00
1.00
1.00
1.00
1.00
Race/ethnicity Non-Hispanic white
1.38 (1.12, 1.70) 1.20 (0.96, 1.49) 0.92 (0.75, 1.12) 0.84 (0.67, 1.06) 1.42 (1.12, 1.78) 1.18 (0.89, 1.57)
Non-Hispanic black
1.12 (0.90, 1.38) 1.10 (0.88, 1.37) 0.91 (0.76, 1.10) 0.91 (0.76, 1.09) 1.24 (0.96, 1.60) 1.22 (0.94, 1.59)
Other
0.87 (0.59, 1.28) 0.84 (0.58, 1.22) 0.82 (0.59, 1.14) 0.82 (0.58, 1.14) 1.24 (0.92, 1.67) 1.20 (0.86, 1.66)
Mexican-American
1.00
1.00
1.00
1.00
1.00
1.00
Family poverty-to-income ratio level, % ⱖ300
1.35 (1.04, 1.75) 1.21 (0.91, 1.61) 1.14 (0.92, 1.42) 1.16 (0.93, 1.46) 1.58 (1.24, 2.03) 1.55 (1.17, 2.06)
200–299
1.26 (0.91, 1.74) 1.21 (0.86, 1.70) 1.09 (0.82, 1.45) 1.12 (0.84, 1.49) 1.08 (0.76, 1.54) 1.08 (0.74, 1.58)
100–199
0.79 (0.58, 1.08) 0.77 (0.57, 1.05) 0.82 (0.65, 1.04) 0.82 (0.65, 1.04) 0.78 (0.58, 1.05) 0.77 (0.57, 1.04)
⬍100
1.00
1.00
1.00
1.00
1.00
1.00
BMIc (percentile) Underweight or normal-weight (⬍85)
1.83 (1.28, 2.61) 1.84 (1.28, 2.66) 1.15 (0.95, 1.40) 1.18 (0.96, 1.45) 1.05 (0.83, 1.33) 1.05 (0.82, 1.34)
Overweight (85–⬍95)
1.48 (0.94, 2.35) 1.50 (0.94, 2.40) 1.18 (0.88, 1.60) 1.21 (0.90, 1.63) 0.96 (0.70, 1.32) 0.98 (0.72, 1.32)
Obese (ⱖ95)
1.00
1.00
1.00
1.00
1.00
1.00
Note: Percentages may not add up to 100.0 because of rounding. a (1) Met aerobic guideline only: defined as performing ⱖ1 hour of aerobic activity daily and did not meet the muscle-strengthening guideline; (2) met muscle-strengthening guideline only: defined as performing muscle-strengthening activities ⱖ3 days per week and did not meet the aerobic guideline; (3) met both guidelines: defined as performing ⱖ1 hour of aerobic activity daily and performing muscle-strengthening activities ⱖ3 days per week; (4) met neither guideline: defined as not meeting either aerobic or muscle-strengthening guideline. b Adjusted for demographic (gender, age group, race/ethnicity, family poverty-to-income ratio level) and BMI variables c BMI estimates were calculated by standard formula and classified based on gender- and age-specific reference data from the 2000 CDC growth charts.
muscle-strengthening guideline. Efforts to increase levels of youth activity currently receive federal support through state, local, and community organizations (www.letsmove.gov; www.saferoutesinfo.org) but the fındings of the present study suggest that more work is needed. Analyzing NHANES 1999 –2006 data for activity levels that meet the 2008 Guidelines may help establish a criterion for evaluating the effectiveness of existing efforts. Additionally, trend analysis over the four waves of data used in this study showed no trends; thus, these fındings can be used as a comparison in evaluating any trends in prevalence over subsequent periods of time. To date, only one previous study9 has examined the prevalence of adolescents who met guidelines for both aerobic and muscle-strengthening activities. In that
study, investigators observed a slightly lower prevalence than reported in the current study using data from NHANES.9 Several factors may explain why the estimates in the current study from the NHANES (16.3%) are slightly higher than those from the previous study that used the 2010 National Youth Physical Activity and Nutrition Survey (NYPANS; 12.2%). The NHANES included questions to identify a range of activities (42 activities) of moderate and vigorous intensities, whereas the NYPANS included one overall physical activity question about aerobic activity. Accordingly, the higher estimates of this study might be a result of more choices on the questionnaire. Second, the NHANES questionnaire was administered via face-to-face interviews and provided opportunities for follow-up probing or clarifıcation, which may inwww.ajpmonline.org
Song et al / Am J Prev Med 2013;44(3):216 –222
crease recall. In the NYPANS, the questionnaire was administered via a self-administered paper-and-pencil questionnaire. Third, participants in NHANES were slightly younger (aged 12–17 years) than those in NYPANS (Grades 9 –12, aged 14 –18 years), and physical activity levels are often higher in younger adolescents.12 Finally, the period of data collection was different. The NHANES is conducted continuously throughout the year, whereas the NYPANS was conducted during the spring, with most data having been collected during the colder months of February and March when physical activity participation is generally lower.13,14 It is important to assess the prevalence of youth who meet both guidelines, as both aerobic and muscle-strengthening activities bring health benefıts. Aerobic activities primarily improve cardiorespiratory fıtness,4 and muscle-strengthening activities also improve cardiovascular fıtness as well as bone mineral density and muscular strength.15–17 It is additionally important to examine aerobic and muscle-strengthening activity separately, as that can help public health offıcials identify demographic or BMI subgroups, for which prevalence of meeting the guidelines may be particularly low for one type of activity, and therefore appropriately target strategies for the needs of particular subgroups. In fact, from adjusted models, the results of the current study suggest that girls and obese adolescents would benefıt more, relative to youth in other groups, from strategies that target aerobic activity, and adolescent girls might benefıt from a focus on musclestrengthening activity. Also, other studies suggest that interventions to increase physical activity self-effıcacy may help girls overcome barriers to physical activity.18,19 Interesting contrasts and similarities were found between this study and previous studies, by subgroup. Findings of this study are consistent with other studies showing boys to be more active than girls,5,6,9,20 and adolescents that are under or at normal BMI levels to be more active aerobically than obese adolescents.5–7,9 By family income levels, the fındings of this study were inconsistent with other studies. No clear pattern was observed in prevalence of meeting guidelines by family income levels, whereas some previous studies5,6 have shown higher levels of aerobic activity among students in lower-income families.5,6 These studies5,6 used accelerometers to estimate physical activity. However, it is unclear if the difference in measurement methodologies would fully explain the differences in fındings by levels of family income.
Limitations First, the reported prevalence of youth who met the guidelines may have been influenced by recall bias due to the diffıculty of recalling the frequency and duration of March 2013
221
specifıc physical activities over a 30-day period, and/or social-desirability bias, because data were collected through face-to-face interviews. It is often noted that self-report data have a potential of overestimation. Two studies21,22 using accelerometer data from NHANES 2003–2006 reported a lower prevalence of meeting the aerobic guideline than the current study, but it is not certain that the relatively higher prevalence in the current fındings was from using self-report data as compared to accelerometer data. Another study that compared accelerometer data to self-report physical activity questions showed an acceptable level of correlation (0.45– 0.51) between accelerometer and self-report questions.23 Second, the data used in the present study were collected using a 30-day-period questionnaire; thus, the estimates of meeting the 2008 Guidelines are not based on actual daily/weekly volume/frequency values as listed in the 2008 Guidelines criteria, but on averaged values. However, these estimates are relatively similar to a previous study (reported during the past 7 days)9 that used the 2008 Guidelines criteria. Third, the muscle-strengthening questions in the NHANES data might not be the most accurate and comprehensive question to capture activities of all the major muscle groups (e.g., legs, hips, and back). Fourth, as noted above, the authors acknowledge that challenges existed in determining the specifıc criteria to use in determining meeting the 2008 Guidelines with the NHANES data. The criteria used in the present study were consistent with those used in previous CDC studies9,24 of physical activity in youth, and the authors think the current paper provides baseline information for meeting the 2008 Guidelines. Lastly, although the determination of having met the 2008 Guidelines was based on data that were collected prior to the establishment of that guideline, the authors believe this presents no substantive problem with the current analysis, as levels of physical activity among U.S. adolescents were quite consistent during the last decade.25
Conclusion The fınding that few U.S. adolescents meet the levels of physical activity recommended in the 2008 Guidelines has important public health implications. Broadly, those implications can be aligned along two prominent settings: school-based or non-school-based. Several strategies and action plans have already been initiated to address each of these settings. Physical activity in schoolbased settings is the focus of the School Health Guidelines to Promote Healthy Eating and Physical Activity,26 and its recommendations include using a coordinated approach to develop, implement, and evaluate school policies and practices. With respect to addressing non-school-based
222
Song et al / Am J Prev Med 2013;44(3):216 –222
settings, the Guide to Community Preventive Services27 recommends evidence-based strategies to increase physical activity such as creating, enhancing access to, and increasing youth awareness of recreational facilities, school playgrounds, or other places for physical activity. Further, the State Indicator Report on Physical Activity28 provides information on urban design, land-use, environmental, and transportation policies that will help increase physical activity among youth, as well as suggestions for developing and maintaining a public health workforce competent in physical activity. In addressing both settings, the National Physical Activity Plan (www. physicalactivityplan.org) identifıes the importance of using a multisector approach, and sustained efforts in multisectoral settings that involve a variety of stakeholders will help to increase the levels of both aerobic and musclestrengthening activities among U.S. youth. The current study examined the prevalence of U.S. adolescents meeting the 2008 Physical Activity Guidelines for Americans. The fındings that less than 20% of adolescents reported meeting both guidelines and nearly 50% reported meeting neither guideline have substantial public health implications. To increase physical activity among youth, there is a compelling need to implement effective programs and/or policies at home, in schools, in communities, and on various healthcare levels. No fınancial disclosures were reported by the authors of this paper.
References 1. Strong WB, Malina RM, Blimkie CJ, et al. Evidence based physical activity for school-age youth. J Pediatr 2005;146(6):732–7. 2. Janssen I, Leblanc AG. Systematic review of the health benefıts of physical activity and fıtness in school-aged children and youth. Int J Behav Nutr Phys Act 2010;7:40. 3. Physical Activity Guidelines Advisory Committee. Physical Activity Guidelines Advisory Committee report, 2008. In Washington DC, 2008. 4. DHHS. 2008 physical activity guidelines for Americans. Washington DC: DHHS, 2008. 5. Nader PR, Bradley RH, Houts RM, McRitchie SL, O’Brien M. Moderateto-vigorous physical activity from ages 9 to 15 years. JAMA 2008; 300(3):295–305. 6. Belcher BR, Berrigan D, Dodd KW, Emken BA, Chou CP, Spuijt-Metz D. Physical activity in U.S. youth: effect of race/ethnicity, age, gender, & weight status. Med Sci Sports Exerc 2010;42(2):2211–21.
7. Eaton DK, Kann L, Kinchen S, et al. Youth risk behavior surveillance, U.S., 2009. MMWR Surveill Summ 2010;59(5):1–142. 8. Grunbaum JA, Kann L, Kinchen S, et al. Youth risk behavior surveillance, U.S., 2003. MMWR Surveill Summ 2004;53(2):1–96. 9. CDC. Physical activity levels of high school students, U.S., 2010. MMWR Morb Mortal Wkly Rep 2011;60(23):773–7. 10. CDC. National Health and Nutrition Examination Surveys (NHANES). www.cdc.gov/nchs/nhanes.htm. 11. Kuczmarski RJ, Ogden CL, Guo SS, et al. 2000 CDC growth charts for the U.S.: methods and development. Vital Health Stat 11 2002; (246):1–190. 12. Wall MI, Carlson SA, Stein AD, Lee SM, Fulton JE. Trends by age in youth physical activity: Youth Media Campaign longitudinal survey. Med Sci Sports Exerc 2011;43(11):2140 –7. 13. Belanger M, Gray-Donald K, O’Loughlin J, Paradis G, Hanley J. Influence of weather conditions and season on physical activity in adolescents. Ann Epidemiol 2009;19(3):180 – 6. 14. Tucker P, Gilliland J. The effect of season and weather on physical activity: a systematic review. Public Health 2007;121(12):909 –22. 15. Benson AC, Torode ME, Fiatarone Singh MA. A rationale and method for high-intensity progressive resistance training with children and adolescents. Contemp Clin Trials 2007;28(4):442–50. 16. Lukacs A, Mayer K, Juhasz E, Varga B, Fodor B, Barkai L. Reduced physical fıtness in children and adolescents with type 1 diabetes. Pediatr Diabetes 2012;13(5):432–7. 17. McCambridge TM, Stricker PR. Strength training by children and adolescents. Pediatrics 2008;121(4):835– 40. 18. Trost SG, Pate RR, Dowda M, Saunders R, Ward DS, Felton G. Gender differences in physical activity and determinants of physical activity in rural fıfth grade children. J Sch Health 1996;66(4):145–50. 19. Lee LL, Kuo YC, Fanaw D, Perng SJ, Juang IF. The effect of an intervention combining self-effıcacy theory and pedometers on promoting physical activity among adolescents. J Clin Nurs 2012;21(7– 8):914 –22. 20. Troiano RP, Berrigan D, Dodd KW, et al. Physical activity in the U.S. measured by accelerometer. Med Sci Sports Exerc 2008;40(1):181– 8. 21. Chung AE, Skinner AC, Steiner MJ, Perrin EM. Physical activity and BMI in a nationally representative sample of children and adolescents. Clin Pediatr 2012;51(2):122–9. 22. LeBlanc AG, Janssen I. Difference between self-reported and accelerometer measured moderate-to-vigorous physical activity in youth. Pediatr Exerc Sci 2010;22(4):523–34. 23. Weston AT, Petosa R, Pate RR. Validation of an instrument for measurement of physical activity in youth. Med Sci Sports Exerc 1997;29(1):138 – 43. 24. Eaton DK, Kann L, Kinchen S, et al. Youth risk behavior surveillance, U.S., 2011. MMWR Surveill Summ 2012;61(4):1–162. 25. CDC. Trends in the prevalence of physical activity. National YRBS: 1991–2009. 26. CDC. School health guidelines to promote healthy eating and physical activity. MMWR Recomm Rep 2011;60(RR–5):1–76. 27. CDC. The guide to community preventive services. www. thecommunityguide.org/pa. 28. CDC. State indicator report on physical activity, 2010. Atlanta GA: DHHS, 2010.
www.ajpmonline.org