Adequate participation in physical activity (PA) during childhood and adolescence is considered essential for good health (3 4,20 25
Low-income minority children living in inner-city areas may be especially at risk for physical inactivity and obesity because they encounter significant barriers to PA such as high crime, neighborhood disorder, and limited or no access to parks and/or recreational facilities (9,10,14,19 11,15 accelerometry to quantify PA (1,2,13,17
Therefore, the aims of this study were to 1) objectively measure the PA characteristics of a racially and ethnically diverse sample of inner-city elementary schoolchildren and 2) examine the influence of sex, race/ethnicity, grade level, and weight status on PA.
METHODS
Participants and settings.
Participants were fourth- through sixth-grade school students from six public elementary schools in Philadelphia. Schools were part of Get Healthy Philly , an ongoing, city-wide, multicomponent initiative aimed at reducing obesity through interventions in schools and afterschool programs, corner stores, the mass media, and the built environment. All assessments were made before the implementation of any school-based initiatives. To be eligible for participation in the current study, schools had to be participating in the larger assessment of weight status in 55 schools from the 25 highest-risk zip codes based on SES and health as determined by the Philadelphia Department of Public Health. The six schools were selected based on their large size (≥700 students or ≥200 fourth to sixth graders). The mean ± SD eligibility for free/reduced meals across the schools was 82.1% ± 7.4%. Fourth through sixth graders were selected based on earlier data indicating that this is a high-risk time for the development of overweight and obesity (7
Instrumentation.
PA was measured using the ActiGraph GT3X+ accelerometer (ActiGraph, LLC, Pensacola, FL). The GT3X+ is a small (4.6 × 3.3 × 1.5 cm), lightweight (19 g) accelerometer-based motion sensor that records time-varying accelerations ranging ± 6g . The accelerometer output was sampled by a 12-bit A/D converter at a user-specified rate (30–100 Hz) and stored in nonvolatile flash memory for subsequent downloading and processing. A sampling rate of 30 Hz was used in the current study.
Accelerometer protocol.
Participating fourth-, fifth-, and sixth-grade students were outfitted with an accelerometer either in the nurse’s office or during a regularly scheduled class period. Consistent with previous studies, monitors were attached to adjustable elastic belts and worn on the right side of the hip. After receiving detailed instructions regarding the care and use of the monitor, participants were instructed to wear the ActiGraph during the waking hours for seven consecutive days.
Data reduction.
Upon return of the GT3X+ unit, ActiGraph propriety software (ActiLife Version 5.8) was used to download and convert the raw acceleration data into activity counts per 15 s (epoch). The resultant data file was saved and subsequently uploaded to a customized Visual Basic EXCEL macro to determine the wear time and time spent in sedentary (SED), light-intensity PA (LPA), moderate-intensity PA (MPA), vigorous-intensity PA (VPA), and moderate- to vigorous-intensity PA (MVPA). Counts were classified into the aforementioned PA intensity categories using the cut points developed by Evenson et al. (5 23 21
Assessment of height, weight, and demographics.
Students were assessed while wearing light, indoor clothing and without shoes. Body weight was measured on a digital scale (SECA Alpha 882 and SECA Large Capacity 634) twice or until two measures were within 0.2 kg on a digital scale. Height was measured twice or until two measurements were within 1 cm on a portable stadiometer (Perspective Enterprises PE-AIM-101). The average of two measures within range was used. Both the scales and stadiometers were calibrated daily. Measurements occurred during the school day in the gymnasium or the nurse’s office and behind privacy screens. School administrative staff provided participants’ demographic information including grade, sex, age, month and year of birth, and race/ethnicity.
Statistical analyses.
Differences between the monitoring sample and students not completing the accelerometer protocol for sex, race/ethnicity, and weight status were tested using χ 2 tests for categorical variables (sex, race/ethnicity) or t -tests for continuous variables (body mass index [BMI] percentile). Group differences in the PA variables were evaluated for significance using a four-way (sex × race/ethnicity × grade level × weight status) factorial ANCOVA. Within each model, daily wear time was included as a time-varying covariate. Preplanned pairwise comparisons were evaluated for significance using single-degree-of-freedom contrasts. Logistic regression was used to calculate the relative odds of meeting PA guidelines according to sex, grade level, race/ethnicity, and weight status. All statistical analyses were performed using SAS software (version 9.2; SAS Institute, Cary, NC). Significance was set at an α level of 0.05.
RESULTS
A total of 510 students were outfitted with an accelerometer. Of this number, 13 students did not return an accelerometer, 18 were returned with less than one valid monitoring day, and 9 students were excluded from the analyses because of biologically implausible values for BMI, providing a final sample size of 470. Mean ± SD number of valid monitoring days was 4.8 ± 1.8 d, with an average wear time of 818.2 ± 102.1 min·d−1 . Participant characteristics for the final monitoring sample are presented in Table 1 . Slightly more than half of the sample was female or Hispanic, and nearly half of the sample was at least overweight. Compared to students not completing the accelerometer protocol (n = 978), the monitoring sample had a significantly higher percentage of female (57.2% vs 48.5%) and Hispanic students (53.4% vs 41.2%) and significantly lower percentage of African American students (35.2% vs 48.2%, P < 0.05). There were no significant differences for mean BMI (21.4 ± 5.4 vs 21.2 ± 5.1) or mean BMI percentile (70.0 ± 28.7 vs 69.0 ± 28.0).
TABLE 1: Descriptive statistics for the accelerometry sample.
The average CPM percentile for the sample was 38.7%, which indicated that, as a group, students were approximately 0.5 SD below the national average for their age and sex. Within the sample, overweight (37.3%, 95% confidence interval [CI] = 31.6%–42.7%) and obese students (31.7%, 95% CI = 27.4%–36.0%) exhibited significantly lower CPM percentile values than their healthy weight counterparts (42.1%, 95% CI = 39.0%–45.2%).
Table 2 displays adjusted means and 95% confidence intervals for the PA variables for groups defined by sex, race/ethnicity, and grade level. With the exception of the sex difference for SED among sixth graders, there were no significant sex, racial/ethnic, or weight related differences in SED and LPA. For boys and girls, SED time increased significantly with grade level, whereas time in LPA decreased significantly with grade level.
TABLE 2: Sex-specific means and 95% confidence intervals (in parentheses) for the physical activity variables by race/ethnicity and grade level.
On average, children accumulated 48 min of MVPA daily. Across all race/ethnicity and grade level groups, boys exhibited significantly higher levels of MVPA than girls did. Hispanic boys exhibited significantly lower levels of MVPA than African American boys, whereas the difference between Hispanic boys and those from the “other” racial/ethnic group approached statistical significance (P = 0.08). There were no significant racial/ethnic differences in MVPA for girls. Fifth-grade boys exhibited significantly lower MVPA levels than sixth-grade boys did. Sixth-grade girls exhibited significantly lower MVPA levels than fourth-grade girls did. Across all sex, racial/ethnic groups, and grade levels, overweight and obese children exhibited significantly lower MVPA levels than children in the healthy weight range did (Fig. 1 ). Expressed as a percentage of monitoring time, children, on average, were sedentary for 63% of the time, in LPA 31% of the time, and in MVPA 6% of the time (Fig. 2 ).
FIGURE 1: Means ± 95% CI for MVPA in healthy weight, overweight, and obese inner-city schoolchildren.
FIGURE 2: Percentage of monitoring time in sedentary, light, and moderate-to-vigorous physical activity.
Results of the logistic regression analysis are shown in Table 3 . Across the entire sample, 24.3% of students met the PA guideline of 60 min or more of MVPA daily. Boys were nearly 6 times more likely than girls were to meet the guideline, fourth graders were 1.5 times more likely than fifth or sixth graders to meet the guideline, and children in the healthy weight range were 2.3 times more likely than overweight and obese children were to meet the guideline. African American children were approximately 1.9 times more likely than Hispanic children were to meet the PA guideline. Children in the “other” race/ethnicity category were also more likely than Hispanic children were to meet the PA guideline; however, this association was not statistically significant because of the small sample size (odds ratio = 1.73, 95% CI = 0.84–3.57).
TABLE 3: Odds ratios for meeting guidelines for daily physical activity.
DISCUSSION
The results of this study indicate that low-income, predominantly African American and Hispanic children, living in an urban environment have much lower levels of PA than the national average. Across the entire sample, only 24.3% met the current public health guidelines for PA. Of concern, the risk for physical inactivity was significantly greater among females, Hispanic children, and those with a BMI in the overweight and obese category.
Our findings highlight the need for effective and sustainable programs to promote PA in inner-city youth. Previous research suggests that inner-city neighborhoods are highly walkable and offer multiple play spaces; yet, safety concerns preclude their use (10,15 6 16 16,22 24 www.cityprojectca.org 8 12,18,26
Relative to other racial/ethnic groups in the sample, Hispanic children in this sample were significantly less likely to meet the 60-min recommendation for daily PA. This finding is in conflict with the results of 2003–2004 NHANES, which used accelerometers to objectively measure PA in a nationally representative sample of U.S. children and adolescents. In that study, Hispanic youth exhibited similar or higher levels of MVPA than white and African American children (21 27
This study had several limitations that warrant consideration. First, the results were obtained from a single city in the northeastern United States. Hence, the findings may not be generalizable to other cities in the United States. Second, given the modest response rate, we cannot rule out the possibility of selection bias. However, we believe that the demographics of the monitoring sample reflected the student enrollment at each of the participating schools. Notably, the sample included significant numbers overweight and obese students, as well as students from racial/ethnic minority groups. Third, waist-mounted accelerometers are unable to account for the increased energy cost associated with walking or running up incline and do not accurately measure the energy costs of static activities such lifting and carry objects (23 accelerometry provides valid estimates of time spent in different levels of PA intensity, they do not provide information about the types of activities being performed and the context in which PA is taking place.
In summary, fewer than one in four inner-city schoolchildren accumulated the recommended 60 min of MVPA daily. Within this group, the risk of physical inactivity was greater among girls, Hispanics, and overweight/obese children. Collectively, these data underscore the need for city-wide initiatives to increase PA in Philadelphia children and families.
Funding for this evaluation was made possible by Cooperative Agreement No. 3U58DP002626-01S1 from the Centers for Disease Control and Prevention, US Department of Health and Human Services; and Get Healthy Philly, an initiative of the Philadelphia Department of Public Health.
The views expressed in this article do not necessarily reflect the official policies of the Department of Health and Human Services; nor does mention of trade names, commercial practices, or organizations imply endorsement by the US Government.
No conflict of interest present for each author.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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