The Association Between Waist Circumference and FITNESSGRAM ...

Pediatric Exercise Science, 2015, 27, 488 -493 http://dx.doi.org/10.1123/pes.2015-0009 © 2015 Human Kinetics, Inc.

ORIGINAL RESEARCH

The Association Between Waist Circumference and FITNESSGRAM® Aerobic Capacity Classification in Sixth-Grade Children John L. Walker and Tinker D. Murray Texas State University

James Eldridge University of Texas of the Permian Basin

William G. Squires, Jr. Texas Lutheran University

Pete Silvius Seguin Independent School District

Erik Silvius HEB Foundation Cardiorespiratory fitness is often assessed through measures of maximal oxygen uptake, sometimes referred to as aerobic capacity (26). The importance of adequate aerobic capacity for optimal health has been observed in numerous studies examining both adults (1,2,20) and children (17,24). An increased risk of overweight and metabolic syndrome in adults can result from a reduction in aerobic capacity from childhood to adolescence (4,11). The FITNESSGRAM® program is a widely used, comprehensive school-based fitness testing program designed to assess students’ levels of health-related fitness that was developed initially in 1982 (26). One of the FITNESSGRAM® assessments offered for measuring aerobic capacity is the 1-mile run. Students’ run times for this test are converted into measures of aerobic capacity (ml·kg-1.min-1) using a validated prediction equation from Cureton et al. (8). Previous research has documented the association between performance on the 1-mile run and Walker and Murray are with the Department of Health and Human Performance, Texas State University, San Marcos, TX. Eldridge is with the Dept. of Kinesiology, University of Texas of the Permian Basin, Odessa, TX. Squires is with the Dept. of Kinesiology, Texas Lutheran University, Seguin, TX. P. Silvius is with the Dept. of Physical Education, Seguin Independent School District, Seguin, TX. E. Silvius is with the Dept. of Wellness, HEB Foundation, Kerrville, TX. Address author correspondence to John L. Walker at [email protected].

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higher cardiorespiratory fitness, along with a better risk factor profile in children and adults (10,23,29). BMI and percent fat based on the sum of triceps and medial calf skinfolds are two FITNESSGRAM® assessments for measuring body size and body composition (26). An advantage of FITNESSGRAM® is the employment of established criterion-referenced standards that represent the appropriate levels of fitness that correspond to optimal health benefits (32,33). Achievement of the health fitness zone (HFZ) indicates the level of fitness that meets the health-related standard. The level of fitness below the HFZ indicates a status that needs improvement. Since 2011, the needs improvement (NI) classification has been divided into two categories for aerobic capacity, body mass index, and percent fat: NI—higher risk and NI—some risk (26). Students whose conditions fall into the NI- higher risk category are advised of potential risk if they remain at that level. Students whose conditions fall into the NI- some risk category are advised that they could reduce potential risks by improving into the HFZ. Determination of pediatric overweight and obesity are often based on body mass index, or BMI (18,21). WC has also been used as a measure of appropriate body size, and is commonly considered as one of the markers for metabolic syndrome (16). High-density lipoprotein cholesterol (HDL-C), fasting triglycerides, systolic/diastolic blood pressure, and fasting glucose level are other markers for metabolic syndrome (16). Fernandez et al. (13) have developed age -, sex -, and ethnicity-specific

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WC percentiles for U.S. children and adolescents that can be used in assessing those at increased risk for overweight, obesity and metabolic syndrome. From a practical standpoint, parents may be more acutely aware of their child’s WC than BMI or the method for calculating BMI, since parents often purchase school clothes for their children, and WC is often used for determining clothing sizes, especially for pants and jeans. There is also recent evidence that WC is a more valid indicator of youth aerobic fitness levels than BMI (3). There is considerable previous evidence regarding the relationship between fitness test scores of aerobic capacity and body size or anthropometric measures (6,9,27), and BMI is now used as a variable in estimating FITNESSGRAM® aerobic capacity from 1-mile run times (8). However, few studies have evaluated the influence of WC on 1-mile performance or aerobic capacity in children and adolescents (3,14). Therefore, the purpose of this study was to determine the association between WC and FITNESSGRAM® aerobic capacity criterion-referenced HFZ classification in sixth-grade children.

Methods Participants Participants were 528 sixth-grade children, 260 girls and 268 boys, that completed the comprehensive FITNESSGRAM® assessment through a continuing university—K-12 partnership that included the collection of yearly comprehensive fitness data through the school district’s physical education curriculum. The sample included 6th grade students from one middle school. The demographics of the sample were representative of that found in the whole district with 66.13% considered economically disadvantaged (or low social economic status), and 61.3% classified as Hispanic, 30.8% White, and 7.1% African American. The data were analyzed retrospectively and approved by the school district, the campus principal, the physical education staff, and by the Institutional Review Board from the university from which the study originated.

Procedures Normal physical education sessions provided the setting for the data collection over a 2-week period, using standard FITNESSGRAM® protocols (26). Teachers and participants were familiar with the components of the FITNESSGRAM® test battery, and university personnel were on hand to assist in the test administration and data collection. Students attended physical education classes for 55 min, every other day and engaged in moderate to vigorous physical activity (MVPA) for approximately 50% of each class period, which meant that students participated in an average of 137.5 min of MVPA in physical education every two weeks, or 16.3% of the daily recommended amount of 60 min for youth (30). Students participated in a FITNESSGRAM®® Friday

program designed by the school’s certified physical education teachers that focused on helping students improve baseline mile run scores and other FITNESSGRAM® assessment scores (22). Students practiced at least one component of the FITNESSGRAM® each week. Each participant completed the 1-mile run after a conditioning program, which included practice trials for learning proper pacing. WC was measured to the nearest 0.1 cm using a steel measuring tape, just above the uppermost lateral border of the ileum at the end of a normal expiration (28).

Data Analysis The dependent variable in this study was aerobic capacity (ml·kg-1.min-1) determined from 1-mile run times using the model developed by Cureton et al. (8): VO2max (ml·kg-1. min-1) = 0.21(age × gender)—0.84(BMI)—8.41(time in minutes) + 0.34(time2) + 108.94, where gender = 0 for girls and 1 for boys. Consistent with FITNESSGRAM® guidelines (26), all 1-mile run times that were slower than 13.0 min were rounded down to 13.0 min for calculating aerobic capacity. There were 68 girls and 39 boys whose run times required this adjustment. Current FITNESSGRAM® criterion standards for distinguishing the HFZ from NI categories were used to classify the participants (26). The independent variable in this study was WC. Descriptive statistics, as well as Pearson Product-Moment correlations were used to determine the linear relationships among the variables. Logistic regression analysis was used to determine the association between WC and the likelihood of being classified within the HFZ category for aerobic capacity. Odds ratios were calculated for WC in both boys and girls. Receiver Operating Characteristic (ROC) analysis was also used to determine appropriate cut-off scores identifying the HFZ and NI—higher risk classifications for WC. ROC analysis generates a curve indicating the diagnostic sensitivity (true-positive rate) and specificity (true negative rate) across the range of possible predictive values for the independent variable (12). For the HFZ classification, the sensitivity for WC represents the likelihood of a positive HFZ classification for an individual at or above that score. The specificity for WC represents the likelihood of a negative HFZ classification for an individual at or above that score. The point at which the sensitivity and specificity are maximized indicates a threshold score where the most accurate classification occurs. The area under the ROC curve (AUC) indicates the overall accuracy of the independent variable in classification, regardless of the cut-off score selected. An AUC of .50 indicates random chance selection, or a very weak predictor. An AUC of .60 to .70 indicates poor accuracy, .70 to .90 indicates moderate accuracy, and .90 to .99 represent a highly accurate predictor. In this study, the AUC indicates the ability of WC to correctly classify participants according to the HFZ standards for aerobic capacity scores. Significance was defined as p < .05 for each statistical test.

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490 Walker et al.

Table 1 Descriptive Characteristics of the Sample Characteristics

Girls (n = 260)

Boys (n = 268)

Total Sample (N = 528)

Mean (SD)

Mean (SD)

Mean (SD)

Age (years)

11.8 (0.6)

11.9 (0.6)

11.8 (0.6)

Height (cm)

153.9 (7.2)

155.2 (8.2)

154.6 (7.7)

Weight (kg)

54.6 (15.9)

55.8 (17.6)

55.3 (16.8)

Body mass index

22.9 (5.9)

22.8 (5.9)

22.8 (5.9)

Waist circumference (cm)

72.5 (13.4)

75.3 (14.8)

73.9 (14.2)

1-Mile run (min)

13.0 (2.4)

11.8 (2.8)

12.4 (2.7)

Aerobic capacity (ml/kg/min)

38.4 (5.4)

41.9 (6.4)

40.2 (6.2)

Table 2 Descriptive Characteristics of the Aerobic Capacity Classifications Healthy Fitness Zone (n = 278)

NI—Some Risk (n = 88)

NI—High Risk (N = 162)

Mean (SD)

Mean (SD)

Mean (SD)

Characteristics Age (years)

11.8 (0.6)

11.9 (0.7)

11.80 (0.6)

Height (cm)

153.0 (7.8)

155.9 (6.5)

156.5 (7.7)

Weight (kg)

43.8 (8.0)

57.0 (6.6)

74.0 (14.2)

Body mass index

18.5 (2.2)

23.3 (1.6)

30.0 (4.4)

Waist circumference (cm)

63.9 (6.6)

74.6 (6.8)

90.1 (10.8)

1-Mile run (min)

11.2 (2.5)

12.8 (2.0)

13.7 (2.2)

Aerobic capacity (ml/kg/min)

44.8 (3.5)

38.8 (0.9)

33.0 (3.7)

Results The descriptive characteristics of the sample are reported in Table 1. For aerobic capacity, 278 (52.7%) participants (117 girls and 161 boys) were classified as within the HFZ, 88 (16.7%) participants (50 girls and 38 boys) were classified as NI—some risk, and 162 (30.7%) participants (93 girls and 69 boys) were classified as NI—high risk. The descriptive characteristics of these three groups are reported in Table 2. Overall, the sample represents a broad range for the variables of interest in the study. The Pearson Product-Moment correlation between 1-mile run times and WC was .66, p < .0001. The correlation between aerobic capacity and WC was -.83, p < .0001. For classifying participants as within the aerobic capacity HFZ versus NI, logistic regression indicated that WC is significantly related to the odds of being classified as within the HFZ for both girls, c2 (1) =190.4, p < .0001, and boys, χ2 (1) = 209.8, p < .0001. The odds ratio for girls was .72, 95% CI = [.66, .79], and the odds ratio for boys was .75, 95% CI = [.69, .81]. This result indicates that as WC increases by one centimeter, the odds that a girl would be classified as within the HFZ for aerobic capacity decreases by 28% (1.00–.72), and for a boy, those odds decrease by 25% (1.00–.75). ROC analysis indicated that WC is a strong predictor of aerobic capacity HFZ classification in girls, AUC = .95

and boys, AUC = .97. These analyses are demonstrated in Figures 1 and 2. For girls, the threshold cut-off score for WC that indicates the best classification accuracy for aerobic capacity HFZ is 68.6 cm, or 27 inches, with sensitivity = 90.1%, specificity = 86.7%, and overall correct classification = 88.9%. Girls whose WC is below 68.6 cm are most likely to be correctly classified as within the HFZ for aerobic capacity, while those with a WC above 68.6 cm are most likely to be correctly classified as NI. For boys, the threshold cut-off score for WC that indicates the best classification accuracy for aerobic capacity HFZ is 76.2 cm, or 30 inches, with sensitivity = 95.7%, specificity = 92.5%, and overall correct classification = 93.8%. Boys whose WC is below 76.2 cm are most likely to be classified as within the HFZ for aerobic capacity, while those with a WC above 76.2 cm are most likely to be classified as NI.

Discussion The intent of this study was to determine the association between WC and FITNESSGRAM®® aerobic capacity HFZ criterion standard classification. This issue is a concern for school-based fitness testing programs, especially those using health-related fitness tests such as FITNESSGRAM®, because an important purpose of those programs is to provide accurate classification of

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Figure 1 — ROC curve for classifying HFZ based on girls waist circumference.

Figure 2 — ROC curve for classifying HFZ based on boys waist circumference.

health-related fitness status for reporting both individual and group performances. Since 2011, FITNESSGRAM® aerobic capacity has been evaluated by converting either 1-mile run times or PACER lap scores into VO2max (ml·kg-1.min-1) through prediction equations (7). As mentioned previously, 1-mile run times are converted into VO2max (ml·kg-1.min-1) from the equation developed by Cureton et al. (8), which includes BMI as a predictor. This is justified, since body size and body composition have been shown to account for significant variation in estimates of VO2max (9,25,27). The same result was found in this study. At least part of the variation in WC is due to abdominal fat; consequently, WC could be considered a field measure of body size and/or body composition. This result might be expected, since another measure of body size, BMI, is used as a predictor variable in estimating aerobic capacity (8). The results of this study appear to confirm that WC and BMI both have a similar effect on aerobic capacity in children.

These results indicate that WC is significantly correlated with estimated aerobic capacity based on 1-mile run times, and is a strong predictor of aerobic capacity classification based on FITNESSGRAM® health-related fitness age- and gender-specific criterion standards. The coefficient of determination (R2) from the regression analysis suggests that after 1-mile run times are converted into values of VO2max, 68% of the variation in VO2max is accounted for by WC. This result appears to indicate that even after performance scores are converted into measures of aerobic capacity, body size (WC) still accounts for more than half of the variation in aerobic capacity. Cureton et al. (5) and Rowland et al. (27) suggested that about half of the variance in aerobic capacity in children can be accounted for by body fatness. The results in this study confirm a very similar relationship regarding WC and aerobic capacity. For those participants whose 1-mile run times were slower than 13.0 min, their performances were rounded

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down to 13.0 min for calculating aerobic capacity, consistent with FITNESSGRAM® procedures (26). For these children, the only source of variation in aerobic capacity for each gender and age category was BMI, since their 1-mile run times were assigned a constant of 13.0 and BMI is the only other predictor variable in the equation. Since BMI and/or percent fat are also assessed as separate components of fitness, it could be argued that these children are being evaluated twice on the same characteristic. Due to the high correlation between WC and aerobic capacity, and the strong association between WC and aerobic capacity classification found in this study, that same argument might be made for all the participants. In addition, the AUC values from each ROC analysis suggest that the reported threshold cut-off scores for WC can be used for classifying aerobic fitness with almost the same accuracy as 1-mile run times. These results support previous research documenting the strong effect that body size/composition has on aerobic fitness performance and assessment (6,8,9,19,25,27). The FITNESSGRAM® HFZ criterion referenced standards are intended to represent levels of fitness that indicate health status in children (32–34) The results of this study appear to support previous research which identifies body size as an important factor in achieving a level of fitness necessary for good health (15,18,28,31). There has been a lack of research that documents the effect of the changes in body size or WC on aerobic capacity classifications, but the results of this study appear to provide insight into this relationship. This study is the first investigation to document the odds of being classified as HFZ based on WC. The odds ratios reported in this study also suggest that waist circumference has a similar association with aerobic capacity classification for both girls and boys. This study also identified the most appropriate threshold scores for WC for aerobic capacity classification. For correct classification within the HFZ, it appears that a WC of 68.6 cm for girls and 76.2 cm for boys provides the highest correct classification. These threshold scores might provide appropriate markers to aid professional practitioners in setting fitness goals for students, or for identifying students who might need intervention. The need for such goals and interventions may increase in the future in light of reported decreases in physical activity during adolescence (10). A primary strength of this study is the large sample size based on individual student data with a high percentage of minority students. Aggregate data has been used in the vast majority of previous FITNESSGRAM® performance research reports (22). This study found that WC measures are significantly related to aerobic capacity, and this is the first study to report the effect of WC on FITNESSGRAM® aerobic capacity criterion-referenced classifications. This study was limited in that it included a sample of 6th-grade students only, primarily between the ages of 11–12 years. This narrow age range may limit the generalizability of the study to other populations, particularly older adolescents. Consequently, these results

might be considered sample-specific, and the relationship between body size/composition and aerobic capacity can vary depending on the demographics of the sample under consideration. Furthermore, the maturation status of the participants was not considered in the analysis, which might have influenced the results. Since FITNESSGRAM® bases its criterion standards on age and gender without regard to maturation status, these results can be applied only to 11–12 year old children within various levels of maturation. Larger samples with a broader range of ages may be necessary to provide more substantial evidence of the influence of WC on aerobic capacity at varying maturation levels, considering the body size and body composition changes that typically occur during adolescence. Another limitation is the use of a field test, in this case the 1-mile run, for estimating aerobic capacity. Laboratory measures of VO2max may provide a more definitive answer to the issue of the effect of body size on aerobic capacity; however, the 1-mile run is a widely used field test of aerobic endurance in youth, and commonly used by FITNESSGRAM® for estimating aerobic capacity and assessing students’ health-related fitness status.

Conclusions Aerobic capacity is an important health-related outcome and an important component of FITNESSGRAM®. WC is a common anthropometric measure of body size used for identifying children at risk for overweight or obesity. This study found that WC accounts for considerable variation in aerobic capacity, and is significantly related to aerobic capacity classification based on FITNESSGRAM® criterion standards.

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