The increasing prevalence of obesity in children in the United States has been well documented (19,29). In 6- to 11-yr-old girls, 27.8% are overweight or at risk of overweight (19). At risk of developing obesity are children of obese parents, where parental obesity more than doubles the risk of adult obesity among children <10 yr of age (34).
The Report of the Surgeon General on Physical Activity and Health (31) emphasized that regular physical activity has important health benefits including reducing the risk of cardiovascular disease and helping to treat and prevent high blood pressure, high cholesterol, and diabetes. Physical activity has been promoted as a lifelong positive health behavior in children and adolescents (5). However, approximately 20% of U.S. children do not exercise vigorously more than twice per week, with the percentages higher in girls (26%) than in boys (17%) (1). Participation in physical activity declines with increasing age in children, particularly in girls (5,21,31). Recently, a report from The National Heart, Lung, and Blood Institute Growth and Health Study (15), found substantial declines in physical activity in girls from 9 or 10 to 18 and 19 yr of age. A higher body mass index was one correlate of this decline. Whether physical activity begins to decline in girls from 8 to 10 yr of age has not been thoroughly investigated.
Fitness is another important factor related to health outcomes, independent of physical activity. Several cross-sectional studies (17,33) have reported fitness levels in girls ranging in age from 7 to 13 yr. However, longitudinal studies examining age-related changes in fitness in young girls are few. A positive relationship between obesity and fitness has been reported (20).
We enrolled multiethnic, normal-weight 8-yr-old girls with or without a predisposition to obesity into a longitudinal study. Predisposition to obesity was defined by parental body mass index (BMI) with LN = girls with two lean (BMI < 25 kg·m−2) parents, LNOB = girls with one obese and one lean parent, and OB = girls with two obese (BMI > 28 kg·m−2) parents. This study was designed to determine whether physical activity, fitness, and sedentary behaviors change in girls from 8 to 10 yr of age and if these changes are different based on whether or not the girls were predisposed to obesity.
Healthy prepubertal girls, 8 yr of age (N = 101), were recruited from the local Houston area to participate in the study. The ethnicity of the girl was determined by the self-reported ethnicity of the mother. All girls were at Tanner stage I at baseline. The girls were screened and had to be below the 90th percentile of their weight-for-height (18) and have a body fat within 12–30% to be included in the study. The reason for this was that the primary aim of the longitudinal study was to study children before they become overweight. This allowed us to examine possible factors that might contribute to their becoming overweight (24, 28), because it is known that children of obese parents are more likely to gain weight (33). We thus enrolled normal-weight 8-yr-old girls with (LNOB and OB groups) or without (LN group) a predisposition to obesity (classified by parental BMI) into a longitudinal cohort study and followed them for 2 yr. Children with cardiovascular disease, anemia, diabetes, significant renal or hepatic disease, hypothyroidism, or musculoskeletal problems were excluded. Their parents provided written informed consent, and the children provided assent to participate in this study, which was approved by the Institutional Review Board for Human Subject Research for Baylor College of Medicine and Affiliated Hospitals.
All measurements were performed at the USDA/ARS Children’s Nutrition Research Center from October 1995 through June 2000 during the school year and the summer, and during the same season yearly for each girl. Anthropometric, body composition, Tanner staging, physical fitness, and physical activity and sedentary measures by questionnaire were taken at 8, 9, and 10 yr of age.
Body weight was measured to the nearest 0.1 kg using a digital balance (Scale-Tronix, Dallas, TX), and height was measured to the nearest 1 cm using a stadiometer (Holtain Ltd., Crymmych, Pembs, UK). Parents were measured using the same techniques at baseline. The body mass index (BMI) was defined as weight/height2 and expressed in kilograms per meters squared units.
Body composition was measured by dual-energy x-ray absorptiometry (DXA, Hologic QDR 2000, Madison, WI, pencil beam mode, software 5.56). For the DXA, the subject lay supine on the bed and was scanned from head to toe in 10–15 min. DXA allows for determination of bone mineral density and for determination of three compartments: lean tissue mass, fat mass, and bone mineral content. For the total body, fat-free mass was defined as the sum of lean tissue mass and bone mineral content.
The mothers were shown pictures of the various Tanner stages (23) and identified their daughter’s stage at each yearly visit.
To assess fitness, V̇O2peak was measured by collecting expired gases with a metabolic measurement cart (Model 2900, SensorMedics Corp, Yorba Linda, CA) during an exercise test on a treadmill (Model Q55, Quinton Instrument Co, Seattle, WA). The treadmill protocol (25) involved a constant speed of 2.5 mph at an initial 0% grade for the first 4 min. The average of minutes 3 and 4 constituted the steady state. The grade was then increased by 2.5% to a maximum grade of 22.5%, when speed was increased by 0.6 mph. Each incremental stage lasted 2 min. V̇O2peak was determined using standard criteria for children, specifically a heart rate > 195 bpm or RQ > 1.0 at peak (10).
Physical activity and sedentary measures: heart rate monitoring and questionnaire.
Heart rate was stored with either a POLAR Vantage XL Heart Rate Monitor (model 61204, Helsinki, Finland) or a Mini-mitter unit (Mini-mitter 2000, Sun River, OR) while the child was at home for 2 d. The watch was placed on the subject’s wrist, and the transmitter was placed on the child’s chest by the investigator to demonstrate how it functioned before leaving the CNRC. The parents were also given instructions on how to start and stop the watch when using the POLAR monitor. The Mini-mitter was programmed for the 2 d that the child agreed to wear the monitor. Heart rate was recorded every minute for two 24-h periods (1 weekday and 1 weekend day).
The resting heart rate (RHR) of all children was measured at baseline while they were in the calorimeter undergoing a basal metabolic rate measurement, as described previously (24). RHR was defined as the average heart rate while lying supine for 30–40 min with minimal activity level. For an index of free-living activity patterns, we calculated the percentage of total minutes recorded for which the heart rate was >125% of RHR and >150% of RHR, as cutoff points used previously (7). A minimum of 1000 min of data from each 24-h period was needed for the data to be used in the analysis. The “active” time refers to the number of minutes the child spent at greater than 125% of RHR or greater than 150% of RHR with the “active” heart rate referring to the heart rate during this time frame. In contrast, “inactive heart rate or inactive time” refers to the heart rate or time below these thresholds. The “active” plus “inactive” times or heart rates constitute the awake portion of the day only, with sleep removed.
The parents were given a questionnaire to evaluate their child’s physical activity over the previous year. Exercise participation was assessed using the Physical Activity Interview for Children (9) with several modifications. Instead of recording an activity only if it occurred >10 times in the previous year, all activities that took place in the previous year were recorded, regardless of the frequency. Activities also were grouped by intensity (3). Activities in the light category included: bowling, fishing, Frisbee, garden or yard work, and horseback riding. Activities in the moderate category included: bicycling (on a trail), dancing, gymnastics, hiking, ice skating, martial arts, rollerblading, snow skiing, soccer, softball/baseball, swimming (recreational), tennis, and walking. Activities grouped in the strenuous category included: aerobics, basketball, bicycling (in the street), running for exercise, swimming laps, and volleyball. This questionnaire was also modified to include the breakdown of activities by the school year (September through May) and the summer (June through August). The number of months, the number of times per month, and the length of time spent on each activity (in hours) were recorded. From this, the overall sum of the hours spent in light, moderate, and strenuous activities were calculated in terms of total hours per year. The sedentary behaviors included the time spent sleeping and watching TV during the school year and during the summer.
Microsoft Access (Windows 95, Version 7.0) was used for database management. Repeated measures ANOVA were used to test for the differences between groups and over time. The interaction term between group (LN, LNOB, and OB) and time (8, 9, and 10 yr of age) was evaluated first. Weight, fat-free mass, Tanner stage, and ethnicity were also used as covariates for the analysis of the V̇O2 data. Significant group or time effects were then explored using post hoc tests (Bonferroni). SPSS for Windows (version 8.0) was used. Data are presented as means ± SD, a two-tailed P < 0.05 was taken as indicating significance.
Ten girls of the original sample of 101 were lost to follow-up because their families had moved from the area. For the 91 girls who completed anthropometric, body composition, and baseline fitness measures, the ethnic breakdown was 48 Caucasian, 27 African-American, and 16 Hispanic. The final sample sizes at year 1 and 2 for the fitness, heart rate monitoring, and questionnaire data are included in the respective tables. Sample sizes were smaller for the heart rate monitoring data due to lack of complete data.
In the entire sample, weight, height, and body fatness increased over the course of the study (28). Weight was 28.0 ± 4.1 kg, 32.7 ± 6.4 kg, and 37.5 ± 8.0 kg at 8, 9, and 10 yr of age, respectively. Body fat by the DXA was 21.7%, 24.1%, and 25.1% at 8, 9, and 10 yr of age, respectively. The weight and fat-free mass of the girls from the three parental obesity groups did not differ at ages 8, 9, or 10. The fat mass and percent fat differed between groups at age 9 and age 10; the OB group had a higher fat mass (P < 0.05) and percent fat (P = 0.05) than the LN group at age 9, and a higher fat mass (P = 0.05) at age 10. The 2-yr changes in weight were different across groups (P < 0.05) controlling for ethnicity and Tanner stage, with the OB group gaining more weight than the LN group.
Descriptive data for the physical fitness tests during steady-state and peak exercise are presented for the girls at 8, 9, and 10 yr of age (Table 1). No significant group by time interactions for any of the physical fitness variables were found (Table 1). However, when controlled for weight and Tanner stage, there was an interaction of group with time (P < 0.05) for V̇O2. Ethnicity was not a significant covariate. No group differences for steady-state exercise were observed after adjusting for weight or fat-free mass and Tanner stage. During steady-state exercise, heart rate, ventilation, V̇O2 (L·min−1), and V̇O2 (mL·kg−1·min−1) were significantly different across time (P < 0.05). V̇O2 (L·min−1) was higher by 10 yr of age than at age 8 and 9 (P < 0.05); however, when expressed per kilogram of body weight (V̇O2, mL·kg−1·min−1), there was a progressive decrease from 8 to 9 yr (P < 0.001) and 9 to 10 yr of age (P < 0.05).
During peak exercise, V̇O2 (L·min−1), V̇O2 (mL·kg−1·min−1), time on the treadmill, and treadmill stage were different across groups (P < 0.05). The peak V̇O2 for the LN, LNOB, and OB groups were 43.3 ± 4.1, 42.0 ± 5.4, 41.6 ± 5.0 mL·kg−1·min−1 at 8 yr, 44.9 ± 4.9, 41.8 ± 4.6, and 40.1 ± 6.1 mL·kg−1·min−1 at 9 yr, and 44.7 ± 5.8, 41.4 ± 5.8, and 39.4 ± 5.9 mL·kg−1·min−1 at 10 yr. Girls with LNOB parents had a lower absolute V̇O2 than the LN girls by 2.5 mL·kg−1·min−1 (P < 0.05). Also, the OB group had a 3.9 mL·kg−1·min−1 lower V̇O2 than the LN group (P < 0.05) and a 1.4 mL·kg−1·min−1 lower V̇O2 than the LNOB group (NS). The girls of LN parents also exercised longer (P < 0.05) than girls with OB parents and reached a higher stage during the treadmill test (P < 0.05) than both the LNOB or OB groups. Further analyses showed that these differences were observed in year 2 between the LN and OB groups (P < 0.05). Also during peak exercise, significant time effects were observed for ventilation, V̇O2 (mL·min−1), and final stage reached on the treadmill. V̇O2 increased significantly over time (all P < 0.001) but not when expressed per kilogram of body weight (mL·kg−1·min−1). Girls were able to reach a higher stage on the treadmill at 10 yr of age compared with 8 yr of age (P < 0.05).
Physical activity measures.
The duration of the weekday heart rate monitoring was 1279 ± 235 min, 1328 ± 113 min, and 1296 ± 178 min at 8, 9, and 10 yr of age. Table 2 shows the activity data from the heart rate monitoring for the weekday and weekend days at both 125% and 150% of RHR. No group by time interactions or group effects were observed. The active heart rate on the weekday was significantly different across time at both intensity levels (125% RHR and 150% RHR) and on the weekend at the lower intensity (125% RHR) (all P < 0.05). Active heart rates were lower at 10 yr of age compared with 8 yr of age (P < 0.05). The percent day active at both intensities and on the weekday and weekend were not significantly different across time.
No significant group × time interactions were observed for any of the questionnaire measures of physical activity or sedentary behaviors (Table 3). The number of team sports participated in was significantly different across groups with the LN group reporting a greater number of team sports than the LNOB group (0.9 ± 1 vs 0.5 ± 0.7, P = 0.012) with the OB group reporting 0.7 ± 0.9. Time spent in sports over the past year was not different across groups or over time. However, time spent in sports in the past week was significantly different across groups (P < 0.05) with the LN group reporting more time spent in sports in the past week (2.3 h·wk−1) than the LNOB group (0.75 h·wk−1, P < 0.05). The OB group reported 1.1 h·wk−1. No other significant group or time effects were observed for the time spent in physical education, time spent playing, number of hours in light, moderate or hard intensity activity separately or combined, number of hours in moderate or hard intensity combined, or time spent watching TV in the school or summer.
V̇O2 at steady-state exercise or peak exercise was not related to the percent day active on the weekend or weekday at either intensity; however, V̇O2 (mL·kg−1·min−1) during steady-state exercise was related to the percent day active on the weekend at 150% RHR (R = 0.199, P < 0.05). The number of hours spent in moderate intensity activity measured by the questionnaire was related to steady-state V̇O2 (R = −0.16, P < 0.05), peak V̇O2 (mL·kg−1·min−1) (R = 0.19, P < 0.05), treadmill time (R = 0.20, P < 0.05), final stage on the treadmill (R = 0.16, P < 0.05), and percent weekday active at 150% RHR (R = 0.16, P < 0.05). Hours in moderate intensity activity were not related to the percent day active on the weekends at either intensity. The number of hours spent in light or strenuous intensity activity as recorded on the questionnaire was not related to any fitness variables or physical activity by heart rate, except for a correlation between reported strenuous hours of activity and peak V̇O2 (mL·min−1, R = 0.16, P < 0.05).
In terms of sedentary behaviors, the time spent watching TV during the school year (but not the summer) was negatively related to peak V̇O2 (mL·kg−1·min−1) (R = −0.17, P < 0.05), time on the treadmill (R = −0.20, P < 0.05), and final stage reached on the treadmill (R = −0.16, P < 0.05). The time spent watching TV during the school year or the summer was not related to the activity data from the heart rate monitoring.
In this study, we examined changes in physical activity and fitness in girls from 8 to 10 yr of age. Girls were measured annually for fitness by a V̇O2peak test, physical activity by heart rate monitoring, and physical activity and sedentary behaviors by questionnaire. These girls had either lean or obese parents. We found that fitness remained fairly constant across time in the girls. In addition, there were no significant changes from 8 to 10 yr of age in physical activity as measured by heart rate monitoring or by questionnaire. However, girls with lean parents were more fit and able to exercise longer than girls with either one or two obese parents.
We reported here changes in fitness over a 2-yr period in girls. Girls maintain their fitness over this time frame, such that absolute oxygen consumption increases but not when expressed per kilogram of body weight. Our most interesting finding is the differences across the groups of girls classified by parental obesity status. The girls with OB parents have the lowest oxygen consumption/kilogram weight and the lowest duration of exercise on the treadmill. Thus, the girls that increased their weight or fat to a greater extent over this time frame decreased their fitness as well. This may be important in designing intervention studies. Because fitness is related to health outcomes such as coronary heart disease, children with either one or two obese parents who are at risk of having low fitness would benefit from interventions designed to increase fitness.
The age-related changes in fitness, specifically in girls from 8 to 10 yr of age, have not been well evaluated. Several cross-sectional studies (13,17,32,33) have examined V̇O2peak using treadmill testing or cycle ergometry, typically in much smaller sample sizes (range 33–70 girls) and with girls of wider age ranges (7–13 yr). One study (32) reported values of 47 mL·kg−1·min−1 in a sample of only 12 girls aged 9–11 from a treadmill test, whereas McMurray et al. (17) reported an almost identical value (V̇O2 = 42.5 mL·kg−1·min−1) to ours in girls (N = 18) aged 7–13 yr. Studies using cycle ergometers to determine V̇O2peak have reported similar values (13,17,33).
We previously used the identical treadmill protocol to report fitness in girls of the same age before and after a strength training intervention (26). By comparison, the V̇O2 was slightly higher at steady-state exercise (by 1.4 mL·kg−1·min−1) but lower at peak exercise (by 3.1 mL·kg−1·min−1) in this smaller group of girls compared with the girls in this study. An extensive literature review points to the variability in V̇O2 values if the test is performed on a cycle or treadmill, as well as the age of the child (16). Ethnicity may also be an important factor, as a lower peak V̇O2 has been documented in black children (12,30). We previously reported no significant differences in peak V̇O2 in black girls compared with white girls (P = 0.12) and Hispanic girls (P = 0.06) after adjustment for fat-free mass when the girls were 8 yr of age, although the values were lower (25). Our longitudinal data agreed with this baseline finding of a lack of a significant difference between ethnicities. Fitness can be improved with training, as physical activity interventions involving an aerobic component have reported positive changes in elementary age children (4,6).
Reviews (11,22) have been written encompassing obesity and physical activity in childhood and adolescence. The decline in physical activity through childhood to adolescence has been studied as well (15,31). For example, the U.S. Youth Risk Behavioral Survey (14) found that 25% of high school students reported no vigorous physical activity and only 24% of high school females meet the recommended levels for moderate activity. A very recent report from the National Heart, Lung, and Blood Institute Growth and Health Study (15) reported a 100% and 64% decline in activity scores in black and white girls, respectively. Interest has focused on the intensity of the activity, as well as the duration.
In our study, both the heart rate monitoring and the questionnaire data together provided information on the intensity and duration of the types of activities that these girls were participating in over the years. The findings from the objective measure (the heart rate monitor) and the self-report (questionnaire) provided us with the same conclusion; that is, activity levels are not changing substantially in this age range of girls. For instance, the percent day active at both intensities and on the weekday and weekend were not statistically different across time, although the values are slightly lower at age 10 compared with age 8.
The recent report from the Institute of Medicine Dietary Review Group (8) recommended 60 min or more of daily moderate intensity physical activity for children. The majority of the girls are not meeting a 60 min·d−1 criteria of moderate to hard intensity activities, with only 42%, 31%, and 43% of the girls participating in 60 or more minutes per day at age 8, 9, and 10, respectively. Again, the activity levels of children may decrease in the middle-school years as spontaneous levels of activity in children decrease, and the time spent during school in physical education may be less. Thus, interventions targeting children after age 10 may prevent any decline in physical activity. Interventions before age 10 would be beneficial if the child is overweight or if one or two parents are obese.
The low but significant correlations between the reported level of moderate intensity activities determined by questionnaire and the fitness test are interesting. The greater the time spent in moderate activity, the higher the peak V̇O2 and the longer time spent on the treadmill. This relationship may be due in part to the possibility that the girls’ parents are better able to recall the moderate, than the light or strenuous, intensity activities of their daughters. The questionnaire also contained a greater number of activities in this moderate category; thus, the questionnaire may be designed to better capture moderate activities (such as a frequent activity like soccer in our girls) rather than light or strenuous intensity activities. The reported hours in strenuous activities was related to absolute peak V̇O2. This relationship is not too surprising as strenuous activities promote increased fitness.
In terms of sedentary behavior, the questionnaire evaluated the time spent watching TV during the school year and the summer. The parents reported these girls were watching an extra hour during the summer of TV (2.3 h·d−1) compared with the school year (1.3 h·d−1). Data from the NHANES III study revealed that watching 4 or more hours per day of TV was associated with greater body fat and BMI than children who watched less than 2 h·d−1 (1). An increase in inactivity (TV/videos) was associated with an increase in BMI in 10- to 15-yr-old girls (2). In our girls, the time spent watching TV does not appear to be excessive. However, our instrument did not ask questions regarding other forms of sedentary activities, e.g., Internet, computer games, talking on the phone, reading, etc. The amount of time spent in these activities could add considerably to the total time engaged in sedentary behaviors.
Some limitations of our study include measuring only 2 d of physical activity by the heart rate technique, the loss of data that often occurs during data collection using heart rate monitors, and an incomplete list of sedentary activities in the questionnaire. Also, there is potential bias in the reporting of the child’s activities by the parents, and this may change as the girl becomes older. It has been shown that in girls of this age, 7 d of monitoring by accelerometry are needed to achieve an intraclass correlation of 0.8 (27). Thus, with heart rate monitors, we would likely need several more days of measurement to achieve a higher reliability.
In conclusion, we found that fitness and physical activity remain fairly constant in girls from 8 to 10 yr of age. However, girls with either 1 or 2 obese parents tend to have lower fitness levels.
We would like to thank the children and parents who participated in the study, the MRU staff and body composition staff for technical assistance, and B. Kertz for subject recruitment.
This study was supported by NIH R29 HD34029 (M. Treuth) and USDA Agricultural Research Service Cooperative Agreement 6250-51000-023-00D/01 (M. Treuth, N. Butte). The contents of this publication do not necessarily reflect the views or policies of the U.S. Department of Agriculture, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government.
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