OBJECTIVE: Physical activity is thought to decline during childhood, but the extent of the decline is unknown. We made objective measures of 2-year changes in physical activity and sedentary behavior in English children who participated in the Gateshead Millennium Study to explore the nature, timing, and extent of changes in physical activity and sedentary behavior before adolescence.
METHODS: We conducted a longitudinal study of 405 children (207 girls), aged 7 years, in 2006/2007 and again 24 months later. Physical activity and sedentary behavior were measured with the Actigraph GT1M accelerometer. Data were analyzed in 2010. Changes in total volume of physical activity (accelerometer counts per minute [cpm]), moderate-to-vigorous–intensity physical activity (MVPA), and sedentary behavior were quantified. Factors associated with changes in physical activity and sedentary behavior were tested by using linear regression. Tracking of physical activity and sedentary behavior over the 2-year period was assessed by rank-order correlation.
RESULTS: Mean daily volume of physical activity declined by 83 cpm (interquartile range [IQR]: −189 to 31) over 2 years; the percentage of daily time spent in MVPA was low at baseline and declined by 0.3% (IQR: −1.4 to 0.9). The percentage of daily time in sedentary behavior was high at baseline and increased from 78.0% to 81.1% of the day (change 3.1% [IQR: −0.3 to 6.0]). The decline in MVPA and increase in sedentary behavior were significantly greater in girls and in those with higher BMI z scores at baseline. Physical activity and sedentary behavior showed moderate tracking over the 2-year period.
CONCLUSIONS: We report here new evidence of low and declining levels of physical activity and MVPA and increasing sedentary behavior before adolescence.
WHAT'S KNOWN ON THIS SUBJECT:
Levels of physical activity during childhood are expected to be quite stable with age. Physical activity is believed to decline markedly at adolescence, particularly for girls.
WHAT THIS STUDY ADDS:
Physical activity declines before adolescence in both boys and girls. Strategies to prevent the decline in physical activity should be considered and started for both sexes before adolescence.
Levels of habitual physical activity and sedentary behavior are now well established as important to both the current and future health of children and adolescents,1,–,3 and attempts to reduce the decline in physical activity in adolescence have been the focus of many public health interventions in recent years.4,5
Most studies of changes in physical activity during childhood and adolescence have been cross-sectional rather than longitudinal. In addition, most previous studies used subjective methods of measurement of physical activity, which do not provide accurate estimates of the amount and intensity of physical activity.6 In addition, longitudinal studies that have used objective methods7,–,17 have usually measured change during adolescence rather than during childhood and have usually focused on girls7,–,9,11,12,15,16 because of concern that adolescent declines in physical activity are especially marked in girls. Little is known about the timing, nature, and magnitude of changes in physical activity levels in childhood, and even less is known about longitudinal changes in levels of objectively measured sedentary behavior. Moreover, results of older studies conducted before the epidemic of pediatric obesity may not provide evidence that is readily applicable to contemporary children and adolescents. Finally, no studies have revealed longitudinal changes in the 3 constructs (total volume of physical activity, moderate-to-vigorous–intensity physical activity [MVPA], and sedentary behavior6), all of which have been identified as important to current and future health.1,2 Understanding the timing, nature, and extent of changes in physical activity and sedentary behavior during childhood is crucial for the development of informed, effective, and evidence-based interventions aimed at preventing obesity and promoting cardiometabolic health. The primary aim of this study, therefore, was to quantify changes in objectively measured habitual physical activity and sedentary behavior over a 2-year period in a sample of English children, before adolescence, by using a longitudinal study design. Secondary aims of this study were to identify factors associated with changes in physical activity and sedentary behavior and to assess the extent to which physical activity and sedentary behavior “tracked” over time.
Participants, Recruitment, and Consent
The Gateshead Millennium Study is a birth cohort study of 1029 infants (born in Gateshead, northeast England) and their families recruited in 1999–2000. The details of the aims, design, and sampling of this birth cohort were recently published.18 In brief, the sample population resided in Gateshead and was predominantly (98%) from the white ethnic majority group and socioeconomically representative of northeast England. For the present study, all families who had not previously opted out of the cohort were sent a letter and information leaflet inviting them to take part. Baseline measures were collected between October 2006 and December 2007, when the children were aged 6 to 8 years (hereafter described as aged 7 years), and follow-up data were collected 24 months later (October 2008 to September 2009), when the children were aged 8 to 10 years (hereafter described as aged 9 years). A favorable ethical opinion was given by the Gateshead and South Tyneside Local National Health Service Research Ethics Committee for baseline data collection at 7 years of age and by the Newcastle University Faculty of Medical Sciences Ethics Committee for follow-up data collection at 9 years of age. Informed written consent was obtained from the parent/main guardian of each child, and children provided their assent to participation.
Objective Measurement of Physical Activity and Sedentary Behavior
Habitual physical activity and sedentary behavior were measured by using Actigraph GT1M accelerometers (Actigraph, Pensacola, FL). The Actigraph is a small, unobtrusive, and lightweight monitor worn on the right hip on an elastic waistband. The Actigraph has high validity (relative to reference measures of physical activity, direct observation, and energy expenditure), high reliability, and low reactivity in children19 and has been used successfully in pediatric physical activity studies over the past 10 years.6
Children were asked to wear the accelerometer during waking hours for 7 days and remove them only for bed and water-based activities such as showering, bathing, or swimming. Families were also given a log sheet to note when the monitor had been worn and removed each day. The accelerometers were set to record data in 15-second sampling intervals (epochs) throughout the period in which the accelerometer was worn. Before analysis, the data were collapsed to 60-second epochs. Data were reduced manually by using accelerometry output and log sheets, as described previously,20 to delete occasional periods of nonwear time (corresponding to times noted by the parent as unworn).
Three constructs were calculated from the physical activity recorded with the accelerometer: total volume of physical activity, expressed as mean counts per minute (cpm); MVPA, equivalent to an energy expenditure in excess of ∼3 times the energy expended at rest (expressed as minutes/day and the percentage of daily time spent in MVPA); and sedentary behavior, equivalent to no translocation of the trunk (percentage of daily time spent in sedentary behavior).
To quantify MVPA and sedentary behavior, evidence-based cut off points from a body of pediatric calibration and validation studies were applied to accelerometry output.6 A cut point of 3200 cpm21 and above was used to quantify MVPA, and a cut point of 1100 cpm and below was used to quantify sedentary behavior.22 The sedentary behavior actually measured in this study can be defined best as no movement of the trunk.6,22 Sedentary behavior is a more complex construct than this and one that is still evolving,23 and other aspects of sedentary behavior are important to health outcomes (notably television-viewing23) but were not measured in this study. Because the Actigraph cut points used in this study were derived from studies using Actigraph model 7164, and the more recent Actigraph GT1M model has been shown to have a consistent bias of 9% relative to model 7164,24 a +9% correction was applied to the raw accelerometry data before applying cut points.
Previous analyses of accelerometry data have shown that 3 days of accelerometry with a minimum of 6 hours recording per day provided acceptable reliability.25,26 Analyses on the current data set confirmed that reliability was high over any 3 days for each of the 3 constructs that were measured, and there were no differences when weekend days were included/excluded.27 As a result, measures were eligible for inclusion in our study if at least 3 days of accelerometry of at least 6 hours/day26 were obtained at both baseline and follow-up measures. For children from the United Kingdom, there are small but significant seasonal variations in objectively measured habitual physical activity28,29; therefore, follow-up measurements were made as close to the baseline time of year as practicable to match baseline and follow-up measures for season.
Anthropometrics and Descriptive Characteristics of Study Participants
Height was measured to 0.1 cm with a Leicester portable height measure (Chasmors, London, United Kingdom), and weight was measured to 0.1 kg in light indoor clothing and percentage body fat was estimated with a Tanita TBF300MA (Chasmors). Waist circumference was measured to 0.1 cm at the minimum circumference between the lowest rib and the iliac crest. BMI (kg/m2) was calculated for each child, and z scores were estimated to UK population reference data.30 Socioeconomic status was reported by using Townsend scores at birth,18 an area-based measure derived from the UK census in 1991. The child's postal code (zip code) at birth was linked to census data for the enumeration district to provide a Townsend score, a well-validated measure of socioeconomic deprivation.31 The Townsend score is based on the percentage of unemployed economically active 16- to 65-year-olds, household overcrowding, and not owning a car. The distribution of these scores in the northern region is known, so the scores could be divided into 5 ordered centiles (each comprising 20% of the population) that corresponded to the quintiles for the general population.
The sample size for our study was fixed by the size of the recruited cohort and the extent of sample attrition over the 2-year period. However, the sample size was similar to or larger than that of most previously reported longitudinal studies (eg, Janz et al10 and Metcalfe et al32). In addition, the accuracy of both the reported levels of physical activity and sedentary behavior and the changes in levels over the 2-year period are increased by the use of accelerometry and the longitudinal design of the study.
Data were checked for normality, and physical activity data were found to be skewed. Therefore, longitudinal changes in all 3 constructs over the 2-year period (calculated by subtracting 7-year data from 9-year data) were tested for significance by using Wilcoxon signed-rank tests. Mann-Whitney U tests were used to assess the significance of differences between boys and girls in levels of physical activity and sedentary behavior.
Linear regression analysis was used to detect factors at baseline (age 7 years) that were associated with changes in total volume of physical activity, MVPA, and sedentary behavior. The proposed candidate explanatory variables agreed on in the analysis plan were sex, physical activity at baseline (cpm and percentage of time spent in MVPA), baseline sedentary behavior (percentage of time spent sedentary), socioeconomic status (SES) (Townsend score quintiles), and child BMI z score at 7 years of age as an index of weight status. Spearman rank correlations between baseline and follow-up measures were used to assess the extent to which tracking occurred over the 2-year period for all 3 constructs. SPSS 17 (SPSS Inc, Chicago, IL) and Stata 10 (Stata Corp, College Station, TX) were used for statistical analyses. Significance was set at P < .05.
Characteristics of Study Sample and Dropout, Including Age and Accelerometry Variables
Characteristics of study participants are listed in Table 1. For practical reasons, only children still living in the region at the time of follow-up measures were recruited, and only children who had both baseline (age 7 years) and follow-up (age 9 years) measurements were included in our analysis (n = 405). A total of 105 children studied at baseline were lost to follow-up because they did not have consent to take part or were unable to be traced (n = 55), were missing accelerometry log sheets (n = 9), were not wearing the accelerometer (n = 6), did not reach the minimum wear criteria (n = 8), lost their accelerometers (n = 14), or had malfunctioning accelerometers (n = 13).
Participants followed up to 9 years of age were not significantly different to those lost to follow-up with respect to initial levels of physical activity and sedentary behavior, BMI, or SES. At baseline, when children were aged 7 years, mean duration of accelerometry was 6.4 days and 11.1 hours/day; at follow-up when children were aged 9 years, mean duration of accelerometry was 6.1 days and 11.3 hours/day.
Overall physical activity levels were low (median: 26 minutes of MVPA daily at 7 years of age and 24 minutes at 9 years of age [both sexes combined]). Only 26 of 405 (6.4%) children achieved a mean of 60 minutes of MVPA daily at 7 years of age and only 23 of 405 (5.7%) at 9 years of age. Nine children (2.2%) achieved an average of 60 minutes of MVPA at both time points.
Longitudinal Changes in Physical Activity and Sedentary Behavior
Changes in total volume of physical activity, MVPA, and sedentary behavior over the 2-year period are summarized in Table 2. In summary, over the 2-year period the total volume of physical activity declined significantly, MVPA declined significantly, and sedentary behavior increased significantly (Table 1). There were significant differences between boys and girls for change in the 3 constructs of physical activity (mean cpm, P = .020; MVPA minutes/day, P = .012; percentage of MVPA, P = .006) but no difference in the percentage of time spent sedentary (P = .274) (Table 2).
Factors Associated With Longitudinal Changes in Physical Activity and Sedentary Behavior
The regression analysis of factors associated with change in total volume of physical activity, MVPA, and sedentary behavior is shown in Table 3. In summary, after adjustment for baseline levels of physical activity and sedentary behavior (which had significant influences on changes in these variables), longitudinal declines in total volume of physical activity (accelerometry cpm) were greater in girls than boys but were not related to BMI z scores or SES. Longitudinal declines in MVPA were significantly greater in girls than boys and in those with higher initial BMI z score but were not related to SES. Longitudinal increases in sedentary behavior were significantly greater in girls but not related to BMI z score or SES. There was no interaction between sex and any baseline variable.
Tracking of Physical Activity and Sedentary Behavior Over a 2-Year Period
Rank-order correlations between baseline and follow-up measures of total volume of physical activity, MVPA minutes/day, percentage of time spent in MVPA, and percentage of time spent in sedentary behavior were r = 0.54, 0.57, 0.58, and 0.57, respectively (all P < .001).
Main Findings, Implications, and Comparisons With Other Evidence
We found low levels of habitual physical activity and high levels of sedentary behavior in participating children at 7 years of age, with declines in physical activity and increases in sedentary behavior by 9 years of age. Recent evidence, based on contemporary samples of children and adolescents that have used accelerometry with evidence-based cut points,6 suggests that levels of physical activity are typically very low and levels of sedentary behavior very high. In addition, small numbers of longitudinal studies of children with objective measures10,17 and 1 recent cross-sectional study with objective measures33 suggested that declines in physical activity may typically occur well before adolescence. Our results, therefore, add to the body of evidence that low levels of physical activity are established early in childhood and decline further during childhood and before adolescence.
The 2-year changes in physical activity and sedentary behavior observed in this study were significant and, although relatively modest in magnitude, are a great cause for concern because of the low level of baseline physical activity and high level of baseline sedentary behavior. In this study, unfavorable changes in physical activity and sedentary behavior occurred in both sexes, but these changes were generally more severe in girls than boys and in those with higher baseline weight status. Identifying factors associated with declines in objectively measured physical activity and with increases in objectively measured sedentary behavior would be useful to inform the development of future public health interventions aimed at modifying these behaviors.9 Our results suggest that public health interventions aimed at preventing declines in physical activity should be targeting elementary school–aged children, because unfavorable trends in health behaviors seem to be already established before puberty. In Scotland, for example, as in some other countries, public health policy is based on the assumption that physical activity is high during childhood but declines markedly only during adolescence. Prevention of declines in physical activity in adolescent girls is a Scottish public health priority,34 but according to our results, we would question the assumptions on which this policy was based.
Tracking of physical activity and sedentary behavior was moderately strong in this study, which is broadly consistent with other recent studies14,35 of tracking of physical activity and sedentary behavior measured objectively during childhood. However, even for the most physically active children, levels of objectively measured physical activity at both time points were low (Table 2).
Study Strengths and Weaknesses
There are several methodologic strengths to this study: The study sample was relatively large and representative of children recruited from the Gateshead Millennium Study longitudinal birth cohort study; measurements were made during a period of relative social and educational stability; physical activity and sedentary behavior were objectively and accurately measured; the longitudinal design allowed the measures to be repeated after a 2-year period; and, in terms of SES, the sample was representative of families living in northeast England.18
Potential weaknesses include the issue of generalizability to other settings and samples, and there was some sample attrition from baseline to the 2-year follow-up. However, there was no difference in SES, BMI, BMI z score, or physical activity/sedentary behavior levels between those who participated in the follow-up and those who did not.
There has been no consensus on the appropriate accelerometer cut points to use for children to quantify intensity of physical activity. The value of 3200 cpm21 used in our study was based on a previous pediatric study in which accelerometer output was calibrated against energy expenditure and is similar to the cut point suggested by a number of other pediatric calibration studies.6 Other studies have used both higher and lower cut points (eg, 2000 cpm36), but these cut points have not usually been based on calibration studies that aimed to identify the most appropriate cut point for MVPA.6 The 1100-cpm cut point used to quantify sedentary behavior in our study is higher than that used by some other pediatric studies (some have used 100 cpm to define sedentary behavior) and was based on a calibration study on younger children,22 but empirical evidence suggests that it is appropriate for older children.6 The use of different cut points has a moderately large impact on objective measurement of MVPA and a less marked effect on measurement of sedentary behavior in children.6
We used individualized manual accelerometer data reduction in the present study, as in previous studies6: we chose not to exclude strings of zeros unless they corresponded with reported nonwear time, despite parents not being with their children during the entire monitoring period. This decision was made on the basis of trying to include as much data as possible, although it may reduce the proportion of time spent in MVPA and increase the proportion of time spent in sedentary behavior. The absolute time spent in MVPA should not be so greatly affected.
However, taking into account these limitations, the reported findings were consistent with several other recent studies that also used objective methods to measure physical activity. In addition, the levels of total physical activity and MVPA observed in the present study were similar to those observed in recent studies of both slightly younger and slightly older samples within the United Kingdom.14,17,28,32,37,38
These findings also question the hypothesis that physical activity levels only decline markedly during adolescence, especially taken alongside the recent studies reporting the levels of objectively measured physical activity during adolescence in the United Kingdom.39 Longer-term follow-up will be required to investigate the extent to which the patterns observed in the present study are generalizable to other groups of children and to assess whether most of the decline in physical activity that occurs before adulthood occurs before, rather than during, adolescence.
The low levels of childhood MVPA and high levels of sedentary behavior reported here present a major public health concern. These findings, if replicated in other groups of preadolescents, suggest that efforts to prevent declines in physical activity should be directed at children not just at adolescents.
This work was supported by grants from the Scottish Government Chief Scientist Office (grant CZH/4/484), the UK National Prevention Research Initiative (incorporating funding from British Heart Foundation, Cancer Research UK, Diabetes UK, English Department of Health, Economic and Social Research Council, Food Standards Agency, Medical Research Council, Research and Development Office for the Northern Ireland Health and Social Services, Chief Scientist Office of the Scottish Government Health Directorate, Welsh Assembly Government, and World Cancer Research Fund), and Gateshead PCT. The funding bodies had no role in the decision to publish or the content of the article. The cohort was first set up with funding from the Henry Smith Charity and Sport Aiding Medical Research for Kids.
Members of the Gateshead Millennium Study Core Team are Ashley Adamson, Anne Dale, Robert Drewett, Ann Le Couteur, Paul McArdlev, Kathryn Parkinson, Mark Pearce, John Reilly, and Charlotte Wright.
We acknowledge the support of an external reference group in conducting the present study. We appreciate the support of Gateshead Health National Health Service Foundation Trust, Gateshead Education Authority, and local schools. We thank members of the research team for their effort. We especially thank the families and children who participated in the Gateshead Millennium Study.
- Accepted September 20, 2010.
- Address correspondence to Laura Basterfield, PhD, Institute of Health & Society, Human Nutrition Research Centre, Newcastle University, M1.151 Leech Building Medical School, Framlington Place, Newcastle Upon Tyne NE2 4HH, United Kingdom. E-mail:
Each author is responsible for the reported research; each author has participated in the concept and design, analysis and interpretation of data, and drafting or revising of the manuscript, and each author approved the manuscript as submitted.
FINANCIAL DISCLOSURE: The authors have indicated they have no financial relationships relevant to this article to disclose.
- MVPA =
- moderate-to-vigorous–intensity physical activity •
- SES =
- socioeconomic status
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- Copyright © 2011 by the American Academy of Pediatrics