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Cardiorespiratory fitness (CRF) has been shown to be negatively associated with metabolic risk factors in youth. On this basis researchers have examined relationships between physical activity (PA) and CRF, showing time spent in moderate-to-vigorous (MVPA) intensity activity to be associated with increased CRF. However with the recent appreciation of sedentary behaviour (SB) as an independent risk factor for health in children and adults, cross-sectional studies in children have reported SB to be independently (controlling for PA, BMI, Gender etc.) and negatively associated with indirect measures of CRF.
To elucidate the direction of this relationship longitudinal studies are required. No extant studies have examined independent relations between SB and change in CRF during childhood. Therefore Mitchell et al. sought to determine if time spent in screen-based SB was independently associated with changes in CRF in children from ages 11 to 13.
What did they do?
The authors reported a final sample of 2,097 boys and girls. CRF was measured using a 20m shuttle run test at ages 11 and 13. Number of laps completed was used as an indirect measure of CRF. At both time points participants self-reported the previous two days screen-based SB (i.e. TV/DVDs, Computer, & Game Console use) and time spent in VPA using the Self-Administered Physical Activity Checklist (SAPAC).
Gender, socioeconomic status (SES), household education level (HEL) and body mass index (BMI) were measured as covariates. Longitudinal regression models were used to determine if screen time was associated with changes at the 10th, 25th, 50th, 75th and 90th shuttle run lap percentiles from age 11 to 13, controlling for covariates.
What did they find?
Screen time increased by 0.3 h from ages 11 to 13, with the greatest increase seen for the children at the 75th percentile, but change in screen time did not differ by gender. The number of shuttle run laps was greater in boys compared to girls at baseline. In addition, there was an increase in number of laps from age 11 to 13 in boys, but not girls. Therefore associations between SB and CRF were examined for boys and girls separately. In boys screen time was negatively associated (adjusted for BMI, HEL and VPA) with changes in laps at the 25th, 50th and 75th shuttle run percentiles, but not at the 10th or 90th percentiles. The associations were progressively stronger from the 25th to the 75th percentiles.
In girls screen time was negatively associated with changes in laps at the 50th, 75th and 90th percentiles. As with boys, the strength of association increased from the 50th to the 90th shuttle run percentile in girls. Additionally, these associations were not ameliorated when controlling for MVPA, however this data was not reported.
What’s the take-home message?
Despite differential results for change in CRF between boys and girls, it appears for most children (those above the lower tail of the CRF distribution) that increased time spent engaged in screen-based SB may lead to lower CRF, regardless of how much PA they engage in. However the negative associations reported were not stable across the CRF distribution. That is, there appeared to be no significant relation at the lower end (10th percentile) in boys and girls, and at the upper end (90th percentile) in boys. The authors speculated that genetic predisposition may account for this non-uniform association, with some individuals at the lower end of the CRF distribution displaying a resistance to age and SB driven changes in CRF.
Whilst this is the first longitudinal study of SB-CRF relations in children, and included a large sample size, there are limitations. PA, SB and CRF were measured using indirect methods. Objective measurement of PA and SB and direct measurement of CRF may alter associations, as previous investigation using accelerometer measured SB in European adolescents, did not find the association between SB-CRF to be independent of MVPA.
Where possible, future research should employ objective/direct measures of PA, SB, and CRF and seek to replicate these findings in other ethnicities and age ranges.
Mitchell, J.A., Pate, R.R., & Blair, S.N. (In Press). Screen-Based Sedentary Behavior and Cardiorespiratory Fitness from Age 11 To 13. Medicine and Science in Sports & Exercise, Published Ahead of Print, DOI: 10.1249/MSS.0b013e318247cd73.
About the author: Ash Routen is in the final months of his doctoral studies at the University of Worcester, UK examining the impact of pedometer interventions on habitual PA in kids, with an interest in the assessment of body composition and objective physical activity measurement in kids. He can be found on Twitter @AshRouten.