Active Voice: Children Spend Most of their Time in Sedentary Behavior

By Jonathan A. Mitchell, Ph.D.

Viewpoints presented in SMB commentaries reflect opinions of the authors and do not necessarily reflect positions or policies of ACSM.

Jonathan A. Mitchell, Ph.D., is a Postdoctoral Fellow at the University of Pennsylvania. He is funded on a Ruth L. Kirschstein National Research Service Award. His research focus includes sedentary behavior epidemiology. He is particularly interested in studying the amounts of time youth spend in sedentary behavior and if high levels of sedentary behavior are independently associated with health outcomes. In the June issue of Medicine & Science in Sports & Exercise® (MSSE), he and his co-investigators presented findings of a prospective study of sedentary behavior patterns and socioeconomic correlates in children ages 12-16.

Being physically active is beneficial to children’s health and development, and children are recommended to spend at least one hour per day in moderate-to-vigorous-intensity physical activity (MVPA). However, consider a hypothetical child who meets this guideline and sleeps for nine hours per day. This child has 14 waking hours remaining in his/her day that could be spent in either light-intensity physical activity or sedentary behavior. Few research studies have quantified the time children spend in sedentary behavior during their waking hours and this served as motivation for our recent study, published in MSSE, which describes levels of sedentary behavior in a cohort of children at ages 12, 14 and 16. The children wore activity monitors at each age for one week, except when they were asleep or when they were participating in water activities (bathing or swimming). We found that the children at age 12 spent approximately 54% of their monitored time in sedentary behavior, and this increased to approximately 65% of their monitored time at age 16. We also found that the children experienced fewer breaks in their sedentary behavior from age 12 to 16. To accommodate the increase in sedentary behavior, we found that time spent in light-intensity physical activity decreased from age 12 to 16 (with little to no change in MVPA).

So why are these descriptive sedentary behavior data important? In children, there is emerging evidence that more sedentary behavior is associated with obesity and metabolic risk factors (see Mitchell, 2012). Importantly, the associations observed in those studies remain after adjusting for levels of MVPA, which suggests that meeting the physical activity guideline does not fully compensate for the deleterious effects of too much sitting. Similar findings have been observed in adults; and one study even found that fewer breaks in sedentary behavior were associated with poorer metabolic health (see Healy, 2008). In addition, adults who report more time spent sitting have an increased risk of early mortality, independent of physical activity levels (see Matthews, 2012). It is very likely that the adults included in those studies were raised in environments with fewer opportunities for sedentary behavior compared to contemporary children. Therefore, the present generation may be especially at risk of poorer health in adulthood, if high levels of sedentary behavior continue over their lifespan.

These data serve as evidence for the need to reduce the time children spend in sedentary behavior. Electronic media are widely available to children and recommendations to limit television viewing and other screen-based sedentary behaviors are important approaches that could help to reduce leisure-time sitting. Outside of leisure time, reductions in sedentary behavior could be achieved by introducing adjustable school desks that permit children to stand while they work. Active transport, such as bicycling or walking, could also help to reduce sedentary behavior by reducing the time spent sitting in the car. These and other solutions need careful consideration if we are to address the adverse health outcomes associated with too much sitting.