Despite recent progress towards stabilization in the prevalence of childhood overweight and obesity in the U.S.  and other countries [2–4], many children still have excessive body weight. In 2007-2008 in the U.S., around 17% of 2-to-19-year-old children had a body mass index (BMI) at or above the 95th percentile of the U.S. growth charts while 32% were overweight or obese (BMI ≥ 85th percentile) . Childhood obesity has considerable adverse consequences for children's physical health, persistence of obesity into adulthood and health later in life . In response to these patterns, prevention of childhood obesity has become a national priority in many countries.
Childhood obesity may lead to impaired cognitive and physical development , which can translate into deleterious social and economic consequences such as social exclusion, diminished school performance, and ultimately poorer labor market outcomes . Mechanisms of these effects are still incompletely understood. One mechanism involved in these observations could be through the inhibiting effect of obesity on children's physical development. Overweight and obese children unable to successfully engage in physical challenges may resist participating in physical activities and overall learning solicitations. Furthermore, parents, caregivers and teachers may be less likely to encourage obese children to engage in physical activity based on their perceptions that the child has limited physical abilities . Impaired physical development could trigger a cycle of physical activity avoidance and reduced social interactions, which could lead to further reduction in physical fitness of obese children . This, in turn, could contribute to negative health and weight outcomes [10, 11].
Prior research on the relationship of childhood obesity with motor skill development has produced mixed results. Two studies showed more limited motor skills (gross and fine skills evaluated together) among obese boys compared to normal weight peers, but these results were not shown in girls [12, 13]. Several studies assessing overall gross motor skills found impaired skills in obese children regardless of gender [14–18] or only in boys . This was also the case for object-control skill components in both girls and boys [14, 17]. For fine motor skills, results are more mixed due to a lower number of studies, which usually suggest no negative association with obesity until 9 years of age . Comparisons across these cross-sectional studies are limited due to differences in the methods used, especially for motor skill assessment. In addition, previous studies were based on rather small sample sizes (from one hundred [14, 15, 17, 18] to less than 700 children , except for one large survey in Germany)  and/or biased samples (with no random selection in representative samples).
As a result, available data on the relationship between childhood obesity and motor skill development at early ages remains inconclusive . One study using a longitudinal design and controlling for reverse causality showed that childhood overweight contributed to a delay in motor development, but the survey sample was limited to low-income African-American infants from 3 to 18 months of age . Furthermore, an interventional study of children in an obesity treatment intervention showed that reduced mean body weight was accompanied by improved gross motor coordination performance .
Our study tests the hypothesis that fine and gross motor skills are inversely associated with BMI z-scores and obesity in young American children. We estimate cross-sectional associations of fine and gross motor skills with BMI z-scores and obesity accounting for individual differences in the learning and family environment and socio-demographic characteristics of preschoolers (4 year-olds) and kindergarteners (5-6 year-olds) residing in the United States.