Timmons et al. BMC Public Health 2012, 12:284 http://www.biomedcentral.com/1471-2458/12/284 childhood [9]. Health-related fitness denotes characteristics that are related to the prevention of disease or the promo- tion of health [10], such as aerobic fitness. Very little is known about health-related fitness during the early years. Thus, an inactive lifestyle, including low fitness, combined with excess adiposity during the preschool years may ac- celerate the development of cardiovascular disease. The need to understand the health impact of physical activity has never been greater. Studies that assess physical activity among preschoolers using objective measures (i.e., accelerometers) are usually cross-sectional in nature and typically report very low levels of physical activity and very high levels of sedentary time [1,11-13]. If we accept that contemporary preschoolers have low levels of physical ac- tivity and that physical activity is needed for optimal health, then it follows that inactive preschoolers are at risk for health ‘deficits’ during the early years. However, in a previous review, we found very little evidence linking physical activity with health in children ages 2 to 5 years [1]. At that time, we concluded that the scientific evidence was too weak to determine how much physical activity a preschooler needs. Moreover, there are only a small number of longitudinal studies using accelerometers to measure physical activity of preschoolers (e.g. [14-17]), and none of these have linked physical activity levels with health out- comes such as obesity, health-related fitness, or vascular structure and function. The goal of the Health Outcomes and Physical activity in Preschoolers (HOPP) study is to describe how the prevalence and patterns of physical activity in preschoolers are associated with a broad spectrum of health outcomes. This longitudinal, observational study will follow 3-, 4-, and 5-year-old boys and girls for 3 years with annual assessments of physical activity, body composition, health- related fitness, vascular structure and function, and health- related quality of life, along with motor skills and parents’ beliefs about physical activity for their preschooler. Specifically, our objectives are: 1) To use high-frequency accelerometry to assess physical activity prevalence and patterns in 3- to 5-year-old children. We hypothesize that how preschoolers accumulate their physical activity (pattern) will be as important as how much physical activity (prevalence) they accumulate, in terms of positive relationships with health outcomes; 2) To determine the relationships between health outcomes and physical activity in preschoolers throughout the preschool years. We hypothesize that those children who engage in more bouts of moderate- to-vigorous physical activity (MVPA) and who are able to maintain participation in MVPA over the 3-year study period will have more favourable outcomes for body composition, health-related fitness, vascular structure and function, and health-related quality of life; Page 2 of 7 3) To examine factors relevant to support a physically- active lifestyle of preschoolers. We hypothesize that motor skills and parents’ beliefs will be strong predictors of physical activity at each annual assessment, but strongest at younger ages. Methods/design Study design The HOPP study is a prospective cohort study to evaluate the relationship between physical activity and health out- comes. At each assessment, we also examine potentially modifiable factors (parents’ beliefs and motor skills) that could serve as targets for future interventions to influence preschooler physical activity. Data collection occurs at study visits through questionnaire and non-invasive physical mea- surements. All study procedures have been pilot tested and modified as necessary for the main study [18-21]. The Hamilton Health Sciences/Faculty of Health Sciences Re- search Ethics Board has provided ethical approval to the study. Participants Participants in the HOPP study are recruited from the south-central Ontario region, including the City of Hamilton and surrounding area. Eligibility for participa- tion is based on medical criteria and not demographic factors. Children with a physical disability or known motor delay or a diagnosed medical condition (e.g., dia- betes, arthritis, asthma, cystic fibrosis) are ineligible. Our decision to exclude children with a medical condi- tion is based on the need to first understand the associa- tions between physical activity and health outcomes in typically developing preschoolers – an understanding that is far from complete. Recruitment Strategy We have implemented a community-based recruitment strategy. This strategy works with the Ontario Early Years Centres (Government- funded centres offering services related to early child- hood development), preschools and daycare centres in the City of Hamilton and surrounding area, and local school boards. To improve participant retention, we provide parents with yearly reports of key results and study progress. When we send the report, we request in- formation about re-location, etc., and ask about any foreseeable changes to their address. Study protocol Participants and their families follow the same procedures at each annual assessment. We conducted a number of smaller pilot studies [18-21] to develop the protocol, which consists of 2 laboratory visits and 7 consecutive days of physical activity monitoring. Parents visit our laboratory where the study is explained in detail and any questions