Timmons et al. BMC Public Health 2012, 12:284 http://www.biomedcentral.com/1471-2458/12/284 and generally require too many assumptions to be prac- tical. It is more typical to calibrate the size of the sample to the complexity of the analysis by ensuring that the ratio of observations to the expected number of estimated para- meters in the analysis is at least 5–10. For this longitudinal analysis, we will have 400 children assessed on 3 occasions, yielding 1,200 observations for all variables in the analysis, and a ratio well above the 5–10 recommendation, allowing for the potential of missing data. Discussion The HOPP study is different from other longitudinal studies of physical activity in preschoolers because it will determine the relationship between being active and being healthy. Many preschoolers worldwide are suffering the obesity epidemic, which may be accelerating the onset of cardiovascular disease. In recent years, several correlates or potential determinants of preschooler physical activity have been identified [1,12,13], yet we know very little about the health benefits of physical activity during the early years [1]. The goal of the HOPP study is to provide evidence linking physical activity and health outcomes in preschoolers, and to examine these relationships over time. Although the goal of the HOPP study is to determine the relationship between physical activity and health outcomes during the early years, it is important to assess factors that could serve as targets for future interventions to influence preschooler physical activity. For example, the dependence of young children on their parents demands an assessment of the extent to which parents’ beliefs about physical activ- ity influence their preschooler’s physical activity levels. Pre- schoolers are likely to develop physical activity-related cognitions such as attitudes; however, through their belief- based actions, parents are in a position of strong control to enable or impair preschooler physical activity behaviours [45]. Motor skill proficiency is also correlated with physical activity levels among preschoolers. However, the causal re- lation between motor skill proficiency and physical activity remains unclear; are preschoolers more active because of better motor skills or do they have better motor skills be- cause they are more active? The longitudinal design of the HOPP study will be able to address some of these issues. We also propose that understanding the pattern of phys- ical activity may be as important as how much physical ac- tivity preschoolers accumulate on a daily basis. Physical activity patterns can be described as how physical activity is accumulated (i.e., frequency and duration of activity). To accurately measure physical activity patterns of preschoo- lers, it is essential to utilize objective methods with the high-frequency recording capacity needed to capture their movement behaviour [18]. A novel aspect of the HOPP study is the use of high-frequency accelerometry, which will allow us to explore relationships between the frequency Page 6 of 7 and duration of bouts of physical activity at different inten- sities with a broad range of health outcomes. For example, total physical activity, representing overall energy expend- iture, may be more related to adiposity whereas bouts of moderate-to-vigorous physical activity, representing greater cardiovascular stress, may be more related to vascular structure and function. The HOPP study will be able to characterize how the prevalence and patterns of physical activity of preschoo- lers are associated with key health outcomes throughout the early years. We plan to disseminate knowledge to the public and among stakeholders responsible for early child development. We expect our findings will hold the potential to shape public health policy for active living during the early years, a critical time to develop these behaviours and for preschoolers to focus on the ‘busi- ness’ of growing up. Abbreviations HOPP: Health outcomes and physical activity in preschoolers; MVPA: moderate-to-vigorous physical activity; FFM: fat-free mass; DXA: dual energy x-ray absorptiometry; HR: heart rate; ECG: electrocardiography; BOT- 2: Bruinitisky-Oserestky test of motor proficiency – Version 2. Competing interests There are no competing interests. Author details 1Child Health & Exercise Medicine Program, Department of Pediatrics, McMaster University, 1280 Main Street West, Hamilton, ONL8S 4K1, Canada. 2Department of Kinesiology, McMaster University, 1280 Main Street West, Hamilton, ONL8S 4K1, Canada. 3Departments of Family Medicine & Psychiatry and Behavioural Neurosciences, McMaster University, 1280 Main Street West, Hamilton, ONL8S 4K1, Canada. Authors’ contributions BWT conceived and designed the study and drafted the manuscript. NAP was involved in drafting the manuscript. MJM contributed to the design of the study and was involved in drafting the manuscript. SRB contributed to the design of the study and was involved in drafting the manuscript. JC contributed to the design of the study and was involved in drafting the manuscript. All authors read and approved the final manuscript. Funding The HOPP study is funded by the Canadian Institutes of Health Research (CIHR Award #: MOP 102560). BWT is supported by a CIHR New Investigators Award. JC is supported by an endowed professorship through the department of Family Medicine, McMaster University. Received: 1 April 2012 Accepted: 17 April 2012 Published: 17 April 2012 References 1. Timmons BW, Naylor PJ, Pfeiffer KA: Physical activity for preschool children: How much and how? Can J Public Health 2007, 98(Suppl 2): S122–S134. 2. Janssen I, LeBlanc AG: Systematic review of the health benefits of physical activity and fitness in school-aged children and youth. Int J Behav Nutr Phys Act 2010, 7:40. 3. Strong WB, Malina RM, Blimkie CJ, Daniels SR, Dishman RK, Gutin B, et al: Evidence based physical activity for school-age youth. J Pediatr 2005, 146:732–737. 4. Canning P, Courage ML, Frizzell LM, Seifert T: Obesity in a provincial population of Canadian preschool children: differences between 1984 and 1997 birth cohorts. Int J Pediatr Obes 2007, 2:51–57.