This study has several strong points. From an ethical point of view, this general intervention means that is that no children, such as the obese, are stigmatized, as all children are “treated”, regardless their physical state. Positive aspects, from a researcher’s point of view, are that it has many participants, making it possible to analyze a relatively large number of variables and covariates in multivariate analyses without exhausting the dataset. The study runs over a long period, which makes it possible to study the long-term course of life-style and life-style conditions. Specifically, it will answer some of the most important questions in the prevention of future life style diseases, such as type II diabetes and cardio vascular disease, such as: “Is it possible to change risk factor levels in children and adolescents through public intervention in schools?”, “Does additional physical education at school result in an overall increased physical activity?” and “Might increased quality of the physical activity, with activity tailored to the biological age of the children, have an effect on outcome?”
Further, it is the first study to include momentary assessment data (SMS-track) of children’s level of physical activity and musculoskeletal complaints and injuries. As these SMS data have a very high participation rate (>93% every week) over several years, it will be possible for the first time to study not only the incidence of musculoskeletal problems in children, but also the age of onset and the course over the years.
In addition, other state of the art measurements (DXA scans to measure body composition and accelerometers to measure physical activity measurements) together with high quality physical measurements with some of the best available methods for epidemiological studies are performed regularly. This together with very the high participation rates in all measurements make the study unique. The study will therefore provide credible new information on many aspects concerning childhood and adolescent health, and the implications for the future health in adulthood.
The study is also unique because it is a “study of a natural evolution” in a community introducing an intervention independently of researchers, a so-called natural experiment
. The role of the researcher is to measure the effect of the change introduced into the community but not to set up the experiment. It is community-based, meaning that its heterogeneity reflects that of the general population and not some possibly highly selected study sample. This heterogeneity requires a large number of participants, which fortunately was obtained in this study.
However, a potential problem with this is that interesting subgroups in the general population may be relatively small, such as the obese, as opposed to intervention performed by researchers, where the inclusion to the study would require a certain weight. Any effect of the extra physical intervention on such small subgroups may therefore be diluted on a group level, if the effect is only slight or none in children of normal weight.
Another potential limitation is that, although the two types of schools are well matched, making comparisons between them credible, and although the parental demographic profile of participants resembles that of its target population, some generalizability of the data may not extend beyond that type of population (small town with surrounding rural district in Denmark).
Another issue that needs careful consideration is the interpretation of data. When many analyses are performed, even if the study sample is large, spurious results are likely to occur. Therefore, unusual, single, or unexpected results must be carefully considered in relation to the profile of other results and their plausibility.
Nonetheless, the results obtained in this study are likely to have an impact on public health in relation to life-style related disorders in early life.