Canada's first set of physical activity guidelines for children and youth were introduced in 2002 [1
]. The basic recommendation within these guidelines was that children and youth, independent of their current physical activity level, should increase
the time they spend on moderate-to-vigorous intensity physical activity by 30 minutes per day, and over a 5 month period progress to adding an additional 90 minutes of daily physical activity. Recently, a narrative literature review was conduced to provide an update on the evidence related to the biological and psycho-social health benefits of physical activity in school-aged children and youth which has accumulated since the publication of Canada's guidelines [3
]. This narrative review explored whether Canada's physical activity guidelines for children and youth are appropriate, and made recommendations as to how the guidelines could be modified to reflect current knowledge.
Several other narrative reviews have examined the relation between physical activity and health in school aged children, a small sample of which are referenced here [4
]. Although informative, narrative reviews have severe limitations. First and foremost, it is uncertain as to whether all of the relevant scientific evidence has been examined. The authors of a narrative review may be exclusive with the materials they review, and these materials may have been selected and interpreted in a biased manner. Thus, the reader is faced with uncertainty and doubt when interpreting a narrative review. The reader may be better served when the choices made in the review are explicit, transparent, clearly stated, and reproducible. This can be achieved through a systematic review. Systematic reviews attempt to reduce reviewer bias through the use of objective, reproducible criteria to select relevant publications, to synthesize and critically appraise the findings from these publications, and to employ defined evidence-based criteria when formulating recommendations [9
The purpose of this report was to: 1) perform a systematic review of the evidence informing the relation between physical activity and health in school-aged children and youth, defined here as those aged 5-17 years; and 2) make recommendations on the appropriate volume, intensity, and type of physical activity for minimal and optimal health benefits in school-aged children and youth. A previously developed evaluation system was used to set the level of evidence and grade for the recommendations. This report was part of a much larger project around Canada's physical activity guidelines, and comparable systematic reviews for adults [10
] and older adults [11
] have also been published in the journal. Additional details on the scope and purpose of the larger project [12
] and the interpretation of the recommendations from an independent expert panel [13
] can also be found elsewhere in the journal.
Overview of existing physical activity guidelines for children and youth
Before conducting the systematic review, this paper provides a brief overview on existing physical activity guidelines for school-aged children, as well as an explanation of the scientific evidence that informed the guideline development process.
The publication of Canada's physical activity guidelines for children and youth in 2002 represented a joint effort of the Canadian Society for Exercise Physiology and Health Canada. Two sets of guidelines were published, one for children aged 6 to 9 years [2
] and a second for youth aged 10 to14 years [1
]. In addition to the physical activity guides, which highlighted the recommended physical activity levels for these two age groups, a number of other promotional and educational packages were developed, including family booklets [14
], teacher booklets [16
], as well as physical activity magazines for children [18
] and youth [19
The key recommendations within Canada's child and youth physical activity guides are:
1) Increase the time currently spent on physical activity by 30 minutes per day, and progress over approximately 5 months to 90 minutes more per day.
2) Physical activity can be accumulated throughout the day in periods of at least 5 to 10 minutes.
3) The 90 minute increase in physical activity should include 60 minutes of moderate activity (e.g., brisk walking, skating, bicycle riding) and 30 minutes of vigorous activity (e.g., running, basketball, soccer).
4) Participate in different types of physical activities - endurance, flexibility, and strength - to achieve the best health results.
5) Reduce non-active time spent on watching television and videos, playing computer games, and surfing the Internet. Start with 30 minutes less of such activities per day and progress over the course of approximately 5 months to 90 minutes less per day.
Many other countries and organizations have developed physical activity recommendations for school-aged children and youth, as recently summarized [3
]. With few exceptions, these countries and organizations recommend that children and youth participate in at least 60 minutes of moderate-to-vigorous intensity physical activity on a daily basis. One of these recommendations was published in 2005 as part of a systematic review that linked physical activity to several health and behavioural outcomes in school-aged children and youth [20
]. This systematic review was sponsored by the U.S. Centers for Disease Control and Prevention (CDC) and was developed by a multidisciplinary expert panel. The expert panel considered over 850 articles published in 2004 or earlier, identified by computerized database searches and by searching the bibliographies of the panellists' own libraries [20
]. Based on conceptual definitions and inclusion and exclusion criteria developed by the panel, participants systematically evaluated relevant articles (primarily intervention studies) for each of the 14 health and behavioural outcomes considered. On the basis of their reviews, the panel provided a summary of the evidence for strength (strong, >60% of studies reviewed; moderate, 30-59% of studies reviewed; and weak, <30% of studies reviewed) and the direction (positive, null, or negative) of the effects of physical activity on each of the health and behavioural outcomes. The strength of evidence was judged from the statistical significance of the outcomes; it did not include other factors usually considered in systematic review, such as the effect sizes of physical activity and the quality and types of studies.
The expert panel reached the following conclusions: (i) Evidence-based data are strong
to conclude that physical activity has beneficial effects on adiposity (within overweight and obese youth), musculoskeletal health and fitness, and several components of cardiovascular health. (ii) Evidence-based data are adequate
to conclude that physical activity has beneficial effects on adiposity levels in those with a normal body weight, on blood pressure in normotensive youth, on plasma lipid and lipoproteins levels, on non-traditional cardiovascular risk factors (inflammatory markers, endothelial function and heart rate variability), and on several components of mental health (self-concept, anxiety and depression) [20
]. A summary of evidence concerning the health outcomes examined by the expert panel is shown in Table 1 [Additional file 1
]. The amount, intensity, and type of physical activity required to achieve the result, when clear, is also shown in the table.
In 2008 a second systematic review of literature examining the relation between physical activity and key fitness and health outcomes within school-aged children and youth was published. This systematic review was part of the "Physical Activity Guidelines for Americans" project that was undertaken by the Unites States Department of Health and Human Services [21
]. Unlike the 2005 CDC sponsored systematic review that focused on intervention studies, the 2008 review considered both observational and experimental studies. The 2008 systematic review concluded that few studies have provided data on the dose-response relation between physical activity and various health and fitness outcomes in children and youth. However, substantial data indicate that health and fitness benefits will occur in most children and youth who participate in 60 or more minutes of moderate-to-vigorous physical activity on a daily basis. For children and youth to gain comprehensive health benefits they need to participate in the following types of physical activity on 3 or more days per week: vigorous aerobic exercise, resistance exercise, and weight-loading activities.
Although informative, the recommendations made within the 2005 and 2008 systematic reviews did not include a level of evidence or grade, which are now becoming a routine part of evidence based reviews. The level of evidence helps inform the reader about the strength of evidence that informed the recommendation. The grade considers the harms and benefits of implementing the intervention, and informs the reader about whether an intervention should be implemented.
Questions addressed in systematic review
The following questions were addressed in this systematic review:
1) How much (volume) physical activity is needed for minimal and optimal health benefits in school-aged children and youth? To address this question careful consideration was given to whether dose-response relations existed between physical activity and fitness with the various health outcomes, and if so, the pattern of these relations (e.g., linear, or curvilinear relations with large improvements in health occurring with limited increases in physical activity at the low end of the physical activity scale, or curvilinear relations with small improvements in health occurring with increases in physical activity at the low end of the physical activity scale).
2) What types of activity are needed to produce health benefits? Specific consideration was given to what types of activity (aerobic, resistance, etc.) influenced the different health outcomes, and whether more than one type of activity would be needed for overall health and well-being.
3) What is the appropriate physical activity intensity? Attention was given to the intensity of physical activity measured (observational studies) or prescribed (experimental studies). An underlying assumption was that children and youth would prefer lower intensity activities over higher intensity activities. Therefore, for higher intensity activities to be recommended over lower intensity activities there would need to be either: i) no evidence that low intensity activities were beneficial for health and evidence that higher intensity activities impacted health in a favorable manner, or ii) clear evidence that higher intensity activities impacted the health outcomes to a greater extent than lower intensity activities.
4) Do the effects of physical activity on health in school-aged children and youth vary by sex and/or age? Results were examined to see if: i) the moderating effects of sex and/or age on the relations between physical activity and health were explored, and if not, iii) whether there were consistent patterns across studies (either statistically or in order of magnitude) that were suggestive of sex or age differences.