A strength of the present study was our use of heart rate monitoring as an objective measure of physical activity. Importantly, heart rate monitoring avoids problems with recall and subjectivity, and is relatively inexpensive to administer in large-scale studies (25). We consider our use of the PAHR-50 index a further strength, as this measure is specific to more vigorous activity in children, is less likely to be affected by factors such as emotional state and climate, and controls for differences in fitness levels and/or age (12).
In keeping with previous studies, both Pearson and Spearman correlation coefficients were used to assess the extent of tracking. Pearson and Spearman correlations ranged from 0.53 to 0.63 and 0.57 to 0.66, respectively. In absolute terms, these correlations are no more than moderate, however, when compared with other tracking studies of risk factors such as body composition(0.53-0.64)(10), serum lipids(0.38-0.66)(33), and systolic blood pressure(0.25-0.65)(8), it appears that these correlations are typical and supportive of our conclusion that physical activity behavior tends to track during early childhood. In support of our conclusions, Bloom(5) defined a stable characteristic as one that exhibits a correlation of greater than 0.50 for two measures obtained at least 1 yr apart. Furthermore, it is possible that our correlation coefficients underestimated the true level of tracking within the group, given that we used only two to four observations of physical activity per year. A larger number of observations would be expected to produce a more reliable index and higher tracking coefficients.
It is important to note that this study was conducted with several important limitations. Physical activity was measured only in young children, only for 3 yr, with only two to four observations per year. Furthermore, because we chose to examine physical activity between the hours of 3:00 and 6:00 p.m., it is possible that some children may have exhibited different activity patterns at other times during the day. However, within the limitations of the study design, our findings provide support for the hypothesis that physical activity behavior has stability over time. This was evidenced by the 62% agreement for the PAHR-50 index tertiles between year 1 and year 3. Therefore, although we know relatively little about how to promote increased physical activity in children, our results strongly suggest that there is a somewhat stable group of children that should be the focus of such interventions.
The results of this investigation clearly highlight the need for physicians and other primary health care providers to become actively involved in the promotion of physical activity and fitness in children and youth. Given their standing within the community and frequent contact with parents and children, health care professionals are uniquely qualified to deliver a broad range of primary and secondary prevention services designed to reduce the prevalence of negative health behaviors such as inactivity. Along these lines, the American Medical Association recently developed Guidelines for Adolescent Preventive Services (GAPS) (1). This document challenges primary health care providers to make preventive services a greater component of their clinical practice. Specifically, GAPS recommends that physicians regularly counsel adolescents about the benefits of exercise and encourage them to engage in safe exercise on a regular basis (1). Our observation of a relatively stable group of children with low levels of physical activity strongly suggests that these guidelines be extended to include children under the age of 10 yr, and that health care professionals, in coordination with schools and other community organizations, become actively involved in the promotion and assessment of physical activity in young children.
Finally, our conclusion, that physical activity in young children tracks during early childhood, indicates that further research in this area is warranted. Among the key unanswered questions are (a) for how many years is tracking evident? (b) Does physical activity track from childhood to adulthood? (c) What are the genetic, social, and environmental factors that explain the tracking of physical activity behavior? and (d) Does the tracking in physical activity account for the tracking in the physiological risk factors? Furthermore, our conclusion indicates that interventions targeted at promotion of physical activity in low active children are needed and worthy of exploration.
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EXERCISE; HEART RATE MONITORING; HEALTH; RISK FACTORS