This is an excerpt from Biologic Regulation of Physical Activity eBook by Thomas W. Rowland.
This is the originally proposed activity-stat concept: A set point for activity energy expenditure exists, such that any exercise intervention program would be met, or compensated for, by an equivalent amount of reduction of out-of-program activity and that the net change in activity energy expenditure would be zero. That is, in this response, a physical activity intervention serves to perturb the activity energy expenditure homeostatic system, but only temporarily, and such interventions do not act to modulate (change) activity behavior over the long term. Recording of a compensatory decline in physical activity after an exercise intervention would serve to support this concept. Studies addressing this idea have often focused on the pediatric age group.
In 2011, T.J. Wilkin and J.J. Reilly participated in a literary debate in the International Journal of Obesity regarding this question: Can we modulate physical activity in children? In the pages of this argument, they took a stand - Wilkin for the yea and Reilly for the nay - about whether activity compensation offsetting physical activity interventions was likely. Wilkin based his argument against a lasting effect of activity intervention on sustained activity habits (in youth) and in support of the activity-stat hypothesis on the grounds that (a) the research literature indicates that variations in environmental influences on young children (school physical education, geographical location) have little effect on levels of habitual activity and (b) studies indicate that compensatory declines in activity occur after exercise interventions (77). Moreover, although (c) some studies indicate increases in total physical activity after exercise interventions, there is a clear inverse relationship between such an effect of the intervention and the duration that the outcome was measured. That is, a compensatory decline in out-of-program activity occurs that acts to neutralize the effect of an activity intervention, but this may take time.
In conclusion, Wilkin contended,
There is no evidence that we can modulate the physical activity of children, although it can clearly be perturbed. There is a danger that the success of some short term studies in raising physical activity is being misinterpreted as modulation when it is really perturbation which will last only for as long as the environmental disturbance that caused it. (77, p.1275)
Reilly countered that "the body of evidence is inconsistent with the activity-stat hypothesis in its current form and suggests that the emphasis on physical activity in obesity prevention interventions in children should be increased, not reduced" (55, p. 1266). In his rebuttal, he cited systematic reviews that indicate favorable effects of physical activity interventions and the potential for environmental manipulations to promote physical activity. The data concerning heritability he considered to demonstrate only a weak effect. Instead, he supported the role of environmental factors as dominant in influencing habitual physical activity. Reilly thought that the current research literature failed to indicate compensatory decrease in habitual activity with exercise interventions, although he acknowledged that these reports generally measured such responses in terms of days and that "compensation may occur over longer periods" (p. 1267).
It is evident, then, this controversy can be viewed from various vantage points, interpreting the same set of experience in the research literature as supportive to divergent arguments. It is worthwhile to examine some of these points of discussion in more detail.
Wilkin and colleagues presented their arguments (outlined previously) in support of the activity-stat concept, which "would comprise a neuro-humoral feedback loop, with a set-point possibly located in the hypothalamus, able to integrate activity carried out by as yet unknown means, and to control further activity accordingly. If centrally controlled in this way," they reasoned, "we would expect overall physical activity to be independent of environmental opportunity or (within limits) of environmental intervention" (78, p. 1050).
To examine this hypothesis, Wilkin and colleagues devised three studies to assess physical activity levels of youth exposed to differing environmental influences (78). Activity was measured by accelerometer recordings continuously over 7 days. In the first, the physical activity levels of 307 young children (mean age 4.9 years) were compared between weekdays and weekends. In accordance with a central control of activity, average activity did not differ in the two periods. In the second study, levels of daily physical activity in older children (aged 7-11 years) were assessed in three schools with widely divergent hours of physical education (9.0, 2.2, and 1.8 hours per week). When in-school and out-of-school activities were combined, no differences were observed in total daily activity in the three schools. The third investigation revealed similar daily activity levels in children who lived in Glasgow and Plymouth, two cities in Great Britain of differing size, culture, and climate. The authors concluded that "together, the data reported here suggest that children of primary school age display consistency in the amount of physical activity they undertake, independently of opportunity, daily routine, background or culture. Such consistency raises the question of central control" (p. 1053).
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