Pediatric Debate

International Journal of Obesity (2011) 35, 1266–1269; doi:10.1038/ijo.2011.62; published online 15 March 2011

Can we modulate physical activity in children?

J J Reilly1

1University of Strathclyde, Physical Activity for Health Research Group, School of Psychological Sciences and Health, Jordanhill Campus, Southbrae Drive, Glasgow, Scotland, UK

Correspondence: Professor JJ Reilly, University of Strathclyde, Physical Activity for Health Research Group, School of Psychological Sciences and Health, Jordanhill Campus, Southbrae Drive, Glasgow, Scotland G13 1PP, UK. E-mail: or

Received 2 November 2010; Revised 31 January 2011; Accepted 11 February 2011; Published online 15 March 2011.



There is concern that interventions that use physical activity to prevent obesity in children might be undermined by an ‘Activitystat’, which exerts an effect to maintain a low set point for physical activity. The present critique summarises evidence from systematic reviews of interventions, from empirical tests of the Activitystat hypothesis, from studies on the heritability of physical activity in childhood and the physical activity of children of and adolescents across a wide range of physical and cultural environments. This body of evidence is inconsistent with the Activitystat hypothesis in its current form, and suggests that the emphasis on physical activity in obesity prevention interventions in children should be increased, not reduced.


physical activity; physical activity compensation; obesity prevention; children; adolescents; energy metabolism



Physical activity has long been considered as important to obesity prevention.1 One recent challenge to use physical activity interventions to prevent obesity is the view that these will be counteracted by an internal compensatory mechanism exerting an effect to maintain current level of physical activity.2 If it is established that such a mechanism exists, it could bring about a paradigm shift in the design of public health strategies aimed at obesity prevention by reducing the emphasis on physical activity. The present short critique therefore aims to consider the evidence that interventions to promote physical activity during childhood and adolescence might be counteracted by tightly controlled internal regulation of habitual physical activity or physical activity energy expenditure. The critique is based almost entirely on evidence from human children and adolescents.


Recent systematic reviews of interventions aimed at promoting physical activity in children and adolescents

The body of evidence on attempts to promote physical activity in children and adolescents has been reviewed systematically, and appraised formally. For example, van Sluijs et al.3 found 57 eligible studies, 24 rated as high methodological quality; they found that 27 (47%) of studies favored the intervention, concluding that ‘strong evidence was found for the effectiveness of school-based interventions including family or community involvement, and for multi-component interventions’. Dobbins et al.4 found 104 eligible studies, 26 rated as moderate or high methodological quality, concluding ‘there is good evidence that school-based interventions have a positive impact on duration of physical activity with generally no effects on leisure time physical activity’, implying no substantial evidence of compensation for imposed physical activity accrued in the intervention by reductions in physical activity at other times.

Many of the studies included in these systematic reviews used subjective methods of measurement of physical activity, which are prone to imprecision and to intervention-related bias.5 The apparent effect sizes in the studies included in the reviews varied widely, from 2 additional minutes per week in physical education to 283 additional minutes per week in total volume of physical activity for example.4 The studies included in these reviews rarely examined the question of physical activity compensation, and for studies that used subjective methods of measuring physical activity it is doubtful as to whether any compensation would have been identified.5

The National Institute for Health and Clinical Excellence also reviewed the topics of physical activity promotion in children and young people,6 and the potential for environmental interventions to promote physical activity in children and young people.7 Both of these reviews, and the Cochrane review on obesity prevention8 reached optimistic conclusions on the potential for interventions to increase the physical activity levels of children and adolescents.

In summary, recent systematic reviews conclude that interventions aimed at increasing physical activity in children and adolescents are worthwhile. Although acknowledging limitations in the evidence, these are not ‘empty reviews’ and are not consistent with the ‘Activitystat hypothesis’ in its current form.2 Moreover, there is no need to invoke a complex explanation—such as a regulatory mechanism for physical activity—for the apparent ‘failure’ of some previous interventions, as plausible and more parsimonious explanations exist. These include lack of statistical power in some studies; inadequate implementation of the intervention; limitations in current ‘intervention science’ can mean that the behaviours targeted by the intervention may not have been altered meaningfully by the intervention.


The ‘Activitystat’ hypothesis

Recent reviews by Eisenmann and Wickel 9 and by Rowlands10 both concluded that the existence of an Activitystat was far from being established at present.

The classical Activitystat hypothesis suggests that children and adolescents maintain close regulatory control over physical activity.11 The hypothesis of an Activitystat advanced by Wilkin et al.2 implies that the ‘set point’ for habitual physical activity is set ‘low’, and attempts to increase habitual physical activity via interventions are likely to be counteracted by compensatory declines in physical activity. It is worth noting that there are several distinct concepts here; the existence of an Activitystat; the nature of any Activitystat; whether any set point for habitual physical activity (or presumably physical activity energy expenditure or total energy expenditure) is set ‘low’ or ‘high’; the issue of whether any compensation for physical activity in children would lie largely on the energy intake side of the energy balance equation rather than on the energy expenditure side of energy balance.

Regulation of energy balance in humans is dominated by regulation of energy intake,12 but a discussion of compensation of energy intake following changes in physical activity is beyond the scope of this paper. In addition, in modern children and adults there seems to be a loose control over energy balance (positive energy balance in particular).12, 13 In summary, the Activitystat hypothesis as framed currently probably overstates the importance of physical activity/physical activity energy expenditure regulation in humans. In other mammals—notably rodents—the scope for regulation of energy balance by variation in physical activity may be much greater, but evidence should be extrapolated from rodent models to contemporary humans with caution.


Empirical tests of the Activitystat hypothesis in children and adolescents

If increased body fatness reduces habitual physical activity, this suggests that habitual physical activity is not under tight homeostatic control. One recent study in English children suggested that an increase in body fatness by around 10 body-fat percentage units might reduce habitual moderate–vigorous intensity physical activity (measured by accelerometry) by around 10%.14 Although this single study should be regarded cautiously, it seems to suggest that physical activity is not tightly regulated and so is inconsistent with the Activitystat hypothesis.

Various findings from the ‘EarlyBird Study’ were used to support the existence of an Activitystat by Wilkin et al.;2 wide variation in school physical education was not associated with marked variation in habitual physical activity as measured by accelerometry, active commuting to school was not associated with higher levels of habitual physical activity (interpreted as evidence that imposed physical activity would be compensated2); levels of objectively measured total volume of physical activity were almost identical between studies of pre-school children in the United Kingdom (interpreted as evidence of the lack of any environmental influence on physical activity2). However, more parsimonious explanations for these observations exist, and the observations made by Wilkin et al.2 are not consistent with other evidence. For example the body of evidence on active commuting to school suggests that it is associated with higher levels of habitual physical activity,15 including studies in which physical activity was measured objectively;16 increased engagement in physical education has been associated with increased physical activity in other studies;17 real and substantive differences in the physical and cultural environment are associated with marked differences in objectively measured physical activity as described below.

At present only two studies seem to have made more direct tests of the Activitystat hypothesis in children and adolescents. Baggett et al.,18 in an observational study of 6916 adolescent girls in the USA, in which habitual physical activity was measured by accelerometry, found no evidence for the kind of compensation that would be predicted by the Activitystat hypothesis either within days or over two day periods. Dale et al.19 used an intervention that decreased opportunities for physical activity within the school day in 76 US children of primary school age, and found no evidence of compensatory changes during the after-school period (measured by accelerometry). These two studies suggest that no compensatory changes in physical activity occur over timescales of hours–days in children. Although compensation may occur over longer periods, current evidence from direct tests of the Activitystat hypothesis in children and adolescents is inconsistent with the hypothesis.


Genetic versus environmental influence on habitual physical activity of children and adolescents

The Activitystat hypothesis predicts that variation in physical activity between children is explained largely by endogenous—probably genetic—influences, and the contribution of the environment to habitual physical activity of children and adolescents is limited.2 One way of testing this prediction is to examine the heritability of habitual physical activity. Only three twin studies have addressed this issue using objective measures of physical activity, and all have found extremely low heritability of habitual total volume of physical activity,20 physical activity energy expenditure21 and walking.22 Although limited in number, the twin studies are consistent in refuting the prediction that endogenous influences on habitual physical activity level of children are strong, and so are inconsistent with the Activitystat hypothesis. Future research might increase the evidence for genetic influences on physical activity in childhood and adolescence (for a recent review see Eisenmann and Wickel10), and there may be strong genetic predispositions to physical activity, but at present the evidence suggests a dominant environmental influence on habitual physical activity.

Recent observational studies of children across a wide range of physical and cultural environments—which have used objective measurement of physical activity—also suggest that the scope for physical activity is very wide in childhood and adolescence. Ojiambo et al.23 compared habitual physical activity of two groups of Kenyan adolescents from the Nandi ethnic group, from an urban and rural setting. Ojiambo et al.23 found much higher levels of habitual physical activity in the rural sample, associated with urbanisation. It seems implausible that an Activitystat would be ‘set’ differently between two groups of Nandi adolescents, and much more likely that environmental differences between the two samples were responsible for the marked differences in habitual physical activity.

Although levels of habitual physical activity may be broadly similar among children living in similar environments,2 and this has been interpreted as evidence in support of the Activitystat hypothesis,2 a number of studies have shown more substantial differences in objectively measured physical activity between children and adolescents living in quite different physical or cultural environments.24, 25, 26, 27 Comparisons of groups living modern lifestyles (in urban and rural North America) versus those living more traditional lifestyles (for example, from old order Amish and Mennonite communities in North America) report consistently that more traditional lifestyles are characterised by much higher levels of objectively measured physical activity. These observations emphasise the importance of the environment as a determinant of physical activity in children and adolescents, and the great flexibility of child and adolescent habitual physical activity across different environments.23, 24, 25, 26, 27



There is a moderately large, reasonably high quality, diverse and consistent body of evidence that is inconsistent with the existence of an internal regulatory mechanism, which defends a low-set point for habitual physical activity among children and adolescents. Further research would be desirable, but belief in the Activitystat hypothesis as it is expressed currently2 is premature.

Modern children could and should be more physically active; systematic reviews have demonstrated many benefits of higher levels of physical activity across childhood and adolescence;28 objective measures of habitual physical activity and physical activity energy expenditure suggests that children and adolescents were more physically active in the recent past.24, 25, 26, 27, 28, 29

Obesity prevention interventions should not exclude physical activity as an option because of concerns over the possible existence of an ‘Activitystat’. On the contrary, recent evidence on the importance of environmental influences on objectively measured habitual physical activity of children, positive systematic reviews on the efficacy of interventions and continuing improvements in intervention science, all argue for greater efforts to promote physical activity in future obesity prevention interventions.


Conflict of interest

The author declares no conflict of interest.



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This study was funded by Scottish Funding Council.

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