Lubas_2008_A Review of Mediators of Behavior in Interventions to Promote Physical Activity

8/6/2019 Lubas_2008_A Review of Mediators of Behavior in Interventions to Promote Physical Activity

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Review

A review of mediators of behavior in interventions to promote physical activityamong children and adolescents

David Revalds Lubans a , , Charlie Foster b , Stuart J.H. Biddle ca School of Education, The University of Newcastle, Australiab Department of Public Health and Primary Health Care, The University of Oxford, Oxford, UK c School of Sport and Exercise Sciences, Loughborough University, Loughborough, UK

a b s t r a c ta r t i c l e i n f o

Article history:Available online 29 July 2008

Keywords:MediationInterventionPhysical activityBehaviorPsychosocial variables

Background. The effectiveness of interventions to promote physical activity in youths is still developing.To develop a better understanding what works , researchers are now focusing on constructing an evidencebase for mediators of behavior change.

Methods. We reviewed studies that examined the direct effect of physical activity interventions onhypothesized mediators and the relationship between mediators and physical activity in young people (aged5 to 18 years). Studies were identi ed via electronic database searches and scanning references againstpredetermined quality criteria.

Results. We found seven studies that evaluated three mediator groups: cognitive, behavioral andinterpersonal mediators. Self-ef cacy was the most commonly assessed mediator in youth interventions andthere was strong support for its role in mediating the relation between theory-based interventions andphysical activity. There was some support for the importance of behavioral strategies as mediators of behavior, but no support for the mediating in uence of interpersonal factors.

Conclusions. Despite recognition of the importance of mediation studies, few interventions have assessedmediators of physical activity behavior in youth interventions. The small number of studies examiningmediators of behavior and the variability in study design and quality prevent us from forming strong

conclusions regarding the most effective mediators of behavior. 2008 Elsevier Inc. All rights reserved.

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Identi cation of studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Criteria for inclusion/exclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46Criteria for assessment of study quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview of study quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Overview of study ndings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Summary of cognitive mediators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46Summary of behavioral mediators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46Summary of interpersonal mediators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Con ict of interest statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Preventive Medicine 47 (2008) 463 470

Corresponding author. University of Newcastle, Faculty of Education and Arts, Callaghan Campus, NSW 2308, Australia. Fax: +61 2 49217407.E-mail address: [email protected] (D.R. Lubans).

0091-7435/$ see front matter 2008 Elsevier Inc. All rights reserved.

doi: 10.1016/j.ypmed.2008.07.011

Contents lists available at ScienceDirect

Preventive Medicine

j o u rn a l h o m ep ag e : w w w. e l sev i e r. co m / lo ca t e /y p m ed
mailto:[email protected]://dx.doi.org/10.1016/j.ypmed.2008.07.011http://www.sciencedirect.com/science/journal/00917435http://www.sciencedirect.com/science/journal/00917435http://dx.doi.org/10.1016/j.ypmed.2008.07.011mailto:[email protected]


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Introduction

Due to the bene ts of physical activity and concern regarding lowlevels of activity, numerous interventions targeting activity behaviorin youth have been evaluated. However, the majority of these studieshave produced modest results ( Stone et al., 1998; Van Sluijs et al.,2007 ). It has been suggested the lack of effectiveness of youth inter-ventions is, in part, due to a poor understanding of the mechanisms

responsible for behavior change ( Baranowski and Jago, 2005 ). Whileinterventions are generally developed in reference to a theory of health behavior change (e.g., Social Cognitive Theory, Theory of Planned Behavior), few studies have examined possible mediators ineffective interventions ( Baranowski et al., 1998 ). In 2002, Lewis et al.(2002) published an important review of psychosocial mediators of physical activity behavior and found only two studies that examinedmediators in youth interventions.

A mediator can be de ned as an intervening causal variablenecessary to complete thepathway from an intervention to the targetedbehavioral outcome ( Bauman et al., 2002 ). Measurement of thesechange mechanisms is necessary for the systematic progression of physical activity research ( Bauman et al., 2002 ) because it allowsresearchers to determine which components of an interventioncontribute to behavior change. Furthermore, mediation analyses allowresearchers to develop more parsimonious models by eliminatingunrelated mediators from future interventions ( MacKinnon andDwyer, 1993 ).

Randomized controlled trials are regarded as the gold standard

for physical activity intervention design and they also provide avaluable opportunity for the identi cation of mediators of behaviorchange ( Kraemer et al., 2002 ). In its simplest form, testing formediatingeffects is accomplished by adding a mediatingvariable (e.g.,self-ef cacy) to the regression equation of the independent (e.g.,intervention condition) and dependent variables (e.g., physicalactivity) ( MacKinnon et al., 2007 ) (Fig. 1). In mediational hypotheses,it is assumed that the inclusion of a mediating variablewill reduce themagnitude of the relationship between the independent anddependent variables ( MacKinnon et al., 2000 ). However, suppressionoccurs when the inclusion of an additional variable (e.g., mediator)increases the predictive validity of another variable (e.g., intervention)by its inclusion in an equation ( Tzelgov and Henik, 1991 ). There arethree major approaches used to establish statistical mediation, theseinclude the causal steps as proposed by Baron and Kenny (1986) , thedifference in coef cients and the product of coef cients ( MacKinnon,2000 ). These methods are described in detail by MacKinnon et al.(2007) .

While studies often cite a theoretical framework for their inter-vention, they rarely test theef cacy of these modelsusing appropriatestrategies ( Baranowski et al., 1998 ). For example, a study might reportthe effect of an intervention on hypothesized mediators or psycho-social constructs (e.g., Deforche et al., 2004; Parcel et al., 1989; Simonet al., 2004 ), without examining potential mediation pathways. This

type of analysis does not establish that changes in the theoreticalconstructs were responsible for changes in the outcome variable. Theaim of this paper is to review the evidence of mediators of phy-

sical activity behavior change in youth interventions because suchan analysis should further our understanding of interventioneffectiveness.

Method

Identi cation of studies

A comprehensive search of published studies was conducted usingthe computer databases PubMed, Embase, PsychINFO and SPORTSDiscus. Experts in the eld were contacted and the following handselected scienti c journals were searched Psychology of Sport andExercise, Journal of Sport and Exercise Psychology , Preventive Medicineand Health Psychology . The key terms for searches included mediation ,mediator , intervention , and physical activity . Titles, references, andabstracts of articles identi ed were checked for relevance by the leadauthor and references of the full-text articles retrieved were searched.The Quality of Reporting of Meta-analyses statement (QUOROM)(Moher et al.,1999 ) was consulted and provided the structure for thisreview. The ow of studies through the review process is reportedin Fig. 2.

Criteria for inclusion/exclusion

The criteria for inclusion in this review were as follows: (1)quantitative assessment of physical activity, (2) quantitative assess-ment of potential mediators, (3) physical activity interventioninvolving experimental or quasi-experimental design, (4) partici-pants were primary or secondary school age (aged 5 to 18 years), (5)study reported the impact of the intervention on hypothesized me-diators and the relationship between the mediator and physicalactivity after adjusting for the intervention effect, and (6) publishedin English.

Criteria for assessment of study quality

The present authors assessed the quality of the mediationstudies that the met the inclusion criteria. A formal quality scorefor each study was computed (ranging from 0 to 8) by assigning avalue of 0 (no) or 1 (yes) to each of the questions listed. (i) Did thestudy cite a theoretical framework? (ii) Were the study methods/procedures designed to in uence mediating variables? (iii) Werepilot studies conducted/reported to test the effect of the interven-tion on mediators? (iv) Was an objective measure of physicalactivity used? (v) Were the psychometric characteristics of themediator variables reported and were they within accepted ranges(Cronbach's alpha and test retest reliability N .06) (Tabachnick andFidell, 1996 )? (vi) Did the study report a power calculation and wasthe study adequately powered to detect mediation? (vii) Did thestudy use an experimental design? (viii) Was post-interventionphysical activity controlled for baseline physical activity? Studies

that scored 0 3 were regarded as low quality studies, studies thatscored 4 6 were classi ed as medium quality and those that scored7 8 high quality.

In this review, the hypothesized mediators were organized intothree groups: cognitive mediators, behavioral mediators and inter-personal mediators. Cognitive mediators included constructs relatedto participants' thoughts and feelings about physical activity and theirability to overcome barriers to participation. Hypothesized cognitivemediators included self-ef cacy, outcome expectancy, enjoyment(including enjoyment of physical education), perceived barriers,perceived bene ts and attitudes. Behavioral mediators were classi edas strategies used by participants to increase their physical activityadherence. Behavioral mediators assessed were goal setting, commit-ment to planning, stimulus control and counter conditioning.

Interpersonal mediators included constructs related to social supportFig. 1. Overview of mediation analysis.

464 D.R. Lubans et al. / Preventive Medicine 47 (2008) 463 470


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Project FAB (Dunton et al., 2007 ) intervention for adolescent girls,none of the psychosocial variables assessed satis ed the criteria formediation. Taymoori and Lubans (2008) examined potential media-tors in two individually-tailored interventions designed for sedentaryadolescent girls in Iranian secondary schools. Both interventions hadpositive effects on physical activity behavior and theoretical con-structs from the HPM. In the Lifetime Activity Program (LAP) ( Lubansand Sylva, 2007 ) for senior school students, increases in physical

activity among adolescent girls in the intervention were related tochanges in the theoretical constructs. Haerens et al. (2007) examinedpotential mediators in a large multi-component intervention foradolescents. The intervention resulted in signi cant increases inphysical activity. They found that self-ef cacy for physical activity atschool partially mediated the effects of the intervention. The HealthyYouth Places (HYP) study ( Dzewaltowski et al., 2008 ) interventionwasdesigned to increase students' proxy ef cacy or con dence in theirskills and abilities to get others to support their physical activitybehaviors.

Summary of cognitive mediators

Self-ef cacy was the most commonly assessed cognitive mediatorand was included in all of the intervention studies. Self-ef cacy wasfound to mediate changes in physical activity in the LEAP study(Dishman et al., 2004 ), the Iranian girls intervention ( Taymoori andLubans, 2008 ), and the Belgian intervention ( Haerens et al., 2007 ). TheLAP intervention ( Lubans and Sylva, 2007 ) had a signi cant effect onself-ef cacy andthe changes in self-ef cacy were related to changes inphysical activity. In the HYP intervention ( Dzewaltowski et al., 2008 ),proxy ef cacy to in uence school physical activity environmentsmediated the program effects on physical activity at the one yearposttest.

Outcome expectancy and perceived bene ts were assessed in vestudies. Although changes in outcome expectancy/perceived bene tswere related to changes in physical activity in the LEAP ( Dishman etal., 2004 ) and in the Belgian study ( Haerens et al., 2007 ), it was only inthe Iranian girls study ( Taymoori and Lubans, 2008 ) that it satis edthe criteria for mediation. In the Belgian intervention, changes inattitudes were associated with the intervention condition and withchanges in physical activity, but could not satisfy the criteria for fullmediation because the effects of the intervention were not reducedafter controlling for the effect of the mediator. In contrast to expec-tations, changes in attitude were found to have signi cant suppressioneffects on physical activity.

Changes in perceived barriers were not related to changes inphysical activity in the Iranian intervention. Although changes inperceived barriers were related to the intervention condition in theProject FAB study, they were not in the hypothesized direction andwere not related to changes in physical activity. Changes in perceivedbarriers were related to the intervention and changes in physicalactivity in the Belgian study, but could not satisfy the criteria for

mediation because the effects of the intervention were not attenuatedafter controlling for the effect of the mediator. While changes in theenjoyment of physical education were not related to changes inphysical activity in the LEAP study, changes in enjoyment of physicalactivity were. Increased enjoyment in physical activity partiallymediated the effects of the LEAP intervention.

Summary of behavioral mediators

Only two studies assessed potential behavioral mediators ( Dish-man et al., 2004; Taymoori and Lubans, 2008 ). In the LEAP study, theintervention was found to have a signi cant effect on goal setting, butthese changes were not related to changes in physical activity. In theIranian girls' intervention, commitment to planning satis ed all of

the mediation criteria in both interventions. In the same intervention

the two behavioral processes from the TTM could not satisfy any of the mediation criteria.

Summary of interpersonal mediators

Five studies evaluated the impact of interpersonal factors onphysical activity changes ( Dunton et al., 2007; Dzewaltowski et al.,2008; Haerens et al., 2007; Lubans and Sylva, 2007; Taymoori and

Lubans, 2008 ). None of the interpersonal variables could satisfy thecriteria for mediation in any of the studies. In the Iranian intervention,changes in exposure to models were related to changes in physicalactivity, but the changes were not related to treatment condition.

Discussion

Mediation analyses from intervention studies provide researchers,health promoters and educators with evidence about what works forchanging physical activity behaviors. The aim of our review was toidentify mediators of physical activity behavior in youth. Only sevenstudies satis ed the criteria for inclusion in this review and due to thediversity of interventions, methods, and ndings, conclusions aredif cult to draw. The majority of studies involved adolescent girls andbecause determinants of physical activity change over time and aredifferent for boys and girls ( Sallis et al., 2000 ), the majority of theseresults are only generalizable to this group.

Self-ef cacy was the most commonly assessed mediator andreceived the strongest support for mediating the relationship betweentheory-based interventions and physical activity in youth. Although acomprehensive review of physical activity correlates among youthfound that the evidence for self-ef cacy was indeterminate ( Sallis etal., 2000 ), a more recent review of correlates among adolescent girlsfound that self-ef cacy was an important correlate ( Biddle et al.,2005 ). Evidence from this review suggests that interventions toincrease physical activity among youth should target self-ef cacyusing appropriate strategies.

While changes in self-ef cacy partially mediated the effects of theBelgian intervention ( Haerens et al., 2007 ) on total and school relatedphysical activity, signi cant suppression effects for attitudes, self-ef cacy, perceived bene ts and barriers on physical activity changeswere found. As mentioned previously, suppression occurs when theinclusion of an additional variable increases the predictive validity of avariable in the same equation. This explains why no mediation effectswere found even though there were signi cant relationships betweenthe intervention and mediators and between mediators and physicalactivity, after adjusting for the intervention.

Two studies included hypothesized behavioral mediators andonly one study ( Taymoori and Lubans, 2008 ) found that increases inthe use of behavioral strategies mediated changes in physical acti-vity behavior. This is a surprising nding considering the strongsupport for the mediating role of behavioral processes in physicalactivity interventions identi ed in the review by Lewis et al. ( 2002 ).

Although these nding were based on studies with adults, by lateadolescence individuals start to develop adult-like cognitions andstrategies. The examination of behavioral mediators requires furtherattention in this age group. There was limited support for the ef -cacy of interpersonal variables as mediators of behavior change.None of the studies that assessed interpersonal factors establishedmediation.

The overall quality of the studies was moderate. Studies werelimited by the use of measures with unacceptable psychometricproperties, the failure to report a power calculation, and the use of self-report measures of physical activity. Four studies used theproduct of coef cients test to assess potential mediators of behavior.The product of coef cients test can be used to establish mediationeffects in small samples ( Cerin et al., 2006; MacKinnon et al., 2002;

MacKinnon et al., 1995 ), even in the absence of a signi cant effect



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between intervention and outcome. Three studies used the jointsigni cance test to identify potential mediators of behavior ( Dishmanet al., 2005; Dishman et al., 2004; Dunton et al., 2007 ). The joint

signi cance test involves two steps. First, the relationship between a

mediator and an outcome variable after adjusting for the effect of anintervention is examined. Then the relationship between the inter-vention and the mediator is assessed. The joint signi cance test refers

to statistical signi cance of both alpha and beta coef cients from the

Table 3Results from mediation analyses in interventions designed to promote physical activity among youth

Study Mediators tested Effect of intervention on mediators Mediated effects

Hypothesized cognitive mediatorsDzewaltowski et al. (2008) Self-ef cacy, PA proxy

ef cacy school,PA proxy ef cacy parents,PA proxy ef cacy peers

Signi cant effect on self-ef cacy and PA proxyef cacy school.

PA proxy ef cacy school mediated the effects of theintervention between baseline and one year posttest.

No effect on PA proxy ef cacy parents of PAproxy ef cacy peers

Controlling for PA proxy ef cacy school at two yearposttest, PA change decreased in the interventiongroup and increased in the control group no mediation

effect found.Proxy ef cacy school was a mediating variable for theintervention group and a suppressor variable for thecontrol group.

Taymoori and Lubans (2008) Perceived bene ts The HPM/TTM intervention had a signi canteffect on perceived bene ts, perceived barriersand self-ef cacy.

HPM/TTM intervention-changes in perceived bene ts,perceived barriers and self-ef cacy were associated withchanges in PA.

Perceived barriers

The HPM intervention had a signi cant effecton self-ef cacy and commitment to planning,but not on perceived barriers.

HPM intervention-changes in self-ef cacy were associatedwith changes in PA.

Self-ef cacy

Perceived bene ts, perceived barriers and self-ef cacysatis ed the criteria for mediation in the HPM/TTM,intervention.Self-ef cacy mediated the effects of the HPMintervention on PA.

Lubans and Sylva (2007) Outcome expectancy andself-ef cacy

Signi cant effect on self-ef cacy in girls. Changes in self-ef cacy were associated with changes inPA in girls only, but could not satisfy the criteria formediation.

No effect on outcome expectancy in boys orgirls.

Dunton et al. (2007) Self-ef cacy, perceivedbarriers and enjoyment

No effect on self-ef cacy or enjoyment. None of the variables were associated with changes in PAand could not satisfy the criteria for mediation.Changes in perceived barriers were related to

intervention (not in hypothesized direction).Haerens et al. (2007) Attitudes, self-ef cacy,

perceived bene ts andbarriers.

The school only intervention had a signi canteffect on attitudes, self-ef cacy (at home),perceived bene ts and barriers(environmental and motivational).

Changes in all psychosocial factors (except for perceivedhealth barriers) were associated with changes in PA inboth interventions.

The school and parent intervention had asigni cant effect on attitudes and self-ef cacy(at school).

Suppression effects for attitudes, self-ef cacy, perceivedbene ts and perceived barriers on total PA and leisuretime PA were found.Attitudes, perceived bene ts and barriers also suppressedthe effect of the intervention on school PA.Self-ef cacy partially mediated the effect of school PA inthe intervention with parent support.

Dishman et al. (2004) Self-ef cacy and outcomeexpectancy

Signi cant effect on self-ef cacy. Changes in self-ef cacy and outcome expectancy wereassociated with changes in PA.Self-ef cacy partially mediated the effects of theintervention on PA.

Dishman et al. (2005) Enjoyment, enjoyment of PE and self-ef cacy

Signi cant effect on factors in uencingenjoyment of PE.

Changes in enjoyment were associated with changesin PA.

Factors in uencing enjoyment of PE had asigni cant effect on enjoyment of PA andself-ef cacy.

Changes in enjoyment of PE were not associated withchanges in PA.

Factors in uencing enjoyment of PE had asigni cant effect on enjoyment of PA andself-ef cacy.

Increases in enjoyment partially mediated the effect of the intervention.

Hypothesized behavioral mediatorsTaymoori and Lubans (2008) Commitment to planning,

stimulus control andcounter conditioning

Both interventions had a signi cant effecton commitment to planning.

In both interventions changes in commitment to planningwere associated with changes in PA and satis ed thecriteria for mediation.Neither intervention had an effect on

stimulus control and counter conditioning.Dishman et al. (2004) Goal setting Signi cant effect on goal setting.

Changes in goal setting were not related to changes in PAand could not satisfy the criteria for mediation.

Hypothesized interpersonal mediatorsDzewaltowski et al. (2008) PA group norm No effect on PA group norm PA group norm could not sat isfy the cri ter ia for mediation.Taymoori and Lubans (2008) Exposure to models,

social support,interpersonal norms

No effect on any of the interpersonal

mediators.

Changes in exposure to models were associated with

changes in PA in the HPM intervention, but could notsatisfy the criteria for mediation.Lubans and Sylva (2007) Peer support Signi cant effect on peer support in

girls only.Changes in peer support were not associated with changesin PA and could not satisfy the criteria for mediation.

Dunton et al. (2007) Social suppor t No effect on social suppor t. Changes in social suppor t were not associated with changesin PA and could not satisfy the criteria for mediation.

Haerens et al. (2007) Social support No effect on any of the social supportmediators.

Changes in social support were not associated with changesin PA and could not satisfy the criteria for mediation.

SCT = Social Cognitive Theory, TTM = Transtheoretical Model, TPB = Theory of Planned Behavior, HPM = Health PromotionModel, CON = Controlgroup, INT = Intervention group, PA =physical activity.



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regression models described above. Researchers aiming to identifypotential mediators of behavior change should choose a method of mediation analysis that is appropriate for their sample size.

There are limitations in this review that should be noted. First, ourcriterion for assessing the psychometric qualities of the scales usedwas limited.Cronbach's alpha representsone of many indicators of themetric properties of a scale. While internal consistency is oftenreported as a measure of reliability, it provides limited information on

measurement quality and is dependent on the number of scale items.Test retest reliability was also assessed in this study; however, fewstudies reportedreliability coef cients. Furthermore, the quality of themediators assessed could not be determined in studies that did notreport any psychometric properties. Second, we cannot be certain thatstudies included in the review had conducted pilot studies and did notreport them in their mediation analyses.

Future research

Interventions to promote physical activity have traditionallytargeted individuals and focused on increasing knowledge and skillsthrough educational programs ( Dishman, 1994; Kremers et al., 2007;McLeroy et al., 1988 ). More recently, the importance of targeting thephysical environmenthas been identi ed ( Fein et al., 2004; Salliset al.,2001 ). Some school-based interventions that have combined environ-mental changes with educational programs have demonstratedpotential in promoting sustainable behavior change ( Haerens et al.,2006; Sallis et al., 2003; Simon et al., 2004 ). Future studies shouldexplore the impact of environmental interventions by examiningindividuals' perceptions of their environment to determine if changesto the physical environment are accompanied with changes inperception, which in turn contribute to increased physical activity.Recent reviews have identi ed a number of potentially modi ableenvironmental correlates of physical activity in youth populations(Ferreira et al., 2007; Salmon and Timperio, 2007 ). However, thecontribution of such variables in explaining the variance of physicalactivity behavior is much smaller than the contribution of cognitiveand interpersonal variables ( Giles-Corti and Donovan, 2002 ) andassessing these constructs must remain a priority. It may be necessaryfor researchers to evaluate more complicated models which recognizethat the small size of the effects of changes in environmental variableson behavior change may be due to these changes being mediated bycognitive variables (e.g., self-ef cacy).

None of the studies included in this review included an objectivemeasure of physical activity. The measurement of physical activityamong youth using self-report measures is notoriously problematic.While it has been suggested that social desirability may lead youngpeople to overestimate their physical activity ( Warnecke et al., 1997 ),more recently, researchers have found that children and adolescentsunderestimate physical activity of moderate intensity ( Riddoch et al.,2004; Telford et al., 2004 ). Future studies should assess changes inphysical activity using a combination of objective and self-report

measures. This will provide a more accurateassessment of the amountand context of activity and enable researchers to determine the effectof interventions on speci c physical activity behaviors.

Finally, the overall quality and quantity of mediation studies inchildren and adolescents is lacking. We recommend more studiesexamining potential mediators of physical activity behavior change inyouth populations, especially among boys. To improve the quality of studies and to enable readers to evaluate the quality of existingstudies, researchers should use and report their studies using theCONSORT criteria. As the majority of behavior change models havebeen developed for adult populations, they may not be entirelyappropriate for children and adolescents. Future studies may chooseto evaluate the ef cacy of models designed speci cally for youth (e.g.,Youth Physical Activity Promotion Model) using scales that demon-

strate strong psychometric properties. Theory driven interventions

that allow researchers to investigate the effects of manipulatingpotential mediating variables provide the greatest opportunityfor understanding behavior change and should be focus of futureresearch, rather than those that adopt a chance approach to estab-lishing mediation.

Conclusion

There is considerable work to be done in order to improve ourunderstanding of physical activity behavior change in youth popula-tions. While an increased emphasison descriptive longitudinal studiesmay help to identify more highly predictive causal mediators, theglobal pediatric obesity epidemic and general decline in physicalactivity levels require immediate interventions. These interventionsshould be guided by theories of behavior change and involve rigorousmediation analyses to identify important mechanisms for behaviorchange. This will enable researchers to develop more effectiveinterventions and expand our knowledge of how to change behaviorin speci c youth populations.

Con ict of interest statement

The authors have no con ict of interest to declare.

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