Research article
The Physical Environment and Physical Activity: A Critical Appraisal of Review Articles

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Background

Over the last few years an increasing number of studies investigating the association between the physical environment and physical activity have been published. Many reviews have also summarized this emerging body of research, and such review papers are frequently used by public health policymakers and researchers themselves to inform decision making.

Methods

This paper systematically appraises methodologic aspects of literature reviews examining the relationship between physical activity and the physical environment published in peer-reviewed journals between 2000 and 2005. Eleven reviews and their antecedent source papers were examined.

Results

The majority of these reviews omitted between one third and two thirds of the studies that could have been eligible for inclusion at the time they conducted the review. Methodologic information on how the review was conducted was not always provided. Furthermore, in some cases results of a study were reported incorrectly, or physical environmental aspects were conflated with social environmental or cognitive factors. Moreover, when results were reported incorrectly, physical environmental variables were almost always reported as significantly associated with physical activity, when these associations were nonsignificant, or were not assessed as part of the primary study.

Conclusions

Users of reviews in this field should be aware that there are significant methodologic variations among them, and that some reviews may include only a sample of the relevant primary studies. However, this is difficult to determine given the frequent incompleteness of review method reporting. Greater standardization in the reporting of review methods may assist with future efforts to summarize studies of the relationship between physical environments and physical activity.

Introduction

Numerous epidemiologic studies have confirmed the health benefits of regular moderate-intensity physical activity, including walking.1, 2, 3 However, in most Western countries a large proportion of the population is not sufficiently active,4, 5 and the increased prevalence of sedentariness and obesity are a growing public health concern.

A prerequisite for the effective design and implementation of strategies to promote physical activity is the identification of factors that influence adoption and maintenance of an active lifestyle. Recent research has explored the relationship between the physical environment and physical activity. If environmental factors are associated with physical activity levels, then environment-changing interventions might form part of an effective population-level strategy to tackling inactivity and obesity; even if the effects are relatively small, they are likely to reach large groups of people, and are likely to achieve a sustainable public health outcome.6

There have been many studies exploring this physical environment/physical activity relationship, but relatively few intervention studies; instead, the evidence base is largely composed of observational etiologic studies describing the relationship between environmental characteristics and physical activity. The observational evidence base on the physical environment and physical activity is, however, substantial, and growing rapidly.7, 8, 9, 10 Decision making based on observational evidence is particularly difficult, given lack of guidance on what constitutes a good enough study in the absence of experimental evidence.11

Decision makers and practitioners seeking to develop healthy public policy therefore face a significant challenge in accessing and interpreting this growing evidence base. In such circumstances, it is common to rely on summaries and literature reviews that aim to synthesize the findings of primary studies as a guide to evidence-based practice. Such overviews of the evidence can not only guide practice, but can also identify future research needs.12, 13, 14 However, as with primary studies, the quality of review articles can vary significantly, and the quality of the review can be related to its conclusions; in particular, it has been shown empirically that biased reviews over-estimate the effectiveness of interventions and under-report adverse effects. Bias can operate at many levels in a review, but reviews that include an incomplete and unrepresentative sample of the literature, and which do not assess the quality of the studies they include have been shown to lead to erroneous and misleading conclusions.15, 16 For these reasons, the use of systematic reviews that standardize review methods and adopt a more scientific approach has grown rapidly.

These differ from “traditional” reviews in several ways. A systematic review clearly states a specific research question or hypothesis, whereas narrative reviews more often only generally discuss various studies related to a certain topic. Systematic reviews identify a priori inclusion and exclusion criteria for the studies they review, and involve a comprehensive search for relevant literature (published and unpublished). In contrast, traditional reviews often uncover only a limited and potentially biased subset of the literature. Systematic reviews also take account of the robustness of the individual studies when drawing their conclusions, and this is formally assessed. By contrast “traditional” reviews mostly do not differentiate between the methodologic quality of the primary studies when their results are synthesized. Moreover, a systematic review also aims to be transparent and replicable.17, 18, 19

Systematic reviews may synthesize the primary studies narratively, or statistically by means of a meta-analysis, a statistical synthesis of data from separate but similar—that is, comparable—studies.20 However, in research about physical environments and physical activity, the collected data, or the contexts in which they were collected, are often too heterogeneous to allow a quantitative summary.

Previous studies have critically examined the methodology and quality of review articles in the health sciences. For example, Oliver et al.14 compared various aspects of six literature reviews on injury prevention in older people, and noted discrepancies in their findings. They also found that the reviews, although related to the same topic, differed significantly in terms of their scope and search strategies. This led Oliver et al.14 to suggest that “what is ‘known’ about what works in health promotion is heavily dependent on ‘what questions’ have been asked and ‘how’ they have been addressed.”

Other methodologic overviews have been carried out, most often focused on healthcare interventions.21, 22, 23, 24 The authors of these studies reached similar conclusions. For instance, reviews did not search the literature comprehensively,21, 22, 24 or the reporting of inclusion and exclusion criteria was incomplete.21, 22, 23, 24 In addition, in some cases review conclusions were not supported by the data of the primary studies,22, 23, 24 or reviews with a low quality rating more often produced positive conclusions.21 Such critical appraisals of literature reviews have not often been done in public health. No previous study has examined these issues in relation to the physical environment and physical activity.

This study therefore investigated the methodologic aspects of review papers published in peer-reviewed journals between 2000 and 2005 that described the association between the physical environment and physical activity.

Section snippets

Methods

A literature search for systematic and narrative review papers published in English between 2000 and 2005 was conducted using the databases Medline, CINHAL, DARE/EBM, Psychlit, Pub Med, Avery, and Transportation, along with hand searching of reference lists of identified studies. Also, reference lists compiled by the Active Living Research group were analyzed. Reviews were included when authors investigated the relationship between any aspect of the physical environment (built and natural) and

Results

Eleven reviews published between 2001 and 2005 were identified.26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36 These reviews summarized the results of 105 different primary studies. All of the reviews were similar in terms of their stated purposes and in so far as they all summarized studies that used some measure of the physical environment as the predictor variable and some form of physical activity as the outcome measure.

Discussion

Literature reviews are increasingly recommended to policymakers and practitioners seeking to base their decisions on better evidence, and in particular on robust summaries of “the evidence.” We identified 128 primary studies examining the relationship between the physical outdoor environment and physical activity for adults published in English between 1980 and 2005, and a number of reviews that had attempted to synthesize this new body of research (the full list of studies can be found online

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    KG is the recipient of post-graduate scholarships from Sport Knowledge Australia and the Australian Housing and Urban Research Institute.

    MP received funding from the Chief Scientist Office of the Scottish Executive Department of Health.

    No financial conflict of interest was reported by the authors of this paper.

    The full text of this article is available via AJPM Online at www.ajpm-online.net; 1 unit of Category-1 CME credit is also available, with details on the website.

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