Background
Physical inactivity remains a global health challenge. Half of all adults in most developed countries are not meeting the public health physical activity guidelines [
1]. Sufficient physical activity reduces the risk of premature death from all causes and several chronic diseases, such as cardiovascular disease, diabetes mellitus and some cancers [
2]. Numerous physical activity interventions have been investigated to encourage increased levels of physical activity in adults, with positive results unlikely to be sustained over the longer term or on a large scale [
3]. Brief physical activity interventions that are effective, low cost and easily implemented into real-world practice have a wide reach and are able to be sustained over a long time period are needed. ‘Measurement as intervention’ may be a suitable approach.
Measurement reactivity, and the mere-measurement effect, or the question-behaviour effect, have been reported in the physical activity literature [
4,
5]. Measurement reactivity is defined as being present where the act of measurement leads to changes in the people being measured [
4]. The mere-measurement effect, or the question-behaviour effect, refers to the change in behaviour under investigation following measurement of the behaviour and/or related cognitions [
5]. These mechanisms are recognised as potential challenges when calculating sample sizes and interpreting results of controlled physical activity intervention studies due to the likely small increase in physical activity levels in the control group, making it difficult to interpret measurement and intervention effects [
4,
5]. To determine how often this may occur in primary care, Waters et al. conducted a systematic review finding that approximately one third of physical activity intervention studies in primary care have reported improvements in self-reported physical activity among participants who were in the control group [
6].
The Solomon four-group design has been utilised in limited studies to separate intervention and measurement effects [
7]. In a large randomised controlled study in Dutch general practice (
n = 635), different frequencies of physical activity measurement were compared using a Solomon four-group design [
8]. Participants were randomised to three or one physical activity measurements over 6 months. More participants in the three physical activity measurement groups met the physical activity guidelines at the end of the intervention period as compared to the one physical activity measurement group when considering self-reported physical activity, but there was no difference found between groups for a subsample of participants using accelerometry. The authors concluded that the increased frequency of physical activity measurements affected the participants’ self-reported physical activity behaviour. These findings suggest that completing questionnaires about physical activity may not affect objectively assessed physical activity, with more evidence for measurement affecting self-reports of behaviour than objectively measured behaviour [
4], although there have been some reports of measurement reactivity using accelerometers [
9,
10] and pedometers [
11].
Waters et al. and Opdenacker et al. also found that control group changes in physical activity were more likely when follow-up assessments were carried out over a longer period of time [
6,
12]. Waters et al. found in their systematic review that follow-up assessments completed at 9 months as compared to 7 months were more likely to result in a clinically meaningful improvement in physical activity in control groups. Opdenacker et al. found that at 2 years, with a 12-month no intervention follow-up, there was no difference in aerobic fitness between the two intervention groups (structured vs lifestyle) and a control group, concluding this was consistent with the control group improvement in physical activity and this was possibly due to a measurement effect.
In primary care, systematic reviews and reviews of reviews have found that physical activity interventions have a small to moderate positive effect [
13‐
15], although the most effective physical activity elements, including intervention intensity and participant characteristics, in this setting are unclear [
13,
14]. Considering physical activity maintenance interventions, Murray suggested that primary care may not be the most appropriate setting for promoting maintenance in healthy populations, finding limited effectiveness in this population in primary care [
3]. It is important to note that the large majority of the studies included in these reviews used self-report measures. In view of the possible implications of measurement reactivity, or the mere-measurement effect, this may account for some of the small effects.
There is evidence of control group improvements in physical activity, to a similar level as intervention groups in some cases, particularly over the longer term [
6]. This may indicate that with minimal contact and resources, physical activity behaviour change may be achievable. However, synthesis of this evidence to date has focused on self-reported physical activity. The objective measurement of physical activity (e.g. accelerometers, pedometers) is considered a superior method of physical activity measurement compared with self-report measures, with lower levels of variability observed for validity and reliability, despite some pragmatic limitations [
16]. This has resulted in a significant increase in objective physical activity measures used within intervention studies in the last 10 years, increasing from 4% of lifestyle physical activity intervention studies in 2006 to 71% in 2016 [
17]. Currently, there appears to be no research investigating the observed changes in the control group physical activity in trials that have used objective physical activity measurement approaches. The aim of this systematic review is to estimate the effect of control group participation and measurement on objectively measured physical activity in adults. The systematic review will answer the following research questions:
1.
How much does objectively measured physical activity increase (or decrease) within control groups in primary care physical activity intervention trials?
2.
What factors are associated with control group changes in objectively measured physical activity, considering potential factors such as length of trial, frequency of measurement, intensity of physical activity measurements, and participant characteristics?
Discussion
To our knowledge, this systematic review will be the first to examine the efficacy of control group physical activity measurement on objectively measured physical activity levels in primary care. This review will systematically retrieve and examine controlled physical activity intervention studies in primary care, assess risk of bias and synthesise the data using meta-analyses. Findings from this study will inform future physical activity intervention research and practice. If physical activity measurement alone is found to improve objectively measured physical activity levels, it may be a low cost, efficient and effective method to increase a proportion of the populations’ physical activity levels, leading to improved health throughout adulthood and into older age.
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