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Mediators and moderators of physical activity change and interventions
This is an excerpt from Physical Activity Epidemiology-3rd Edition by Rodney K. Dishman,Gregory W. Heath,Michael D. Schmidt & I-Min Lee.
One explanation for the limited success of past physical activity interventions is that even though they may have been guided by theory, most of them did not explicitly target or change the person-level factors (e.g., beliefs, motives, and behaviors) that are presumptive mediators (i.e., causal explanations) of physical activity change (e.g., people’s motives and beliefs) theorized to be causes of physical activity or that modify an intervention effect (Baranowski, Anderson, and Carmack 1998; Dishman 1991; Lewis et al. 2002; Luban et al. 2008; Rhodes and Pfaeffli 2010). That is to say, the studies didn’t identify or confirm mediators of change.
Mediators of Physical Activity Change
Mediators are variables in a causal sequence that transmit the relation or effect of an independent variable on a dependent variable (MacKinnon, Fairchild, and Fritz 2007). Analyses shown in figures 17.16 and 17.17 indicate that increases in self-efficacy (Dishman et al. 2004) and enjoyment (Dishman et al. 2005) partly mediated the effectiveness of a school-based intervention designed to increase girls’ leisure-time physical activity during grades 8 and 9, a period when physical activity begins to decline among adolescent girls.
The prevailing theoretical approach taken by researchers to understand physical activity behavior has been social cognitive theory, which focuses on a person’s self-efficacy (i.e., personal confidence or agency for being active) as it interacts within social experience and structures. Other emphases from about 2005 to 2020 have been on understanding people’s needs for social affiliation, feelings of mastery, and autonomy of personal choice and self-regulation of behavior, as well as deriving enjoyment and pleasure from physical activity. Additionally, the broader socioecological model encompasses the interactions between multiple levels of influence on physical activity (personal beliefs and motives; social relationships and structures; and professional, educational, and governmental organizations that shape policies and laws about health, education, and the environment) (Rhodes, McEwan, and Rebar 2019).
However, the superiority of these different theoretical approaches in changing people’s physical activity has not been established either by prospective cohort studies or by experimental evidence. A meta-analysis of 82 randomized controlled trials of theory-based interventions to promote physical activity among adults found that 31 trials were based on the TTM, 16 were based on social cognitive theory, 8 were based on the theory of planned behavior, and 5 were based on self-determination theory (Gourlan et al. 2016). Another 14 trials were based on 2 theories, with 7 combining 3 to 5 theories. The average impact of the 82 trials was a small (0.31 SD; 95% CI: 0.24-0.37) increase in mostly self-reported physical activity compared with control groups, with no clear advantage in impact for any single theory. However, trials did not directly compare theories as competing arms within the same intervention. Regardless, all single-theory interventions collectively outperformed those based on a combination of multiple theories, which had smaller effects (0.21 SD; 95% CI: 0.11-0.32).
Physical activity interventions based on these theories (see Gourlan et al. 2016) commonly report an overall change in key variables within that theory but do not confirm by empirical analysis that each person’s change in physical activity depended on the person’s change in the variable proposed by the theory to be a causal influence on physical activity. This is to say, that average scores on both physical activity and theoretical cause can each change at the intervention group level without there being a correlation between the intervention and how much the scores change for each person. Also, a systematic review found that none of 23 observational studies that associated social cognitive variables with physical activity among adolescents used an objective measure of physical activity and observed change over more than two years (Plotnikoff et al. 2013).
A systematic review of 22 trials of varying quality designed to change physical activity and presumed mediators of that change in healthy adults found that half the studies did not increase physical activity (Rhodes and Pfaefflie 2010). The other half of the studies reported that the intervention changed in the proposed mediators, but actual tests of that mediated effect were performed in only 6 of those 11 cases, with mixed positive or null findings. Among these few studies, results showing mediated effects on physical activity behavior change were most favorable for the use of self-regulation and limited for the usefulness of changing self-efficacy or expectations of positive outcomes of physical activity.
Moderators of Physical Activity Change
Moderators (i.e., effect modifiers) are variables that are not in a causal sequence but that alter the relation or effect between an independent variable and a dependent variable (MacKinnon, Fairchild, and Fritz 2007). Those constructs help explain how people set and strive to attain specific behavioral goals. Little is known about the role of personality (Wilson and Dishman 2015) or of general motivational traits that may act as moderators of the influence of social cognitive variables on exercise adherence. For example, concepts related to willpower or self-motivation are acknowledged in social cognitive theory as possible influences on persistence during behavior change (Ajzen 2002; Bandura 1997a), but they have been understudied as moderators of exercise adherence (Dishman 2010). In one example, a three-year longitudinal study of South Carolina girls during high school, self-efficacy was stable and moderated the relation between changes in physical activity and perceived social support. Figure 17.18 shows that girls who maintained a perception of strong social support had less of a decline in physical activity if they also had high self-efficacy. However, girls having high self-efficacy had a greater decline in physical activity if they perceived declines in social support (Dishman, Saunders et al. 2009).
Of equal importance, it is not known how presumed mediators of physical activity change may interact to moderate each other’s influence on change in physical activity. Observational findings from the TRACK cohort indicated that physical activity declined less between grades 5 and 7 in children who reported the least decreases in self-efficacy for overcoming barriers to activity and perceived parental support (see figure 17.19). Physical activity also declined less in those who persistently felt they had more parental and friend support for activity compared to those who reported the largest decrease in support from friends (Dishman et al. 2017). After further adjustment for race, the decline in physical activity was less in those who had the largest decrease in perceived barriers and maintained a favorable perception of their neighborhood environment. Changes in enjoyment and social motives were unrelated to change in physical activity (Dishman et al. 2017). However, among boys and girls from the TRACK cohort who were followed from grade 5 through grades 6, 7, 9, and 11, physical activity measured by an accelerometer declined most in students who (1) had bigger declines in self-efficacy and also maintained higher perceptions of barriers to physical activity; (2) had bigger declines in enjoyment and fitness goals (see figure 17.20); and (3) had smaller declines in appearance and social goals (Dishman et al. 2019).
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