Seven relevant RCTs were identified (n=6,894, range 78 to 2,840), one of which had a quasi-experimental design (n=78). One study was of low quality and the others were of medium quality. Two studies appeared to be in the same cohort of participants. A summary of the individual criteria used for validity assessment for each study was provided. Two studies used confirmatory factor analysis to confirm that all scales were unidimensional. Two other studies reported that all their scales had acceptable psychometric properties based on Cronbach’s alpha and test-retest reliability. A fifth study reported acceptable internal consistency. A sixth study provided a reference for a scale validation study but did not report the psychometric properties used in the study. Four studies were reported to have adequate power. The studies were in four countries: Belgium, Iran, USA and UK.
Self-efficacy was assessed in seven studies. The intervention had a significant effect on self-efficacy in both boys and girls in two studies, only in girls in one study of both boys and girls, and in three studies of girls, but no significant effect in the remaining study of girls. Self-efficacy modified the strength of the effect of the intervention on physical activity in four or five of the studies, but significance was not given.
Four or five studies (unclear) reported on outcome expectancy/perceived benefits. Interventions in three of the studies had a significant effect on this mediator, but only one study was considered to provide evidence of a mediated effect on physical activity (significance not given). All three studies that reported on perceived barriers found that interventions had a significant effect on this mediator, but not always in the expected direction and the authors considered that the studies did not provide evidence for a mediated effect of perceived barriers on physical activity.
Enjoyment as a mediator was investigated in two studies: one found no effect; the other found the intervention had a significant effect on enjoyment and enjoyment of physical activity had a mediated effect on physical activity (but not enjoyment of physical education).
Behavioural mediators (two studies):
The intervention in one study had a significant effect on goal setting, but the changes were not related to changes in physical activity. Another study found the intervention had a significant effect on commitment to planning, stimulus control and counter conditioning; only commitment to planning had a mediated effect on physical activity.
Interpersonal mediators (five studies):
Five studies evaluated interpersonal factors (physical activity group norm, exposure to models, social support, interpersonal norms, peer support and social support). Only one study found the intervention significantly affected peer support, which did not appear to have a mediated effect on physical activity.