Several recent physical activity intervention trials have reported physical activity improvements in control group participants. Explanations have been proposed, but not systematically investigated.
A systematic review of physical activity intervention trials was conducted to investigate the frequency of meaningful improvements in physical activity among control group participants (increase of ≥ 60 minutes [4 MET·hours] of moderate-to-vigorous physical activity per week, or a 10% increase in the proportion of participants meeting physical activity recommendations), and possible explanatory factors. Explanatory factors include aspects of behavioral measurement, participant characteristics, and control group treatment.
Eight (28%) of 29 studies reviewed reported meaningful improvements in control group physical activity, most of which were of similar magnitude to improvements observed in the intervention group. A number of factors were related to meaningful control group improvements in physical activity, including the number of assessments, mode of measurement administration, screening to exclude active participants, and preexisting health status.
Control group improvement in physical activity intervention trials is not uncommon and may be associated with behavioral measurement and participant characteristics. Associations observed in this review should be evaluated empirically in future research. Such studies may inform minimal contact approaches to physical activity promotion.