Objective: To describe and explore the effects of contextual and behaviour change technique (BCT) content of control and target interventions in clinical trials.

Design: Review and meta-analysis of 42 trials from a Cochrane review of physical activity in chronic musculoskeletal pain.

Main Outcome Measures: Two researchers coded descriptions of target and control interventions for (a) 93 BCTs and (b) whether target and control interventions shared each of five contextual features (practitioners’ characteristics, patient-practitioner relationship, intervention credibility, superficial treatment characteristics e.g. delivery modality, and environment). Quality of study reporting was assessed. Effect sizes for adherence to physical activity and class attendance were computed (Cohen’s d) and analysed separately.

Results: For physical activity outcomes, after controlling for reporting quality, larger effect sizes were associated with target and control interventions using different modalities (β = −.34, p = .030), target and control interventions involving equivalent patient-practitioner relationship (β = .40, p = .002), and target interventions having more unique BCTs (i.e. more BCTs not also in the control) (β = .008, p = .030). There were no significant effect moderators for class attendance outcomes.

Conclusion: Contents of control conditions can influence effect sizes and should be considered carefully in trial design and systematic reviews.