To identify the minimum effective dose of a multi-behaviour change technique (BCT) intervention to increase physical activity among individuals on primary statin therapy using the time-to-event continual reassessment method (TiTE-CRM).

A large New York metropolitan area healthcare system comprising approximately 85 000 employees and 5.5 million patient encounters annually.

42 participants enrolled in 13 cohorts of 3 participants, 1 cohort of 2 participants and 1 cohort of 1 participant. The sample was composed of 16.7% individuals aged 66 and older (n=7), 64.3% women (n=27), 69.0% white individuals (n=29) and 7.1% Hispanic individuals (n=3).

A variable-duration, four-BCT text message intervention and a 2-week follow-up. Dose assignment relied on TiTE-CRM to adjust the duration of the intervention based on adherence of participants in prior cohorts. Five mechanisms of action (MoAs) were assessed: self-efficacy, intrinsic regulation, discrepancy in behaviour, motivation and barriers to activity.

The primary outcome measure was the proportion of participants who achieved a 2000 step/day increase between baseline and follow-up. The secondary outcomes were within-participant changes in daily steps (examined as a continuous variable at the daily level) and potential MoAs for increased physical activity.

Of the 40 participants who completed follow-up, 7 (17.5%) achieved the goal of 2000 or more steps per day during their follow-up period. Though participants did increase the number of steps they walked during the intervention (B(SE)=373.1 (154.7) steps; p=0.016), there was no association between increased intervention duration and increased daily average steps. The intervention was also associated with increases in self-efficacy (p=0.002), intrinsic regulation (p=0.037), discrepancy in behaviour (p

The results of this trial did not show a traditional dose-response curve to increasing the length of a multicomponent BCT intervention. Results did show that the intervention successfully increased steps during the intervention period and that the benefit of the intervention dwindled during follow-up. Further, potential MoAs for the intervention were confirmed.

NCT05273723.