Adherence to a lower versus higher intensity physical activity intervention in the Breast Cancer & Physical Activity Level (BC-PAL) Trial

Purpose: The first aim is to examine adherence to a lower versus higher intensity physical activity (PA) prescription in breast cancer survivors in the Breast Cancer & Physical Activity Level (BC-PAL) Trial. The second aim is to assess associations between baseline characteristics with mean PA adherence in both intervention groups combined.

Methods: Forty-five participants were randomized to a 12-week, home-based lower (300 min/week, 40-59% heart rate reserve (HRR)) or higher (150 min/week, 60-80% HRR) intensity PA intervention, or no intervention/control. Both intervention groups received Polar A360® trackers and were included in this analysis (n=30). Study outcomes assessed on a weekly basis with the Polar A360® activity tracker throughout the intervention included relative adherence to the prescribed PA interventions (% of PA prescription goal met), and the absolute amount of PA time ≥40% of HRR. Baseline predictors of adherence included demographic characteristics, cardiorespiratory fitness, habitual PA and sedentary time, quality of life measures, and motivational variables from the Theory of Planned Behavior. For our primary aim, a linear mixed model was used to assess the effects of randomization group, time (intervention weeks 1-12), and the interaction of these factors on the natural logarithm of PA adherence. For our secondary aim, the association between each baseline predictor with the natural logarithm of mean weekly PA adherence was assessed, with randomization group added as a covariate.

Results: Higher relative time within the prescribed HRR zone was noted in the lower versus higher intensity PA groups (e^β=3.12, 95% CI=1.97, 4.95). No differences in adherence across time were noted. Social support was inversely associated with relative PA time within the prescribed HRR zone (e^β=0.83, 95% CI=0.72, 0.97) and absolute PA time ≥40% of HRR (e^β= 0.82, 95% CI: 0.71, 0.93). Baseline VO_2max was inversely associated with relative PA adherence (e^β=0.98, 95% CI=0.95, 0.99). No other baseline measures were associated with PA adherence.

Conclusions: There were no significant changes in absolute PA time ≥40% of HRR across time or between groups. However, the lower intensity PA group averaged over 3 times the relative amount of PA within the prescribed HRR zone compared to the higher intensity PA group. Finally, lower peer support and cardiorespiratory fitness at baseline were associated with higher PA adherence.

Implications for cancer survivors: The recent rise in popularity of commercially available activity trackers provides new opportunities to promote PA participation remotely, and these devices can be used to continuously and objectively measure PA levels as an indicator of intervention adherence. Future studies are needed to explore baseline predictors, facilitators, and barriers to sustained activity tracker use to promote PA behavior change and intervention adherence in cancer survivors.

Trial registration: This study was registered at www.

Clinicaltrials: gov (No. NCT03564899) on June 21, 2018.