ABSTRACT BACKGROUND There is a wide variability in measurement methodology of physical activity (PA). This study investigated the effect of different analysis techniques on the statistical power of PA outcomes after pulmonary rehabilitation. METHODS PA was measured in 57 patients with COPD (mean±SD age, 66±7 years; FEV1, 46±17 % predicted) before and after 3 months of pulmonary rehabilitation. The choice of the outcome [steps per day (STEPS), time spent in moderate PA (TMA), mean METs level (METS) and activity time (ACT)], impact of weekends, number of days of assessment, post processing techniques and influence of daylight time (DT) on the sample size to achieve a power of 0.8 were investigated. RESULTS The number of steps and time in activity (1.6-2.3 METs) were the more sensitive outcomes. Excluding weekends decreased the sample size for STEPS (83 vs. 56), TMA (160 vs. 148) and METS (251 vs. 207). Using 4 weekdays (STEPS and TMA) or 5 weekdays (METS) rendered the lowest sample size. Excluding days with less than 8 hours wearing time reduced the SS for STEPS (56 vs. 51). Differences in DT were an important confounder. CONCLUSIONS Changes in PA following pulmonary rehabilitation are best measured for 4 weekdays, including only days with at least 8 hours of wearing time (during waking hours) and considering the difference in daylight time as a covariate into the analysis. CLINICAL TRIAL REGISTRATION www.clinicaltrials.gov; NCT00948623.