How many days of continuous physical activity monitoring reliably represent time in different intensities in cancer survivors

**Background:** Physical activity (PA) monitoring is increasingly used in cancer research, but no consensus exists on how many days of monitoring are needed for reliable estimates in cancer survivors. Extended monitoring periods may increase participant burden and reduce adherence. This study aimed to determine the minimum number of continuous monitoring days required for reliable estimates of time spent at different PA intensities in cancer survivors, and to assess whether reliability differs by sex, age, cancer type, weight status, and education level.

**Methods:** Baseline data were harmonized from two studies (Phys-Can and CRC-NORDIET) involving 984 cancer survivors diagnosed with breast, colorectal, or prostate cancer (stages I–III). PA was monitored continuously for seven days using the SenseWear Armband Mini (SWAM). A valid day required ≥22 hours of wear-time. The first monitoring day was excluded due to inadequate wear-time, leaving six days as the criterion. Participants with ≥22 hours wear-time for all six days (n=736; 74.8%) were included in the reliability analysis. Mean age was 62.6 years (SD ±10.5), mean BMI 26.4 (SD ±4.6). Light PA (LPA) was defined as 1.5–3 METs, moderate PA (MPA) as 3–6 METs, vigorous PA (VPA) as >6 METs, and moderate-to-vigorous PA (MVPA) as ≥3 METs. The intra-class correlation coefficient (ICC) for single measures and the Spearman-Brown prophecy formula were used to calculate reliability for increasing numbers of monitoring days. An ICC >0.80 was considered sufficient for reliable estimates. Analyses were stratified by sex, age (<60 vs ≥60 years), cancer type, BMI (<25, ≥25–<30, ≥30), and education level (low-medium vs high).

**Key Results:** The single-measures ICC [95% CI] was 0.690 [0.660, 0.716] for LPA, 0.606 [0.576, 0.636] for MPA, 0.378 [0.345, 0.412] for VPA, and 0.610 [0.580, 0.639] for MVPA. Using the Spearman-Brown formula, an ICC >0.80 was achieved with two monitoring days for LPA and three monitoring days for MPA and MVPA. No number of days within the six-day timeframe reached an ICC >0.80 for VPA due to large intra-individual variance. In stratified analyses: for LPA, participants with BMI ≥25, low-medium education, colorectal cancer, or age ≥60 years required three monitoring days (one additional day). For MPA and MVPA, participants with BMI ≥25 or breast cancer required four monitoring days. For VPA, reliable estimates were obtained with six monitoring days in females, breast cancer survivors, and participants <60 years, and with four days in participants with BMI ≥30. Excluded participants (n=248) were significantly younger (59.8 vs 62.6 years, p<0.01) and more likely to have breast cancer (p=0.009).

**Clinical Implications:** Shorter monitoring periods (2–3 days) may provide reliable estimates of LPA, MPA, and MVPA in cancer survivors when using continuous wear protocols, potentially lowering participant burden and reducing exclusion due to non-adherence. However, researchers should account for participant characteristics (BMI, cancer type, age, education, sex) that may increase intra-individual variability and require additional monitoring days. Vigorous PA remains difficult to capture reliably and may require longer monitoring periods. The findings support using flexible monitoring protocols tailored to the research question and population, rather than a one-size-fits-all approach. The study also highlights that continuous monitoring yields much higher absolute PA estimates than standard recommendations (e.g., ~1000 min/week of MVPA), suggesting that current PA guidelines may need adjustment when applied to continuously monitored data.