From accelerometer output to physical activity intensities in breast cancer patients

We aimed to investigate accelerometer output corresponding to physical activity intensity cut-points based on percentage of peak oxygen consumption (%VO₂peak) and Metabolic Equivalent of Task (MET) value in women treated for breast cancer.

Laboratory study.

Fifty female patients shortly after completion of treatment for breast cancer were included. VO₂peak was determined during a cardiopulmonary exercise test. Subsequently, patients performed ten activities with different intensities while wearing an accelerometer on the right hip and a mobile oxycon to assess oxygen consumption. We studied the relationship between energy expenditure (expressed as %VO₂peak and MET-value) and accelerometer output (in counts per minute (cpm)) with linear regression analyses. We determined accelerometer output corresponding to physical activity intensity cut-points (40% and 60%VO₂peak; 3 and 6 MET) using regression equations.

VO₂peak was 22.4mL/kg/min (SD 5.2) and resting metabolic rate was 3.1mL/kg/min (SD 0.6). Accelerometer output corresponding to the cut-points for moderate (40% VO₂peak) and vigorous intensity (60% VO₂peak) were 1123 and 1911, respectively. The analyses based on MET-values resulted in accelerometer output of 1189cpm for the moderate (3 MET) and 2768 cpm for the vigorous intensity cut-point (6 MET).

Accelerometer outputs for moderate and vigorous intensity physical activity were lower than commonly used cut-points (i.e. 1952 and 5724 cpm), irrespective of the method used to express energy expenditure (%VO₂peak versus MET-value). Thus, categorizing physical activity intensities based on general-population cut-points, may underestimate physical activity intensities for women treated for breast cancer.