Assessment of two physical activity monitors in COPD patients

We can objectively asses physical activity in COPD patients using accelerometers but we do not known whether different devices give equivalent results. We studied the Actiwatch (AW) worn on the ankle and Sensewear (SW) worn on the R arm. 12 stable COPD patients (2 female, mean(sd) age 68(5)yrs, FEV1 1.0(0.4), 37(14%) predicted) wore both devices for 3-4 days. We applied a threshold (derived from 13 different COPD patients) to AW readings to exclude background vibration. There was no difference between the 29 weekday and 16 weekend recordings in any activity variable. There was no correlation of 6min walk, FEV1 or quadriceps strength and any measure of physical activity. SW step count correlated with AW mean activity score and AW % time moving and this relationship improved when threshold was applied. In general physical activity measured by AW was unrelated to overall energy expenditure derived from the SW. COPD patients have similar levels of activity throughout the week. Activity levels measured by the different devices worn on the lower and upper limb are closely related; this is improved when a threshold is applied to the AW. AW data does not correlate as well with SW derived data such as estimates of energy expenditure or METs; caution should be exercised when interpreting such data.