We have read with great interest the recent article by Walker et al1 and the accompanying editorial by Morgan2 on the measurement of physical activity in chronic obstructive pulmonary disease (COPD). Walker et al compared an activity monitor (DynaPort) with a leg-mounted uniaxial accelerometer (Actiwatch). They concluded that “lower limb activity is the major determinant of whole body activity”. In the accompanying editorial Morgan states that it therefore “does not seem necessary to use overly complicated devices” any more to measure physical activity. The latter statement is, in our opinion, an oversimplification.

First, the validity of the Actiwatch to assess leg activity in these patients seems insufficient. Walker et al reported an inverse and poor relation (r = −0.42) between “leg activity” from the Actiwatch and walking time from the DynaPort. Since the DynaPort showed excellent agreement with video recordings,3 this inverse relation suggests that the Actiwatch does not accurately measure patients’ walking at low walking speed.

Second, movement intensity and overall movement time (including minor movements such as fidgeting) were chosen as the main outcomes from the DynaPort to represent “whole body activity”. These outcomes do not reflect the full scope of information that activity monitors can provide. Activity monitors differentiate between postures (ie, standing, sitting and lying) and movements (ie, walking and cycling) and classify intensities of movements. By being able to measure these outcomes, activity monitors provide information that is easily interpretable both for healthcare providers and patients. In contrast, uniaxial accelerometers register “activity counts” as an abstract overall measure of daily activity that combines intensity and time spent in physical activity.4

In general, both accelerometers and activity monitors can provide useful information depending on their purpose of use. Validated accelerometers are useful as an overall measure of physical activity, discriminating physically active from physically inactive populations. Most accelerometers and pedometers seem, however, not to be sensitive enough to pick up changes in physical activity in slowly moving patients.5 6 Whenever one wishes to quantify daily time spent in different leg activities (ie, walking or cycling) and postures one will have to rely on activity monitors. Facilitating interpretability of results in this way is of special interest when one aims at increasing patients’ awareness of their activity levels.

In summary, the study by Walker et al does not provide enough evidence to allow the conclusion that the Actiwatch accurately measures “leg activity” in patients with COPD. This uniaxial leg accelerometer should therefore not be regarded as a surrogate measure for an activity monitor in this population. Efforts should be undertaken to make activity monitors as user-friendly as possible. These should lead to the next generation of physical activity monitors with larger memory and smaller size that are affordable for use in both research and clinical practice.