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 …