We would like to thank Dr Langer and colleagues for the interest shown in our paper.1 The authors appear to have interpreted our article and the accompanying editorial as suggesting that accelerometers measure exactly the same outcomes as activity monitors. This is not the case, as evidenced by the fact that we used an activity monitor (Dynaport) to validate the recordings made with an accelerometer (Actiwatch). However, the data we presented show a close correlation between measurements of overall activity recorded by the two devices (overall …