A device for monitoring the activity of a user to prevent deep vein thrombosis when travelling on transportation vehicles. The device comprises a carrier (20) for positioning on or adjacent a user, a motion sensor (10) mounted on the carrier (20) and adapted to detect the user performing a predefined motion, processor adapted to filter the motion detected to remove background motion not attributable to the desired exercise and to reset a timer (12) when the predefined motion is detected. An alarm (14) is operated by the processor should the time period elapse without the exercise pattern being detected. The components are all contained in the carrier (20) which is preferably a small container that can be attached to a user's trousers or around the limb of a wearer. Failure to undertake the required motion will cause the alarm (14) to be activated, thus notifying the wearer of the omission.