Activity monitor: how does it work?

The activity monitor is a high-tech body worn sensor. It tells us when you move and when you stop.

It is not designed to measure heart rate (changes in which can be accounted for by a variety of other events, such as fear) so it is not meant to measure ‘fitness’. But it will help researchers understand the amount of energy you use, allowing for important comparisons to be made between different sorts of people.

Don’t be fooled by the bland exterior. At the heart of each monitor sits a tiny machine called a Micro-electro-mechanical-system, no wider than a single human hair. Incredibly, it is able to measure changes in the rate of velocity in three directions – up and down, side to side and forwards and backwards (see below – click to enlarge).

The device is primed and ready to go when you receive it. So you will need to put it on as soon as you can – because it will turn itself off after seven days.

Every which way

The activity monitor measures three directions of movement, and each can be plotted on a chart, building up a picture of movement through-out the day.

If you have your hand by your side and you start to walk, your arm begins to swing, and we can start to monitor all three directions of that movement.

 Walking generates a characteristic humped curve. The more you move your arm the more you get these changes, producing a clear pattern of movement. Brushing teeth produces a very intensive, high frequency pattern as the arm moves backwards and forwards.

Graph showing physical activity

Activity record throughout the day

 

 

 

 

 

 

 

Graph showing rowing

Rowing - note the rhythm of the activity

Scientists will analyse the graphs created from the data and be able to calculate movement for a wide range of activities. However, they do not necessarily want to know what type of activity is being undertaken – they just need a sense of the level of movement.

Energy expenditure

Statistical techniques are used to relate movement to energy expenditure and energy expenditure to disease. For instance, at the most basic level, it will be possible to study people with chronic heart disease – or who go on to develop chronic heart disease – and see the likelihood of that being related to activity. 

These techniques have been validated in other studies, to show they are reliable and produce scientifically useful information.

However, with increasing computer power and more intensive modelling, it is easy to see how particular activities generate their own patterns or footprints. This may allow for more detailed analyses in the future.

Sophisticated programmes can distinguish between rowing and walking, or running and swimming, and would even pick out the high frequency bounces of a bike rattling along a road.  

The activity monitor cannot track location and it does not contain any information that could identify participants. A special code on each device is matched with participants’ ID numbers and the key to this code is known only to a small number of UK Biobank staff. Personal details and data about health and lifestyle are stored separately. Other security mechanisms are also used to protect privacy.

The activity monitor has been developed by the Digital Interaction Group at Culture Lab, Newcastle University, led by Professor Patrick Olivier.