Pedometers, which count the number of steps people take, are used both as a research method to monitor people's physical activity and a way of motivating people to take more exercise. In the course of their studies researchers have noted a phenomenon called reactivity in which people's activity increases in response to the fact that they are conscious of being monitored. This can be useful if the object of the exercise is to get people to become more active but not so good if the purpose is to measure people's activity as it distorts the results. Getting children to become more active is very important to prevent childhood obesity and researchers from Hong Kong University studied 156 children aged between nine and 12 to look at whether rehearsal - a tendency to constantly chew over upsetting experiences - is linked to increased pedometer reactivity. The researchers' theory was that the upsetting effect of being monitored would lead to a greater increase in activity in the children who were prone to more rehearsal as they would spend more time thinking about the implications of being monitored. In week one of the study this was, indeed, the case but by week three the children who 'rehearsed' more were showing a decrease in activity levels from week one and were taking far less exercise than average for children of their age group. The study showed both that the effects of reactivity were short-lived and that children who are sedentary may be more prone to dwell on things.
Ling, F. C., Masters, R. S. and McManus, A. M. , Rehearsal and pedometer reactivity in children. Journal of Clinical Psychology, n/a. doi: 10.1002/jclp.20745