Depending on the size of the reactivity effect, it could affect the validity of conclusions drawn from studies of physical activity. In descriptive epidemiological studies, the mean physical activity level would be over-estimated. If there are systematic individual differences in reactivity (i.e. if some kinds of people react more to measurement than others), this could affect the observed associations between the measure of physical activity and measures of determinants or consequences. It is therefore important to try to estimate the size of the reactivity effect to enhance interpretation of study findings. Finally, a reactivity effect might inform the development of interventions to promote physical activity.

The two versions of the Fenland Study Participant Information Sheet will be very similar to the existing one but with changes to the bullet point currently called “Measure your fitness and physical activity level”. The guiding principle is that, although different participants will be given different information (which is essential to the study design), all information provided to participants is true. In Version A of the Fenland Study Information Sheet, the heading will be changed to “Measure your fitness and heart rate” and the monitor will be referred to as a “heart rate monitor”. In version B, the bullet heading will be left unchanged, the monitor will be described as a “combined heart rate and movement monitor” and the following sentence will be added: “By analysing the information stored in the monitor, the research team will be able to tell exactly how active you have been during these six days”. Corresponding changes will be made to the visit instruction sheet and the Actiheart instruction sheet that are sent to Fenland participants before their baseline visit.

Two weeks before the Fenland baseline measurement visit, participants will be sent an Actiheart and an instruction sheet explaining the purpose of the device and how to wear it. After they have worn the Actiheart for six days, they will be asked to post it back to the measurement centre together with a brief diary sheet noting when they started and finished wearing the monitor and to do this before they attend the Fenland baseline measurement visit. At the measurement visit, participants will be asked to complete a short Actiheart Evaluation Sheet to find out what they thought the device was measuring, whether they read the instruction sheets, whether they thought that they had increased or decreased their physical activity while they were wearing the monitor, and measures designed to assess possible mediators of any reactivity effect. The measurement staff will then fully debrief the participant, explaining the Fenland study and the reactivity component, and ask for written consent for the FMR research team to use the Actiheart data, Actiheart Evaluation Sheet data and relevant baseline characteristics (gender, age, social class, practice, measurement centre).