Modelling physical activity: a multi-state life-table approach

To develop a consistent set of epidemiological estimates (incidence, prevalence, remission, mortality) for physical activity in New Zealand; project these estimates in the light of demographic trends; and predict the effectiveness of different health promotion strategies.

Multi-state life tables were constructed using physical inactivity prevalence data from the 1996/97 New Zealand Health Survey, and estimates of the relative risk of mortality, and of remission rates, from the literature. Statistics New Zealand population projections were used to forecast these multi-state life tables to 2021. Two physical activity health promotion strategies -uptake (remission enhancement) and maintenance (incidence or relapse reduction)--were simulated by changing the relevant epidemiological variables.

The current fatal burden of physical inactivity in New Zealand is estimated to be 2,600 deaths per year (9% of all deaths). By 2021, the prevalence of physical inactivity will rise 4% as a result of demographic trends. Relapse reduction (enabling active people to remain active) is about 50% more effective than uptake enhancement (enabling inactive people to become active) as a physical activity health promotion strategy, but the two approaches are additive. Maximum realistic changes in relapse prevention and uptake enhancement could reduce the prevalence of physical inactivity by about 30%.

Multi-state life table methods can be used to model health risks (such as behaviours), as well as (chronic) diseases. The model has provided valuable insights for policy makers into the burden of physical inactivity in New Zealand, the impact of demographic trends, and the relative effectiveness of different health promotion strategies.