The effects of age, birth cohort and survey period on leisure-time physical activity by Australian adults: 1990-2005

The prevalence of obesity continues to rise with many factors contributing to energy imbalance. Leisure-time physical activity (LTPA) has been proposed as one solution to counteract increasing energy intakes. The present study determined whether age, birth cohort and period of survey had independent effects on time, volume and energy expended in LTPA by Australian adults from 1990 to 2005. Adults were categorised into twelve age groups (5-year intervals from 20-24 years to >75 years), four survey periods (1990, 1995, 2000 and 2005) and fifteen birth cohorts (5-year intervals from pre-1916 to 1985). Time spent in three categories of LTPA was determined and metabolic equivalent (MET) values of 3.3, 4.0 and 8.0 were assigned for walking, moderate and vigorous activities, respectively, to calculate daily volume (MET minutes). Energy expended in LTPA was calculated using estimated BMR (from self-reported weight and published formulae), multiplied by the MET value. Regression models were fitted to the data. Age and period had independent effects on duration, volume and energy expenditure of LTPA for both males (P<0.01) and females (P<0.01), while birth cohort had independent effects for males only such that all three LTPA factors declined with recency of birth cohort (P<0.01). This indicates that more recent birth cohorts of males may need to be targeted to increase LTPA, but as duration, volume and energy expended in leisure time have been declining since 1990, both the sexes may benefit from the promotion of increased LTPA.