Declines in aerobic fitness: are they only due to increasing fatness?

Med Sport Sci. 2007;50:226-240. doi: 10.1159/000101394.


Aims: This study quantifies the cross-sectional and longitudinal relationships between young people's fatness (BMI, skinfold thickness) and fitness (performance on tests of aerobic fitness).

Background: Over the last 20-30 years, young people have become fatter and less fit. It is likely that the decline in fitness is largely due to increases in fatness. There are strong mechanistic connections; within cohorts, variability in fatness accounts for about 20% of variability in running performance; there is a strong correlation between overweight prevalence and relative fitness across specific cohorts from different countries; and secular declines in fitness coincide temporally with increases in BMI.

Methods: Australians aged 10-12 years tested in 1985 were matched for age, sex, BMI and triceps skinfold thickness with their counterparts tested in 1997 (n = 279 matched pairs), and 12-15 year-old tested in 1995-1996 were matched with their counterparts tested in 1999-2000 (n = 2,834 matched pairs). Performance differences on running tests in the matched datasets were compared with performance differences in the complete (unmatched) datasets.

Results: Performance differences persisted even when young people were matched for fatness. Matching for fatness reduced overall performance differentials by 29-61%. Other factors such as reduced physical activity and subsequent training effect are likely to have contributed to the decline.

MeSH terms

  • Australia / epidemiology
  • Body Mass Index
  • Child
  • Cross-Sectional Studies
  • Databases as Topic
  • Exercise*
  • Female
  • Humans
  • Male
  • Motor Activity
  • Obesity / epidemiology
  • Obesity / physiopathology*
  • Overweight
  • Physical Fitness*