Figure 3 -.

Research directions to further unravel the biological regulation of physical activity level. The three major sources of individual differences in physical activity level are genetic variants, environmental influences, and their interplay. Strategies to understand the biology of physical activity regulation and the contribution of genetic and environmental factors are numbered 1 to 9. Studies of humans and of animal models play complementary roles. The ultimate goal is to identify safe and effective environmental and/or pharmacological interventions that can increase the level of physical activity.