Figure 2 -.

Conceptual model for the main physiological systems involved in physical activity and its regulation. 1) The brain is the behavioral control center integrating pre-set information from the activity-stat (see text) with ongoing motivational state. 2) Duration and intensity of physical activity will depend on cardiorespiratory fitness, partly by viscerosomatic signals (e.g. becoming out of breath) that affect motivational state. 3) Muscle is the mechanism of action (effector) and performance capability of this unit, as well as the cardiorespiratory system, is necessary but not sufficient for physical activity. Effects of biological, including genetic/genomic and environmental factors, with many interactive effects, will determine individual differences in the functioning of these physiological systems and hence the level of physical of physical activity.