Physical activity can be added to two factors i.e., biological and psychological, even though we cannot confidently say that which factor actually cause physical activity to vary. Age is a potent, predictor, and the level of physical activity is known to decrease throughout the entire age span, beginning at least with entry into school.
During the school years, the activity level declines about, 50% (Sallis, 1993), and the decline continues until the typical elderly person is almost entirely sedentary (USDHHS, 1996). Females are less physically active than males virtually all ages but many people feel this difference is due to socialization rather than biology.
To understand the role of biological factors in physical activity we shall have to take some examples. A biological reason why adults drop out of vigorous exercise programs is musculoskeletal injuries. In one study, injuries were the most common reason for dropping out (Sallis et at., 1990), and the history of injuries is a good predictor of future injuries. Though most people believe the obese are less active than the normal weight, it is difficult to find evidence to support this notion in either adults or youth. However, recent studies indicate that substantial portion of physical activity may be explained by genetic factors.