A National Institutes of Health study followed girls for a decade beginning at age 8 or 9. Over time, leisure-time physical activity declined dramatically, and the drop off for African Americans girls was steepest. By age 17, more than half of black girls, and nearly a third of white girls reported no leisure time physical activity at all. What causes this decline? "They have found changes in enjoyment of activities, in peer support or social support for physical activity, they found a lot of competing interests, be it part-time jobs, or caring for younger siblings, or other family members," said Clare Lenhart, a public health researcher at Temple.