Jump-In is a school-based, block-randomized, trial of the effects of impact-loading exercise on skeletal development in prepubescent and early pubescent girls. Fourth (n=~200) and sixth (n=~200) grade girls in 14 Tucson (Arizona) schools with no contraindication to physical exercise will be enrolled. Girls in intervention schools will participate in impact activities 3 times per week at school, progressively increasing the number (up to 40) of jumps and their height (from 6 inches to 18 inches per repetition) over the initial 2 months. Thereafter, new activities will be introduced approximately every 2-3 months to maintain interest and motivation, and continually stress the skeleton in novel ways, over 2 years of intervention. Physical and behavioral assessments will be done in both intervention and control groups at baseline, end of the initial school year, and yearly thereafter for 5 years. Assessments include height and weight, selected skeletal lengths, fat, lean soft tissue, bone mineral content and areal density from dual energy x-ray absorptiometry (DXA), and bone geometry (e.g., cortical thickness, periosteal circumference and cortical and trabecular density) and muscle area from peripheral quantitative computed tomography (pQCT). Also, maturation will be assessed via the Tanner Stage (self-assessment against standardized drawings of stages of physical development), diet from the Harvard Youth Food Frequency Questionnaire, and physical activity using pedometers and standardized questionnaires. We hypothesize that girls in intervention schools will demonstrate enhanced skeletal development, including greater mineral mass and density, and enhanced bone geometry, leading to greater bone strength compared to girls in control schools. We further hypothesize that younger (Tanner Stage 1) girls will experience greater adaptations than older girls (Tanner 2 and 3), and that positive adaptations will be maintained throughout the 3-year follow-up period.