Jump-In: Building Better Bones

The Jump-In study will prospectively assess the effects of impact exercise on skeletal development in young girls, including bone mass, bone mineral density, and bone geometry. We hypothesize that girls who regularly participate in impact loading exercise will accrue greater skeletal mass, increase bone density and undergo structural adaptations that in combination will improve bone strength compared to girls who do not participate in impact exercise.

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.

Eligibility Criteria:
Inclusion Criteria:

female; entering 4th or 6th grade at enrollment; and willing and able to undergo all of the testing requirements; no physical conditions that limit participation in physical activity/exercise; able to read and write English.

Exclusion Criteria:

Girls with severe learning disabilities (identified by schools), who are unable to complete questionnaires or otherwise comply with assessment protocols will be excluded. Any subjects (or potential subjects) suffering from medical conditions and/or disabilities that would limit their participation in exercise will be excluded from the study, following the American Academy of Pediatrics (AAP) guidelines. Further, any volunteers who are taking medications that would limit exercise and/or alter bone mineral accrual will also be excluded from the study. Girls with a positive pregnancy screening test at the time of the lab visit before DXA scanning will be excluded.