How growing children's bone mass, muscle mass, and muscle force respond to exercise and nutrition interventions is not yet clear. We investigated the effects of seven months of jumping intervention with (Group EF) or without (Group E) a daily 450 mg calcium snack on calf muscle cross-sectional area, peak muscle force and cortical bone and in 57 girls attending public elementary schools(Group C, controls). A supervised progressive program beginning at 150 jumps/wk on two legs and progressing to 450 one or two-legged jumps/week (ground reaction force at impact / takeoff ranged from 2.5 to 5 times body mass per jump) was administered to 4th and 5th grade girls (age, 9.7 ± 0.6 years; height, 140.8 ± 8.4 cm; weight, 38.36 ± 9.83 kg; pubertal development, 65% Tanner stage I, 35% Tanner stage II). Peripheral quantitative computed tomography (Norland XCT2000, 12 detectors, software v5.40) was used to image a transverse section of the right leg 66% of the distance from the medial malleolus to the medial condyle of the tibia. Scans were analyzed for muscle area, and cortical bone (tibia + fibula) tissue areas. Peak jumping force and power were assessed using a Stratec force plate during five successive maximal-effort jumps off the right leg. Mean values are shown in the table (* indicates significant difference, pre vs. post). Intervention significantly increased peak muscle power in E anf EF, but not in C. Cortical bone and muscle areas rose (6–9%) significantly in E and EF, but not in C (2–3%). We conclude that jumping intervention increased calf muscle cross sectional area and, through an increase in peak power, increased regional (leg cortical) bone mass, regardless of calcium intake.Table