OBJECTIVES: To assess adaptations of the lower leg muscle-bone unit in 8- to 12-year-old children following a randomized controlled jumping exercise intervention for 9 months. METHODS: Twelve boys and 10 girls (INT) performed a supervised jumping protocol during the first 10 min of their regularly scheduled physical education class twice a week, while 11 boys and 12 girls (CON) completed the regular curriculum. We assessed maximum voluntary ground reaction force during multiple one-legged hopping (Fm1LH), and tibial bone strength/geometry by peripheral quantitative computed tomography (pQCT) at the 4-, 14-, 38- and 66%-site pre, intermediate, and post intervention. RESULTS: Whether increases in Fm1LH (+2.1% points, p = 0.752), nor changes in bone strength/geometry (+1 to +3% points, 0.169 < p < 0.861), were significantly different for INT relative to CON. The relationship between Fm1LH and volumetric bone mineral content at the 14%-site (vBMC14%) was very strong for both groups, pre and post intervention (0.51 <= R2 <= 0.88). However, changes in Fm1LH and vBMC14% were not correlated. CONCLUSIONS: In children, growth and exercise did not increase maximum muscle force and bone strength in proportion to each other, meaning that the adaptive processes were not tightly coupled or follow different time courses.

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