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"Bounce at the bell" : the effects of a 7-month intervention of brief bouts of moderate intensity exercise on bone mass, bone structure and bone strength in children MacLean, Leslie Bryant


INTRODUCTION: Adult bone mass is a function of the amount of bone gained during the years of growth, and the amount of bone lost with advancing age. Therefore, childhood bone mineral accrual is critical for bone health. The current cost of osteoporosis in Canada is estimated to be $1.3 billion per year. There is recent evidence that load-bearing exercise elicits a positive osteogenic response and that childhood is an opportune time to deliver an exercise program to optimize this response. No studies have been conducted to determine whether frequent, brief bouts of jumping exercise have an osteogenic effect in children. OBJECTIVE: To examine the effects of a 7-month program of jumping on bone mass, strength and structure in early pubertal children. METHODS: There were 51 children in the exercise group (mean age 10.2 ± 0.4 years, mean height 139.8 ± 7.0 cm, and mean weight 35.7 ± 9.1 kg) and 75 children in the control group (mean age 10.2 ± 0.4 years, mean height 140.6 ± 6.1 cm, and mean weight 35.4 ± 8.0 kg). We administered physical activity (PA), calcium intake (CA) and maturity questionnaires at baseline and final. We also assessed bone mineral content (BMC) at the total proximal femur (PF) and its subregions - trochanteric (TR), intertrochanteric (IT) and femoral neck (FN), as well as at the lumbar spine (LS), and total body (TB) using dual-energy x-ray absorptiometry (DXA, Hologic, QDR 4500). Using hip structural analysis (HSA) at the PF, areal bone mineral density (aBMD), subperiosteal width (SPW), estimated cortical width (AVG CORTEX) and bone cross-sectional area (CSA) - which is equivalent to cortical area and endosteal diameter - were measured at the narrow neck (NN), intertrochanteric (IT), and femoral shaft (S) regions. Section modulus (Z, a representative of bone strength) was then estimated from these measurements. The 7-month exercise intervention consisted of brief bouts of 5-10 jumps (interspersed 3 times throughout the school day) 5 days/week. Analysis of covariance (covariates: baseline maturity, age height, change in height, and average physical activity score) was used to determine differences between intervention and control groups in all the previously mentioned bone parameters. RESULTS: There were no differences between groups for age, HT, WT, maturity level, Ca or PA at baseline or for change in these parameters across the trial period. BMC at the IT (3.5%, p=0.005) and PF (2.6%, p=0.012) increased significantly more in intervention children when compared with control children, but BMC was not significantly different between groups at other sites. There were no differences between groups for bone structure and bone strength, with the exception of NN SPW which changed significantly more in control (2.1%, p=0.014) compared with intervention children. CONCLUSIONS: A progressive program of 5 - 10 jumps, 3 times throughout the day elicits a positive osteogenic response at the IT region and the PF. Further study is required to investigate whether or not these changes are maintained 12 months after withdrawal of the exercise stimulus.

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