Graduate Thesis Or Dissertation
 

Kinetics and kinematics of prepubertal children participating in osteogenic physical activity

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  • Introduction: Recent reports in exercise related bone research have shown increased bone mineral content (BMC) at the femoral neck for prepubescent children participating in exercise programs consisting of repeated drop landings from a height of 61 cm. Increases in BMC from this type of exercise are believed to be the result of both high rate and magnitude of loading at the proximal femur. However, the dynamic characteristics associated with these landings in children have not been studied. Purpose: To describe the dynamic characteristics of children during landing and to quantify the forces associated with an activity associated with increases in bone mass. Methods: 13 prepubescent children (males=8, females=5, age 9.3 ± 0.7 years) who had previously completed drop landings over a 7 month period as part of an exercise intervention to increase bone mass participated in this research. Each subject performed 100 drop landings onto a force plate from a height of 61 cm. Ground reaction forces and two-dimensional kinematic data were recorded. Hip joint reaction forces were calculated using inverse dynamics based on a four segment rigid body model. Vertical ground reaction force and displacement data were fit to two single degree of freedom models, the Voigt and standard linear solid (SLS). The goodness of fit was quantified using the standard deviation of the error (SDE) between the experimental and the predicted data. Results: Peak vertical ground reaction forces were 8.5 ± 2.2 (mean ± SD) body weights (BW) while hip joint reactions were 6.0 ± 1.8 BW. Loading rates for ground reaction forces during initial impact were in excess of 470 BW/s. Across 100 jump trials, ground reaction forces changed significantly for 5 subjects (4 increase, 1 decrease, p<0.05) but were unchanged as a group. The SLS and Voigt models replicated the displacement traces well (SDE=0.003 m and 0.001 m respectively). However, in fitting force data, the SLS outperformed the Voigt model (SDE=580 N and 493 N respectively), but slightly under-predicted peak forces by 13%. Conclusion: Comparing force characteristics from drop landing to force characteristics known to be osteogenic, we can see how drop landings contribute to the osteogenic stimulus. The models used to represent children during drop landing closely fit displacement data, but did not replicate the time history of the impact force peaks thought to be important to osteogenesis. Quantification of exercises known to increase bone mass provides a basis on which to develop and implement additional exercise interventions for the purpose of increasing bone mass.
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