Ground Reaction Forces and Kids

[Bill Hartman]

Jumping improves hip and lumbar spine bone mass in prepubescent children: a randomized controlled trial.

Fuchs RK, Bauer JJ, Snow CM.

Bone Research Laboratory, Oregon State University, Corvallis 97331-3303, USA.

Physical activity during childhood is advocated as one strategy for enhancing peak bone mass (bone mineral content [BMC]) as a means to reduce osteoporosis-related fractures. Thus, we investigated the effects of high-intensity jumping on hip and lumbar spine bone mass in children. Eighty-nine prepubescent children between the ages of 5.9 and 9.8 years were randomized into a jumping (n = 25 boys and n = 20 girls) or control group (n = 26 boys and n = 18 girls). Both groups participated in the 7-month exercise intervention during the school day three times per week. The jumping group performed 100, two-footed jumps off 61-cm boxes each session, while the control group performed nonimpact stretching exercises. BMC (g), bone area (BA; cm2), and bone mineral density (BMD; g/cm2) of the left proximal femoral neck and lumbar spine (L1-L4) were assessed by dual-energy X-ray absorptiometry (DXA; Hologic QDR/4500-A). Peak ground reaction forces were calculated across 100, two-footed jumps from a 61-cm box. In addition, anthropometric characteristics (height, weight, and body fat), physical activity, and dietary calcium intake were assessed. At baseline there were no differences between groups for anthropometric characteristics, dietary calcium intake, or bone variables. After 7 months, jumpers and controls had similar increases in height, weight, and body fat. Using repeated measures analysis of covariance (ANCOVA; covariates, initial age and bone values, and changes in height and weight) for BMC, the primary outcome variable, jumpers had significantly greater 7-month changes at the femoral neck and lumbar spine than controls (4.5% and 3.1%, respectively). In repeated measures ANCOVA of secondary outcomes (BMD and BA), BMD at the lumbar spine was significantly greater in jumpers than in controls (2.0%) and approached statistical significance at the femoral neck (1.4%; p = 0.085). For BA, jumpers had significantly greater increases at the femoral neck area than controls (2.9%) but were not different at the spine. Our data indicate that jumping at ground reaction forces of eight times body weight is a safe, effective, and simple method of improving bone mass at the hip and spine in children. This program could be easily incorporated into physical education classes.

Bill Hartman, President, Brian Grasso Fan Club

[hard_rox]

Our data indicate that jumping at ground reaction forces of eight times body weight is a safe, effective, and simple method of improving bone mass

bill, excuse my ignorance, but what exactly does this mean?

[Bill Hartman]

Ground reaction force is the force produced by the ground upward against your feet when standing. Otherwise you'd fall through!

So in this study they had kids jump downward from a 61 cm box. The force at ground contact was 8 times their body weight. 90 pound kid = 720 pounds of force. The cool thing about the study is that is shows the force absorption capabilities of the study group and that this form of training under appropriate supervision is beneficial. While there is no form of training that is 100% safe, the fear of serious injury (the ol' growth plate concerns) is most likely exaggerated. Increased bone mineral content is especially an issue for girls as the density of their bones gained at a younger age carries over quite well to adulthood.

Bill