HOW DO GROUND REACTION FORCES FROM DROP LANDINGS COMPARE TO THOSE FROM RUNNING?

Abstract

At the same time that we are developing practical exercise interventions for increasing and maintaining bone mass in growing children, we are also interested in the mechanisms of the osteogenic response. One important issue in this regard is knowing how the impact forces from our exercise intervention compare to those from the normal activities of daily living in this age group. PURPOSE: Our objective with these pilot experiments was to compare in a small cohort of children the ground reaction forces from our jumping protocol with those from running. METHODS: Sixteen children of ages 9.0 ± 1.2 years (6 boys, 10 girls) performed 15 drop landings from a height of 61 cm onto two force plates. They also ran 10 times across the force plates at an average speed of 4.0 ± 0.5 m/s (within 10% of the 50th percentile running speed for 9 year olds). Vertical Ground Reaction Forces (GRF's) were collected at a sampling rate of 1000 Hz. The loading rate was defined as the slope of a line from 20% to 80% of the rising side of each force peak. RESULTS: For Drop Landings, the maximum GRF and loading rate acting at each foot were 4.9 ± 0.8 BW and 368 ± 103 BW/s, respectively. For Running, the maximum GRF and loading rate were 3.0 ± 0.5 BW and 171 ± 73.6 BW/s, respectively. These represent a 63% increase in peak force (p < 0.001) and a 2.2-fold increase in loading rate (p < 0.001) when jumping is compared to running. CONCLUSIONS: We conclude that the impact loads from our jumping protocols differ significantly in both magnitude and rate when compared against running, a common activity of daily living for children in this age group. Given the magnitudes of the observed differences, it is attractive to speculate that it is the difference in loading rates, rather than magnitude, that is the major stimulus for osteogenesis in our exercise interventions. Supported by NIH AR 45655-01