1573 While exercise can easily be prescribed for improving cardiovascular and muscle health, there is currently no prescription for improving bone health. Turner (1998) identified strain rate as the key variable related to bone changes in animals. While strain rate cannot be ethically measured in humans, the external forces (GRF's) and rate of force application that cause bone deformation in humans can be measured. We have previously suggested that loading rate serve as an analog to strain rate in the search for variables related to bone change in humans. However, while we have previously reported forces and loading rates at the ground acting at each foot and hip joint reaction forces (HJRF's) based on combined two-footed landing forces (Bauer, 2001) we have never studied loading rates at the hip joint calculated from forces applied to each individual foot. PURPOSE: To answer: 1. What are the HJRF's and HJRF loading rates from drop landings? 2. How do the HJRF's compare to GRF's? 3. How do the HJRF loading rates compare to GRF loading rates? METHODS: Prepubertal children (n = 12; 7.8 ± 0.3 yrs;24.8 ± 3.6 kg; mean ± SD) performed 10 landings from 61cm onto 2 force plates. 3D kinematic data were combined with GRF data to calculate HJRF's. Drop landings have two characteristic force peaks from toe (F1) and heel (F2) contact. Four variables were quantified at F1 and F2: max GRF, max GRF loading rate, max HJRF, and max HJRF loading rate. In addition, HJRF variables were expressed as a percentage of the GRF variables to describe the attenuation of the GRF variables to the hip. RESULTS: HJRF's and loading rates at F1 were 1.8 ± 0.5 BW and 310 ± 70 BW/s, respectively and at F2 were 4.5 ± 0.8 BW and 457 ± 129 BW/s respectively. HJRF's were 70.0% ± 5.8% of GRF's at F1 and 87.6% ± 1.6% of GRF's at F2. HJRF loading rates were 100.3% ± 3.9% of GRF loading rates at F1 and 101.5% ± 3.5% of GRF loading rates at F2. CONCLUSIONS: Since loading rates at the hip were similar to those at the ground, these results provide evidence that simple GRF loading rates may be a good marker to more easily quantify the dose response of bone to mechanical loading. NIH AR 45655–01, BRL Clinical Program.