It has been postulated that muscle and bone form an operational unit; that is, factors that affect muscle will also influence bone. It also is thought that muscle action is paramount in affecting bone adaptation; if so, then it would be expected that muscle development (or lean tissue as a surrogate of muscle) during the growing years should precede bone mineral accrual. We have shown previously that this is the case for total body mass; however, in theory this relationship should be even stronger at the extremities where muscle action is more isolated. The purpose of this study was to investigate the relationship of the timing of bone-free peak lean mass accrual (PLM) to peak bone mineral mass accrual (PBM) at the arms and legs. Subjects (70 boys and 67 girls) were measured annually (DXA: Hologic 2000 QDR in array mode). Whole year velocity values were calculated for bone-free lean tissue and bone mineral content. The mean age of peak tissue accrual was then calculated as the mean of the peak. Dependent t-tests were done to test for differences between the age of PLM and PBM (p<.05). As shown in the following table, PLM occurred prior to PBM at both sites in both boys and girls; however, this difference was not statistically significant at the legs in girls. These preliminary data support the hypotheses that PLM (a surrogate measure for muscle mass) precedes PBM and that muscle and bone development are closely related.