Bone loss from the lower extremities during bed rest is thought to be due to unloading, which stimulates remodelling activity and thus increases bone resorption, with the bone formation rate unchanged. One would expect, thus, that upon re-loading bone resorption is suppressed and that bone losses would stop. However, in the long term bed rest study (Watanabe, 2004), the distal tibia bone mineral content (BMC) was lower after 14 days recovery than at the end of 90 days bed rest. The Berlin BedRest (BBR) study has shown that resistive vibration exercise (RVE) is an effective countermeasure to preserve bone during 56 days of strict bed rest (Rittweger, 2004). Twenty young healthy males between 24 and 43 years were randomized into either the control (Ctrl) or the exercise group (RVE), the latter performing 4 sets of ~80 s RVE for 6 min twice a day (except on Sundays). BMC of the distal tibia epiphysis (4% of its length) was assessed by peripheral quantitative computed tomography (pQCT) with an XCT2000 (Stratec, Germany) on the 55th day of bed rest (BR55), and on days 4, 14, 28 and 90 of the recovery period (R+4, R+14 etc). Mean values (±SD) of the BMC changes from baseline are given in the table. A repeated measures ANOVA (Greenhouse-Geisser method) yielded a significant time effect (p = 0.01), but no time*group interaction (p = 0.16). Post-hoc testing with the paired-samples t-test (Bonferroni correction) showed no difference between changes on R+4, R+14, R+28 and R+90, but significant differences between the changes on BR55 and on R+4. To interprete these results, the time delay between osteoid lay-down and matrix mineralistation has to be considered. As the pQCT technique is x-ray based, it assesses only bone mineral, but not un-mineralized tissue. It probably takes 14 days for the first phase of mineralisaton (Martin, 1998). As a consequence, the ongoing loss of BMC after re-ambulation must be due to a still increased bone resorption. Thus, it seems as if active osteoclasts were more or less insensitive to increased loading. Possibly, their activity might be pre-programmed already when they are recruited.