Interruption of disuse by short duration walking exercise does not prevent bone loss in the sheep calcaneus

The effect on calcaneal bone loss of interrupting immobilization by daily periods of normal walking was investigated in adult female sheep. The left calcaneus of 28 sheep was protected from normal loading by placing an external fixator across the hock joint, from the tibia to the metatarsus. In vivo strain gauge recordings from 3 animals showed that, at a test location on the calcaneus, this resulted in a significant reduction in principal strain magnitude during walking at 1.5 m s-1 from a peak of -228 x 10(-6) before application of the fixator, to 71 x 10(-6) with the fixator in place. In addition to reducing peak strains during normal activity, the fixator also abolished the high magnitude transient strains (peak -1147 x 10(-6)) normally experienced when the sheep made sudden movements. Thirteen animals (group 1) had fixators applied, and were given no further treatment. In a further 6 animals (group 2), the fixator was removed for 20 min per day, during which they walked on a motorized treadmill at 1.5 m s-1. In the remaining 6 sheep (group 3), incomplete fixator bars were applied to allow uninterrupted joint movement. Over the 12-week period of the experiment, dual photon absorptiometric scans showed that the bone mineral content (BMC) of the calcanei of group 1 fell by 22%. In group 2, where immobilization was interrupted by daily walking, the BMC fell by 21%. In group 3, with incomplete bars, there was no significant reduction in BMC.(ABSTRACT TRUNCATED AT 250 WORDS)