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 × 10−6 before application of the fixator, to 71 × 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 × 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. There was no significant difference between groups 1 and 2, although in both, the loss of bone associated with the fixator was highly significant (p< 0.0001). Measurements of the calcanei post-mortem revealed that the patterns of bone loss from the cortex and the trabeculae of the calcanei in groups 1 and 2 were not significantly different. The lack of difference between these two groups suggests that the magnitude and distribution of the strains engendered by the short period of walking exercise did not provide a sufficiently strong stimulus to modulate the loss resulting from the reduced strains for the remaining time. Since the exercise-induced strains were similar in magnitude and rate as those experienced during walking in control animals, this suggests that either the character or the duration of these strains are unimportant in maintaining bone mass. In other experiments we have shown that similarly short periods of more unusual distributions of applied strain have not only prevented bone loss, but also caused significant increases in bone formation. The data presented here suggest either that the strains involved in normal controlled locomotion have little affect on limb bone architecture, or that they need longer periods of time than unusual strains to have their effects.