Electrical stimulation of leg muscles increases tibial trabecular bone formation in unloaded rats

Rat head-down hindlimb suspension (HS) has been shown to induce hindlimb cancellous bone loss. As HS is known to associate unloading with progressive disappearance of hindlimb muscle contractions, we investigated whether persisting muscle motion could modify suspension-induced bone disorders or even prevent them. Chronic electrical stimulation (ES) was applied to leg muscles of rats during 3-wk hindlimb suspension, the lack of support for hindlimbs maintaining a hypodynamic situation. The histomorphometric characteristics of the proximal tibial metaphysis were analyzed. At the end of this protocol of combined suspension and stimulation, trabecular bone loss remained similar to that of nonstimulated HS animals. However, trabecular bone cell activity parameters showed greater bone formation after muscle stimulation in unloaded animals, with significantly increased osteoblastic, osteoid, and mineralizing surfaces. In addition, periosteal mineral apposition rate and cancellous bone formation rate, markedly decreased by suspension, were not significantly different in suspended stimulated compared with normal loaded animals. This enhanced formation activity could be related to persistence of muscle activity, as shown by partial preservation of muscle mass. However, direct electrical effects on bone cannot be excluded. Thus, despite muscle stimulation, with enhanced bone formation, isolated suppression of hypokinesia has not been able to counteract bone effects of unloading. This finding supports the hypothesis of the importance of mechanical loading to maintain bone architecture.