Intramedullary pressure and matrix strain induced by oscillatory skeletal muscle stimulation and its potential in adaptation.

Intramedullary pressure (ImP) and low-level bone strain induced by oscillatory muscle stimulation (MS) has the potential to mitigate bone loss induced by disuse osteopenia, i.e., hindlimb suspension (HLS). To test this hypothesis, we evaluated (a) MS-induced ImP and bone strain as function of stimulation frequency and (b) the adaptive responses to functional disuse, and disuse plus 1 and 20 Hz stimulation in vivo. Femoral ImP and bone strain generated by MS were measured in the frequencies of 1-100 Hz in four rats. Forty retired breeder rats were used for the in vivo HLS study. The quadriceps muscle was stimulated at frequencies of 1 and 20 Hz, 10 min/d for four weeks. The metaphyseal trabecular bone quantity and microstructure at the distal femur were evaluated using microCT, while bone formation indices were analyzed using histomorphometric technique. Oscillatory MS generated a maximum ImP of 45+/-9 mmHg at 20 Hz and produced a maximum matrix strain of 128+/-19 microepsilon at 10 Hz. Our analyses from the in vivo study showed that MS at 20 Hz was able to attenuate trabecular bone loss and partially maintain the microstructure induced by HLS. Conversely, there was no evidence of an adaptive effect of stimulation at 1 Hz on disused skeleton. The results suggested that oscillatory MS regulates fluid dynamics and mechanical strain in bone, which serves as a critical mediator of adaptation. These results clearly demonstrated the ability of MS in attenuating bone loss from the disuse osteopenia, which may hold potential in mitigating skeletal degradation imposed by conditions of disuse, and may serve as a biomechanical intervention in clinic application.