Osteoporosis, induced by aging and long-term disuse, often occurs together with muscle loss. Musculoskeletal disuse causes severe physiologic changes and it has been proposed the synergistic effects of muscle function and bone adaptation. Bone fluid flow has been shown to be induced during mechanical loading, and is proposed to be a critical mediator of bone adaptation. The skeletal muscle may serve as a muscle pump that may mediate bone mechanotransduction via modulation of intramedullary pressure. Thus, muscular stimulation is proposed to be used to simultaneously treat both muscle and bone loss, but the optimal parameters required for such treatment is unclear. Studies have separately investigated the optimal signal parameters for bone or muscle. Insertion of recovery periods during high frequency stimulations have shown potential to reduce muscle atrophy by minimizing fatigue and mimicking physiologic contractions, and demonstrated enhancement of bone remodeling. Our preliminary research has indicated that dynamic muscle contractions within an optimal frequency range can significantly recover disuse induced bone loss. However, the optimal rest periods required to prevent muscle fatigue during stimulations are not clear. The overall objective of this study was to evaluate optimized dynamic muscle stimulations at relatively high frequency, e.g., 20 Hz, and to test the role of varying the rest duration on muscle mass and bone morphology in a functional hind limb disuse mouse model.

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