Trumble, Dennis R., and James A. Magovern. A permanent prosthesis for converting in situ muscle contractions into hydraulic power for cardiac assist. J. Appl. Physiol. 82(5): 1704-1711, 1997.The key to utilizing muscle power for circulatory support lies with the development of a practical scheme by which contractile energy may be collected and efficiently delivered to the bloodstream. This work describes initial in vitro testing of a prototype muscle energy converter (MEC) designed to transform the power of in situ muscle contractions into hydraulic form. The MEC resembles a simple piston pump and is designed for implant beneath the humeral insertion of the latissimus dorsi muscle. Bench tests were conducted to measure component function and to characterize device performance under various hydraulic loads. Under simulated muscle-pull conditions, MEC energy transfer capacity was found to be 170 mJ/stroke while operating at peak efficiencies (i.e., >98% of input power converted into hydraulic energy and preload work). Transfer efficiencies dropped from 96 to 38% as mean generated pressures increased from 23 to 36 N/cm² due to metal bellows flexion. These results demonstrate that a significant amount of contractile energy can be efficiently transformed to hydraulic power via this mechanism.