Harnessing skeletal muscle for circulatory support would improve on current blood pump technologies by eliminating infection-prone drivelines and cumbersome transcutaneous energy transmission systems. Toward that end, we have built and tested an implantable muscle energy converter (MEC) designed to transmit the contractile energy of the latissimus dorsi muscle in hydraulic form. The MEC weighs less than 300 g and comprises a metallic bellows formed from AM350 stainless steel actuated by a rotary cam (440C) attached to a titanium rocker arm (Ti–6Al–4V). The rocker arm is fixed to the humeral insertion of the muscle via a looped artificial tendon developed specifically for this purpose. The device housing (Ti–6Al–4V) is anchored to the ribcage using a perforated mounting ring and a wire suture. Lessons learned through seven previous design iterations have produced an eighth-generation pump with excellent durability, energy transfer efficiency, anatomic fit, and tissue interface characteristics. This report describes recent improvements in MEC design and summarizes results from in silico and in vitro testing. Long-term implant studies will be needed to confirm these findings prior to clinical testing.