The insertion of unconditioned latissimus dorsi muscle at the humerus was reattached in anesthetized goats to the first stage piston of a two stage mechanical-to-hydraulic energy convertor for a skeletal muscle powered ventricular assist device. To study a range of forces, pistons of different cross sectional areas were evaluated during thoracodorsal nerve stimulation. Each energy convertor piston was coupled hydraulically to an actuator on a Thoratec VAD (Thoratec Laboratories, Berkeley, CA) in a mock circulatory loop. Maximum force (70.1 +/- 10.8 N) was greatest for the largest piston, and stroke length (4.0 +/- 0.7 cm) was greatest for the smallest piston. However, maximum stroke work (1.2 +/- 0.5 J) and muscle powered VAD ejected stroke volume (45 +/- 17 ml) were greatest for the middle size piston. These results are consistent with a biomechanical model of whole muscle contraction that predicts that there is an optimum force that produces maximum cycle work. Thus, with a two stage energy convertor, by changing the ratio of the cross sectional areas of the energy convertor and muscle powered VAD actuator pistons, the effective mechanical advantage for the muscle can be optimized to produce more work output and muscle powered VAD flow. Skeletal muscle powered devices using such an energy convertor could provide completely implantable circulatory support free from batteries and other power conditioning hardware required with electromechanical systems.