One of the problems for this is then how do you mount the heart? The human body isn't great for hardpoints. Yes, you can screw in stiffening devices, but for so

One of the problems for this is then how do you mount the heart? The human body isn't great for hardpoints. Yes, you can screw in stiffening devices, but for something that is meant to move, it gets a lot harder. Cranking a box requires that the box be locked in place, or it will move with the crank. The 'heartbox' in such a case can't be easily attached to ribs, as these move relative to each other, or to the spine. And that's the end of hardpoints in the chest. You might be able to put the pump somewhere else (pelvis?) but now you're affecting other organs as well. On top of all this, skeletal muscle tires quickly. A pump bulb arrangement might be better than a crank, but it's still the case that voluntary muscle movement isn't really designed for extended rhythmic use (eg: runners have to train a lot to be able to sustain running for any length of time)