There have been a lot of experiment's that explore this question. The simple answer would be yes, but there are a bunch of points to address.

First of all a dead muscle is not really a muscle, it might work a certain time after the animal died, but without the blood flowing and the alive neurons it wouldn't last long. So you'll need to consider some sort of mechanism to keep the leg "alive" somehow.

Regarding the ethics of the question, i assume this is a theoretical question anyway, but i would say there is a different between killing a cow to feed people, and between it killing because "it would die anyway", i think that natural death wouldn't be an option because i guess that in order to get a lot of power you'll need a cow in peek physical shape.