There's three types of muscles: smooth, skeletal, and cardiac. They share a lot in common but are very distinct to each other. Might not be an apples to apples comparison because fatigue is a notion taken from our experience of skeletal muscles which is not cardiac muscle. It would be interesting to compare the energy spend of each, or rather the power each is capable of. As enduring as the heart is its likely limited in its power output because its just circulating fluid against resistance and gravity. I don't know how much energy is required for that. Bench pressing a hundred pounds, what's the power in that and how many heart beats would equal that work?