Does Superman always hold back
@MB25: Superman also goes all out on Darkseid (or at least, it hasn't been said that he doesn't, and would be incredibly stupid otherwise), and he once went all out on Wonder Woman, although he thought she was Doomsday at the time. I think it's also fair to say he went all out on the Imperiex Probes. That's all I can think of at the moment.
With Thor, I believe the "holding back" applies mostly when he's on earth and concerned about injuring mortals. His adventures in Asgard may not suffer from this rule. I'm pretty sure he has also gone "all out" on his powerful foes like the Destroyer, Mangog, and the Celestials. He probably uses all his strength when he fights Hercules too, although they don't actually try to kill each other in those matches. All of their encounters that I've seen stress that they are physically extremely even. Thor also killed Loki once.
Thor hit Galactus with a Godblast in Thor 161, way back in 1969 I think. It was later stated in Thor Annual 14 (so like, 20 years later they finally explained it) that it only worked was because it was channeling the power of Odin, and because Galactus had become exhausted during his battle with Ego.
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