lol, Zeke pretty much said what I was going to say. Mike too. ^^ I just think the sheer exhaustion from saving Simba from the stampeding wildebeests and the trek up the side of the cliff would have drained even poor Muffy of most of his strength. Then, combine that with the shock of having his brother tearing into him with his claws, which is why he cried out, more so in, shock, than in pain I think, but yeah.
In Simba's case, he had already witnessed Scar slapping his mother, so perhaps he was already enraged past the point of feeling pain. Like Zeke said, it was probably an adrenalyn rush that caused Simba to be able to leap up from where he was positioned at, seeing as he did have a small portion of his frame already over the edge, unlike Muffy, who was already slipping down the cliffside.
Good points though Kirstin. ^^