I found some more info on lactic acid- from here- http://outside.away.com/outside/bod...research-3.html



The most important difference between the dogs and people, though, may have to do with energy—how sled dogs get it and how they use it. Physiologists refer to energy sources as "substrates," and there are three basic kinds: carbohydrates, fats, and proteins. Fats have big advantages over carbs. First, they contain about twice the caloric density, so a gram of fat can supply a lot more energy than a gram of carbs. Second, they burn "cooler." But human muscle relies primarily on glucose, a carbohydrate that's stored in muscles as glycogen, becoming glucose again when it's used. Glucose burns "hot" compared with fat. "It's like the difference between regular ethyl and nitro fuel in a hemi," Bielitzki says. "You can use nitro once in a while, but you can't go forever without burning out the engine."

Fast-twitch muscles—like those used in sprinting—tap glycogen reserves in the muscles, turning it into glucose and burning that as an organic compound called pyruvate. That burning can work anaerobically, without oxygen, which is good because people are not as aerobically efficient as dogs and our systems can't deliver that much oxygen to muscle cells. But we can't burn up all the pyruvate, so it "overflows," leading to a buildup of lactic acid.



the article later states:
Most important, though, the dogs rebuild their glycogen stores. It's likely that they manage this miracle by literally switching much of the fuel they use from glucose to fat. No cell burnout, no lactic-acid buildup, no long-term depletion of stored glycogen.