How can sled dogs run more than 100 miles a day for weeks on end, while humans couldn’t possibly recover from such abuse of their muscles? A study from Ohio State University shows why (Medicine & Science in Sports & Exercise, August 2005). How long you can exercise a muscle depends on how long you can keep stored sugar, called glycogen, inside that muscle. Muscles burn carbohydrates, fats and protein for energy during exercise. They get these sources from both the bloodstream and from the muscles themselves. However, when a muscle runs out of its stored sugar, it hurts, becomes more difficult to coordinate and requires far more oxygen than usual. So a limiting factor in how long you can exercise a muscle is how much sugar you can store in a muscle, how quickly you use it up, and how quickly you can restore sugar the sugar in your muscles.
Humans take a long time to restore muscle glycogen. Top marathon runners restore muscle glycogen in anywhere from a day to several days. This study shows that sled dogs can restore muscle glycogen almost as quickly as they are fed. They were able to restore more than 50 percent of their resting muscle glycogen after two consecutive 100-mile runs even when fed a low-carbohydrate, high-fat diet. Humans could never replace muscle glycogen that fast.
The only way that you can teach your muscles to store more glycogen and preserve it better is to train by running (or cycling or swimming) lots of miles and doing long depletion runs taking more than three hours at least once a week. Doing too many depletion runs will delay recovery of muscle glycogen so that you will not be able to do the very fast short interval runs that teach your brain and muscles how to run faster.
Note: It has been brought to my attention that this article may appear to support dog sledding practices. My intent was to provide useful information about human muscles and human training techniques. I have no expertise in dog racing or veterinary medicine.