Biancone, cold with 2 year olds, my theory why

[Gander]

Just like humans. When you run on a treadmill, you use different muscles than you do when you run on the road. When you run on a sandy beach, you use different muscles than you do when you run on a grassy soccer field.

As far as training goes, it all depends on the person running. There are olympic marathoners who have trained exclsuively on treadmills, and do not run outside until a few weeks before the actual race. Conversely there are those who never use treadmills, rather do all their running on the same surface the marathons are run on.